Monarch Parasites & Enemies

Natural Enemies and Other Invertebrates

Are aphids on milkweeds harmful to monarchs?

No, aphids eat the milkweed plant, not monarchs! Ants that may be tending the aphids can sometimes attack small caterpillars on the plant, but the aphids themselves do not harm monarchs. 

How are monarchs toxic to predators?

Monarchs become toxic to predators by sequestering or storing toxins from the milkweed plants that they eat. Milkweed contains toxins called cardenolides, or cardiac glycosides, which are toxic to predators. This makes monarchs very distasteful or unpleasant to predators. Some predators have evolved ways to avoid or tolerate these toxins, such as the bird predators found in the Mexican overwintering colonies. 

What invertebrate predators do monarchs have?

Invertebrate predators such as ants, spiders, and wasps attack monarch larvae on milkweed plants. Wasps have been observed feeding on monarch abdomens at a California overwintering site, and fire ants have been suggested as a major predator of monarch larvae in Texas. Other research suggests that wasp predators may be sensitive to the chemical defenses of monarch larvae, and that wasps fed monarch larvae with high cardenolide concentrations had lower reproductive potential and more deformities in their nests than wasps that preyed upon less toxic caterpillars. Some invertebrate predators, such as ladybug larvae or lacewing larvae prey on monarch eggs. 

Small white worms (maggots) emerged from my monarch pupa, what are they?

These are tachinid fly larvae. Soon after they emerge from the monarch caterpillar or pupa, they will turn to dark brown pupae. An adult tachinid fly lays her eggs on a monarch caterpillar, then the tachinid larvae burrow inside the monarch caterpillar and develop within it. They emerge from late stage monarch caterpillars or early pupae, killing the monarch. 

Are there parasitoids that affect monarchs?

Yes, there are a few. Tachinid flies are the most commonly known parasitoid of monarchs. Adult flies lay fly eggs on monarch caterpillars, which then burrow into the caterpillar and develop inside. After feeding on the growing monarch, they emerge from the large monarch caterpillar or early pupa as larvae and then pupate soon after, killing the monarch. Research in the Monarch Lab suggests that the species Lespesia archippivora (La) is the most important monarch tachinid parasitoid. 

Less is known about the extent to which other parasitoids attack monarchs, but at least one wasp in the family Braconidae has been reported in monarchs. The closely-related queen, Danaus gilippus is parasitized by two Chalcid wasps,Brachymeria annulata and B. ovata, as well as L. archippivora. Current research in the Monarch Lab demonstrates that a wasp in the family Pteromalidae and the same superfamily, Chalcidoidea, as the two Chalcid wasps found in queens, could be an important pupal parasitoid.

Are all milkweeds toxic?

Yes, all milkweed species contain cardenolides, or cardiac glycosides, making them toxic. Species do, however, differ in level of toxicity. For example, common milkweed, Asclepias syriaca,  has a much lower cardenolide concentration than tropical milkweed, Asclepias curassavica