IPM for Various Larvae and Moths
Note: The IPM approach uses pesticides as a last resort. The article below offers some good information on the insects' life cycles. See also Pest Management.
Sustainable Practices for Vegetable Production in the South
Dr. Mary Peet, North Carolina State Univ.
Most of the following insect descriptions are based on information in Insects and Related Pests of Vegetables in North Carolina. References to other materials are cited individually.
Aphids feed on most vegetables. They damage plants by sucking plant sap, spreading viruses, and excreting a sticky 'honeydew' that coats the plant. Sooty molds grow in this honeydew, reducing photosynthesis and making produce unattractive. The complicated aphid life cycle involves many forms. Green peach aphids overwinter as tiny black eggs deposited near dormant buds of peach, plum, cherry and other Prunus species. In the spring, wingless females hatch, feed on the new tree leaves, and give birth to live female nymphs. After two or three generations of nymphs, a winged 'spring migrant' form emerges and flies to potatoes and other summer hosts. Spring migrants produce more colonies of wingless females. When population densities get high, winged female 'summer dispersal forms' appear. These forms are most important in the spread of viruses. Even if no viruses are present, heavy feeding causes young leaves to crinkle, cup and curl downward and older leaves to wilt and die. Aphids most frequently feed on older leaves which are hard to reach with foliar sprays. With cooler temperatures in the fall, migrant females are produced that fly to Prunus trees and produce egg-laying females that mate with males and deposit overwintering eggs.
The following guidelines are suggested for monitoring green peach aphid populations on potatoes. One leaf should be picked from the lower third of a plant every 6 feet for 300 feet or halfway into a field. In southwestern Idaho, treatment is recommended when more than 40 wingless aphids are found in a 50-leaf sample for two consecutive weeks of sampling. In central and eastern Idaho, treatment is recommended at populations of 10 or more wingless aphids. Thresholds apply only to fields that are not near peach orchards, unsprayed peach trees, or in fields with many volunteer potato plants. In these fields, it may be necessary to use systemic insecticides early to control aphids, followed by foliar sprays. Systemic materials are broken down in the plant. If applied preplant on a slow emerging crop, they may be broken down before the plants emerge.
Thus when delayed shoot emergence is anticipated, systemics can be sidedressed after 75 percent of the plants have emerged but before the plants are 6 to 8 inches tall. Planting virus-free seed potatoes also reduces viruses present and their potential to be spread by aphids.
The following procedures have been developed to scout for aphids on staked tomatoes in North Carolina. The third leaf from the top of the plant on 10 consecutive plants in five locations per field is inspected to determine whether any non-winged aphids are present. Treatment thresholds depend on the type of insecticide to be used. Narrow spectrum insecticides, such as insecticidal soap, are applied when 25 percent of the leaves are infested. Broad spectrum insecticides are applied when 50 percent of the leaves are infested.
Aphids are also an important pest of cole crops. In broccoli and cauliflower, the presence of aphids in the heads makes the crop unmarketable. Early damage to the growing point of a cabbage plant distorts the head. Aphids can also be a problem on older cabbage plants. Aphids appear first on borders of the field and will generally be found there if they are present in the field at all. Scouting should take place at least twice a week and should cover all quadrants of the field. For all cole crops, aphid infestations should be treated when 1 to 2 percent of the plants are infested, and treatments should be repeated whenever aphids reappear on the crop. Small colonies can sometimes be destroyed by natural or introduced predators and parasites, especially in the spring. While both insecticidal soap and predators sometimes control aphids, neither is always effective.
Experiments in Texas tested the use of insecticidal soap and inundative releases of Chrysoperla carnea to control aphids on broccoli.
Plots were treated when aphid populations reached 10 aphids per plant. Release rates for the C. carnea were 100,000 eggs per acre. Insecticidal soap was sprayed four times during the season. Neither the C. carnea nor the insecticidal soap reduced aphid levels below those in untreated plots.
Several natural parasites and predators of aphids offer a degree of control unless killed by insecticides.Secondary outbreaks of spider mites may occur when their natural enemies are destroyed by insecticides applied to control aphids. Applying carbaryl to control chewing insects can also result in increased aphid populations.
Cabbage maggots destroy the roots, particularly of seedlings, causing the plant to become stunted and wilt. In addition to the root damage, the plants may become more susceptible to diseases as pathogens enter through lesions left by the maggots. Cabbage maggots are more likely to be a problem in cool areas and in winter or spring crops. Because maggot populations are often clumped in the field, many samples are necessary. Digging up the top 1 inch of the soil in a 5-inch diameter circle around a plant and mixing the soil with water in a jar is the best way to detect maggots. The small white maggot eggs will float to the surface.
Rove beetles (ground beetles) are an important natural enemy of cabbage maggots. They eat eggs and parasitize pupae. Two nematode species ('Hb' and 'Hc') reportedly attack maggot populations in the soil, but their effectiveness has not been tested in controlled experiments.
There are three types of cabbage worms: cabbage loopers, diamondback moth larvae, and imported cabbageworm. The most critical worm-free periods are young seedlings and heading plants. Insect damage to the growing point can cause multiple heading. In cole crops sold for the fresh market, insects or insect damage make the produce unmarketable unless occurring only on outer (wrapper) leaves which can be trimmed off. In cabbage for processing minor damage may not significantly reduce marketable yield.
