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BACILLUS THURINGIENSIS (B.T.)
By CARRIE SWADENER |
Bacillus thuringiensis (B.t.) is a live microorganism that kills
certain insects and is used to kill unwanted insects in forests, agriculture, and urban
areas.
In a purified form, some of the proteins produced by B.t. are acutely toxic to mammals.
However, in their natural form, acute toxicity of commonly-used B.t. varieties is limited
to caterpillars, mosquito larvae, and beetle larvae. B.t. is closely related to B. cereus,
a bacteria that causes food poisoning and to B. anthracis, the agent of the disease
anthrax. Few studies have been conducted on the chronic health effects, carcinogenicity,
or mutagenicity of B.t. People exposed to B.t. have complained of respiratory, eye, and
skin irritation, and one corneal ulcer has occurred after direct contact with a B.t.
formulation. People also suffer from allergies to the "inert" (secret)
ingredients. People with compromised immune systems may be particularly susceptible to
B.t.
Viable B.t. spores are known to exist for up to one year following application. Insect
resistance to B.t. has been well documented. Genetic engineering may greatly expand use of
B.t., speeding up the development of more resistance.
Large-scale applications of B.t. can have far reaching ecological impacts. B.t. can reduce
dramatically the number and variety of moth and butterfly species, which in turn impacts
birds and mammals that feed on caterpillars. In addition, a number of beneficial insects
are adversely impacted by B.t.
B.t. is less toxic to mammals and shows fewer environmental effects than many synthetic
insecticides. However, this is no reason to use it indiscriminately. Its environmental and
health effects as well as those of all other alternatives must be thoroughly considered
before use. B.t. should be used only when necessary, and in the smallest quantities
possible. It should always be used as part of a sustainable management program.
As hazards of conventional, broad acting pesticides are documented, researchers look for
pesticides that are toxic only to the target pest, have less impact on other species, and
have fewer environmental hazards. Bacillus thuringiensis (B.t.) insecticides result from
this research. However, there is evidence suggesting that B.t. is not as benign as the
manufacturers would like us to believe, and that care is warranted in its use.
B.t. is a species of bacteria that has insecticidal properties affecting a selective range
of insect orders. There are at least 34 subspecies of B.t.' (also called serotypes or
varieties) and probably over 800 strain isolates.2 B.t. was first isolated in 1901 in
Japan from diseased silkworm larvae. It was later isolated from Mediterranean flour moths
and named Bacillus thuringiensis in 1911.3 It was not until 1958 that B.t. was used
commercially in the United States.4 By 1989, B.t. products had captured 90-95 per cent of
the biopesticide market.5
Bacillus thuringiensis products available in the United States are comprised of one of
five varieties of B.t.: B.t. var. kurstaki and var. morrisoni which cause disease in moth
and butterfly caterpillars; B.t. var. israelensis which causes disease in mosquito and
blackfly larvae; B.t. var. aizawai which causes disease in wax moth caterpillars); and
B.t. var. tenebrionis, also called var. san diego, which causes disease in beetle
larvae.6,7 Other strains of B.t. have been discovered that exhibit pesticidal activity
against nematodes, mites, flatworms, and protozoa.5
B.t. products are used to control moth pests in fruits, vegetables, and beehives; blackfly
and mosquito pests in ponds and lakes; and several beetle pests in vegetables and shade
trees.6 (See Fig. 1,2, and 3 for more details.) Common brand names include Dipel, Foray,
Thuricide (all B.t. kurstaki), Vectobac, Mosquito Attack (all B.t. israelensis), and
M-Trak (B.t.tenebrionis).6
Mode of Action
When conditions for bacterial growth are not optimal B.t., like many bacteria, forms
spores. Spores are the dormant stage of the bacterial life cycle, when the organism waits
for better growing conditions. Unlike many other bacteria, when B.t. creates spores it
also creates a protein crystal. This crystal is the toxic component of B.t..
