What Eats Monarch Caterpillars?

Did you know that fewer than one out of every ten eggs laid by a female monarch will survive to become an adult butterfly? Monarchs have many natural enemies. Predators such as spiders and fire ants kill and eat monarch eggs and caterpillars. Some birds and wasps feed on adult butterflies. These predators are easy to see, but monarchs are also attacked by parasites, living things that actually live inside the monarchs own bodies. Parasitic insects called parasitoids frequently kill monarchs. Other parasites are extremely small and can only be seen with a microscope. Just like humans, monarchs can get sick and die from diseases caused by parasitic bacteria, viruses, fungi, and protozoans.

Protozoan parasites such as Ophryocystis elektroscirrha and a microsporidian Nosema species have been reported to infect both wild and captive monarch butterflies. Dark spots are lesions containing thousands of developing parasite spores just underneath the monarchs cuticle.

One sign that monarch larvae could be infected with a pathogen is if they stop eating and hang from the host plant (or side of a container) by their prolegs, with the anterior and posterior ends drooping downwards. Dead larvae and pupae often turn dark brown or black within a few hours of death; this can be a sign of bacterial decay.

Do monarch caterpillars have any predators?

Invertebrate predators such as ants, spiders, and wasps attack monarch larvae on milkweed plants (Prysby 2004). … Birds such as black-backed orioles and black-headed grosbeaks are common predators at monarch overwintering sites. These species can eat large quantities of monarchs without getting poisoned.

How can I protect my monarch caterpillars from predators?

You can use mesh bags with drawstrings to enclose either entire plants or just a few branches or vines. You can either make mesh bags, or buy them. Keep in mind that butterflies will not be able to lay any more eggs on the plant while it’s socked in.

What killed my monarch caterpillar?

A: There are many diseases and parasites that kill monarchs, including viral, protozoan, fungal, and bacterial infections. These often kill the caterpillars just before they pupate, or during the pupa stage.

What animals eat monarch butterflies?

Assassin bugs feast on monarch caterpillars..Birds (Black-backed orioles and black-headed grosbeaks are common predators for butterflies overwintering in Mexico.).Chalcid Wasps (monarch chrysalis parasite).Lizards..Mice will eat chrysalides..Spined Soldier Bug- Predatory Stink Bugs..Toads.

The same misinformation we embraced last century continues to mislead new generations through shows like Wild Kratts: Voyage of the Butterflies. In this episode, a spider cuts a monarch from its web, refusing to eat the milkweed-laced butterflyessentially spinning science into science fiction!

Monarchs become toxic to predators by sequestering toxins from the milkweed they ingest as larvae, and are brightly colored in both the larval and adult stages to warn predators of this toxicity. Despite the fact that milkweeds are assumed to convey some degree of protection from generalist predators and parasitoids, monarchs of all life stages are vulnerable to predation and disease.

Protozoan parasites such asOphryocystis elektroscirrha and a microsporidian Nosema species have also been identified in wild and captive monarchs (McLaughlin and Myers 1970, Leong et al. 1992;1997, Altizer and Oberhauser 1999, O. Taylor, personal communication). Many researchers are currently exploring the role of parasites and infectious diseases in regulating insect population size (E.G. Faeth and Simberloff 1981, Bowers et al. 1993, Jaenike 1998).Monarch larvae are generally found singly on milkweed plants, unlike the large aggregations of adults in overwintering clusters.

Lower larval density in milkweed patches reduces the chance of diseases, such as nuclear polyhedrosis virus and Pseudomonas bacteria, spreading between larvae. After host pupation, the parasite undergoes sexual reproduction and forms dormant spores around the scales of the developing adult butterfly (McLaughlin and Myers 1970).

Ophryocystis Elektroscirrha OE) is a single celled organism called a protozoan. A protozoan isnt a plant or an animal but a single celled organism that shares characteristics of both.

Other birds, rodents, wasps, parasitoids, dragonflies, mantids and even automobiles will kill or injure adult butterflies. This is mainly due to year round availability of milkweed, the Monarchs host plant and their caterpillars food source.

