Predators and Parasites on the Prowl

Female tachinid fly laying her eggs on the skin of a moth larva.
Photo by Robert W. Mitchell

Sucking the life from their victims, devouring the eggs, eating the young--you don't have to go to Africa to see it. You can explore the predator/prey interactions that monarchs face in your own backyard and you'll be amazed at what you see.

I certainly was! Right before my eyes, my monarch caterpillar suddenly split open--and out crawled 3 maggots. It was like science fiction....The white larvae, I learned later, were tachinid flies. The adult tachinid fly lays its eggs on monarch larvae--and the young flies develop inside. Just before my monarch became a chrysalis, the fly larvae emerged. (I was curious, so I kept them. They developed into adult flies within a few days....Not as nice as monarchs, maybe, but fascinating.)

Stink Bug
Photo by FAIRS

Keep in mind: A single female monarch butterfly can lay several hundred eggs. If they all survived, the world would be overcome with monarchs. Predators play a significant role in keeping the balance--but nobody really knows their impact on overall monarch abundance. This summer, you can help scientists study these questions. See:

Predator Watch

Michele Prisby, U. of MN, demonstrates the exclosure she invented to contain monarch larvae.

You can also simply find a few milkweed plants and standby. Spiders, ants, predacious wasps, milkweed bugs, stinkbugs, mites and many others are known to prey on, or parasitize, monarch eggs and larvae. Sketch the different creatures you find on milkweed and try to identify them. Build an exclosure around several milkweed plants. Do more monarch larvae seem to survive when protected inside the exclosure? Try to quantify your observations.

Protection in the Bag!
A Monarch Watch contributor suggested protecting monarch eggs and larva by using mesh paint filter bags to cover the milkweed plants. The bags are about 5 gallon size, available at paint stores, and inexpensive.

Before covering the milkweed, carefully look the plant over and remove any existing potential predators. Then, gently pull the bag over the plant, and secure the bag at the base of the stem with a twist tie. But be careful--check the bag periodically, because one larva may eat the whole plant before you know it and need to be moved to another plant!

Monarch larvae develop through 5 stages, called "instars." Learn how to recognize each stage:

Photos & Information About Some Monarch Larvae Parasites & Predators

Copyright 2002 Journey North. All Rights Reserved.
Please send all questions, comments, and suggestions to our feedback form