In California, up to nine small caterpillar larvae per plant can be tolerated when the plants are midway between thinning and heading, but only one larvae per 25 plants can be tolerated during heading. In young plants, 25 individual plants chosen randomly should be examined per a field of up to 80 acres. When the crop is heading, five plants from five different locations in each quadrant of the field should be examined. For plants with holes in the outer leaves, heading leaves must also be checked for worms. The outside edges of the field should be checked separately for invading species.
Although the same scouting procedures are recommended for all three types of cabbageworms, it is also important to identify the species of caterpillar since insecticides are not equally effective on all types. Cabbage looper. Cabbage loopers are the most destructive of the cabbageworms. One looper larvae does approximately three times the damage of one imported cabbageworm larvae and can consume almost 20 times as much foliage as a diamondback moth larvae. The larval stage of a green moth, cabbage loopers move in a doubling motion and eat holes in leaves. As loopers mature, they move deeper into the cabbage heads. In their overwintering sites in the southern-most tier of the southern states, pupae can be destroyed by plowing under crop residues.
In applying the techniques described above to loopers, work in North Carolina suggests beginning control procedures for fresh market crops when an average of 1 to 5 larvae are found per 10 plants or when 1 or 2 feeding holes per plant are found. In addition to the field scouting methods described above for all types of worms, adult looper stages can be monitored with pheromone traps or black light traps. Bt is most likely to be effective on young, active looper larvae exposed during time of application. See the Bt section of this chapter for more details on the use of Bt. Consult Cole Crops Publication for cole crop cultivars with some looper resistance.
Diamondback moth larvae.
These small larvae wiggle actively in response to touching. They may hang from a leaf by a silk thread. They are highly destructive, feeding on all crucifer plant parts, but preferring the underside of leaves. The adult can be monitored with pheromone traps. Pheromone-impregnated strips are also being tested that disrupt diamondback mating. Control measures include use of resistant cultivars, chemical insecticides, and destruction of crop debris. Bt is effective on diamondback moth larvae but resistance has been found in several places, including Texas and Florida. All chemical controls, including Bt, should be rotated to decrease the opportunity for resistance to develop. With all insecticides, thorough coverage is required for best results. A spreader-sticker may be useful on cabbage.
Diamondback populations are also sensitive to the weather. Dry weather necessitates higher insecticide rates and scheduling of sprays every 4 days, while heavy downpours can reduce diamond-back moth and larvae populations, decreasing the need to apply insecticides. Several Bt formulations can be used on diamondback moths. 'Dipel', 'Javelin' and 'Biobit' can be used on populations which have not developed resistances to other insecticides. If growers have used non-Bt insecticides before with limited success, the insect population may be resistant. If regional entomologists confirm that resistance has developed, Bt formulations such as 'MVP', 'Xentari' and 'Agree' should be substituted.
Trap cropping with white mustard or rape strips has been an important method of controlling diamondback moth in some areas. In India, strips of mustard are planted every 15 to 20 rows of cabbage to attract diamondback moths. The larvae on the trap crop became heavily parasitized while the cabbage is only lightly infested. In order to trap new populations of diamondback moths as they enter the field, however, the mustard must be grown throughout the cabbage season.
White butterflies lay rocket-shaped, white to yellow-orange eggs singly on leaf surfaces of cole crops. These develop into large, velvety green larvae which lie along leaf veins for camouflage. Damage is easy to see, however: leaves eaten from the margins inwards. Scouting and control are similar to that for cabbage looper, discussed above.
Although not as effective as Bt, extracts of tansy have been shown to reduce egg-laying by two-thirds and slow the development of the imported cabbageworm. The tansy extract was made by grinding 5 g of fresh tansy leaves in a blender with 100 ml of distilled water and filtering the mixture through two layers of cheesecloth.
Colorado Potato Beetle.
The voracious appetite and impressive reproductive capacity of the Colorado potato beetle (CPB) make it an important pest of vegetable crops. Both adults and larvae feed on the leaves. Potato, tomato and related weeds are quickly reduced to stems and skeletonized leaves. In potato, the CPB also transmits spindle tuber disease and bacterial ring rot. In tomatoes, defoliation increases the incidence of sunscald on exposed fruit. A serious and longtime pest elsewhere in the United States, CPB populations have recently increased in the southern states. In North Carolina, for example, potato fields are now scouted for this pest beginning in April.
Over a 4-week period, a female CPB can lay over 300 bright orange, oval eggs in orderly rows of 10 to 30 on leaf undersides. Eggs hatch into larvae in 4 to 9 days. Larvae molt 4 times. About 3 weeks later, larvae go through a final molt and enter the soil to pupate. They emerge as adults in only 5 to 10 days. Two to three generations can be produced in one year, and adults overwinter in the soil.
Mulches, trenches, and no-till are promising techniques for reducing beetle immigration from neighboring fields. In Virginia, a 2-to-4-inch-deep straw mulch around the field margins at the time potato shoots emerged lowered CPB populations because it prevented newly emerged beetles from walking into the field. No-till planting into killed rye also lowered Colorado potato beetle populations in Virginia tomato fields. Acting as a physical barrier, the rye lowered the speed at which invading beetles established themselves. Migrating Colorado potato beetles can also be trapped by lining a 1-foot trench between the crop and the overwintering site with black plastic. Rotation is also helpful in reducing CPB levels.
Rotation to non-susceptible crops delayed the arrival of the first generation of adults by 7 to 10 days in New Jersey. Research in Massachusetts showed that rotating fields out of potatoes for a year slowed colonization by several weeks and lowered Colorado potato beetle densities. In planning rotations it appears to be important to rotate to fields some distance away. In North Carolina, researchers found that adult and egg mass distributions were clumped near the field edge closest to fields planted to potatoes the previous year.
