7898

• UCREFRP Catalog Number: 7898
• Author: Swadener, C.
• Title: Insecticide Fact Sheet,
• USFW Year: 1994
• Copyright Material: YES

tl~

.lOURNAL OF PESTICIDE REFORM/FALL 1994. VOL.14, NO.3

Figure 1
B.t. Used in U.S. Agriculture

Acres treated:
. 300,000 - 400,000

'. '. . 200,000 - 300,000

11 100,000 - 200,000

ij5J L than 100 000
lEI ess ,

o Minor amount

. Sources:
U.S. Dept of Agriculture. National Agricultural Statistics Service. Economic
Research Service. 1994. AgriculturaT chemical usage: 1993 field crops
sunvnary. Washington, D.C. (March.). , '. .
U.S. Dept. of Agriculture. National Agricultural Statistics Service. Economic
Research ServiCe. 1993. AgriculturaT chemical usage: Vegetables 1992
summary. Washington, D.C: (June.)
U.S. Dept. of Agriculture. National Agricultural Statistics Service. Economic
Research ServiCe. 1992. Agricultural chemical usage: 1991 fruits and nuts
summary. Washington. D.C. (June.),." , ' , . '.

B.l is widely used in cotton production in Texas, Mississippi, and Louisiana as well as in the
production of fruits and vegetables in California, Arizona. and Aorida

the insect's digestive enzymes break down
the crystal structure and activate B.t.'s in-
secticidal component, called the ddta-en-
dotoxin. The ddta-endotoxin binds to the
cells lining the midgut membrane and cre-
ates pores in the membrane, upsetting the
got's ion balance. The insect soon stops
f~g and "Sta.iVes to death.
If the insect is not susceptible to the
direct action of the ddta-endotoxin, death
occurs after B.t. starts vegetative growth in-
'side 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 characteris-
, tic is its selectivity; only certain insects are
susceptible to the ddta-endotoxin. Scien-
tists have identified at least 29 diRerent crys-
tals arid ddta-endotoxins.5 Each is effective

against specific insects. Each variety of B.t.
ean produce one or more of these toxins.?
Alkaline (basic; pH greater than 7) solu-
tions activate the ddta-endotoxin, and dif-
ferent varietic!s may require different pHs.9
Certain enzymes must also be present in
the insect's gut to break the crystal into its
:oxic e!ementll8 In addition, ce..rN2in c("l1
characteristics in the insect gut encourage
binding of the endotoxin and subsequent
pore formation.? 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 mi-
crobe to cause disease) and infectivity (the
ability of the organism to reproduce within
the body.) The United States Environmen-

tal Protection Agency (EP A) requires no
testing of B.t. for carcinogenicity, mutage-
nicity, or chronic toxicity.IO

Laboratory Tests of
Acute Toxicity

,. '. Each of the more than 800 strains of
BaciUus thuringimsis may exhibit different
toxicity to insects, rodents and humans.
This fact complicates any discussion about
the toxicity ofB.t. The following are sum-
maries of the acute toxicity data available
for two commonly used commercial vari-
eties ofB.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 labora-
toty animals, researchers did not observe
any adverse effects aner feeding large
doses. I 1-13
Other types of exposures have some
acute effects. Rats who breathed air con-
taining B.t.k. spores ~rienced respira-
torydepression,I4 and B.t.k. spores injected
into rats' veins aggravated preexisting dis-
ease.15 Both B.t.k. and Foray 48B are irri-
tating to rabbit skin,16 and Foray 48B is
moderatdy 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 oc-
curred when B.t.i. was injected into the ab-
domen or the brain. In one study conduaed
on rats, 79 percent mortality occurred after
a single injection into the brain,17 Effects
other than morcility ean also occur. For
example. in mice injected with a B.t.i. sus-
pension, 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 me skin resulted in abscesses
in mice. I? Rabbits' eyes are irritated by
B.t.i.18 The irritancy of B.t.i. to eyes de-
pends on the physical characteristics of the
formulation; a city, dusty formulation with
smaller particles is less irritating and cleared
nom me eye more quickly than a clumped
formulation with larger particles. I?
In a purified form, B.t.i.'s endotoxin is
clearly toxic to mammals. When the delta-
endotoxin from B.t.i. was injected intrave-
nously into mice, they exhibited rapid pa-

14

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