Monarch Disease

What are some reasons a caterpillar might die?

There many reasons why a caterpillar might not survive to adulthood. It can be very difficult to determine exactly what happened to a particular larva, but these are some of the reasons one may die.

There are many diseases and parasites that affect monarchs. Infection can be transmitted between adults and/or larvae, especially if the monarchs are in close proximity. For more information on the risks of rearing monarchs in close proximity to one another, see our handout: Rearing Monarchs For Research: A Guide for Enthusiasts

The genetics of each monarch can also determine the outcome of each monarch, some may have a genetic reason they did not survive.

The contamination of milkweed can also reduce the odds of monarch larvae successfully making it to and survive adulthood. If the milkweed the larvae ingest carries pesticides, bacteria, or other harmful organisms, this may also be a reason monarchs might not survive.

Monarchs also have many predators that are part of their natural ecosystem.

Why am I not seeing monarch caterpillars at my site?

If it's monarch season and you're not seeing monarchs at your site, there are a few things that could be happening. Monarch larvae have natural predators that may be getting them before you can spot them. To learn more about monarch natural predators, you can visit the Monarch Lab website.

It's also possible that adult monarchs aren't laying any eggs at your site. If this is the case, you can work to create a more diverse and extensive garden with more milkweed plants. The more milkweed you have at your site, the more likely it is that you'll find eggs and larvae! For more information on creating monarch habitat, check out the MJV 'Create Habitat for Monarchs' page.

What are common monarch diseases or problems?

It can be difficult to identify exactly what the problem with any specific monarch is, but here are a few common ailments that affect monarchs.

Monarch caterpillars often turn black or darkish in color when they are sick with bacterial infections. This is often referred to as 'black death.' There are a number of other bacterial infections or viruses that monarchs can get as well. If the chrysalis has darkened, it could either have an infection, like previously mentioned, or it could have been parasitized by a fly or a wasp.

Monarchs also may have O.E. (Ophryocystis elektroscirrha). OE is a protozoan parasite that infects monarchs. You won't be able to tell if a monarch has OE until it's in the pupal, or even adult, stage. Sometimes adult monarchs infected with OE appear normal. You cannot be sure that a monarch is infected without testing it.

If you suspect a monarch has OE or another disease, contact Project Monarch Health with your observation. 

What is OE (Ophryocystis elektroscirrha)? Why am I seeing adult monarchs with deformed wings?

Ophryocystis elektroscirrha (OE) is a debilitating protozoan parasite that infects monarchs. Infected adult monarchs harbor thousands or millions of microscopic OE spores on the outside of their bodies. When dormant spores are scattered onto eggs or milkweed leaves by infected adults, monarch larvae consume the spores, and these parasites then replicate inside the larvae and pupae. Monarchs with severe OE infections can fail to emerge successfully from their pupal stage, either because they become stuck or they are too weak to fully expand their wings. Monarchs with mild OE infections can appear normal but live shorter lives and cannot fly was well as healthy monarchs. For more information on OE and the higher risk of OE associated with planting tropical milkweed, read our handout Tropical Milkweed and OE: Potential Risks for Monarchs.

I've heard of people bleaching their milkweed and monarch eggs while raising monarchs. Should I do this?

Sanitizing containers between individuals is one of the most important steps in keeping monarchs healthy in a captive rearing environment. We do not recommend bleaching monarch eggs or milkweed plants.

It is common and encouraged to rinse milkweed leaves with water before feeding them to caterpillars. This simple step is typically sufficient to remove debris or small organisms that may come in on wild-collected leaves.

Some people attempt to bleach monarch eggs in an effort to destroy Ophryocystis elektroscirrha (OE) spores on the outside of the eggs. While this may have limited effectiveness, OE is a naturally occurring parasite in monarchs and not something that can or should be eliminated entirely from the population. Instead, the focus should be on preventing its spread in a rearing environment.

By sanitizing containers between uses and ensuring individuals are not housed together in ways that allow direct exposure, home rearers can greatly reduce the risk of inadvertently spreading OE. The goal is not to eliminate this naturally occurring parasite but to manage rearing conditions responsibly so monarchs remain as healthy as possible.