Fields further than 750 feet from fields planted to potatoes the previous year had lower densities of potato beetles than closer fields. Since the beetles walk to the field, the field edges should be monitored closely. The first sprays can be applied only to the edges of the fields. Monitoring should begin early both to detect immigration from neighboring fields and because a Virginia study showed potatoes to be most sensitive to defoliation by beetles in the early, pre-bloom period. Yield was significantly reduced when 20 percent of the foliage was removed before blooming. Up to 30 percent of the foliage could be removed during the bloom period without serious effects on yield.
After bloom, potatoes were much less sensitive to beetle damage and up to 50 percent of the foliage could be lost in dry weather or up to 60 percent in normal weather without loss of yield. The authors of the Virginia study suggest a monitoring sample of at least 5 potato plants from each of 10 sites, following a V shaped path through the field. If treating with Bt, fields should be resampled after two sprays. In California, monitoring in potatoes consists of tagging 10 egg masses and beginning to spray when 50 percent have hatched. In tomato, an action threshold for Colorado potato beetle is an average of two larvae or overwintering adults or three summer adults per plant when sampling 20 plants in each of 10 locations.
In potatoes, chemical controls include soil-applied systemic insecticides early in the season to kill overwintering adults and foliar treatments later in the season to kill emerged adults and larvae. The CPB continues to develop resistance to most insecticides, making it important to monitor for resistance, use labeled rates, and rotate classes of insecticides. If pyrethroids are to be used as part of a rotation, they should be applied early in the season since pyrethroids work best in cool weather.
Two types of Bt are effective against the larval stage of the Colorado potato beetle: Bt var. tennebrionis and Bt var. san diego . Both are more effective on small larvae and should be applied no later than 6 days after peak egg hatch. Peak egg hatch is determined either by collecting eggs from the field and monitoring them in a shaded, ventilated, outdoor container or by determining when 190 degree days have passed from appearance of the first egg mass. Another method is to treat with Bt if 10 percent of the plants in the field have at least one egg mass. Bt is more effective at higher temperatures. Temperatures over 80 degrees F caused mortalities of second instar larvae of 98 percent or more while treating at 65 degrees F caused only a 45 percent mortality. Since complete coverage of Bt is essential for it to be effective, drop nozzles are often used. Spray additives have no effect, however. Propane flamers have also been used to 'toast' overwintering CPB adults, in hope of destroying them before they lay eggs. The flamer is used after shoot emergence but before shoots are 6 to 8 inches tall. In experiments on Long Island, New York, best control was obtained on warm, sunny, calm days when the beetles were actively feeding on top of the plants. The flamer was used repeatedly on the field edges during the time the overwintering beetles migrate from the edge of the field. One or two passes in the field during that time also controlled overwintering beetles. In addition to killing larvae, the flamer reduced egg hatch by 35 percent.
Corn earworms are pests of many crops including corn, beans, soybean, cotton, and tomato. Corn. In corn, the night-flying, light-brown or buff colored adult moth typically lays its eggs on the silks. The tiny, dome-shaped eggs hatch in 2 days in warm weather but take up to 10 days to hatch in cool weather. The young larvae work their way down the silk to feed on the ear. Sometimes eggs are laid on the tassels and the larvae migrate to the ears.
After 12 or 13 days, larvae leave the ears by boring out the side or by crawling out the tip. They fall to the ground and pupate 3 to 5 inches deep in the soil.
Except for the overwintering forms, pupation requires only 12 days, for a total life cycle of about 30 days. The adult form, or moth, can emerge as early as late March, and be present as late as mid-November, producing at least three generations each year in North Carolina. Earworms may be abundant on any planting, but are usually more serious late in the season.
The following treatment schedules were developed in Ohio and are presented in Table 4.4 as an example of an IPM strategy to monitor corn earworm with phermones. Spray frequency depends on temperature and moth counts from a mesh trap with a pheromone lure. Lures should be replaced every two weeks and old lures removed from the field.
Days between sprays based on corn earworm moth counts and air temperatures
|Days between sprays|
|# of moths in 22-inch trap||at temperatures above 80 degrees F||at temperatures below 80 degrees F|
*(if other pests are present)
In applying insecticides to control the corn earworm in corn, a boom-type sprayer with drop nozzles is much more effective than a mistblower (airblast sprayer) because boom sprayers directed the spray to the ears while the mistblowers sprayed the entire crop canopy. In a Massachusetts study, crop losses to earworms were 1 percent using boom sprayers compared to 10 percent using mist blowers.
One-time, per-acre treatment cost for oil on corn (compare with $80/acre for sprays over 13 weeks for conventional treatment)
|Labor, 8 hrs @ 5.00/hour||40.00|
|Oil, 2 gal @ 6.00/gallon||12.00|
|Possible additional expenses:|
|Pyrethrum, 8 oz @ 78.00/gallon||4.88|
|Bt, 1 oz @ 10.00/pint||0.65|
Prior to the development of synthetic insecticides after World War II, oils were sometimes used for earworm control. Experiments in Oklahoma tested the conditions under which oils and oil-pesticide blends could be used to control corn earworm on small acreages. Injecting oil into the neck of the ear (where the silk emerges from the husk), with a standard oiling can was more effective than simply depositing it on top of the silks. Time of application was also critical. Oil had to be applied 2 or 3 days past the full-brush stage (when silks are at maximum extension from the eartip). Oil applied too early inhibited pollination, but oil applied too late was ineffective. In this experiment, mineral or vegetable oils applied to sweet corn ears reduced corn earworm damage to the eartip but not below the eartip.
