From: archiel@sun_archielcom (Archie Lachner)
Subject: Re: Bt (was huge green bugs...)
Date: Fri, 14 Aug 1992 18:45:21 GMT
Organization: mentor
Keywords: 
Followup-To: 

In article <1992Aug13.213007.6066@iscsvax.uni.edu>, klier@iscsvax.uni.edu writes:
|> 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.
|> 

I haven't read anything here that will cause me to stop using BT.  I've never
had the experience of spraying BT on a crop once and then not have that crop
bothered again by the same caterpillar pests.  I guess that the BT that I've
been able to obtain is not as robust as that for which you cite evidence of
residual effect.  I have never observed any residual effect after spraying
with BT, but it has never failed to take care of an immediate problem.

I guess I take my gardening a little more seriously than you do.  I put a fair
amount of time and effort into it, and it ruins my day when I just end up "feeding
the bugs."  I firmly believe that use of biological controls is a
reasonable compromise between "slob gardening" (your term) and use of "hard"
pesticides.

Anyway, thanks for the information on the subject just the same.

By the way, where in Iowa is UNI (or have I somehow gotten the state wrong)?
-- 
Archie Lachner
Mentor Graphics Corporation
Wilsonville, Oregon
archie_lachner@mentorg.com