One way to see if your Monarch has OE is to carefully press a clear piece of tape on its abdomen and check under a microscope. The University of Minnesota has a citizen scientist program called the Monarch Larva Monitoring Project , which you can be a part of. During the Monarchs late instar or pupa stage the fly maggot will drop out on tendrils looking like white strings.


Parasitoids are parasitic insects such as small flies and wasps that lay eggs on other insects. These special parasites only live on a single host, eventually killing it. Parasitoid larvae eat their host from the inside out, usually emerging from the remains of the host as a pupa or adult.Tachinid flies and braconid wasps are two parasitoids that feed on and kill monarchs. These parasitoids lay their eggs on the caterpillars. Tachinid fly larvae feed on monarch caterpillars, but usually don’t kill their hosts until just before the caterpillars pupate. When a parasitized caterpillar hangs upside down in the pre-pupal “J”-shape, several tachinid fly larvae or maggots will come out of the monarch caterpillar. The fly maggots drop to the ground on long, gel-like threads.Braconid wasps do not parasitize monarchs as often as tachinid flies. When braconids do attack monarchs they can produce as many as 32 tiny adult wasps from a single butterfly. Very little is known about how frequently various invertebrate parasites and predators harm monarchs in different parts of their range.

Solution 1

Plant placement can reduce predator traffic. Monarch eggs and caterpillars on single milkweed plants often go undetected…so do plants in partial shade.Many species of ants will feed on both monarch eggs and caterpillars. They also feed on other butterfly caterpillars:What’s worse, is ants share this strange symbiotic relationship where they protect milkweed-destroying aphids in exchange for their sweet secretions! Nature is nothing if not strange…In our northern climate, we have several ant species in our garden and I’m sure they eat some monarch eggs and caterpillars. However, they have not proven to be a nuisance so we leave them alone, as we do with many insects that butterfly gardeners consider pests.If your garden pests are innocent until proven guilty, a healthy ecosystem should develop that can support both monarchs and their predators.And as always, you always have the option of bringing in a few eggs or small caterpillars to save them from the surprisingly long list of monarch predators…But, if their numbers start getting out of control:

Ant Solution 1

Mix 1 Tbsp. of powdered sugar with 1 part baking soda. The powdered sugar attracts them, and the baking soda disrupts their digestive tracks, eventually killing them. Place the mixture in jar lids near the plants they are invading.

Ant Solution 2

If you know of other effective solutions for keeping ants at bay in the garden, please post in the comment section below…

Mantis Solution 1

MBG Community member Roberta C. and others have reported using“Use coffee grounds, recycled works fine, not decaf, in your dirt around your milkweed. Ants depend on their scent trail to survive and make it home, there’s something in coffee grounds that disrupts their trail or scent so they avoid coffee grounds. Grams used it in her flower beds and veggie garden.”You can also pour the grounds on their anthill to encourage them to move their base of operations…Mantids are skilled hunters that eat a variety of insects including monarchs. Mantids are sometimes used as biological pest control. The nymphs eat aphids and leafhoppers, but the adults will go after larger prey:

Wasp Solution 2

Relocate monstrous mantids to less butterfly-friendly plants.If you raise monarch butterflies, there is a great lesson to be learned from this disturbing video:Wasps are an issue for monarchs across the globe, from the US to New Zealand.If you raise monarchs, they needIf you’ve read other articles on this blog, you know I’m a proponent of nurturing a healthy ecosystem which includes monarchsWasps are always welcome in our garden as beneficial pollinators, but unfortunately, their children have to eat…caterpillars.Remember, one monarch lays hundreds of eggs so predators are aHowever, that doesn’t mean you need toWe also had a paper wasp nest on our house, conveniently built right above one of our milkweed patches.

Wasp Solution 3

This is one of the only times I’m on board with using pesticides in the garden, but please read the label carefully and try to keep the spraying directly on the nest:Find Wasp and Hornet Killer SprayLast season we sprayed an underground yellow jacket nest in our raised beds. We sprayed early morning during a light rain, and I never saw another wasp emerge from the nest.