Although oil suppressed pollination of the kernels at the tip of the ear, this did not appear to affect marketability. Addition of Bacillus thuringiensis (Bt) or pyrethrum was more effective than the oils alone in some but not all trials. Costs were about the same or less than those for the conventional treatment . Problems with this method are identifying the proper time of oil application, loss of developing and therefore untreated secondary ears and the difficulty of applying oil to ears of very short cultivars. Costs from the Oklahoma study are similar to those reported by a Florida grower (See Grower Example 2).
Grower Example 2
|Wade Howell, who raises 40 acres of sweet corn on his organic vegetable farm in
Florida estimated $50/acre for labor plus $12.00 for materials to apply 1.5 cc of mineral
oil with the Bt formulation, Dipel and ryania to the ears at the stage when the silk just
starts turning brown.
Using a veterinary vaccination pump connected to the oil mixture, he reported one person could treat an acre a day.
Tomatoes. When the corn earworm attacks fresh market and processing tomatoes, it is called the tomato fruitworm. In fresh market tomatoes, affected fruit can be removed in the grading line, but canners often reject even lightly infected loads of processing tomatoes because it is difficult to prevent insect contamination in canned goods. Fruitworms also feed on leaves where they can cause distortion. They also bore into stems.
Tomato fruitworm losses ranged from 5 to 25 percent in Florida and were approximately 4 percent in Georgia in 1987. In North Carolina fruitworms are considered the most common and potentially the most damaging insect pest of tomatoes.
Within 1 to 6 days after hatching from eggs laid on the leaves and fruit, fruitworm larvae bore into the stem end of green fruit. Normally, ripe fruit are not attacked. The deep, watery fruitworm holes contaminated with their feces are easily distinguished from the shallower and drier holes made by armyworms. A single larvae can injure several fruit before pupating. While overwintering pupae can be destroyed by disking tomato plants and destroying infected fruit, adults move in from corn fields the next season and lay eggs on tomato leaves and fruit. The moths actually prefer corn to tomatoes for egg-laying but will visit tomatoes early in the season before silking and after the corn silks have all dried up.
Sprays are not effective once larvae penetrate the fruit so weekly monitoring for the small, white fruitworm eggs is necessary. In North Carolina, scouting recommendations for fields of less than 5 acres are to examine leaves from the top of the plant down to the most recent fully expanded leaf in up to 10 consecutive plants on up to five locations. For larger fields, one sample site should be added for each additional 3 acres. If any eggs are present, the field should be treated. Pheromone traps placed on the border of the field can be used as a check on egg sampling.
Vegetation within 3 feet of the trap should be removed. Trap catches of more than 20 moths per week indicate potential for egg laying on tomato but treatments should be based on visual inspection for eggs rather than solely on trap counts.
Synthetic chemicals called for by egg mass monitoring should be applied every 7-to-10 days.
Biological controls for the tomato fruitworm include Bt and Trichogramma wasps. Bt must be reapplied after 5 to 7 days. Trichogramma is a parasitic wasp which lays its eggs in the eggs of a number of insects, including fruitworms. Trichogramma must be released when fruitworm eggs are first found and then twice weekly for the next five weeks. In southern California, control of tomato caterpillar pests was achieved by releasing 15,000 wasps/acre twice a week for 10 weeks.
Beans Corn earworm can also be a problem on processing snap beans. In Arkansas, Oklahoma and Missouri, earworm is the major insect pest of the spring snap bean crop. Suggested monitoring practices for larvae were 300 sweeps of a standard insect net. Sampling began at first bloom, and continued every 3 to 4 days. Sprays were recommended if any corn earworm larvae were detected.
The cowpea curculio, the most damaging pest of southern peas, punctures the pod, leaving a small scar on the pod and a speck on the peas. They attack later plantings more than earlier ones. Crop rotation and destroying crop residue will lower curculio populations.
European Corn Borer.
European corn borer (ECB) is a pest of many crops including corn, peppers, potato, and snap bean. In corn, mature European corn borer larvae overwinter in stalks, ears, stubble and other plant residue left in the field. Adults emerge and lay eggs in masses on leaf undersides. In 3 to 10 days, larvae hatch and feed on the leaf surface. This is the only time they are susceptible to chemical control. Within 2 to 5 days they enter the midribs of leaves and bore into the stalk or ear.
Borer-weakened stalks frequent-ly fall over and in standing stalks, movement of plant nutrients is disrupted, reducing yields. The 2 to 5 days during which larvae feed on the leaf surface is the only time they are susceptible to chemical control so early season scouting for egg masses on the leaf underside or using black lights to monitor adult moth populations is critical. In Ohio, adults appear after 450 degree days (base of 50 degrees F).
Chemical control must begin 7 to 10 days after a moth flight or 5 days after eggs are found. Bt shows some promise for ECB control and natural parasites also exist in corn-growing areas.
Peppers. In peppers, European corn borer is particularly destructive because it is a season-long pest, especially in corn production areas and is not easily detected in the grading line. In pickled peppers, worms may not be noticed until the consumer opens the jar to find a worm floating on top. For this reason, processors may reject a whole load based on evidence of borer feeding.
In pepper fields in the south, up to 4 or 5 European corn borer generations occur each year. Eggs are laid in masses of 20 to 30 on the underside of pepper leaves. The small larvae feed externally 2 or 3 days and then bore a small hole into the fruit where they feed for several weeks before exiting to pupate in the soil. Population increase is most rapid after mild winters and in wet summers. During the period when eggs hatch, however, dry conditions favor survival.
