Monarch butterflies are famously toxic, but a surprising number of predators eat them anyway. Birds, mice, insects, and spiders all prey on monarchs at every life stage, from egg to adult. Fewer than one in ten monarch eggs survives to become an adult butterfly, and predation is the biggest reason why.
Why Monarchs Are Toxic but Not Invincible
Monarch caterpillars feed on milkweed and store the plant’s cardiac glycosides in their bodies. These toxins carry through to the adult butterfly and can cause vomiting and heart problems in animals that eat them. The bright orange-and-black coloring serves as a warning to potential predators.
But this defense is far from foolproof. Many predators have evolved ways to tolerate, avoid, or simply ignore the toxins. Others prey on eggs and young caterpillars before they’ve accumulated enough toxin to matter.
Birds That Eat Adult Monarchs
The most significant predators of adult monarchs are birds at the overwintering colonies in central Mexico, where millions of butterflies cluster in fir trees each winter. Research at five overwintering sites found that individual birds from several species have learned to get around the monarch’s chemical defense. Two species do the most damage: black-backed orioles and black-headed grosbeaks.
These two birds use completely different strategies. Grosbeaks tear off the wings and eat the abdomen whole. They appear to have some physiological tolerance for the toxins. Orioles are more surgical: after removing the wings, they gut the abdomen and eat only the insides, discarding the outer shell, which carries a higher concentration of cardiac glycosides. Orioles also reject individual monarchs that have especially high toxin levels, suggesting they can detect how poisonous a given butterfly is.
Mice at Overwintering Sites
Black-eared mice migrate into monarch overwintering colonies in Mexico and feed on the butterflies throughout the winter. A population of 75 to 105 mice can kill roughly 400,000 to 570,000 butterflies in a single colony over the 135-day overwintering season. That represents 4 to 5.7% of a colony covering one hectare. The mice forage among the carpet of butterflies that accumulate on the forest floor, picking off individuals weakened by cold temperatures.
What Preys on Caterpillars
Monarch caterpillars face a gauntlet of threats. Fire ants swarm and kill them. Paper wasps snatch them off milkweed leaves. Spiders catch them in webs or ambush them directly. Even ladybugs eat monarch eggs and newly hatched larvae.
Praying mantises eat monarch caterpillars using a particularly clever technique. Chinese mantises and other species remove the caterpillar’s gut before eating the rest of the body, discarding roughly 59% of the cardiac glycosides in the process. Because these toxins are water-soluble and don’t pass through the mantis’s gut lining, the small amount remaining in the body tissue does little harm. Mantises use this same gutting behavior even on non-toxic caterpillars, suggesting it originally evolved to discard indigestible plant material.
Parasitoids: Killed From the Inside
Some of the deadliest threats to monarch caterpillars aren’t predators in the traditional sense. Parasitoid insects lay their eggs on or inside the caterpillar, and their larvae slowly consume the host from within.
Tachinid flies are the most common parasitoid. A female fly lays eggs directly on the caterpillar’s skin. The maggot hatches and burrows inside, feeding on the caterpillar’s tissues without killing it right away. The host typically survives until it’s ready to form a chrysalis. Then, as the caterpillar hangs in its characteristic J-shape, the fly larvae emerge and drop to the ground to pupate. No parasitized monarch survives. Across a large study spanning 1999 to 2005, about 13% of wild monarch larvae were parasitized by tachinid flies, though in some local populations the rate reached as high as 90%.
Older caterpillars are more vulnerable. The likelihood of parasitism roughly doubles with each successive growth stage, and larger caterpillars tend to harbor more fly larvae, sometimes up to ten per host. Interestingly, fifth-instar caterpillars (the final and largest stage) can sometimes shake off the adult flies before they lay eggs.
Braconid wasps also parasitize monarchs, though less frequently. When they do, a single caterpillar can produce as many as 32 tiny adult wasps. Tiny parasitoid wasps in the genus Trichogramma target monarch eggs themselves, accounting for about 5% of egg-stage deaths in field studies.
What Eats Monarch Eggs
Monarch eggs are small, exposed, and lack the toxin levels of later life stages, making them easy targets. A study published in Scientific Reports tested dozens of arthropod species and found that 16 different types consistently ate monarch eggs. The list is broader than most people expect: earwigs, crickets, katydids, lacewing larvae, grasshoppers, plant bugs, and certain ants all consumed eggs in more than half of trials.
Spiders, despite their reputation as generalist predators, were surprisingly uninterested in eggs. Orb-web spiders ate zero eggs in 28 trials, and jumping spiders ate none in 23 trials. Crab spiders consumed just one egg out of 34 attempts. All three spider families did eat newly hatched caterpillars, though, suggesting the movement of a tiny larva triggers a predatory response that a stationary egg does not.
How This Adds Up
The cumulative toll is staggering. In field studies tracking monarchs from egg to the third caterpillar stage (roughly halfway to adulthood), only about 13.4% survived. The losses came from every direction: arthropod predators, parasitoids, plant damage from wind or wildlife, and even rabbits browsing the milkweed plants the eggs were laid on. Plant damage and egg parasitism alone accounted for 16% of all recorded deaths, with the rest driven largely by predation.
By the time you factor in the remaining caterpillar stages, the chrysalis period, and the hazards of adult life, fewer than one in ten eggs produces a butterfly that takes flight. The monarch’s famous toxicity improves those odds, but it’s a numbers game. Monarchs survive as a species not because they’re untouchable, but because females lay hundreds of eggs across many milkweed plants, ensuring that enough make it through.