After the insect ingests B.t., the crystal is dissolved in the insect's alkaline gut. Then
the insect's digestive enzymes break clown the crystal structure and activate B.t.'s
insecticidal component, called the delta-endotoxin. The delta-endotoxin binds to the cells
lining the midgut membrane and creates pores in the membrane, upsetting the gut's ion
balance. The insect soon stops feeding and starves to death.
If the insect is not susceptible to the direct action of the delta-endotoxin, death occurs
after B.t. starts vegetative growth inside the insect's gut. The spore germinates after
the gut membrane is broken; it then reproduces and makes more spores. This body-wide
infection eventually kills the insect.8
Factors Affecting Selectivity
One of B.t.'s most desirable characteristic is its selectivity, only certain insects are
susceptible to the delta-endotoxin. Scientists have identified at least 29 different
crystals and delta-endotoxins. Each is effective against specific insects. Each variety of
B.t. can produce one or more of these toxins.7 Alkaline (basic; pH greater than 7)
solutions activate the delta-endotoxin, and different varieties may require different
pHs.9 Certain enzymes must also be present in the insect's gut to break the crystal into
its toxic elements.8 In addition, certain cell characteristics in the insect gut encourage
binding of the endotoxin and subsequent pore formation.7 The age of the insect is also a
factor, the younger larvae being more susceptible than older larvae.8
Health Effects Testing
Since B.t. is a live microbial organism, testing for the possible hazards of B.t. is
conducted differently chat for conventional pesticides. Microbial toxicity is described
using pathogenicity (the ability of the microbe to cause disease) and infectivity (the
ability of the organism to reproduce within the body,) The United States Environmental
Protection Agency (EPA) requires no testing of B.t. for carcinogenicity, mutagenicity, or
chronic toxicity.
Laboratory Tests of Acute Toxicity
Each of the more than 800 strains of Bacillus thuringiensis may exhibit different toxicity
to insects, rodents and humans. This fact complicates any discussion about the toxicity of
B.t. The following are summaries of the acute toxicity data available for two commonly
used commercial varieties of B.t..
Bacillus thuringiensis var. kurstaki (B.t.k.): B.t.k. and commercial products containing
B.t.k. generally have low oral acute toxicity to rats. In tests with laboratory animals,
researchers did not observe any adverse effects after feeding large doses.
Other types of exposures have some acute effects. Rats who breathed air containing B.t.k.
spores experienced respiratory depression,~4 and B.t.k. spores injected into rats' veins
aggravated preexisting disease.~5 Both B.t.k. and Foray 48B are irritating to rabbit skin,
and Foray 48B is moderately irritating to rabbits' eyes.
Bacillus thuringiensis var. israelensis (B.t.i): In studies assessing B.t.i.'s acute
toxicity to mammals, morality only occurred when B.t.i. was injected into the abdomen or
the brain. In one study conducted on rats, 79 percent morality occurred after a single
injection into the brain. Effects other than morality can also occur. For example, in mice
injected with a B.t.i. suspension, spleens became enlarged.
B.t.i. is irritating to both eyes and skin. Injection of both viable and inactivated
B.t.i. spores under the skin resulted in abscesses in mice. Rabbits' eyes are irritated by
B.t.i.l8 The irritancy of B.t.i. to eyes depends on the physical characteristics of the
formulation; a dry, dusty formulation with smaller particles is less irritating and
cleared from the eye more quickly than a clumped formulation with larger particles.
In a purified form, B.t.i.'s endotoxin is clearly toxic to mammals. When the
delta-endotoxin from B.t.i. was injected intravenously into mice, they exhibited rapid
paralysis, followed by death within 12 hours. When the same dosage was injected under the
skin of suckling mice, death occurred in 2-3 hours. The delta-endotoxin also caused
destruction of rat, mouse, sheep, horse, and human red blood cells.~9 When a small protein
isolated from the endotoxin was administered to mice at sublethal levels, mice suffered
from severe hypothermia and their heart beat slowed.20
Acute Toxicity to Humans
Bacillus thuringiensis var. kurstaki: There have been few experimental studies assessing
the toxiciy of B.t.k. to humans. Most information comes from occupational exposures, or
from exposures occurring during large-scale B.t.k. programs.
