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Bacillus thuringiensis (Bt)

How To Decide Which Bt (Bacillus thuringiensis) Product To Use For Worm Control In Vegetables , #3-5-93 (Updated 6/96)

Geoff Zehnder
Extension Vegetable Entomologist

Bill Moar
Research Entomologist

Bacillus thuringiensis (or Bt) insecticides have been around for over 20
years, ever since DiPel was introduced by Abbott Laboratories. Early Bt
formulations gave acceptable worm control under optimal conditions, but
were generally not considered as effective as synthetic insecticides. This
situation has dramatically changed in recent years due to the introduction
of new Bt strains and formulations that are highly effective, and because
insects in some areas have become extremely resistant to synthetic
insecticides, particularly the pyrethroids. In addition, some synthetic
insecticides used in vegetables and/or specialty crops are being lost
because the manufacturers are not willing to spend the money necessary for
re-registration. As a result, Bts actually have replaced synthetic
insecticides for control of worm pests in some vegetable-growing areas. The
purpose of this article is to explain how Bts work, and to present some
guidelines for choosing the best Bt strain or formulation for your
particular vegetable insect problem.

What is Bacillus thuringiensis (Bt)?

Bt is a naturally-occurring bacterium that is found throughout most regions
of the world. It occurs naturally in soil and in other common environmental
habitats where insects are found. The primary component of Bt that is toxic
to insects is a crystalline protein (toxin). The Bt toxin is harmless to
humans, birds and other mammals, but is lethal to many species of pest
insects. Bts also are harmless to bees and other beneficial insect species
(e.g., predators and parasites). Different strains of Bt exhibit
specificity to caterpillar (worm) species in the Order Lepidoptera, or to
beetles or flies in the Orders Coleoptera and Diptera, respectively.
Recently, "high-tech" Bts have been developed in which toxin-producing
genes from different strains have been incorporated into one Bt product.
These "transconjugated" Bt strains have activity against two or more insect
pests (e.g., European corn borer and Colorado potato beetle).

How Bt Works

Bt must be eaten to cause mortality. The Bt toxins dissolve in the insect
gut and become active in the presence of high pH (insect gut contents are
highly alkaline). The toxins then attack the lining of the insect gut,
rupturing the cell walls and allowing the gut's highly alkaline contents to
spill into the insect's circulatory system (hemocoel). Bt spores germinate
in the intestinal tract, resulting in septicemia and death. Within a short
time after ingestion of a lethal dose of Bt, the insect stops feeding.
Death of the insect follows within three or four days, as the insect
succumbs to a combination of septicemia and lack of food. Although insect
death does not occur immediately following ingestion of a lethal dose of
Bt, treated plant parts will not be damaged because insect feeding will
stop.

This mechanism of action of Bt partly accounts for its specificity. The
biochemical sequence of events initiated in the insect gut after digestion
of the Bt toxin protein depends upon the binding of the toxins to specific
molecules, called receptors, in the insect gut wall. A good analogy for
this process is a key fitting into a lock that allows a door to be opened.
Each insect pest species possesses gut wall receptors (locks) that will
bind or match only certain, specific toxin proteins (keys). This is why it
is important for the farmer to match the target pest species with a
particular Bt toxin protein which is specific for that insect.

Which Bt Product Should You Choose?

There are at present two Bt strains available for control of worms
(caterpillars) in vegetables; Bt kurstaki and Bt aizawai. Each strain
contains a different combination of toxin proteins, and some protein
combinations are more effective than others, depending on the target insect
pest species. The strain name is located on the product label, under
"Active Ingredient:" Examples of product trade names containing proteins
from the kurstaki strain are Javelin (Sandoz Corp.), MVP (Mycogen Corp.),
Dipel (Abbott), Cutlass (Ecogen Corp.), and Biobit (Dupont). Some trade
names for products containing proteins from the aizawai strain are Agree
(Ciba-Geigy) and XenTari (Abbott). Both strains have activity against a
wide range of worm species, including loopers, imported cabbageworm,
hornworms, european corn borer, armyworms (beet and fall), diamondback moth
larvae and fruitworm.