Parasites and Disease

Parasitoids are specialized insects that lay eggs in or inside other insects and develop by feeding in or on a host organism, causing its eventual death. Both fly and wasp parasitoids lay their eggs on monarch larvae, but the most important larval parasitoid is probably a fly species in the family Tachinidae. This family includes about 10,000 species, most of which parasitize Lepidoptera (butterflies and moths), although they also parasitize Hymenoptera (ants and bees), Heteroptera (true bugs and their relatives), Coleoptera (beetles), Diptera (flies and mosquitoes), Dermaptera (earwigs), Orthoptera(grasshoppers and crickets), Chilopoda (centipedes), as well as some scorpions and spiders.The best-studied monarch parasitoid is the tachnid flyLess 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 (Arnaud 1978). The closely-related queen, Danaus gilippus is parasitized by two Chalcid wasps,Brachymeria annulata and B. ovata (Prudic and Olson 2005), as well as L. archippivora (Arnaud 1978). Research in the Monarch Lab demonstrates that the wasp Pteromalus cassotis (in the family Pteromalidae and the same superfamily, Chalcidoidea, as the two Chalcid wasps found in queens) could be an important pupal parasitoid (Oberhauser et al.). P. cassotis wasps are tiny, and over 200 can emerge from one monarch pupa. Research has also shown that a closely related generalist parasitoid (Pteromalus puparum) will attempt to parasitize monarch pupae under lab conditions, but their offspring fail to develop in monarch hosts. We are currently investigating the role of monarch‘s sequestered cardenolides in these host-parasitoid interactions (Stenoien et al.). Recent studies have documented a pupal parasitoid of monarchs,

Ophryocystis elektroscirrha

Perhaps the most-studied parasite of monarchs is a protozoan parasite calledWhile it is often not fatal, OE can have negative effects on survival, mass, and life span of monarchs. Heavily infected adults have difficulty emerging from their pupal cases and expanding their wings, although adults with low parasite loads appear normal (McLaughlin and Myers 1970; Leong et al. 1992). High parasite doses decrease larval survivorship from hatching to eclosion, and heavily captive adults are smaller and shorter-lived than uninfected adults (Altizer and Oberhauser 1999). Researchers in Sonia Altizer’s lab at the University of Georgia are studying rates of parasitism by OE and its effects on monarchs. For more information about this disease and how you can join in this research, visit monarchparasites.org.There is a higher occurrence of this parasite in populations that do not migrate, such as the one in southern Florida. The eastern migratory population has the lowest occurrence of OE, likely due to the fact that infected monarchs are less likely to make it to their overwintering destinations in Mexico and therefore will not reproduce and spread the parasite. Recent studies about OE and exotic milkweed describe how the year-round presence of tropical milkweed in some parts of the US may be facilitating the spread of this parasite. For more information, please read our Potential Risks of Growing Exotic Milkweeds for Monarchs flyer.


Invertebrate predators such as ants, spiders, and wasps attack monarch larvae on milkweed plants (Prysby 2004). Only about 5% of monarchs reach the last larval instar. Wasps have been observed feeding on monarch abdomens at a California overwintering site (D. Frey, personal communication), and fire ants have been suggested as a major predator of monarch larvae in Texas (Calvert 1996). 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 (L.S. Rayor, personal communication) than wasps that preyed upon less toxic caterpillars. In a laboratory experiment, one lacewing larva was observed consuming 40 monarch eggs. Chinese mantids and paper wasps have also been observed preying on immature monarchs.1) A spined soldier bug preying on a monarch larva (photo: Duane Miller); 2) Ants have been known to predate monarchs (UMN Monarch Lab)Adults face less danger of being eaten by predators during the breeding season, but there is a much greater risk of being eaten by bird predators in overwintering locations. Birds such as black-backed orioles and black-headed grosbeaks are common predators at monarch overwintering sites. These species can eat large quantities of monarchs without getting poisoned. This may result from the decay of toxins inside the monarchs’ bodies during the many months of migration and overwintering, or from the specific feeding behavior of the birds. Orioles slit open the monarchs’ abdomens before feeding, avoiding most of the toxin-rich cuticle. Grosbeaks, which eat the entire abdomen, can tolerate higher levels of cardenolides in their digestive tracts. Research has shown that predation by these two bird species accounts for over 60% of the total monarch mortality during overwinter (Calvert et al. 1979). In some colonies, up to 9% of the butterflies are eaten by birds during the winter, and this number can be up to 15% when the forest is disturbed by logging, making it easier for the birds to reach the branches on which monarchs cluster.Predation by birds is one of the most important natural causes of monarch mortality during the winter. Two bird species, black-headed grosbeaks and black-backed orioles, are the main predators. (Photo: Lincoln Brower)


