Whether you are raising monarch butterflies in a classroom, or just observing them in your backyard butterfly garden, you've probably noticed that a percentage of your monarch caterpillars never reach adulthood as a butterfly. Some just seem to disappear, while others show visible signs of disease or parasitism.
After several years of raising bumper crops of monarchs in my own milkweed patch, I started to notice a decline in the health of my caterpillars. This past summer, nearly all the monarch caterpillars in my yard slowly turned black, and then died. I also found black monarch chrysalides. A healthy chrysalis does turn dark just before the adult butterfly is ready to emerge, but this was different. These chrysalides were solid black, and just didn't look healthy. I couldn't see the monarch's wing marking through the pupal case. The adult butterfly never emerged. Why were my monarchs turning black?
You can see what a healthy monarch chrysalis looks like in these time lapse photos of a monarch butterfly emerging.
Symptoms of Butterfly Black Death
Butterfly enthusiasts sometimes refer to this condition as the "black death." One day, your caterpillars are munching away on their milkweed, and the next, they turn lethargic. Their colors seem a little off – the black bands look wider than usual (as in the inset photo above). Gradually, the entire caterpillar darkens, and its body appears deflated. Right before your eyes, your monarch caterpillars turn to mush.
Signs that your caterpillars will succumb to black death:
- discoloration of the cuticle (skin)
- watery droppings
- shriveled tentacles
Causes of Black Death
In most cases, black death is caused by either a bacterium in the genus Pseudomonas or by the Nuclear polyhedrosis virus.
Pseudomonas bacteria are ubiquitous; they're found in water, in soil, in plants, and even in animals (including people). They prefer moist environments. In humans, Pseudomonas bacteria may cause ear, eye, and urinary tract infections, as well as other hospital-acquired infections. The opportunistic Pseudomonas bacteria typically infect caterpillars that are already weakened by other diseases or conditions.
The Nuclear polyhedrosis virus is usually deadly to monarchs. The virus resides within the catepillar's cells, forming polyhedra (sometimes described as crystals, but this is not quite accurate). The polyhedra grow within the cell, eventually causing it to burst open. This is why the infected caterpillar or pupa seems to dissolve – the virus ruptures the cells and destroys the cellular structure of the insect. Fortunately, the Nuclear polyhedrosis virus does not reproduce in humans.
Tips for Preventing the Black Death in Your Monarchs
If you're raising monarch butterflies in a classroom or in your backyard butterfly garden, there are a few things you can do to lower the risk of your monarchs succumbing to the black death. The Pseudomonas bacteria like moist environments, so keep your breeding environment as dry as possible. Watch for condensation in breeding cages, and let milkweed plants dry thoroughly before watering them again. If you see any signs of sickness in a caterpillar (lethargy, discoloration, etc. as listed above), isolate it from the other caterpillars. Be vigilant about removing sick caterpillars from your breeding area to keep infections from spreading to healthy larvae.
- Monarch Predation by Invertebrates, Parasitoids and Disease, Monarch Watch. Accessed January 7, 2013.
- Nuclear Polyhedrosis - A Basic Description, International Butterfly Breeders Association. Accessed January 7, 2013.
- Pseudomonas Infection, Medscape Reference. Accessed January 7, 2013.
- Pseudomonas, Southern Illinois University. Accessed January 7, 2013.
- Parasites and Natural Enemies, MonarchLab, University of Minnesota. Accessed January 7, 2013.