Edit: NVM, NWI

As I said, the ingredients list solids, spores, and toxins. I just don't know what "solids" is. No, the solids don't break the intestinal wall down. The toxins do. They are manufactured by the bacteria, so it would seem there is no reason for the bacteria to be kept alive. Um, what is supposed to be keeping them alive for the years that the stuff is sitting on my shelf? That culture medium isn't going to do the job in my bottle. Bacterial lifetime is of order half a day. The spores may, however, develop into living bacteria, once in the caterpillar, but it's not clear what role they play. They may be just going along for the ride. Solids are probably dead bacteria, but nobody is saying. I have a question in to Southern Ag.

OK, talked with Darrin who is the chemist at Southern Ag insecticides. They market Bt as "Thuricide". Very interesting. Here's the deal.

First of all, Bacillus thuringiensis is not, biologically, all that far from bread mold!

The solids in their stuff are DEAD BACTERIA. Even the spores are dead. No alive stuff in their Bt. They cultivate the bacteria, and then raise the temperature so the bacteria die. When they die, they form tiny crystals (that's the Cyt proteins), and these crystals are, when ingested by caterpillars, what punctures their intestines and kills them. So the "toxin" in their Bt is these Cyt crystals. Now, the shelf-life of this stuff is a couple of years, if treated right, but you're supposed to use it within 12 hours of mixing with water, which you do for spraying. The reason for that is that the crystals tend to rapidly soften when they're diluted in water. The crystals do soften naturally with time (which is what the shelf-life is about) but it happens a lot faster on mixing with water. As noted, the "solids" in their ingredient list are the dead bacteria. Not easy to strain those out, leaving the Cyt crystal toxin behind.

Of course, Bacillus thuringiensis lives naturally in soil, but it stays there only because it regenerates. The lifetime of the bacteria in soil is on the order of a week, and for the same reason that mixing with water incapacitates the toxins, those crystals only are in soil for a day or so.

Bt is actually 50-year old insecticide. It's been around for a long time! Originally sold as "DiPel", a name that is still used as a trademark. There is a tendency for cats to develop some resistance to it. That's why Bt is marketed with different bacterial strains, though Thuricide is precisely one strain. If resistance does develop, you either switch to a different strain (which might be significantly more expensive) or just not use it for a year or two.

On doing some research, I note that several extension sites warn that Bt should not be mixed with high pH water, though that's not on the instructions for Thuricide. My tap water pH is 9.2, so that's a big issue for me. The high pH "activates" the toxins, and you want that to happen in the cat gut, which is alkaline, and not in the solution you're spraying. I believe that's one reason why Bt isn't toxic to humans, because our guts are highly acidic. Not sure how this "activation" happens, but it has to do with solubility. Darrin says that many insecticides are pH sensitive in this way.

Hey thanks for the info! I had never really thought about it, but now I wonder if high humidity affects the crystals softening? I use Thuricide and noticed the last couple times I sprayed, it didn't seem to make a dent in the caterpillars at all.

I used to have a Bt powder - wonder if that might be a better choice?

Well, the humidity is always kind of high around here, and it's always worked well for me. But who knows, maybe humidity makes it less effective. Yes, the powder is, I think, DiPel, so Thuricide is just a liquid form of DiPel. Never used powder Bt, but I find that spraying both the tops and bottoms of leaves is important, and I'm not sure how easy it is to dust the bottoms of leaves.

If you have a puffer/squeeze bottle, it's not hard to hit the undersides of plants with powder.

But on the bottom of leaves, the stuff is less likely to stick.

I have been using thuricide for a while - for me it has worked great. I have been following this thread - thanks for the info!!!!!!!!!