Enthusiasts often run out of leaves for caterpillars and rush to the nursery to buy more plants. Then comes the BIG question: How long should I test it with one caterpillar before placing more caterpillars/leaves together? You won’t like the answer.
First, to be 100% sure, until it becomes an adult and flies away without problem. If a caterpillar is exposed just before pupating, some insecticides won’t affect them until they emerge as adults. If some pupae are exposed, it won’t affect them until they emerge as adults. The adults will expand and dry their wings, but when they start to fly around, they go into spasms, usually eventually dying with their wings folded over their legs.
Next, at least into chrysalises. Some insecticides (such as used in some oral flea/tick medications) will affect them only when they begin to pupate. The result is deformed or incomplete chrysalises.
If you don’t have that much time and MUST feed the leaves to your caterpillars, wait three days if at all possible. The certified organic pesticide, Bt (a natural soil-dwelling bacteria), takes three days to do it’s job. Three days after caterpillars eat it, the caterpillars die.
Many insecticides will cause almost immediate vomiting (green fluid that stays green) and sometimes convulsions. This isn’t a pain reaction. It’s a nerve reaction. The insecticide acts on certain nerves, causing the caterpillars to jerk around.
Bottom line – the longer the better, up to the adult emerging and flying away.