One case of B.t.k. infection resulted from a farmer splashing a B.t.k. formulation, Dipel,
in his eye. The man developed an ulcer on his cornea from which positive B.t.k. cultures
were taken. Another man working on a spray program splashed B.t.k. on his face and eyes.
He then developed skin irritation, burning, swelling, and redness. B.t.k. was cultured
from a sample taken from his eye.22 Ground-spray applicators using Foray 48B reported
symptoms of eye, nose, throat, and respiratory irritation. The frequency of their
complaints was found to be related to the degree of exposure. Workers with similar
preexisting health problems were more likely to report adverse effects from the ground
spray.23
A woman exposed to an B.t.k. formulation as a result of drift went to the hospital due to
burning, itching-and swelling of her face and upper chest. She later exhibited a fever,
altered consciousness, and suffered seizures.24 No B.t. was cultured from tissue samples,
but her doctor believed that B.t. was the cause of the clinical symptoms.25
Monitoring studies following large-scale B.t. spray programs have shown that exposed
people carry B.t. in their tissues. For example, more than 11 percent of nasal swab
samples taken from patients surveyed by doctors in Vancouver (Canada) following a gypsy
moth spray program were found to contain B.t.k.23 B.t. was also found in cultures taken
from patients in Lane Couny, Oregon following a gypsy moth spray program there. Monitoring
studies also show that exposed people report a variety of health problems that they
believe to be associated with B.t. exposure.22 For example, during the Vancouver spray
program, almost 250 people reported health problems, mostly allergy-like or flu-like
symptoms. During a Washington gypsy moth spray program, over 250 people reported health
problems and 6 were treated in emergency rooms for allergy or asthma problems.26
Physicians have so far been unable to definitively link B.t. exposure to these health
problems.22.23,26
Bacillus thuringiensis var. israelensis: There has only been one case of documented
adverse effects of B.t.i. on humans. This case involved a researcher who accidentally
injected himself with a mixture of B.t.i. and another kind of bacteria commonly found on
human skin.20 He suffered from a toxic reaction and irritated lymph vessels. When these
two bacteria were later injected into rodents the combination was consistently lethal, but
each bacteria injected separately caused only slight inflammation.8
Special Concerns about B.t. Toxicity
Exotoxins: The earliest tests done regarding B.t.'s toxicity were
conducted using B.t. var thuringiensis, a B.t. strain known to contain a second toxin
called beta-exotoxin. The beta-exotoxin is toxic to vertebrates, with an LD50 (median
lethal dose; the dose that kills 50 percent of a population of test animals) of 13-18
milligrams per kilogram of body weight (mg/kg) in; mice when injected into the abdomen. An
oral dose of 200 mg/kg pet day killed mice after eight days.20 Beta-exotoxin also causes
genetic damage to human blood cells.27 B.t. formulations containing beta-exotoxin have not
been used in most countries 20 although. attempts are currently being made to register
beta-exotoxin as an insecticide in the United States.8 Another toxin produced by B.t. is
the alpha-exotoxin that is highly acutely toxic to mice.20 Current B.t. production methods
are such that alpha-exotoxin is not a "significant component" of B.t.
formulations.