The first generation ECB moths are usually not a serious problem on peppers but second and third generation moth activity may necessitate weekly sprays to protect developing fruit, especially in processing peppers. Scouting for egg masses is difficult because the plants are packed together tightly and egg masses can be anywhere on the undersides of the leaves. For this reason, adult populations are usually monitored, rather than the eggs or larvae. In New Jersey, the action threshold is when an average of 5 moths per night are caught in black light traps. In North Carolina, the recommendation is to begin treatments 7 to 10 days after finding 25 moths in a black light trap over a 5-day period or about 5 days after egg masses are found.
Potatoes. The larvae of European corn borer damage potatoes by feeding in the stems, on exposed tubers, and occasionally on tubers still in the soil. In the fall, potato residue and weeds should be removed or destroyed by plowing, or shredding to reduce overwintering sites.
Three monitoring strategies have been developed in North Carolina: tag 10 egg masses and spray when 50 percent have hatched, as indicated by a black head; spray when 5 out of 25 potato stems show entry points; or spray when 25 or more adult moths are caught in black light traps in a 5-day period. In most years, even high levels of European corn borers do not cause significant yield losses of potatoes.
Snap Beans. ECB larvae bore directly into the pod, making feeding damage difficult to detect. As with peppers for processing, entire loads of beans for canning may be rejected at the plant. Beans are susceptible to borers from the bud stage until one week before harvest. Monitoring must begin when beans are in the late bud to early bloom stage, followed by insecticide application if black light traps capture an average of 5 moths/ night.
Unlike the true armyworm, which feeds primarily on corn and other grasses, the fall armyworm will feed on just about any plant. Damage is especially severe to late sweet corn and field corn, but the fall armyworm will eat kale, collards, turnip greens, cabbage, broccoli, spinach, snap beans, tomatoes, soybeans, potatoes, sweetpotatoes, cucumbers, and many ornamentals.
Each female moth deposits approximately 1000 pinkish-white eggs in clumps of 100-200 on corn leaves. In warm weather, these eggs hatch in three to four days and the armyworms reach maturity in two to three weeks. Full-grown armyworms are about 1.5 inches long and vary in color from light tan or green to almost black.
Generally speaking, the critical time to prevent damage to corn is before the tassels and ear appear.
Unfortunately, however, by this time, the worms are feeding deep within the leaf whorls where they are difficult to detect. As the tassels emerge and ear shoots are formed, the armyworms move in back of the ear shoots and may completely destroy the young ear before silks are formed. Once in back of the ear shoot or inside the ear, they cannot be controlled and the crop will be heavily damaged. Potential yields of well over 100 bushels per acre of field corn can be reduced to 10 to 15 bushels or less by fall armyworms.
In Florida, insecticides are applied by air everyday from tassel until harvest for fall armyworm control. In NC, the most effective method to prevent armyworm damage is harvesting before August. In Virginia sweet corn should be harvested before army worms reach uncontrollable levels the second week in August.
These small, black, jumping beetles are a common problem for eggplant but they can also feed on many other vegetable crops, including tomatoes and young cole crops, leaving small, round 'shotholes' on the leaves. Flea beetles are especially damaging to Chinese cabbage, which is sometimes used as a trap crop to keep flea beetles off other crucifers. Planting after soil warms up allows the crop to outgrow flea beetle damage. Good sanitation practices and elimination of weed hosts will lessen damage. New Bt formulations are also available. Flea beetle attack is sudden and can destroy young plants, so fields should be scouted daily. Three to four generations can be produced annually. Sometimes just treating the outside rows of a field is effective, since flea beetles migrate in from weedy areas.
Flea beetle larvae also damage plant roots. Flea beetle larvae leave holes and ragged tunnels in sweetpotato storage roots, especially in weedy fields. In Virginia, populations high enough to yield 3 to 4 adult flea beetles in 100 sweeps of a net through the field produced enough larvae to cause severe economic damage to a sweetpotato crop.
Larvae also feed on the roots of tomatoes and other crops but generally do not cause significant damage. Sweetpotato cultivars with flea beetle resistance are listed in the Sweet Potato Crop Profiles.
Vegetable leafminers feed on squash, okra, pea, radish, tomato, bean, cabbage, turnip, potato, spinach, watermelon and pepper. Leafminer larvae tunnel inside leaves to feed. Photosynthate area is reduced, and tunnels offer entryway for diseases.
Early infestation reduce yield and fruit size, and expose tomato and pepper fruit to sunscald if leaves drop off. Mature larvae emerge from leaf tunnels and drop to the soil. Except for Southern Florida and Texas where they breed and feed year-round, they overwinter as pupae. They emerge from pupation as small black and yellow flies.
The entire life cycle can take place in just two weeks in warm weather. At least five generations can occur each summer. Tomato cultivars with curled leaves, such as the VF145s, are less susceptible to leafminer damage, as are Butternut 23 and Cozella squash.
Leafminers are often controlled by naturally occurring parasitic wasps. In fields treated for pinworms or other insects with insecticides such as methomyl, parathion and azinphosmethyl, however, the natural wasp predators are often destroyed, leaving the field vulnerable to infestation by leafminers, which are not controlled by these insecticides. These parasitic wasps are also killed by weekly carbaryl applications.
