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Christy's on Bt

Square Foot Gardening List - http://www.flinet.com/~gallus/sqft.html

There are a few irregularities in Christy's post to Cyn on the subject of 
Bacillus thuringiensis (Bt).

Christy said,
  BT (Bacillus thuringiensis) is a microbial insecticide that is marketed as
>Dipel, Thuricide and Attack.

So far, so good.

But there are some inaccuracies in this paragraph:

>It is a virus that when ingested by the worms
>kills them. It is approved for organic gardening (food crop) use in
>California. It breaks down quickly and is only effective for a few days, so
>you may need to properly time (look for eggs) your application, or spray more
>than once to get those in the process of hatching.

Bt is a bacterium, not a virus.  It forms crystalline spores that are 
shaped like elongated diamonds, with points.  These points are what causes 
the insect gut to liquefy.  It's a mechanical thang.

Bt occurs naturally in some parts of the country; thus it's not quite right 
to stipulate that it "breaks down quickly."  Some of it is there all the 
time.  Just not enough to handle society's insecticidal needs all by itself.

One of the reasons Bt, though much safer than contact insecticides, is 
tough to use properly is that timing is of the essence--but not because it 
breaks down. Bt works only if ingested, so it's got to hit leaf structures 
that form part of the target insect's natural diet, and stick to those 
leaves long enough for the target bugs to wander by and chow down.

If you get a heavy rain within 24 hours of having sprayed Bt, forget about 
it.  You've got to reapply.  This is not such a hassle when you're talking 
about your garden.  But state and federal agencies that spray entire 
hardwood forests with Bt from airplanes (which has been very common in the 
U.S. Northeast for the last 15-18 years because of gypsy moth) are in a 
mess if they have to respray because of the cost.

It is not appropriate to spray based on having seen eggs hatch, as Christy 
suggested above.  Most target insects have more than one larval stage, and 
the trick is knowing at which instar the little "worms" do the most eating 
and therefore are likeliest to take up the Bt.  For gypsy moths, for 
instance, it's about instar #4.  Wait a couple more instars (as you might 
have to do, for instance, if it's raining on the days you need to spray) 
and you're out of luck again.  The bugs are still "worms," but they're 
about to pupate in coccoons prior to emerging as adults, and they've 
stopped eating!!!

Christy is right about this:
>  It takes the worms a couple
>of days to die (their insides liquefy, eeech!) but they usually stop doing
>damage soon after becoming infected.

>I especially like not having to go eye to eye with those big tomato worms when
>I'm picking. I think I read somewhere that BT even kills mosquito larva, but I
>wouldn't know what to spray it on to kill them?

Well, I don't know anything about Bt on mosquitoes.  Originally, Bt was 
billed as solely killing the larval stages of lepidopts (butterflies and 
moths).  In recent years, I have seen posts on the 'Net suggesting that it 
works on some beetles.

If you've got a bag of Bt around the house, check the label.  The U.S. 
Environmental Protection Agency requires that all pesticide labels clearly 
indicate which pests the product can be used against.  It's a violation of 
federal law to use any insecticide against a pest not specifically OK'd on 
the label.  Even if it works against them!

Make no mistake:  using Bt against appropriate pests is much less harmful 
to other insects and aquatic invertebrates than using a contact insecticide 
like malathion, chlorpyrifos, or Sevin.  (Bt is also less dangerous to 
mammals, including human pesticide applicators, than the contact 
pesticides.)  But Bt is also tricker to apply correctly and, as Christy 
indicated, you must be prepared to postpone your gratification long enough 
for some vegetation consumption to take place in order for it to work at all.

--Janet


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