Ophryocystis Elektroscirrha OE) is a single celled organism called a protozoan. A protozoan isn’t a plant or an animal but a single celled organism that shares characteristics of both.The parasite OE only infects milkweed butterflies. Worldwide all Monarch populations are infected with OE, which suggests this parasite evolved with Monarchs. Monarchs east of the Rocky Mountains have the lowest rate of infection, west coast Monarchs a much higher rate and non-migratory populations in tropical areas have the highest rate. This is mainly due to year round availability of milkweed, the Monarch’s host plant and their caterpillars food source.OE needs a live host to live on and survives between infections as spores. The spores are dormant cells among the scales on the Monarch. Most of these spores are found on the butterfly’s abdomen. They are tiny and can only be seen with a microscope. The spores are scattered by infected adult Monarchs and can also be spread by mating and casual contact.The spore is passed from one Monarch generation to another by the caterpillar eating milkweed exposed to infected Monarchs and is also passed from the female to the egg shell while laying the eggs. Dormant spores are spread from the egg laying female onto the egg and the milkweed plant. When the caterpillar breaks out of the egg and eats the shell it ingests the spores, which reproduce in its body. Caterpillars also eat the spores scattered on the milkweed. During digestion the spores open and release the protozoa, which reproduce. Before the butterfly pupates this parasite releases spores that can survive nearly anywhere. When the pupa ecloses the Monarch is covered with spores. The parasite has done its damage and reproduces no more. The cycle perpetuates.Infected chrysalides won’t have uniform color. There may be mottled or spotted. Heavily infected Monarchs may not emerge or if they do they may be deformed or too weak to hold on to their case. Many will not survive. Mildly infected butterflies will be smaller and weaker. They will have a hard time holding on to fluids they need to survive. The males will be less able to compete for mating; the females however will still reproduce normally. Many infected butterflies will show no signs of infection at all.One way to see if your Monarch has OE is to carefully press a clear piece of tape on its abdomen and check under a microscope. The University of Minnesota has a citizen scientist program called the Monarch Larva Monitoring Project, which you can be a part of.

Tachinid Fly

The worst caterpillar parasitoid is the Tachinid fly. These flies in the family Tachinidae parasitize many arthropods. A particular Tachinid fly, Lesperia archippivora, is a major parasitoid of many Lepidoptera.Tachinids parasitize butterflies by laying eggs on the caterpillar larva. After the eggs hatch the tachinid larva bores into the butterfly larva and completes its development inside the new host. During the Monarchs late instar or pupa stage the fly maggot will drop out on tendrils looking like white strings. Then the fly larva will pupate and complete metamorphosis within 2 weeks.The tachinid pupa is reddish brown, elongated ovoid around 3/8” long. There can be as many as 8 tachinid maggots parasitizing each caterpillar.The visual signs of Tachinid parasitization are many. The silky white strings show the maggot has left the caterpillar or pupa. Infested pupa will be discolored with brown, black or rust spots or you may see small holes in it. The pupa may be lighter in color, shriveled or the rings at the top may turn black.The Tachinid fly itself looks pretty much like a housefly except it has red eyes and a hairy back. T