Ants and Aphids for Caterpilliars?

Four monarch caterpillars recently hatched on my milkweed, and I've noticed some ants and aphids on the plant, too. Do monarchs need these insects, or should I get rid of them?

—Mrs. John Mecozzi, Camarillo, California

Tom: Many butterflies in the hairstreak and blue families have a cooperative relationship with ants. The ants protect the caterpillars from predators, and in return, the ants feed on a sugary solution the caterpillars secrete. However, monarch caterpillars don't secrete a sugary solution, and they don't need protection, either, because the milkweed they eat produces toxins that ward off predators. So while the ants and aphids don't pose a threat to the caterpillars, they could be a problem for your milkweed. You can deal with the pests by applying a little soapy water to the plant when the caterpillars aren't present.