To monitor pole tomatoes for leafminers, 12-to-15-inch trays should be placed randomly in the field. Mature larvae will drop off plants and pupate in the trays. In California, recommended treatment threshold is when an average of 10 pupae per tray per day accumulate over 3 to 4 days. In Florida, the recommended threshold on 2 acre plots was a total of 25 mines after checking the terminal 3 leaflets of 6 plants, with at least 6 containing live larvae. In NC, the leafminer is not considered a major pest of tomatoes, so no threshold levels have been developed. Growers are advised to check for mined leaves but to apply insecticides only when populations become large. On small plantings, removing and destroying leaves containing live larvae keeps populations at a manageable level.
Mexican Bean Beetle.
Adult Mexican bean beetles look like lady beetles but have 16 black spots on their back. Adults and larvae feed on snap bean leaves, leaving a bronzed lacey leaf. The adults overwinter in weeds, hedges and in crop residue. Although variable, damage is generally most severe in July and August. In the mid-Atlantic region, the treatment threshold is one beetle per plant. Alternatively, thresholds are 20 percent defoliation on pre-bloom plants or 10 percent defoliation at bloom. Tolerant cultivars are listed in Crop profiles: Beans.
The pepper maggot overwinters in the soil in the pupal form. Larvae feed inside the pepper fruit causing the fruit to turn red prematurely, and decay, or drop off. Pepper maggots are detected by checking for dropped or deformed fruit, and using yellow sticky cards to trap adult flies. Control by rotating fields.
Pickleworms migrate north from Florida and other tropical areas each year, becoming a severe late-season problem in many cucurbit production areas of the South. The larvae burrow into flowers, then migrate into the developing fruit. Once they are in the fruit, they are hard to detect until they emerge, leaving ragged frass-filled tunnels behind.
Pickleworm populations can be lowered by planting early, plowing deeply before planting and rotating crops. Chemical control measures must be started as soon as pickleworm adults appear since insecticides cannot reach larvae inside the flower and developing fruit. Pheromone lures for adult pickleworms and monitoring guidelines are under development.
Researchers in South Carolina found that in 42 cucumber fields over 2 years, where moths and larvae were present the adults were trapped before or during the same week that larvae were first detected in the crop. Withholding of spray treatments until moths were caught saved an average of one treatment per field with no loss of control. Resistant cultivars of cucumber and squash are listed in the respective pages.
Seed Corn Maggots.
These tiny, white, legless maggots reduce stands of beans, corn, melons, cabbage and cucumber by attacking seeds and seedlings. Maggots hatch from fly eggs laid in decomposing plant material including organic amendments and cover crop residues (if less than 30 days has elapsed since the residues were incorporated).
Squash bug adults overwinter in crop debris. After emerging, adults lay orange-yellow eggs in rows on the undersides of leaves. After hatch, larvae feed on the fruit and undersides of leaves of most cucurbits. A severe infestation causes rapid wilt. Usually there are one to one and a half generations per year.
Since the presence of egg masses is highly correlated with the presence of larvae, monitoring for egg masses has been found to be an effective means of timing insecticide applications in Oklahoma. The few chemical controls available for squash bugs are most effective if applied to newly hatched larvae. Sixteen to twenty plants were sampled twice weekly, and the first insecticide was applied when an average of one egg mass was found per plant. This was followed up with treatments every 208 degree days (base of 56 degrees F) (See Appendix 2 for a description of heat units). Yields were equivalent to those from plots receiving weekly sprays. Other control measures include rotation and destroying crop residues.
Squash Vine Borer.
Squash vine borer is a pest of cucurbits, particularly squashes. Small, flattened brown eggs are deposited singly on leaf petioles, stems, and fruit.
Soon after they enter the stem or fruit to feed, the larvae extrude sawdust-like frass from bore-holes in the stem or fruit. Damaged stems wilt and die and fruit are unmarketable. The larvae burrow into the soil to pupate and can overwinter in the soil as either larvae or pupae. There can be two generations per year.
Because they feed inside the plant, squash vine borers are very difficult to control. Injecting Bt or other insecticides in the stem, placing collars on the stems, and crushing the eggs are too time consuming on large acreages. Once vine borers become established in a field, growers unwilling to use chemical insecticides may find it necessary to switch to other crops.
Striped and Spotted Cucumber Beetle.
Striped and spotted cucumber beetles are closely related species which appear in late April in North Carolina. Adult cucumber beetles feed on the leaves and stems of young cucumber and squash plants while larvae burrow into the roots. If conditions are such that plants become established quickly, cucumber beetles do less damage. Even where direct feeding damage is minimal, however, adult beetles can transmit bacterial wilt or mosaic virus as they feed. This is a particular problem in fall crops where the populations of infected plants and insects have built up over the summer.
When larvae of the spotted cucumber beetle scar sweetpotato roots, they are referred to as the southern corn rootworm.
In fall cucurbit crops in North Carolina, the threshold population for control is 5 beetles in 10 feet of seedlings. Seedlings are often sprayed with carbaryl or Thiodan at emergence, however, to prevent damage. Cucumber beetles can be trapped with sticky adhesives such as Tanglefoot . The traps should be 6-by-8-inch yellow boards attached to stakes 2 feet above ground level. To attract beetles to the adhesive-covered surface, some type of attractant is needed. Eugenol, a compound found in allspice oil, bay oil, and clove oil, is an effective bait and can be purchased commercially in small quantities.
