Even without facing the contemporary perils of climate change, habitat loss, and pesticide, a monarch egg has roughly a 5 percent chance of surviving to the adult stage. Why is survival so low, even in the best of natural circumstances?

The largest losses occur at the egg stage, due to predation by insects, desiccation, or dislodgment from milkweed leaves by rain and wind. Monarch caterpillars and chrysalises — although somewhat less vulnerable — are also preyed on by invertebrate predators and parasitoids.

However, for centuries the species has flourished in spite of these vulnerabilities, offset by the ability of a single female to lay 300-500 eggs

What are the strengths that the monarch brings to the table?

While monarchs are susceptible to predation from mammals and birds as well as insects, parasites, and parasitoids, they receive powerful protection from consumption of milkweed.

Milkweed is the monarch’s secret weapon. Monarch butterflies lay their eggs on milkweed plants and only milkweed plants. When the caterpillars hatch, they eat milkweed and only milkweed. The milkweed plant has a toxic milky sap in its leaves and stem. The sap contains cardiac glycosides or cardenolides that taste bad, not only making the milkweed unappealing to many animals but also lending a foul taste to the caterpillar that consumes it.

And the vivid markings of the monarch caterpillar and butterfly are thought to be aposematic (or “warning!”) coloration, clear signs for predators not to eat them because they have sequestered away plant poisons.

This system works best for defense against vertebrates, notably birds and small rodents. When these animals consume a monarch caterpillar, they ingest the whole body and thus receive all the toxic cardenolides. In contrast, insects are capable of selectively eating the parts of the caterpillar that contain the least milkweed sap. This explains why insects are the more powerful predators.

Vertebrate Predators

Vertebrate predators — birds, rodents and other small mammals — pose relatively little threat to Western monarchs. ·One reason for this is that birds are relatively vulnerable to the poisonous chemicals present in the sap of milkweed. Mammals and birds are likely to become sick and vomit after consuming monarch caterpillars, pupae, or butterflies.  

In Marin, vertebrate predators can be problematic primarily at overwintering sites along the coast. Mia Monroe, who monitors these sites as part of the Xerces Western Monarch Count, notes that she has seen monarchs occasionally facing attacks from black-headed gros-beaks, jays, and hawks (personal communication, 2025). Squirrels and mice can also invade overwintering clusters.

The most effective way to offset predation in overwintering sites is by providing ground cover, understory plants, and roosting trees where the monarchs can take cover from harmful vertebrates.

Invertebrate Predators

In contrast to vertebrate predators, insects are often big milkweed fans. Spiders, ants, wasps, stinkbugs, thrips, white flies, and aphids are all major predators. It seems as if insects are capable of selectively eating the parts of the caterpillar that contain the least milkweed sap, whereas birds typically consume the whole thing and thus ingest all the cardenolides.

In Marin, the aphid is one of the biggest consumers of milkweed. While there are various aphid species in Marin, it is the oleander aphid (Aphis nerii) that most frequently targets milkweed. The oleander aphid is an invasive insect equipped with piercing, sucking mouth parts that enable it to pierce the stem or leaf of a milkweed plant and suck out the milky sap. And like monarchs, they actually sequester milkweed cardenolides as a protection against predators! This allows them to feed nonstop, except when they encounter their only sworn enemy, the Lysiplebus testaceipes (no common name), a tiny wasp capable of injecting eggs into the body of an aphid, where it hatches and feeds on its still-living prey! Read more about this amazing creature in Eric Lee-Mader’s wonderful book, The Milkweed Lands.

Parasitoids

Parasitoids are specialized insects that lay eggs on a host caterpillar or inject their eggs inside it. Newly hatched parasitoids feed on the host from the inside out, eventually causing its death. Fly and wasp parasitoids lay their eggs on monarch caterpillars. Most problematic are flies in the family Tachinidae. Various species of tachinid flies lay eggs on monarch caterpillar skin (integument) which then hatch and bore into the caterpillar. These parasitoids are apparently unaffected by sequestered cardenolides, perhaps because parasitoid larvae move freely inside the monarch body, perhaps avoiding the most toxic parts.

Some strategies for preventing parasitoids from harming monarchs include reducing high-density caterpillar populations that attract parasitoids as well as planting an abundance of milkweed to help reduce the concentration of caterpillars on any one plant.