But does diotamaceous earth work on caterpillars or is it mainly a deterrent?

DE will kill them, if it gets on them. That's why uniform application is important. BT is sprayed on as a liquid, so the coverage is more complete than dusting. Dust has to be applied to the tops and bottoms of leaves. BT holds up better in the rain than DE. DE doesn't work when it is wet.

I'm confused because Safer sells a brand of dust that is BT not meant for spray. Also, the DE that I got said it can be mixed 20% with water and sprayed also ! /-:

Both come in at least a couple of different forms - dust, powdered, or spray - but the choice isn't about the form but which you use for which pest. DE dust is far more common and more commonly used than DE spray. Bt is the best for caterpillars as it kills rather than just deters. Dust or liquid spray Bt is your choice.

Dave

Thank you for that explanation. While I'm worried about standard leaf-eating caterpillars, the most urgent concern is root maggots that I found on three cucumber seedlings (Bonnie brand) that I've never had a problem with ever so I'm thinking they were already there (eggs or baby maggots) when I got them.

Z

Sorry for the confusion. You want BT, and spray is, in my view, your best option for that. I use both DE dust and BT spray (for different pests). DE dust is a real PIA to apply. If you blow it in bulk, most of it doesn't land on the plants. For DE, which is very cheap, that's not a big problem. Spray is point and shoot, covering large areas at once, and you can cover many square yards of foliage with just a tiny bit of concentrate.

Thank you. Yes I applied some DE earlier tonight and it is messy and the powder-on-plant look is not that great. For now I just have DE but next up will be getting some BT!

Z