A new bait product, AdiosTM, is also available. This product uses cucurbitacin, a secondary compound found naturally in cucumbers, to attract adult beetles, which are then killed by the carbaryl in the bait. The bait can be diluted with water but should not be mixed with other products. Bait products may have an advantage over other insecticides in preventing the spread of bacterial wilt since beetles presumably die before feeding on and infecting the plant. Baits are also less damaging to beneficials. Growers may soon be able to use bioengineered cucurbits with resistances to the most common mosaic and yellows viruses, which will make it less vital to control fall cucumber beetles.
Adult weevils are antlike and very small with dark metallic blue heads and wings and reddish orange bodies and legs. Adults and larvae feed on storage roots both before and after harvest. Deep-rooted cultivars such as Porto Rico show less damage. The sweetpotato weevil is present in some Louisiana production areas but has not yet become established in other production areas in the South. Use of seed stock from non-infested locations is critical.
Thrips are a problem in eggplants as well as in onions, beans, crucifers, and cucurbits. They will also feed on almost any flowering plant and often are brought to the field from infected greenhouses. Western flower thrips transmit tomato spotted wilt virus, a serious disease in tomatoes.
Almost invisible, these tiny narrow-bodied yellowish insects feed out of sight in the flower but the damage caused can blemish the tomato fruit sufficiently to reduce it in grade. The entire life cycle takes 12 days in hot weather or up to 44 days in cool weather. Eggs are hidden within plant tissue, usually near the flower or buds. Several generations of larvae feed on the developing tissues, then move down the plant to pupate in the soil.
The pupal stages does not feed and develops in the soil, in plant litter, or on protected places on the plant. Thrips also feed on eggplant. As with tomato, superficial scars on the eggplant fruit may make it unmarketable.
Thrips can be monitored by examining flowers or leaves, using sticky traps or by shaking the flowers over a white cloth or card. Affected plants appear stunted and distorted with silvery to black spots on the leaves. Nearby small grains and some weeds are alternate hosts, and should also be monitored. Thrips are especially likely to invade vegetables when adjacent grain fields are harvested.
Thrips are difficult to control because pesticides do not reach their feeding places deep in the flower or bud. Soaps and oils have been used with varying success to control thrips. Insecticidal soap has reduced thrips feeding damage by as much as 49 percent in experiments in Canada. However, it had no effect on transmission of tomato spotted wilt virus. The type of feeding that transmits the virus is apparently not the same as the feeding that causes visible damage. Oil, Wilt-Pruf and Dow Corning 36 (polydimethyl siloxane emulsion) significantly reduced transmission of tomato spotted wilt virus by thrips although oil had no effect on feeding activity. Dow Corning 36 was the only product that also reduced the number of thrips nymphs.
Any insecticides used should be applied every other day for 10 days and materials should be rotated to avoid buildup of resistance by thrips. In greenhouses, biological control programs using insidious plant bugs and predaceous mites have been used effectively and fine-mesh screening will also exclude thrips.
The tomato pinworm is a serious pest in warm-winter areas such as Florida and California because populations are already established in the field. The pinworm does not overwinter further north than Florida, but populations brought in on transplants can increase rapidly.
The adult moth lays eggs on the leaf underside. The first two larval instars tunnel within the leaves, creating a blotchy mined area. The third instar larvae emerge from within the leaves to feed on the leaf surface, protecting themselves in a leaf which they hold together with webbing. The larvae which crawl into the fruit near the stem attachment and tunnel through the fruit walls cause the most economic damage.
The small pinholes left as they exit the fruit act as entry points for decay organisms, especially when overhead irrigation is used. Fourth instar larvae mature and drop into the soil to pupate. Adults cannot emerge from pupae buried more than 2 inches deep, so crop rotation and disking after harvest offer some control.
In California, pheromone traps are used to monitor populations of tomato pinworm moths, but in North Carolina and Florida, pinworm larvae populations are monitored by inspecting the leaves. Florida recommendations are to sample the lower leaves of 6 to 12 plants on every 2 acres. If the number of larvae per plant average more than 0.67, pre-treatment of the field is recommended. In North Carolina, recommendations are to inspect foliage, particularly the lower leaves, and treat when the larval population averages 0.5 per plant. Established populations must be treated weekly to protect the fruit once threshold levels are reached.
Mating disruption with pheromones has shown promise as a control strategy in areas with established populations. In areas such as North Carolina and Virginia where pinworms come in on transplants and occurrence is sporadic, fewer applications may be necessary and pheromone control strategies are not recommended.
Wireworms are the larval form of a dark-brown click beetle. After a short pupation, adults emerge and females lay about 175 eggs. To escape the hot, dry summer and cold winter, wireworms burrow deep into the soil. The orange-brown larvae, which take 4 to 5 years to mature, are very destructive when they feed on developing potato tubers, seed pieces, and roots. Young potato tubers injured by wireworms often become misshapen.
Wireworms also feed on sweetpotato roots. They pose the greatest threat in dry years, in heavy soils, and in fields that were previously grassy and undisturbed. Wireworm larvae leave irregular dark and fibrous holes 1/4-inch deep in the roots. Resistant sweetpotato cultivars are listed in Crop Profiles: Sweetpotato. Susceptible cultivars planted in the same field with resistant cultivars have less damage than when only susceptible cultivars were planted. Small roots, such as found with higher plant populations, received less damage than large roots.
Controls include use of granular insecticides applied at planting time. Insecticides should be applied when the population level at the 6-inch depth reaches one larvae per square foot of soil. Once wireworm populations have been reduced, they usually remain low. Crop rotation will help prevent high populations. In fields that already have high wireworm populations, grain rotations are recommended.
