What Eats Monarch Caterpillars? Predators and Defenses

Monarch caterpillars face a surprisingly long list of predators despite being loaded with toxic chemicals. Wasps, ants, spiders, beetles, crickets, birds, and even parasitic flies all prey on or exploit monarch larvae at various life stages. The caterpillars’ famous chemical defense, built from toxins absorbed through milkweed, deters many would-be attackers but is far from foolproof. Some predators have evolved workarounds, others simply target the youngest caterpillars before they accumulate enough poison, and a few just don’t seem to care about the toxins at all.

How Milkweed Toxins Build the Caterpillar’s Chemical Shield

Monarch caterpillars earn their unpalatability by eating milkweed, which contains bitter compounds called cardenolides. These chemicals interfere with a sodium-potassium pump found in the cells of most animals, which makes them toxic to the hearts and muscles of many predators. Monarchs tolerate cardenolides because they carry a mutation in that same pump, rendering them largely immune to the toxin’s effects. But tolerating a poison and weaponizing it are two different feats. Monarchs go a step further: they actively sequester cardenolides and store them in their outer tissues, right where a biting predator would encounter them first.¹

The strength of this shield depends on what the caterpillar eats. Milkweed species vary more than tenfold in cardenolide concentration. On highly toxic milkweed, adult butterflies eventually max out and can’t store more than a certain amount, ending up with lower concentrations than the leaves themselves. On milder milkweed, monarchs actually concentrate the toxins, ending up more toxic per gram than the plant.² Not all cardenolides are equal, either. Some milkweed compounds containing nitrogen or sulfur get broken down by the caterpillar’s own metabolism into less toxic forms rather than being stored intact, weakening the defense.³ The most abundant toxin in one well-studied milkweed species, a compound called voruscharin, isn’t sequestered at all. Instead, caterpillars convert it into other compounds, and the conversion itself slows their growth.⁴

Even soil quality plays a role. When milkweed grows in soil fertilized with nitrogen and phosphorus, monarch caterpillars feeding on those plants sequester cardenolides less efficiently and grow more slowly.⁵ So the chemical shield a caterpillar ends up with is shaped by the species of milkweed it eats, the particular toxins in those leaves, and even the dirt the plant grew in.

The Invertebrate Predators That Ignore the Warning

The sheer variety of invertebrates that eat monarch eggs and tiny first-instar larvae is wider than most people realize. Lab feeding trials have identified at least 16 different arthropod groups that consistently consume monarch eggs and 11 that eat newly hatched caterpillars. The list includes grasshoppers, crickets, tree crickets, damsel bugs, stink bugs, katydids, lacewing larvae, earwigs, plant bugs, lady beetles, and multiple ant species.⁶ Many of these are generalist predators that happen to share the milkweed plant with monarchs. Lady beetles, for instance, are drawn to milkweed by aphid colonies and then opportunistically eat monarch eggs they encounter along the way.⁷

Among the most devastating invertebrate predators of larger caterpillars are paper wasps. The European paper wasp, an invasive species now common across much of North America, is a ruthless hunter of monarch larvae. These wasps attack second through fifth instar caterpillars. Smaller caterpillars are simply bitten and carried off whole. Third and fourth instars get a grislier treatment: the wasp slices open the gut, discards it on the leaf, then macerates the rest of the body into a ball to fly back to its nest. Larger caterpillars sometimes require multiple trips.⁸

This gut-removal behavior is telling. By discarding the digestive tract, where cardenolide concentrations are highest, the wasps may be reducing their own exposure to the toxins. The same study found that nearly all fifth-instar caterpillars escaped wasp attacks through violent thrashing or dropping off the plant, suggesting the defense improves with size. But the damage to younger caterpillars is severe, especially in urban and suburban areas where paper wasp densities are high. In one survival experiment, only 45% of caterpillars placed in suburban areas survived just six hours of exposure to wasp foraging, and paper wasps accounted for 85% of the deaths.⁹

Vertebrate Predators and How Some Birds Beat the Toxins

Birds are the predators that the monarch’s entire warning-color strategy is theoretically aimed at. The bright yellow, white, and black stripes of the caterpillar (and the orange and black of the adult butterfly) serve as a visual signal: eating me will make you sick. For most bird species, this works. A single unpleasant experience with a cardenolide-laced monarch teaches many birds to avoid the pattern in the future.

