Monarch butterfly caterpillars are sometimes an unusual color, shade, or has unusual markings. What causes it? We wish we had all the answers. Having raised hundreds of thousands of Monarch butterflies, we have documented a few unusual caterpillars and have identified the cause of a few.
The photo above shows normal color variation that is caused by the amount of light in which the caterpillar has been living. The lighter caterpillars were outdoors on plants that had very few leaves left to protect them from the sun. The darker ones were raised in a closed lab in rearing containers with very little light. All of these caterpillars are healthy.
Caterpillars raised together in the same rearing container should be the same basic color. If they are collected from a garden, some from the bottom of leaves on the lower part of a wide-leafed milkweed plant, their black bands will be wider. If they are collected from the same garden, from the top of the same plant, in bright sun, their white bands will be wider. This difference will be normal because they were in different rearing conditions in the same garden on the same plant. In a rearing container, the vast difference in level of light won’t be present to cause the difference in band width.
Bacterial infection as well as other diseases sometimes cause the caterpillar’s white bands to become dusky. They look somewhat dull. The bright white is not present on their bodies. Bacteria will sometimes enter the hemolymph/blood of a caterpillar. They do not have blood vessels as do many animals. Their hemolymph is free flowing in their bodies. The dark line down the back of the caterpillar is its heart. Hemolymph is pumped out of the heart at the head of the caterpillar and flows freely though its body to the rear of the caterpillar. At the bottom of its heart, holes draw hemolymph into the heart and it is pumped back up to its head area again. If bacteria is in the hemolymph, it will cause the caterpillar to appear dusky/dark. In the photo above, two caterpillars are side by side. The one on the left is healthy. The one on the right is not healthy. A disease or parasite can cause the gut wall to be breached and gut bacteria to enter the hemolymph of the caterpillar, resulting in bacterial infection.
One week we saw, out of the thousands of caterpillars in the labs, several dozen dark black Monarch caterpillars. Over a period of several weeks, we saw dozens more as we raised roughly ten thousand Monarchs. Those few dozen had black lines that were wider and messy. There wasn’t a clear clean line between the black and yellow bands on the caterpillars. It was as if someone had colored them with water color and the black paint bled into the yellow paint. The white bands were missing. These caterpillars were found in the same containers, with the same food, same amount of light, and same temperatures as those that were normal colored. Concerned, we separated the dark caterpillars from the normal ones and raised them in the same area, same shelves, but in a totally separate rearing container. Although some did well, pupated normally, and emerged normal, a percentage of them died. As we found more caterpillars with the messy wide black bands, we raised more in separate containers. The overall slowness of their growth, slightly lower appetite level, and failure to thrive (not to mention death rate) caused us to *how to euthanize diseased caterpillars all of them. It has been suggested that the cause of the abnormal color may have been genetic. If so, we believe it is a genetic disease or genetic weakness.
Totally opposite of the wider messy black bands, we have also seen a short time when caterpillars stayed almost white as they grew and molted into third instar. They looked healthy other than their light color.
Again, light colored Monarch caterpillars are normal in direct sun. But these caterpillars were found in the same containers, with the same food, same amount of light, and same temperatures as those that were normal color. We separated out the smaller whiter caterpillars into their own rearing containers. As their siblings ate and grew in their containers, the white ones simply failed to grow and eventually most of them died. They never looked diseased. Other than the extreme lightness of the caterpillars, they seemed normal. Again, we *euthanized the white caterpillars when we saw a few over the next few weeks.
Although people often believe that if a butterfly emerges and looks and acts normal, it is healthy. This isn’t always the case. Many unhealthy butterflies are flying about, laying eggs with pathogens inside the eggs that will weaken or kill the offspring before they become adults.
*A gardener or butterfly enthusiast does not need to euthanize caterpillars that they believe are not sick. Quite a few gardeners have found odd caterpillars in their gardens. A butterfly farm that recognizes issues with a caterpillar should be responsible and euthanize those that may possibly be diseased.