Also for squash it says you need to wait 3 days before harvest. They also say it can be effective against bugs for up to 3 months. What? It also says to not get it wet. Does dew count as getting it wet?

Sevin is a broad spectrum insecticide and will kill beneficial insects.

True; It is short life contact insecticide. Its effective span is mitigated by water, so the dust will remain effective until it gets wet ( a heavy dew will do it) When wet it biodegrades rapidly. Liquid emulsion is used on insect pollinated crops by spraying in late evening after bees quit working. It is ineffective when they return in the morning. Dust is another matter as it does not degrade until wet.. Bees can pick up the dust along with pollen and carry it back to their hives. Advice: do not use dust on insect pollinated plants. Dust is also harder to control so if carbaryl (SEVIN) is needed, use a spray form. While listed as a broad base insecticide it is not effective on all insect pests.

Thanks recently someone here said it broke down by exposer to light (or maybe air). It got a heavy dew this morning but is still working good - but lots is under leaves where it likely did not get wet.

Carbaryl has a half-life of 1-30 days in pond water. Degradation there is largely from bacterial breakdown. So I suspect wetted dust will work for a little while. I think that's the case for many chemical pesticides.

That's why concentrated pesticides, that are diluted with water for spraying, should be used promptly. The ready-to-use versions aren't packaged diluted in water. Most likely ethylene glycol.

So I applied it 3 days ago, it is still working (no live cucumber beetles and new dead ones every day). According to PHI I can eat my squash. This seems insane to me? Still killing bugs but safe to eat?

Whatever you've put Sevin on, it would be very smart to wash before eating. I think the PHI refers to what is actually absorbed in the plant. That's why it's different for different plants. The PHI *assumes* that you've washed the stuff off the surface.