But at the monarch’s overwintering sites in central Mexico, two bird species feed freely on adult monarchs by the millions. Black-headed grosbeaks appear to have evolved a degree of insensitivity to the toxins themselves, while black-backed orioles use taste to sort through butterflies and selectively eat those with lower cardenolide loads.¹⁰ These strategies target adult butterflies rather than caterpillars, but they illustrate an important point: the cardenolide defense is not an impenetrable wall, and evolution keeps finding ways around it.

Studies using artificial caterpillar models placed on milkweed plants in the field confirm that birds do attack caterpillar-shaped objects on milkweed, though at lower rates than arthropods. In one large deployment of over a thousand model caterpillars in Ohio, about 2% showed signs of avian attack compared to roughly 3% from arthropods.¹¹ Mammalian attacks were vanishingly rare in the same study. These models can’t replicate the taste or toxicity of real caterpillars, so the numbers likely overestimate how often birds follow through on a real monarch. Still, they confirm that arthropods are the primary predators in the field and that avian predation, while real, is secondary.

Parasitoids That Kill from the Inside

Some of the most insidious threats to monarch caterpillars aren’t predators in the traditional sense. Parasitoid flies and wasps lay their eggs on or inside caterpillars, and the larvae develop by consuming the host from within. The tachinid fly Lespesia archippivora is the best-documented parasitoid of monarchs. Female flies deposit eggs on the caterpillar’s skin, and the fly larvae bore inside to feed. Most of these flies emerge during the monarch’s fifth (final) larval stage, killing the caterpillar just before it would have pupated. One study collected over 400 individual flies from 124 parasitized monarch larvae, with higher survival for fly broods that emerged during the fifth instar compared to those emerging earlier.¹²

Monarchs also contend with a protozoan parasite called Ophryocystis elektroscirrha, often abbreviated OE. This single-celled organism spreads primarily from mother to offspring: infected female butterflies scatter parasite spores on milkweed leaves when they lay eggs, and newly hatched caterpillars ingest the spores while eating. First instars are the most vulnerable, with as little as one spore capable of producing a detectable infection in the resulting adult butterfly. Older caterpillars are less susceptible simply because they need more spores relative to their body mass to become meaningfully infected.¹³

The milkweed species a caterpillar feeds on can dramatically affect OE’s impact. Infected monarchs raised on milkweed with high cardenolide levels suffered about a 30% reduction in lifespan, while those on low-cardenolide milkweed lost roughly 55% of their lifespan. The toxic milkweed didn’t prevent infection, but it slowed the parasite’s ability to replicate inside the host.¹⁴ This dual-use benefit of cardenolides, deterring both vertebrate predators and internal parasites, partly explains why monarchs have evolved such an intimate dependence on milkweed.¹⁵

Behavioral Defenses Beyond Chemistry

Monarch caterpillars don’t rely on toxins alone. They also use physical behaviors to survive encounters with predators and to maximize their toxin intake in the first place.

One of the most distinctive behaviors is vein-cutting or “trenching.” Before feeding on a section of milkweed leaf, caterpillars chew through the leaf veins to cut off the flow of latex, a sticky white sap that can gum up their mouthparts or even trap tiny larvae. This behavior was originally interpreted purely as a way to disarm the plant’s physical defense.¹⁶ But more recent research reveals an additional purpose that surprised researchers: older monarch caterpillars actually drink the latex that pools at the cut site. Fifth-instar caterpillars were observed gulping latex immediately, and their regurgitate afterward had cardenolide concentrations ten times higher than those of a related butterfly species that avoids the toxins.¹⁷ Younger caterpillars rejected latex when it was applied to their mouthparts, but the frequency of drinking increased steadily with age. This suggests that as caterpillars grow large enough to tolerate the physical stickiness of latex, they actively seek it out as a concentrated source of defensive chemicals.

