Just as humans can contract disease so can Monarch butterflies. Bacterial and viral deaths are very common in this particular species so if you have caterpillars that suddenly go limp or begin to ooze then it is possible that they've contracted a disease. If you have a pupa that suddenly gets brown or black, then chances are it has contracted a bacterial infection.


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Black Death…just the phrase sounds ominous and describes it all. If you've ever had a caterpillar suddenly turn black and ooze, then you have experienced Black Death. When you were thinking, "Hooray! My pupa has started getting dark so my butterfly is getting ready to eclose!" only to find that the entire chrysalis has gone dark, your pupa is a victim of Black Death.

What is this hideous end that comes to the Monarch? Why does it occur? What went wrong?

Black Death

Black Death is usually caused by either a bacterial (Pseudomonas) or viral (Nuclear polyhedrosis) infection. The caterpillar suddenly becomes lethargic and stops eating and then sort of deflates turning a browish-black colour.

Pseudomonas bacteria is commonly found in the soil. In fact, it is the bacteria responsible for 'swimmer's ear' in humans! This bacteria thrives in warm, moist environments. This is why it is absolutely critical that you keep the caterpillars' habitat dry (i.e., away from windows). If you see moisture building up then it is imperative to remove the larvae and sanitize all surfaces, drying them and replacing the leaves so that there is less chance for the bacteria to grow.

If you've had a caterpillar regurgitate some liquidy stuff or emit a runny scree (poop) then you should immediately remove this guy from the others and get rid of the leaves with which it has been in contact. Other caterpillars that eat leaves that have been contaminated with the scree from the infected caterpillar are likely to contract the Nuclear polyhedrosis virus. Even having them walk through the scree or regurgitation invites potential infection! This particular virus is one that has been used as a bio-chemical for pest control. Of course, for those of us who raise insects, it is NOT a particularly positive pest control!


Sometimes the pathogen doesn't show up until the butterfly is in the pupal stage. Then, while you are thinking that you are going to have a beautiful butterfly eclose, instead, you have a strangely-coloured chrysalis that doesn't quite look like what you'd expect. The pupa turns dark but the wings are not visible. Instead, the markings are just blah. If you let it go longer, be careful! If the chrysalis falls and splatters, the odour is putrid. It is the decaying that smells so horrific. You definitely don't want the liquid to touch any surface as this will be a contaminant…

So, what is the lesson here? Keep all surfaces in the caterpillar habitat DRY. Make sure if a Monarch larva spits up or has runny poo, it is taken away from its brothers and sisters. Be diligent in your sanitizing when rearing Monarchs. And remember: DRY, DRY, DRY.


Being careful about the plants you feed your caterpillars is very critical. It is safest to feed plants that you have raised yourself and kept free from any pesticides/insecticides/fertilizers simply because Monarchs tend to be very susceptible to poisoning.

If you purchase a plant from a store, even if the store is supposedly 'organic,' be careful! Most stores will have treated the plants with a systemic insecticide. This means that the plant absorbs the chemicals through its root system and the chemical is now throughout the plant! It will take about two months for the chemicals to work their way out of the plant…and, any caterpillar that EATS the leaves of a plant that has been treated, will more than likely DIE!

For example, if you notice your caterpillars doing a weird twitching, moving, dance and/or they start throwing up or having what looks like diarrhea, chances are very likely they have been eating Milkweed that has been treated with something. There is not much hope for the poor caterpillar…although, you could try to feed it untreated leaves.

The bottom line…plant your own Asclepias (Milkweed). It is safer for everyone!