This is tricky, as you should not use any substances that could harm other soft-bodied insect, like the caterpillars. Normally we could recommend using a sprayer or hose to spray down the plants and blast some of the aphids off, but in this case, you may inadvertently get rid of eggs or caterpillars, too. Perhaps you can do this before the monarchs arrive?
Destroying the aphids by hand is likely the best way to ensure that butterfly eggs and caterpillars are not damaged in the crossfire. You could also look into purchasing ladybugs online and setting them loose in your garden. If there are plenty of aphids, the ladybugs should stick around, though there’s no guarantee they won’t venture elsewhere.
This is tricky, as you should not use any substances that could harm other soft-bodied insect, like the caterpillars. Normally we could recommend using a sprayer or hose to spray down the plants and blast some of the aphids off, but in this case, you may inadvertently get rid of eggs or caterpillars, too. Perhaps you can do this before the monarchs arrive?
Destroying the aphids by hand is likely the best way to ensure that butterfly eggs and caterpillars are not damaged in the crossfire. You could also look into purchasing ladybugs online and setting them loose in your garden. If there are plenty of aphids, the ladybugs should stick around, though there’s no guarantee they won’t venture elsewhere.