I've removed the footnotes, to shorten the posting. I can also send
these to those who want them.
Roberto Verzola
--------------------------------------
Bacillus thuringiensis (B.t.).
Carrie Swadener. Journal of Pesticide Reform, Volume 14, Number 3,
Fall 1994.
Northwest Coalition for Alternatives to Pesticides, Eugene, OR.
Bacillus thuringiensis (B.t.)
by Carrie Swadener
Introduction
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 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.1 (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
down 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.5
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 that 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.10
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.11-13
Other types of exposures have some acute effects. Rats who breathed
air containing B.t.k. spores experienced respiratory depression,14
and B.t.k. spores injected into rats' veins aggravated preexisting
disease.15 Both B.t.k. and Foray 48B are irritating to rabbit skin,16
and Foray 48B is moderately irritating to rabbits' eyes.12
Bacillus thuringiensis var. israelensis (B.t.i.): In studies
assessing B.t.i.'s acute toxicity to mammals, mortality only occurred
when B.t.i. was injected into the abdomen or the brain. In one study
conducted on rats, 79 percent mortality occurred after a single
injection into the brain.17 Effects other than mortality can also
occur. For example, in mice injected with a B.t.i. suspension,
spleens became enlarged.18
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.17 Rabbits' eyes are irritated by B.t.i.18 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.17
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.19
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 toxicity 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.21 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 County, 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 per 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 countries20 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.8
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.28,29 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.28 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.31 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 I 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.8 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.38
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,41 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,
lowered 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 magnitude 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.51 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 and genetic engineering with B.t.
include the following:7
* 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.t.k. cell that only affected
butterflies and moths. The resulting cell showed insecticidal
properties against beetles, butterflies, and moths. Since 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-endotoxin 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 products to be 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 been
engineered to produce the B.t.i. crystal proteins.
* Over thirty different crops have been 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.61,62 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 moth
(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 about 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 still 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.71 A
recent (1994) study in four different Oregon plant 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.75-77
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 be 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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