Related Bacteria: B.t. belongs to a small group of closely related
Bacillus species, including B. cereus, a bacteria that is an agent of food poisoning, and
B. anthracis, the pathogen of the virulent animal disease, anthrax. These three bacteria
are so similar it has been theorized that they are all varieties of the same species.2529
If B. cereus is cultured with B.t. cells, genetic material is transferred to the B. cereus
cells that allows B. cereus to produce B.t.'s crystal proteins.25 Transfers of genetic
material between B. anthracis and B.t. have also occurred.30
A toxin produced by B. cereus that causes diarrhea in monkeys is also produced by certain
strains of B.t.,30 although this toxin is not likely to be present in B.t. spore
formulations.28 Human volunteers suffered from nausea, vomiting, diarrhea, colic-like
pains, and fever after eating food contaminated with one B.t. strain, B.t. var. galleriae.
These examples indicate the close relationship between B.t. and disease-causing pathogens.
Increased Susceptibility: People with compromised immune systems or
preexisting allergies may be particularly susceptible to the effects of B.t. In mice with
reduced immune function, the dose required to kill more than 50 percent of the mice when
injected was several orders of magnitude smaller than the highest dose tested in normal
mice.32 Mice with impaired immune function also showed higher mortality than regular mice
when one dose of B.t.i. was injected into the abdominal cavity.33 Although no definite
cases have been reported of B.t. infecting humans with compromised immune systems, the
Oregon Health Division suggested before a B.t.k. spray program that individuals with
physician-diagnosed causes of severe immune disorders may consider leaving the area during
the actual spraying.+34
A memo from Novo Nordisk, the manufacturer of Foray 48B, states that the amount of the
spray a person would be exposed to would be too small to develop new allergies. However,
"It is possible that someone that already has developed an allergy to one of the
components of Foray 48B or has asthma . .. could be affected by exposure to small
quantities of Foray 48B."35-The 1991 Material Safety Data Sheet for Foray 48B states
"Repeated exposure via inhalation can result in sensitization and allergic response
in hypersensitive individuals."36
Contaminants: In the mid 1980s, several B.t. products were contaminated
with other bacteria, including Streptococcus faecium and S. faecalis 37 While B.t.
products are routinely monitored for bacterial contaminants, 2 the risk of contamination
with a disease-causing bacteria is always present.25
Inert Ingredients
All B.t. products contain ingredients other than B.t.. These are identified only as
"inert" ingredients and are called trade secrets by the manufacturers of the
products. The "inert" ingredients are potentially the most toxic components of
the formulations. For example, during the 1992 Asian gypsy moth spray program in Oregon, a
woman who was exposed to Foray 48B had a preexisting allergy to a carbohydrate that was
present as an inert ingredient. Within 45 minutes of exposure, the woman suffered from
joint pain and neurological symptoms.
Because "inerts" are called trade secrets, there is little public information
about their identity, but the information that is available indicates they could cause
health problems. Foray 48B has contained sodium hydroxide, sulfuric acid, phosphoric acid,
39 methyl paraben, 40 and potassium phosphate, as "inerts." While these
ingredients make up less than 10 percent of Foray 48B, 39 they pose hazards. Sodium
hydroxide, more commonly known as lye, causes "severe corrosive damage to the eyes,
skin, mucous membranes and digestive system ..'. Breathing sodium hydroxide dust or mist
leads in mild cases to irritation of the mucous membranes of the nose ... and in severe
cases to damage of the upper respiratory tract."42 Sulfuric acid and phosphoric acid
are both corrosive. Sulfuric acid can cause severe deep skin burns and permanent loss of
vision. When inhaled as a mist, sulfuric acid may cause severe bronchial constriction, and
bronchitis.43 Phosphoric acid is an irritant to skin and mucous membranes, and its vapors
may cause coughing and throat irritation.43 Both methyl paraben and potassium phosphate
were once registered by EPA as pesticide active ingredients.44
Sodium sulfite has been identified as an inert ingredient of the B.t.k. formulation Dipel
8AF.45 Up to ten per cent of asthmatics (about one million people in the United States)
may react to sulfites, particularly those people who are treated with steroids.42 Symptoms
of exposure in those sensitive to sulfites usually involve the respiratory system, and can
also include nausea, diarrhea, lower~ed blood pressure, hives, shock, and loss of
consciousness.42
Environmental Fate
Very little is known about the natural ecology of B.t. It occurs naturally in many soils.
