What I am thinking about here with the bees comes from reading Jaycox's
book Beekeeping in the Midwest which lists the Bacillus thuringiensis in
his "relatively non-toxic group" of pesticides. What I don't know for sure
is what form of the Bt toxin is expressed by the corn pollen. Is it the
protein crystal associated with the Bt spores or is it the transformed
delta-endotoxin. If it is the delta-endotoxin, then it would not have to
be transformed in the larvae's gut and the posion could then potentially be
more active in a variety of non-target species than the standard protein
crystal/digestion mode of action.
See below. I don't know the answers so this why I am asking the questions.
Has anybody done real tests on this and not just assumed the problem away
because they already "know" Bt only affects lepidoptera? Mike Miller
"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"
Bacillus thuringiensis (B.t.).
Carrie Swadener. Journal of Pesticide Reform, Volume 14, Number 3,
Fall 1994. Northwest Coalition for Alternatives to Pesticides, Eugene, OR.
actors Affecting Selectivity
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