From: klier@iscsvax.uni.edu
Newsgroups: rec.gardens
Subject: Re: Bt (was huge green bugs...)
Date: 13 Aug 92 21:30:07 -0500
Organization: University of Northern Iowa

In article <1992Aug13.004625.25484@news.mentorg.com>, archiel@sun_archielcom (Archie Lachner) writes:
> In article <1992Aug11.202848.6039@iscsvax.uni.edu>, klier@iscsvax.uni.edu writes:
> |> Nope, my premise was that BT is a _living organism_.  It can spread.  It is
> |> not selective as long as the host has a certain gut pH.  You can put it
> |> on your tomato hornworms, but you might accidentally get other species,
> |> some of them ones you wouldn't intend.  
> |> 
> |> There's a big difference in my mind between using a non-selective, pesticide
> |> that can actually INCREASE in the environment-- remember BT is living --
> |> and using a carefully targeted, biodegradable, non-living pesticide --
> |> especially when there are simple tactics like caging a couple of plants,
> |> handpicking, or trap crops, that allow me both to harvest a crop and
> |> have the non-target species.
> 
> I was under the impression that BT degraded and died after a few days exposure
> to the elements (sun light, etc.).  If what you say is true, many of the organic
> gardening suppliers, magazines, etc., have been leading us astray on the safety
> of BT.  Anybody out there with definitive information about BT's persistence
> in the environment.

Bt is _Bacillus thuringensis_; Dipel is _B. thuringensis_ var. _kurstakii_
spores. Bacillus spores are incredibly tough-- the wall consists primarily
of peptidoglycans and calcium dipicolinate in the core.  There is very little
water, and the combination of the lack of water and Ca dipicolinate
means that some spores can withstand boiling water for more than an hour;
if you want to sterilize spores with dry heat, better think about 120oC
for several hours.

The insect ingests a Bt spore, and it produces a new cell of Bacillus
thuringensis, and also starts producing 4 toxins.  The toxins eventually
cause the caterpillar to "get the slows" and eventually die.  The live
Bt cells in the caterpillar gut will produce more spores as conditions
become unfavorable for its growth.  Those spores can survive in the soil
or on the plant surface and be passed on to another caterpillar.

The _toxins_ are pretty labile, and I suspect that the suppliers are
talking about toxin degradation, not spore degradation.  I don't have
the field trial evidence immediately at hand (see refs. below), but
I do know that one dusting of Dipel supresses caterpillar populations
for several years.


Yes, there is a difference between spreading a broad-spectrum "hard"
insecticide (say DDT) and spreading a biological insecticide like Dipel.
Both are fairly broad-spectrum, which means they will hit non-target
organisms.  The DDT will eventually decompose, but the breakdown products
are likely to be hazardous themselves.  When Bt spores die, all you've
got is a very non-hazardous dead spore.

But if you think about using insecticides (or other pesticides) in an
evolutionary context, you start realizing that the more exposure pest
species have to a pesticide, the more likely that we're going to be
(unconsiously) selecting a resistant strain of the pest.  Just like
the DDT-resistant mosquitoes we developed in the course of attempting
to "wipe out" malaria...

So the more we use our best pesticides, the more likely the bugs are
to become develop a resistance.  When I have the choice of using
mechanical control of a pest (water sprays, exclosures, hand-picking,
encouraging predators), I'd much sooner do that and save the heavy
ammunition for the time that it is _really_ needed.

I garden for my enjoyment.  I'm not a truck gardener, nor a farmer.
I'm not going to starve if all my tomatoes are wiped out by some
plague or pest (might have in grad school, but there I hedged my
bets by planting 6-10 different and unrelated cultivars).  I can
tolerate a fair amount of damage before it becomes anything like
an economic problem for me.  Likewise for my neighbors, who are 
also not dependent on their gardens for subsistence.  I'm certainly
not endangering local cash crops: the fields to my north are corn
and soybeans, and don't share the insect or disease problems of 
my garden.

At the same time, as a biologist, I've been taught to think of the
interconnectedness of living organisms, and of living organisms to
environment.  I can't say I'm particularly fond of tomato hornworms,
but I recognize that they play a distinct role in the local ecology.
The eat some things.  They pollinate other things.  They are closely
related to other Sphinx moths that are known to be crucial to survival
of other species.  I just don't feel like taking the chance that
applying dipel for hornworm control could knock out local populations
of, say, Great Poplar Sphinx, or Cherry Sphinx.  Not when I can
cage a couple of tomatoes in egg-laying season, pick a few caterpillars,
or just buy some tomatoes in the farmer's market if they did manage
to kill all my tomatoes (unlikely).

I also know that if we get a population explosion of good-sized caterpillars,
the birds will be happy to help in the caterpillar control efforts.
Populations of species that have high reproductive rates (r-selected
species in ecologese) have boom and bust population cycles.  Even if I
don't do anything, the populations won't stay high.  Disease, predators,
and famine will bring the populations back down.


This is getting too long, so I'll stop here.

Kay Klier   Biology Dept  UNI



recent books dealing with microbial pesticides (mostly compendia and
  symposium proceedings)

Safety of microbial insecticides.  1991

Biology of bacilli: Applications to industry.  1992

Bacterial control of mosquitoes and black flies  1990

Natural toxins:  Animal, plant, microbial.