In one study, B.t. was isolated from 70 per cent of soil samples taken from around the
world, and was most abundant in samples taken in Asia. More than half of these isolates
were undescribed varieties of B.t. 46 B.t. has also been isolated from insect bodies, tree
leaves and aquatic environments.7 It has even been recovered from paper.47
Soil: B.t. generally persists only a short time in soil. The half life of the insecticidal
activity (the time in which half of the insecticidal activity is lost) of the crystal is
about 9 days.48 However, small amounts can be quite persistent. In one experiment, B.t.
spore numbers declined by one order of rnagnitude after 2 weeks, but then remained
constant for 8 months following application.49
B.t. does not appear to move readily in soil. In one study, two varieties of B.t. were
applied in adjacent plots, but did not become cross-contaminated, indicating that B.t.
does not move laterally in soil.2 8 Other studies found that B.t. was not recovered past a
depth of 6 centimeters after irrigation, and that movement beyond the application plot was
less than 10 yards.7 50
Foliage: B.t. deposited on the upper side of leaves (exposed to the sun) may remain
effective for only 1-2 days, but B.t. on the underside of leaves (i.e. protected from the
sun) may remain active for 7-10 days.2 8 It is possible for it to be significantly more
persistent, however. Viable spores of B.t.k. were recovered from white spruce foliage one
year after application.5' In one experiment conducted in Japan, B.t. persisted for two
years in a citrus orchard and remained toxic to caterpillars.52
Water B.t.k. has been recovered from rivers and public water distribution systems after an
aerial application of Thuricide 16B. Standard water treatment processes are not adequate
to destroy B.t.k. spores.53
B.t.i. spores and crystals bind readily to sediments in the water column, 54,55 which
reduces their efficacy by making them inaccessible to mosquito and blackfly larvae.
In one test, B.t.i. was applied to water, then allowed to contact mud particles. Over 99
percent of the B.t.i. spores were found in the mud, rather than in the water, after 45
minutes. The B.t.i. retained viability and toxicity for at least 22 days, killing 90
percent of the mosquito larvae when the mud was stirred and reintroduced to the water
column.54
In another experiment, viable cells were recovered from the water for up to 200 days and
in the sediment for up to 270 days after application.55
Air B.t.k. has been found to drift over 3,000 meters downwind during an aerial
application. The distance B.t.k. is capable of drifting depends upon the amount and method
of application, 56 as well as the climatic conditions. B.t. thuringiensis was measured in
air for up to 17 days following an application.4
Biotechnology
Examples of genetic manipulation ant genetic engineering with B.t. include the following
In the agricultural product Foil, the gene for a toxin with activity against beetles was
transferred through conjugation (sexual reproduction in bacteria) to a B.tk. cell that
only affected butterflies and moths. The resulting cell showed insecticidal properties
against beetles, butterflies, and moths. Since
" During the 1992 Asian gypsy moth spray program in Oregon, a woman who was exposed
to Foray 48B had a preexisting allergy to a carbohydrate that was present as an inert
ingredient. Within 45 minutes of exposure, the woman suffered from joint pain and
neurological symptoms."
EPA considers the organisms resulting from conjugation to be genetically manipulated
rather than genetically engineered, Foil was registered for use in the U.S. in 1990.
Pseudomonas fluorescens cells can be engineered to produce the B.t. delta-endotoxins
without production of a spore. The crystal protein remains inside the P. fluorescens cell
wall. In the products MVP and M-Trak, the P. fluorescens cell is killed after it produces
the crystal protein. When the product is applied, the delta-endotoxin remains protected
within the now dead cell wall. In this way, the B.t. delta-endotoxin retains its
effectiveness for two to three times longer than other B. t. formulations. MVP and M-Trak
were the first genetically engineered produces to k registered by EPA, since the
transgenic organism was not alive when released into the environment.
