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Natural Enemies of Monarchs 


Did you know that fewer than one out of every 10 monarch eggs 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 also suffer attacks from parasites, organisms that live inside the monarchs’ bodies. Some parasites that kill monarchs are insects themselves; these are called parasitoids. Other parasites are extremely small and can only be seen with a microscope. Just like humans, monarchs can get ill or die from diseases caused by parasitic bacteria, viruses, fungi, and protozoans.



Parasitoids are insects that parasitize other insects. Parasitoids such as small flies and wasps lay eggs on other insects and eventually kill their host. Parasitoid larvae eat their host from the inside out, usually emerging from the remains of the host once it becomes a pupa or adult.

tachinid fly

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Monarch Tachinid J

Monarch caterpillar parasitized by tachinid fly larvae (note the white threads produced by the flies as they exit the host). (Photo by S. Altizer)

Monarch Pupa Tachinid

Tachinid fly larvae recently emerged from a monarch chrysalis. A single monarch can host 8 or more tachinid fly larvae (photo by S. Altizer).

  • Tachinid flies commonly feed on and kill monarchs. These parasitoids lay their eggs on monarch caterpillars. Tachinid fly larvae feed from inside the caterpillar, but usually don’t kill the host until just before the caterpillar pupates. 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. The fly maggots drop to the ground on long, gel-like threads.​

hornworm braconid wasp larvae

Tomato hornworm parasitized by braconid wasp larvae (photo by R. Bartel).

hornworm braconid wasp larvae

Tomato hornworm parasitized by braconid wasp larvae (photo by R. Bartel).

  • 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.

Parasites and Infectious Diseases


Parasites are small living things that live in or on another living thing, called the host. Have you ever had a tick on your leg or felt sick from the flu? Ticks and flu viruses are both examples of parasites. Parasites usually obtain resources from their hosts, which often makes the host sick or weak -- causing disease. Sometimes the parasite damages the host to the point of death. The parasite benefits from the food and shelter that it steals from the host.

Some parasites have a very close relationship with their host. These obligate parasites cannot lead independent lives but must live in the host to grow and reproduce. Obligate parasites often produce resistant structures like spores to survive in the environment while it waits for the next host.


Pathogens are microbial (single-celled) parasites that cause harm to their hosts. These can include viruses, bacteria, fungi, and protozoa. Most pathogens and parasites enter insects when they eat. Others infect insects through pores or joints in the exoskeleton. Larger and multicellular parasites are known as macroparasites, which include nematode worms. Researchers are currently exploring how parasites control insect population sizes. 

Nematodes From Beetle

Parasitic nematodes from the body cavity of a forked fungus beetle (100x) (Photo by C. Howell).

Here are some examples of monarch parasites:

  • Nuclear polyhedrosis virus

  • Beauvaria bassiana

  • Pseudomonas bacteria

  • Ophryocystis elektroscirrha (OE)

  • Microsporidian Nosema

Pupa OE

Monarch pupa infected with the protozoan O. elektroscirrha. Dark spots are lesions containing thousands of developing parasite spores just underneath the monarch's cuticle. Photo by S. Altizer.

Monarch Larva Beauveria

Monarch larva infected with the fungal pathogen Beauvaria bassiana. White growth is fungal mycelium and conidia (vegetative and dispersal stages). Photo by S. Altizer.

Signs of infection for Monarchs


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. Sometimes, monarch larvae or pupae appear to die for no apparent reason. This does not mean that a parasite killed them; other causes of death include ingestion of chemical toxins, a wound that became infected by opportunistic bacteria, or thermal stress caused by conditions that are too hot or too cold.

Disease July 07-1

Monarch pupa that showing brown, black discoloration following death (photo by C. DeCurtis).

Jane Arnold Dead Larva

Monarch larva hanging by prolegs shortly after death (photo by J. Arnold).

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