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.

Ants attacking monarch larva on milkweed (larva is spitting green fluid from its mouth). Photo by S. Altizer and M. Prysby.
Assassin bug attacking monarch larva. Photo courtesy of Monarchs in the Classroom.
Anole lizard eating an adult swallowtail. Photo by J. Cheek.



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.

Tachinid fly larvae next to the monarch chyralis they emerged from. A single monarch can host 8 or more tachinid flies. Photo by S. Altizer.
Monarch caterpillar parasitized by tachinid fly larvae (note the white threads produced by the flies as they exit the host). Photo by S. Altizer.


Tomato hornworm parasitized by braconid wasp larvae. Photos by R. Bartel.


Click here to see a video clip of a tachinid fly (R) laying eggs on a tomato hornworm

Parasites and infectious diseases

Have you ever had a tick on your leg or seen a dog with fleas? Ticks and fleas are examples of parasites. Parasites are small living things that live in or on another living thing, called the host. Parasites usually obtain resources from their hosts, and the host in turn is harme]d by the parasite. Sometimes the host is damaged so much that it dies. The parasite benefits from the food and shelter that it gets from the host.

Some parasites have a very close relationship with their host. These obligate parasites can not lead independent nonparasitic lives. They must live in the host to grow and reproduce. Obligate parasites often produce resistant structures like spores to survive between hosts.

Parasites can be microscopic like viruses and bacteria or much larger like mites. A pathogen is a term used for microbial (single-celled) parasites that cause harm to their hosts. Most pathogens and parasites get into insects when they eat. Others infect insects through pores or joints in the exoskeleton. Many researchers are currently exploring the role that parasites and diseases play in controlling the size of insect populations.

Parasitic nematodes from the body cavity of a forked fungus beetle (100x). Photograph by C. Howell.
Adult fly stuck to a leaf and covered in hyphae from a parasitic fungus. Photo by A. Pedersen.

Several disease causing organisms can infect monarchs. Nuclear polyhedrosis virus and Pseudomonas bacteria are two monarch pathogens. Protozoan parasites such as Ophryocystis elektroscirrha and a microsporidian Nosema species have been reported to infect both wild and captive monarch butterflies.

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

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.

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. Often times, monarch larvae or pupae die for no apparent reason. This does not mean that a pathogen has killed them; other causes of death could 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.

Monarch larva hanging by prolegs shortly after death. Photo by J. Arnold. Monarch pupa that showing brown/black discoloration following death. Photo by C. DeCurtis.