B.t.i used to control mosquito and blackfly larvae that live on the water surface begins
to sink, away from the target larvae, within 24 hours. Bacteria that naturally live on the
water surface ( in the same environment as mosquito or blackfly larvae), have ken
engineered to produce the B.t.i. crystal proteins.
Over thirty different crops have ken engineered to produce the B.t crystal protein
throughout their plant structure. Any pest that feeds on any part of these plants will be
exposed to the B.t. delta-endotoxin, and those susceptible to the toxin will be killed.
Clearly, the possibilities for the genetic engineering of B.t. delta-endotoxins seem
endless. However, researchers know so little about the ecology and genetic stability of
B.t., that the potential ecological effects of these transgenic organisms are impossible
to predict with certainty.
Resistance
Scientists once thought that the mode of action of B.t. was complex enough to prevent the
development of pest insect resistance. However, time and further research proved this to
be untrue. Eight insect species have been studied because of their ability to develop
resistance to B.t. 57 The Indian meal moth, a pest of grain storage areas, was the first
insect to develop resistance to B.t k.58 in laboratory experiments. Resistance progresses
more quickly in laboratory experiments than under field conditions due to higher selection
pressure in the laboratory.59 No indications of insect resistance to B.t. were observed in
the field, until the development of resistance was observed in the diamondback moth in
crops where B.t. had been used repeatedly. Since then, resistance has been observed in the
laboratory in the tobacco budworm, the Colorado potato beetle and other insect species.57
The gypsy moth also shows potential for developing B.t. resistance.60 Some insects, such
as the diamondback moth and the tobacco budworm, exhibit resistance to multiple B.t.
strains.6~ 62 Development of resistance occurs faster when larger amounts of a pesticide
are used, so that use of crop plants genetically-engineered to produce the B.t. toxin
could dramatically increase the number of B.t.-resistant insects.
B.t.'s Ecological Impacts
Some of the most serious concerns about widespread use of B.t. as a pest control technique
come from the effects it can have on animals other than the pest targeted for control. All
B.t. products can kill organisms other than their intended targets. In turn, the animals
that depend on these organisms for food are also impacted.
Beneficial insects: Many insects are not pests, and any pest management
technique needs to be especially concerned about those that are called Beneficials, the
insects that feed or prey on pest species. B.t. has impacts on a number of beneficial
species. For example, studies of a wasp that is a parasite of the meal math (Plodia
interpunctella) found that treatment with B.t. reduced the number of eggs produced by the
parasitic wasp, and the percentage of those eggs that hatched.63 Production and
hatchability of eggs of a predatory bug were also decreased.63 On collards, aphid-eating
flies in the family Syrphidae were reduced by Dipel treatment.64 Both B.t.tenebrionis and
Dipel have caused mortality of predatory spider mites.65 Dipel also has caused mortality
of the cinnabar moth, used for the biological control of the weed tansy ragwort.66
Finally, B.t.i. has caused mortality of a moth (Synclita obliteralis) that helps control
aquatic weeds in Florida.67
Other insects: Many insects that do not have as directly beneficial
importance to agriculture are important in the function and structure of ecosystems. A
variety of studies have shown that B.t. applications can disturb insect communities.
