Here we have 2 types of cabbage worms, eggs laid from the cabbage moth (brown) and the cabbage butterfly (white w/black spots on wings). My spring cole crops (broc/cabbage) are long gone now but if I see the white cabbage butterflies flying around in this heat they're most likely laying eggs somewhere. Don't really know if the worms can survive the summer heat but would assume that they would crawl deep into the shadier parts of the plant and continue to feed on them.

I'd think any temp. high enough to kill an insect would also not be so great for the plants either, so I wouldn't count on that.

"When exposed to temperatures of just less than 100° F, an insect’s growth will slow. At temperatures of 100° F to 120° F, it will die in a day. Above 120° F, an insect will die in minutes."

https://www.qualityassurancemag.com/article/-pest-management--treating-with-heat/

I'm getting temps around 100F and, with enough water, my collards are doing fine.

I've got grasshoppers chewing on my collards now.

when it's hot and dry during the summer/ going into fall I just try to keep the collards alive by watering. Then when frost comes , that slows the hoppers down and then I have a winter supply of collard greens at least till it gets too arctic cold for them.

Okie HU

i can't grow any of the cole crops without row covers which protect them from the cabbage butterflies and flew beetles. I usually sow my collards around mid July--but I've never had the kind of heat we're getting now.

I always have started my winter savoy cabbage July 1 (actually moved it up from mid June when fall weather got hotter and hotter a few years ago), but this year I'm afraid it was waay too soon. The plants, sown in a little pots July 1, already have their first true leaves (!) by July 10 and will need to be potted up by the end of this weekend. Very fast growth. I'm afraid that the same thing will happen to them as happened when I realized that mid June was too early--the fall was so hot that they grew and grew until they burst, so no cabbage to harvest in winter.

Re cabbage butterflies, Bt works great. That's what I was asking about. When I can stop applying it. I believe it may work somewhat on flea beetles as well. So it is said.