Why Are My Monarchs Turning Black?

Signs of Viral or Bacterial Infections in Monarch 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 most important insect species. Whether you are 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 your 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 – the black bands look wider than usual. 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:

  • 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. You could see an overall decline in the health of your caterpillars. Some or nearly all of the monarch caterpillars in your yard could slowly turn black and then die. Chrysalis discoloration is another sign that things are very wrong. A healthy chrysalis does turn dark just before the adult butterfly is ready to emerge, but an unhealthy one turns solid black, and adult butterflies never emerge from them.

What Causes Black Death in Butterflies?

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 caterpillar'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 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, such as raising them in a ventilated mesh cage.
  • Keep the cage out of the sun.
  • Vacuum up any frass (butterfly poop) and old milkweed leaves, and wipe down and dry the cage daily.
  • Rinse milkweed cuttings and leaves with water before feeding.
  • Watch for condensation in breeding cages, and let milkweed plants dry completely.
  • If you see any signs of sickness in a caterpillar (lethargy, discoloration, etc. as listed above), isolate it from the other caterpillars.
  • Remove any chrysalides that are turning black.
  • If you have evidence that your butterflies are suffering from black death, disinfect the cage with a 5-10% bleach solution before 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. The only part of that is due to the "black death." Other parasites affecting monarchs include tachinid fly infections, Ophryocystis ​elektroscirrha (OE), and Trichogramma and Chalcid wasps. But the most serious threats to monarchs come from human activities, from insecticide and herbicide use to loss of habitat.

Today there are several monarch preservation opportunities for students and ordinary citizens to take part in, from monitoring and reporting infestations like the black death, to tracking migrating butterflies, to getting grants to start new backyard gardens to promote butterfly health.