OE in Monarch Caterpillars

"I live just outside of Orlando and have been a butterfly garden enthusiast for a few years. This year I have collecting cats from my and a relatives butterfly gardens' milkweed plants and raising them in butterfly pavilions. I feed them milkweed from my yard and that I purchase from a local nursery. I have had some issues with OE and need some advise. My wish is to contribute to the Monarch conservation movement and want to make sure I have proper information. Is there any way to tell if a cat I collect has OE? I've read info about testing adult butterflies and of the bleach solution for eggs but little about the cats themselves which is usually when I get them. Any help you could provide would be greatly appreciated."

There isn’t a way to tell definitely if a cat has OE or not without killing it and checking it through a high powered microscope. Some people report that OE in cats will cause splotching but we found that other things also cause the same type of splotching. We did isolation tests with spotted/splotched larvae. This was during a time of feeding larvae milkweed infected with plant pests like thrips. The Monarchs with spots and splotches emerged without OE. Out of the thousands we raised each week, none had OE.

(OE is the abbreviation for Ophryocystis elektroscirrha, a protozoa disease that affects Monarch, Queen, and Soldier butterflies and other butterflies in the Danaus family. OE is MUCH easier to say than Ophyrocystis elektroscirrha!)

It is the cats that contract the disease – the cat has to eat spores to become infected. One cat can’t give it to another cat – the spores are only transferable from adult butterflies to another butterfly or object – as surface contamination. When a cat EATS the spore, it contracts OE. A pupa or adult cannot contract the disease although an adult can become surface contaminated by mating with an infected adult butterfly. Somewhat like touching a door handle after someone with cold pathogens on their hands touch it – the pathogens can contaminate surfaces such as bodies and plant leaves. Spores fall off like pixie dust when infected adult butterflies fly.

You can tell, sometimes, if a pupa is heavily infected. It does show splotching that I’ve not seen otherwise. Any I’ve had with these splotches (pupae) were OE infected.

The photo above is OE infected. Notice the splotching?

The photo below is of pupae that are not infected. Notice the smoother green color near the cremaster area?


Of course, a light infection won’t show so it always pays to check adults to be sure.

On the encouraging side, it is reported that in the southern tip of Florida, 85% of all Monarchs are reported to be OE infected. If it was as deadly in the wild as we are often led to believe, the population in southern Florida should be extinct by now. BUT it is deadly in nature … we’ve seen milkweed gardens in Florida that were full of dead pupae – from OE. We know it is deadly and in a rearing operation where Monarch butterflies are kept together and bred together, infection can build up if it is not eliminated. That’s why it’s so important for those who raise Monarchs and Queens and mate captive breeding stock to check for OE and to disinfect eggs. We don’t raise them like nature with a 98% death rate BEFORE they become adults – we raise them to have a high survival ratio!

I imagine you already do this but just in case, remember to always disinfect a pavilion after one set of cats are raised in it. OE is only one disease and is one of the least dangerous dangerous. Some other diseases are far worse than OE and they can be transferred from cat to cat.

You can wash milkweed with a light bleach solution and rinse thoroughly before putting cats on it. That will help with surface contamination of milkweed leaves – one of the primary sources of infection. The other source is spores on the eggs themselves. When the cat eats its way out of the egg, it eats spores that the adult female Monarch butterfly left on the egg surface when she laid the egg.

The same goes for Queens too. Queen cats are so cool – one of my very favorite larvae.