Research following large-scale B.t. applications to kill gypsy moth larvae in Lane County,
Oregon, found that the number of oak-feeding caterpillar species was reduced for three
years following spraying, and the number of caterpillars was reduced for two years.68
Similar results were found in a study of caterpillars feeding on tobacco brush following a
B.t.k. application to control spruce budworm in Oregon.69 In untreated areas, the number
of species was above 30 percent higher, and the number of caterpillars 5 times greater,
than in B.t.k.-treated areas two weeks after treatment. The number of caterpillars was
sill reduced in treated areas the following summer. In Washington, B.t. applications in
King and Pierce counties to kill gypsy moths reduced spring moth populations by almost 90
percent.70 In addition, one rare species appeared to have been eradicated from the
treatment zone, and moth populations were "heavily impacted in an area more than
double that which was actually sprayed" as moths moved into the treatment zone from
surrounding areas.70 In West Virginia, applications of Foray 48B reduced the number of
caterpillar species and the number of caterpillars. The year following application, the
number of moth species and the number of moths were both reduced. A recent (1994) study in
four different Oregon plane communities found that total weight of caterpillars was
reduced between 90 and 95 percent by B.t. treatment; the number of caterpillars was
reduced by 80 percent; and the number of caterpillar species was reduced by over 60
percent.72
Aquatic insects are also affected by B.t. treatments. Canadian studies
found that certain stream insects (Simulium vittatum and Taeniopteryx nivalis) were killed
by applications of Thuricide and Dipel respectively.73,74 Midges (chironomids) have
repeatedly been shown to be killed by B.t.i.77
Birds: Because many birds feed on the caterpillars and other insects
affected by B.t. applications, it is not surprising that impacts of B.t. spraying on birds
have been documented. In Lane County, Oregon studies of chickadees following a gypsy moth
spray program found that birds nesting in B.t.-treated areas brought fewer caterpillars to
their nests than did birds nesting in untreated areas. The birds were able to-find other
food, so that nesting success was not significantly impacted.78 In New Hampshire, when
B.t.-treatment reduced caterpillar abundance, black-throated blue warblers made fewer
nesting attempts and also brought fewer caterpillars to their nestlings.79 A Canadian
study found that numbers of caterpillars, followed by numbers of two species of warblers
and a thrush, were reduced by B.t. treatment. In addition, there were fewer spruce grouse
chicks in B.t. treated areas, and the chicks in those areas grew more slowly than chicks
in untreated areas.80 ,
There is also some evidence that B.t. can be directly toxic to birds. A study of the
effects of application of Dipel to ringneck pheasant eggs found that hatching was only
half as successful as hatching of untreated eggs. Because the Dipel was applied with a
spreader-sticker compound (Plyac) the decrease in hatching may k a result of the Plyac and
not the B.t. product.81
Other animals: Because shrews often feed on caterpillars, impacts from
B.t. treatments are likely. A study in northern Ontario (Canada) found that treatment with
Dipel changed the structure of the shrew population. Adult males emigrated, so that the
proportion of juveniles increased. The juveniles and adult females who did not emigrate
shifted from a diet of caterpillars to alternative prey.82
Foray 48B at high concentrations (about 3 percent) is acutely toxic to rainbow trout,
probably because the product is highly acidic.83
B.t.i. treatments can also affect other animals. Low concentrations of B.t.i. endotoxins
decrease the weight of tadpoles and delay their metamorphosis.84 The B.t.i. formulation
Vectobac is acutely toxic to fathead minnows, probably because "inerts" in the
product deplete the dissolved oxygen in water.85 The B.t.i formulation Teknar was acutely
toxic to brook trout fry, probably because of xylene used as an "inert" in the
product.86
Comparison with synthetic insecticides: Where comparative studies have been done, the
ecological impacts of a B.t. treatment are almost always less than those of synthetic
insecticides. For example, B.t. treatment of collards caused less of an increase in aphid
numbers than did treatment with carbaryl, which killed many aphid predators.64 Vectobac
was much less acutely toxic to an estuary fish than other mosquito insecticides including
temephos, fenoxycarb, diflubenzuron; and methoprene.87
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Citation : Swadener, Carrie, 1994, " Bacillus Thuringiensis (B.T.)", Vol. 14,
No. 3, Fall 1994, pp. 13-20
Copyright © 1994 Northwest Coalition for Alternatives to Pesticides.
Ecological Agriculture Projects, McGill University (Macdonald Campus)
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