Although 600 species of grasshoppers occur in the United States, common pest types are the differential, the redlegged and the two-stripped. They are general feeders and occur annually in forages and some vegetables such as asparagus, but infestations are not usually serious enough to require control.
Grasshopper eggs, about the size of rice kernels, occur in oval, elongate or curved pods made out of soil particles. Eggs may be white, yellow green, tan or brown. Nymphs are white immediately after hatching, but develop adult coloration. These three grasshopper species overwinter in the soil as eggs and hatch from April to June. Nymphs feed and grow for 35 to 50 days, molting five or six times during this period before becoming adults and mating.
Development is most rapid when the weather is warm and not too wet. Adults feed after mating and in about 2 weeks females lay eggs in pods containing 25-250 eggs, depending the species. Except for the redlegged grasshoppers, which have two generations per year, the economically important species generally only have one generation annually.
Whitefringed Beetles feed on at least 385 plant species, but are most likely to damage plants with taproots (rather than fibrous roots) and smooth, broad, leaves, such as corn, potato, velvet bean, okra, cowpea, sweetpotato, melons, bean, cauliflower, and cabbage. As foliage feeders, damage is relatively inconspicuous, consisting of sawtooth cuts on the leaf margins. On sweetpotatoes, however, beetle feeding results in roughened holes and characteristic surface channels with rough ridges.
The beetles usually overwinter as larvae (called grubs), although eggs may survive the winter in protected locations. The 1 mm-long oval white eggs turn pale yellow before hatching. Larvae are slightly curved and yellowish-white with a light-brown head. Legless, they can measure up to 1/2 inch in length. Larvae feed on sweetpotato roots until mature, then burrow 2 to 6 inches into the soil. After a pupation period of 13 days, typically ending in July and early August in North Carolina, adults emerge.
The adults are flightless black beetles about 1/2 inch long which have two lengthwise stripes and a marginal fringe of white hairs. Females deposit eggs in clusters of 15 to 20 in the soil around the base of host plants. Eggs hatch 17 days later, unless temperatures are too low and larvae feed until slowed by cold weather. There are no males. Only one generation occurs each year.
Populations increase in areas of summer legumes, but grasses, including corn and small grains provide little food for adults, and thus make a good rotation crop for sweetpotatoes. Other cultural control practices include avoiding infested fields, using tolerant cultivars, and incorporating crop debris. Few chemical options are available for control.
The silverleaf whitefly is a worldwide pest in tropical and subtropical areas and in greenhouses. It is becoming an increasingly important pest in vegetable production in the southern states and has been reported in most families of vegetables. Tomato is the most frequently reported host, however. Like aphids, whitefly damage is threefold. Sap-feeding by the adult and nymph stages weakens the plant, rains honeydew on the plants so that sooty mold develops, and introduces viral diseases. In southwestern states, the silverleaf whitefly is a vector for several important viral diseases of lettuce and melons. Chlorotic spots sometimes appear at the feeding sites on leaves and heavy infestation causes leaf wilting. In relation to its economic impact, relatively little is known about economic thresholds or physiological aspects of damage. When silverleaf whitefly feeds on certain curcurbits, it induces a disorder called squash silverleaf. This disorder includes a yellowing of leaves followed by a progressive silvering of the leaf tissues between the veins. Finally the whole leaf surface can be bleached, and fruit quality and yields decrease. This disorder is not well understood but is not caused by feeding of the sweetpotato whitefly, which is otherwise closely related. What is now called the silverleaf whitefly (Bemisia argentifolii) was previously called Bemisia tabaci, biotype B. The sweetpotato whitefly is now known as Bemisia tabaci Biotype A. Feeding by the sweetpotato whitefly causes some chlorosis, but not the characteristic silvering.
The time required for the light-colored eggs, which are attached to the undersides of new leaves, to develop into adults varies from 16 to 38 days, depending on temperature, humidity, and host plant. Generally females lay 80 to 100 eggs, but there are reports from Israel that repeated insecticide application produced a super-female strain which lays up to 300 eggs. Forms that hatch, called crawlers, feed on the leaf underside, then molt into scalelike, sucking nymphs. The nymphs are glassy to opaque yellowish with a flattened scalelike body. After several molts, the nymph becomes a nonfeeding, plump, light-colored pupa, within which an adult whitefly develops. Adults are slightly yellower and smaller than other whiteflies, and appear more slender.
Several factors make control of silverleaf whiteflies difficult. The eggs and older immature forms are resistant to many aerosol and insecticide sprays. Adults are extremely resistant to dry pesticide residue.
Pesticides will not be effective unless applied to leaf undersides and re-applied regularly so that all the immatures are exposed as they develop into adults. The cycle cannot be broken if any of the immature stages escape exposure after turning into adults. Some of the pyrethroid pesticides are somewhat more effective and need not be applied as often. Neem seed extract is not as acutely toxic as some of the synthetic pesticides, but has the advantage of being toxic to young nymphs, inhibiting growth and development of older nymphs, and reducing oviposition by adults. One study showed that Sunspray oil and a broad-spectrum pyrethroid product were effective at killing adults both when wet and after drying. Both repelled adults for 5 to 7 days, preventing feeding and reducing numbers of whitefly eggs.
Other materials tested, Ultra-Fine Spray Oil, an insecticidal soap, and a plant-derived surfactant (sucrose ester extract of Nicotiana gossei), garlic-based repellent, were less effective, and the garlic barrier product was no more effective than water.
See NCSU Extension IPM section for this and more data.