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