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A few weeks ago I sprayed my broccoli and bok choy plants with bacillus thuringiensis in an attempt to control caterpillar numbers. I'm pleased to see no new caterpillars (pieris rapae larvae) since then even though I have seen eggs laid on leaves.

However, bacillus thuringiensis is said only to persist on the leaves for a few days so why is the effect lasting so long? I think the recommendation is to respray after three weeks so it's not just me.

Do the bacteria numbers increase in the soil after application, or is there some other mechanism that suppresses the larvae?

Ps: diy expansion of the bacteria using a milk culture

  • I've been reading and amazingly no one points out the differences in terminology, what I've been reading is that is IS a mixture of bacteria, spores, toxin crystals. There is a two fold kill; one by the crystals and the other by the proliferation of the bacteria via spores. The article I read was one that was posted by you or me? The best bit of information I've read since you asked this question. There is no problem with the Bt taking off in the soil, nor is there any indication the Bt you sprayed is still working. It could simply be your caterpillar turned into a butterfly and flew away – stormy Feb 2 '18 at 1:08
  • @stormy Maybe you read my answer? – Graham Chiu Feb 2 '18 at 1:22
  • Well, maybe that is where I read that? Hey, I was heavily into Bt stuff when I got waylaid by my stolen truck and different cop departments and then I tried to finish and I'll just go delete my answers/comments. What a day. Sorry.. – stormy Feb 2 '18 at 1:26
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Most commercial preparations of BT contain both the spores of bacillus thuringiensis and the crystal toxin produced by the bacterium.

The commercial product contains about 2.5 x 10 to the 11th viable spores per gram. Typical agricultural formulations that contain spores and protein crystals include wettable powders, spray concentrates, liquid concentrates, dusts, baits, and time release rings source

It's the actual insecticidal crystal protein that is required to kill the insect, and this dissolves in the highly acidic pH of the insect gut activating the poison.

BT mechanism

Now the bacterium and toxin only persists for about 4 days with a bacterium half life of about 3.8 hours on leaf tissue due to the sterilizing effect of UV, and presumed breakdown of the crystals in the environment.

But the infected insects constitute another source of the bacterium since as they die their bodies become infected with the bacillus and spill their contents. So, I suspect that is why the effect appears to persist for a few weeks.

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