A good way to learn more about OE and to get a better sense of infection rates in the wild and in your home rearing activities is to participate in Project Monarch Health.

For more information, check out our Rearing Monarchs For Research: A Guide for Enthusiasts handout.

Why are caterpillars disappearing from my milkweed patch?

There are a number of potential reasons for this. If disappearing caterpillars are 5th instar caterpillars, odds are they moved away from your milkweeds to find a safe place to form their chrysalis (they typically do not pupate on milkweed plants). If earlier instars or eggs are disappearing, a more likely culprit is a monarch predator. You can read more about natural enemies on our website. Researchers agree that less than 10% (with some studies as low as 2%) of monarch eggs reach adulthood. 

How can I tell if a butterfly has O.E.?

Unless your vision compares to a microscope, you can't tell with the naked eye whether or not a monarch is infected with O.E. As a caterpillar, the parasite is located within the gut and would be impossible to detect without killing the larvae to obtain a sample of the gut contents. On adult butterflies, O.E. spores are dormant and reside on the outside of the body, usually the butterfly's abdomen. You can't see these dormant spores without observing a sample of the butterflies abdominal scales through a microscope. Learn more about obtaining scale/parasite samples from an adult butterfly by visiting Project Monarch Health, www.monarchparasites.org. Observing these samples through a microscope is the most definitive way of diagnosing O.E., but there are some other symptoms associated with infection by this parasite that we often look for. Heavily infected adults can have trouble emerging from their chrysalis and may fall and thus, can be highly deformed as adults. With heavy infections, you can also see spores under the chrysalis skin before the butterfly has emerged - they appear as small black dots. These symptoms often suggest an O.E. infection, but not always. It is also important to note that not all butterflies that have an O.E. infection will show these symptoms. Some appear to be perfectly healthy to the naked eye, but are still infected with (and possibly spreading) the parasite to their offspring.

What should I do if I suspect a monarch I'm raising has OE or some other disease?

It is best to keep monarchs in individual containers, since disease can spread quickly when multiple individuals are housed together. Ophryocystis elektroscirrha (OE) can spread if adults share space with eggs or caterpillars, and caterpillars kept in the same enclosure are more likely to transmit viral or bacterial pathogens, many of which are fatal. If you notice an individual that appears sick, handle it last and take care to avoid contaminating other containers.

Diseases and natural enemies are a natural part of monarch ecology, so if an individual dies, it can be returned to the outdoors. If an individual is still alive but clearly showing symptoms that would prevent it from surviving in the wild, you may choose to euthanize it—freezing is one common and humane method.

For more information please see our Rearing Monarchs For Research: A Guide for Enthusiasts handout.

In places where tropical milkweed is native, do monarchs have a high incidence of OE as well?

The short answer is 'sometimes but not always.' The long answer is that monarchs breeding year round on tropical milkweeds in some places like Australia, Hawaii, Bermuda, South Florida do have very high infection rates with OE.  In Hawaii it is very variable - some local sites on some islands have low prevalence and some have nearly 100%.  In Australia it can be variable too. Other locations such as Belize and Costa Rica, where monarchs breed on A. curassavica, are on the lower end of the spectrum with about 20-30% heavy infection rates.  Further research is needed into why this variability exists.

Does OE affect humans or pets?

Luckily, OE only affects a few butterfly species. It does not affect humans in any way and there is no chance that it could adapt to infect humans.  This pathogen is not closely related to any human pathogen and it has only been reported to infect insects.  The spores require a very precise pH shift to activate in larval guts, which our bodies fortunately do not have.  It is a very good question, though, as some insects vector medically important pathogens to humans.

How do you check the butterflies for OE infection?

Sampling involves capturing or raising adult monarchs and pressing clear tape against the butterfly's abdomen to collect parasite spores. MonarchHealth is a project in which volunteers sample wild monarch butterflies to help track the spread of this protozoan parasite across North America.

Is there a benefit to keeping OE infected monarch butterflies alive?

From the standpoint of the individual butterfly staying alive for as long as possible it great! However, this may not be beneficial from a population level perspective because individuals with OE will have a prolonged period to spread the disease to their offspring, other adults, and to milkweed plants.