Are there situations where one strain has an advantage over the other for
control of worm populations? The toxin proteins in the aizawai strain have
demonstrated better control of armyworms (e.g., beet and fall armyworm),
and also diamondback moth larvae that have developed resistance to proteins
contained in the kurstaki strain of Bt, or to pyrethroid insecticides.
Because insect pests like the diamondback moth are capable of developing
rapid resistance to insecticides, including Bts, experts recommend that Bt
applications should be alternated with synthetic insecticides (e.g.,
pyrethroids) so that resistance to any one class of insecticide does not
develop. Based on experiments to study the development of insect resistance
to Bts, some researchers have recommended that the kurstaki strain should
be used before the aizawai strain.

The aizawai strain should be used if the target insect population has
already developed resistance to the kurstaki product, and it is no longer
effective. This has been proposed as a strategy to minimize Bt resistance
development. This is because most aizawai strains contain proteins similar
to those in the kurstaki strains, in addition to unique toxin proteins.
Therefore, use of a product with the aizawai strain may result in an insect
pest population with resistance to toxin proteins contained in both
strains.

Another Bt strain, tenebrionis, is available for control of Colorado potato
beetle larvae on potato, tomato and eggplant. Product trade name examples
include M-Trak (Mycogen Corp.), Foil (Ecogen), and Novodor (Entotech). The
Foil product is an example of a "transconjugated" strain, which contains
genes for both Colorado potato beetle- and European corn borer-specific
toxins.

A Few Tips on Bt Application

It is important to remember that most Bt formulations have better activity
against young compared with mature insect larvae. This means that the
initial Bt application should be made immediately before or just after the
insect eggs have hatched and the larvae are still small. This is
particularly critical with an insect like tomato fruitworm, which bores
into the plant after hatching. The best way to apply Bts at the proper time
is to monitor plants for worm eggs. Pheromone traps are useful to determine
when adult moths are in the area and when egg laying is likely to occur.
Our research last year in Cullman demonstrated that Bt sprays were very
effective against fruitworm in tomato if the sprays were made at the first
sign of worm eggs on the leaves. Please contact me for information on
identification and sampling of worm eggs in vegetables.

It is also important to remember that insects must eat Bt-treated foliage
for activity to occur. Therefore, good spray coverage of the plant is
essential for satisfactory control. This is best accomplished using
adequate spray volume (at least 40 gpa) and pressure (at least 150 psi).
For vegetables like cabbage, we have had excellent results using a three
nozzle per row sprayer arrangement; one over the top and one on each side
of the row directed into the plant. Reports from Georgia indicate that a
five nozzle per row arrangement is superior even to three nozzles per row
for worm control with Bt.

In summary, Bt products offer an effective alternative to synthetic
insecticides for control of certain pest insect species in vegetables. Worm
control with Bt will be enhanced by proper identification of the target
insect species, and selection of a Bt strain with activity against that
insect (remember to read the label to identify the Bt strain contained in
the product, and for a list of pest insects controlled). It is also useful
to alternate Bts with synthetic insecticides to delay the development of
insect resistance to any one material. Finally, proper timing of the
initial Bt spray against small larvae, and adequate spray coverage of the
plant are important for effective control.
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Use pesticides only according to the directions on the label. Follow all directions, precautions, and restrictions that are listed. Do not use pesticides on plants that are not listed on the label.

The pesticide rates in this publication are recommended only if they are registered with the Environmental Protection Agency or the Alabama Department of Agriculture and Industries. If a registration is cancelled, the rate listed here is no longer recommended. Before you apply any pesticide, check with your county Extension agent for the latest
information.

Trade names are used only to give specific information. The Alabama Cooperative Extension System does not endorse or guarantee any product and does not recommend one product instead of another that might be similar.