Edith Smith, the founder of the Association For Butterflies
http://www.forbutterflies.org/conservation.htm and a high volume monarch breeder also recommends rinsing the Malathion off treated plants. However, after conducting some tests, Edith found monarch caterpillars weren't harmed even if the treated plants weren't rinsed. On Dec. 19 & 20, 2007, Edith wrote the following on the "'Butterfly farmers and enthusiasts mailing list'" <butterflyfarming@butterflymarket.com>
"We use Malathion all the time. We have dunked milkweed into mixed Malathion and fed it directly to caterpillars, they pupated, and emerged with no problems. We don't guarantee it to other people, of course, as you never know what other 'inert' ingredients are in some Malathion mixes."
"Since we use Malathion and recommend it for milkweed aphids, we did a test with a few caterpillars to be sure that what we recommended wouldn't harm them if they didn't rinse well or something like that. Of course, you never know about the carrier agents in sprays ... sometimes a carrier or 'inert' agent might do harm."
"I have talked with people who thought their caterpillars were diseased so they sprayed them with Malathion and it never hurt them. They were shocked... two ways. One was that it didn't hurt them, the other way was that they were wrong; their caterpillars weren't diseased!"
~Edith