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Old August 20, 2016   #7
gorbelly
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Location: Southeastern Pennsylvania
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Originally Posted by jillian View Post
Applied DE to the stems and all was well in the beginning. Then the wilting began . I just pulled them and here is what I found

In all 4 plants. Maybe it is my growing conditions, timing, or some other error on my part but I am finished with squash. I will be quite content purchasing mine from produce stand. The SVB's have won the battle
FWIW, DE isn't really for soft-bodied insects. It works on hard-shelled insects because it abrades their exoskeletons and removes the waxy coating that protects them, then they desiccate. It doesn't always work well on soft, flexible-skinned pests like caterpillars and grubs.

And for people who don't like to use pesticides because of possible harm to non-pest organisms--DE is actually more broad-spectrum than Bt. DE will harm anything that has an exoskeleton, which means most pollinators and predatory insects. But Bt is very specific. BtK harms only moth and butterfly larvae, i.e., caterpillars, including the squash borer caterpillar. (BtI harms only the larvae of long flies like mosquitoes, blackflies, etc.). So it's actually less of a "pesticide" than DE and, unlike DE, it will actually work on borers if you spray the plants at the right time. The caterpillar has to eat the Bt for it to work--it's not a contact pesticide--and Bt doesn't kill its targets immediately. So Bt has to be on the vine when the borer starts boring into it so that the borer actually eats the Bt. It won't prevent the borer from boring, but it will hopefully kill the borer before it can eat through a lot of the vine and do enough damage to hurt the plant. If your timing is off, though, and the Bt has worn off or gotten washed off when the borer hatches and starts eating into the vine, it won't work.

In my experience, you should apply it at a higher concentration than the bottle indicates to the bases of the vines every few days once you start seeing eggs and reapply immediately if it rains. If you have a lot of plants, you won't get them all, as there will still be windows of opportunity for borers to get in if they hatch right after the Bt was washed off by rain or if you missed a spot or something. But it should help more than DE.

I've actually been wondering whether it might work to spray the Bt, then coat with vaseline or something on top after it dries to protect it from wearing off/washing off so quickly. But I have no idea whether that would inactivate the bacterium or what.

Spinosad is somewhat more broad-spectrum than Bt, so I use it sparingly. I think only at the bases of squash plants is a good use for it, since it doesn't come into contact with pollinators at all and covers a limited area on the plant.
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