Why Are Monarch Caterpillars Turning Black?

How Infections Are Threatening These Majestic Butterflies

Monarch caterpillar.
Monarch caterpillars with unusually dark coloring may be infected. Debbie Hadley

Black death in monarch butterflies (Danaus plexippus) is one of several recent threats to the one of our most popular and revered insect species. Whether you're raising monarch butterflies in a classroom, observing them in your backyard milkweed garden, or taking part in one of the habitat restoration projects, you may have noticed that a percentage of monarch caterpillars never reach adulthood as a butterfly. Some just seem to disappear, while others show visible signs of disease or parasitism.

Symptoms of Butterfly Black Death

One day, your caterpillars are munching away on their milkweed, and the next, they turn lethargic. Their colors seem a little off. Their black bands appear wider than usual. Gradually, the entire caterpillar darkens, and its body looks like a deflated inner tube. Then, right before your eyes, the caterpillar turns to mush.

Signs that your caterpillars will succumb to black death:

  • lethargy, refusing to eat
  • discoloration of the cuticle (skin)
  • watery droppings
  • regurgitation
  • shriveled tentacles

Even after several years of raising bumper crops of monarchs in your own milkweed patch, you may still be in danger of infestations. In the worst case, a catastrophic parasite infestation can occur, leading to an overall decline in the health of your caterpillar population. What are the signs? Some or nearly all of the monarch caterpillars slowly turn black and die. Chrysalis discoloration is another thing to look out for. While a healthy chrysalis does turn dark just before the adult butterfly is ready to emerge, an unhealthy one turns solid black—and adult butterflies never emerge from them.

What Causes Black Death in Butterflies?

In most cases, black death has two causes: a bacterium in the genus Pseudomonas and the Nuclear polyhedrosis virus. Pseudomonas bacteria prefer moist environments and are pretty much ubiquitous. You can find them in water, in soil, in plants, and even in animals (including people). In humans, Pseudomonas bacteria may cause ear, eye, and urinary tract infections, as well as other hospital-acquired infections. Pseudomonas is an opportunistic bacteria that typically infects caterpillars that are already weakened by other diseases or conditions.

The Nuclear polyhedrosis virus is almost always deadly to monarchs. It resides inside the caterpillar's cells, forming polyhedra (sometimes described as crystals, although this is not entirely accurate). The polyhedra grow within the cell, eventually causing it to burst open. This is the reason infected caterpillars or pupa seems to dissolve as the virus ruptures the cells and destroys the structure of the insect. Fortunately, the Nuclear polyhedrosis virus does not reproduce in humans.

Tips for Preventing Black Death in Monarchs

If you're raising monarch butterflies in a classroom or in your backyard butterfly garden, there are several precautions you can take to lower the risk of black death.

  • The Pseudomonas bacteria like moist environments. Keep your breeding environment as dry as possible. Raised cages constructed of ventilated mesh are a good choice.
  • Keep the cage out of the sun.
  • Vacuum up any frass (butterfly droppings) and old milkweed leaves. Wipe down and dry the cage daily.
  • Rinse milkweed cuttings and leaves with water before feeding.
  • Watch for condensation in breeding cages. Be sure to let milkweed plants dry completely before use.
  • If you see any signs of sickness in a caterpillar (lethargy, discoloration, etc.), isolate it from the other caterpillars.
  • Remove any chrysalides that are turning black.
  • If you have evidence that your butterflies are suffering from the black death, disinfect the cage with a 5 to 10 percent bleach solution prior to raising any more.

Citizen Scientists and Preserving Monarchs

The monarch butterfly population has crashed in recent years, experiencing an 80 percent decline in North American populations over the past few decades. Only part of this downturn is due to the "black death." Other parasites affecting monarchs include tachinid fly infections, Ophryocystis ​elektroscirrha (OE), and Trichogramma and Chalcid wasps. Unfortunately, the most serious threat to monarchs comes from human sources including insecticide and herbicide use and loss of habitat.

Today there are several monarch preservation opportunities for students and ordinary citizens to take part in that offer opportunities from monitoring and reporting infestations, to tracking migrating butterflies, to getting grants to launch new backyard gardens and promoting butterfly health.

Sources