When a predator approaches, many caterpillars drop off the plant entirely. This dropping behavior is context-dependent and surprisingly sophisticated. Caterpillars adjust their willingness to drop based on temperature and body size, both of which affect their odds of surviving on the ground versus staying on the leaf. Smaller caterpillars are more vulnerable to heat on the ground surface but also more vulnerable to predators on the plant, so the decision involves a tradeoff. Research has found that monarch caterpillars generally make dropping decisions consistent with what would maximize survival under the specific conditions they face, though at very high temperatures they may drop even when it’s not clearly advantageous, likely because heat makes them more reactive.¹⁸

Fifth-instar caterpillars also thrash their bodies violently when attacked. Against paper wasps, this thrashing is effective enough that nearly all late-stage caterpillars survive encounters that would be fatal for younger, smaller larvae.⁸

An Evolutionary Arms Race in the Predators’ Genes

The cardenolide defense strategy has been so successful that it has shaped the genetics of the predators and parasites that attack monarchs. At least four distantly related groups of monarch enemies have independently evolved changes in the same molecular target that cardenolides attack, the sodium-potassium pump encoded by a gene called ATPalpha. The black-headed grosbeak, which feeds on adult monarchs in Mexico, carries amino acid substitutions in two versions of this pump that are associated with reduced sensitivity to the toxins. Similar substitutions appear in rodents, a parasitoid wasp, and a nematode that attacks caterpillars.¹⁹

This convergent evolution across birds, rodents, wasps, and worms is striking. The same handful of mutations keep appearing independently in unrelated lineages, suggesting that there are very few molecular paths to cardenolide resistance. It also underscores that the monarch’s toxin defense is not static. It exists within an ongoing evolutionary tug-of-war, with attackers continually evolving resistance and monarchs relying on high enough concentrations, warning coloration, and behavioral defenses to stay ahead.

When the Habitat Itself Changes the Odds

Where a monarch caterpillar grows up can matter as much as what it eats. Urban pollinator gardens planted with milkweed might seem like ideal monarch habitat, but they can function as ecological traps. Paper wasp densities tend to be higher in suburban and urban settings, where the wasps nest on buildings and fences. In one study, sentinel monarch caterpillars placed on milkweed in urban pollinator gardens suffered significantly more predation than caterpillars placed in rural meadow habitats, with over 52 hours of observation revealing no predation from anything other than paper wasps in urban gardens.⁸

Pesticide exposure adds another layer of risk. Neonicotinoid insecticides used on crops can drift onto nearby milkweed plants. Monarch caterpillars that feed on milkweed contaminated with clothianidin, a common neonicotinoid, can experience reduced body size even at concentrations as low as one part per billion.²⁰ Smaller caterpillars are more vulnerable to predation, less able to thrash off wasps, and carry fewer cardenolides. So pesticide drift may indirectly increase predation pressure even without directly killing caterpillars.

New Arrivals and How They Respond to Warning Signals

As invasive species spread across North America, researchers are watching to see whether newcomers will become significant monarch predators. The Jorō spider, a large orb-weaver from East Asia that has rapidly colonized the southeastern United States, was an early concern. But feeding trials found that Jorō spiders attacked monarchs only about 20% of the time, compared to 86% for similarly sized gulf fritillary butterflies and 58% for tiger swallowtails. Some spiders even cut monarchs out of their webs and dropped them rather than eating them.²¹ The spiders had no evolutionary history with monarchs, yet they recognized them as unpalatable, likely through chemical cues rather than visual ones. This finding is somewhat reassuring: warning signals can work even on predators encountering the signal for the first time in evolutionary terms.

The European paper wasp is a starkly different case. It arrived in North America in the late twentieth century and has become one of the most significant predators of monarch caterpillars in many regions, particularly in the suburban landscapes where people are most likely to plant milkweed. Unlike the Jorō spider, paper wasps are not deterred by cardenolides in any obvious way. They gut the caterpillars, discard the most toxic tissue, and feed the protein-rich remains to their own larvae. Whether this gut-discarding behavior is a learned response to toxicity or simply how wasps process any large soft-bodied prey is still debated, but the practical outcome is the same: paper wasps eat monarch caterpillars at high rates regardless of the chemical defense.

The contrast between these two invaders illustrates a recurring theme in monarch ecology. The caterpillar’s defenses, chemical and behavioral, work well against many threats but are far from universal. Success depends on the type of predator, the caterpillar’s age and size, the milkweed species it grew on, and increasingly, the habitat that humans have shaped around it.