Butterfly diseases are all over in nature. Quite often, we bring those butterfly diseases into our homes when we bring in wild caterpillars to raise them indoors. (The good news is that humans and our mammal pets cannot catch their diseases.) Let’s look at some of the diseases that are brought in with wild caterpillars.

Nuclear Polyhedrosis Virus (NPV) is probably the deadliest virus to attack butterfly and moth caterpillars.

NPV causes the caterpillar to climb to a high spot, hang in an l or inverted V shape, and die. The caterpillar literally dissolves inside. When the skin/cuticle of the caterpillar ruptures, the liquid contents splash all over, spreading millions of virus particles over the surrounding host plant, rearing container, and other items. The stench is horrific.

If a caterpillar lies at the bottom of a rearing container (instead of crawling higher in the container) and dies either firm or mushy, it is not infected with NPV. NPV turns the caterpillar to liquid.

Disease is the reason we always recommend that everyone disinfect every rearing container between batches of caterpillars. Even if the first batch didn’t die from a disease, they may have left pathogens and the new batch may become sick at a younger age and die from the disease.

Some pesticides do not always directly kill caterpillars. Aria is a pesticide that kills aphids by reducing the amount of food they can uptake. Although aphids can return and safely feed on the plant within a few weeks, the plant affects pupation of Monarch butterfly caterpillars for eight weeks or more.

Bt (Bacillus thuringiensis) is a soil dwelling bacteria. It occurs naturally in soil. Because it is totally organic, it can be used to treat Certified Organic milkweed and other host plants. Nurseries growing parsley, fennel, dill, and carrots often use Bt on their Certified Organic plants. Many nurseries use it on milkweed because they believe in organic pest control but they consider the ‘worms’ on milkweed plants as undesirable. Bt causes the caterpillar to stop eating after it has eaten only a few bites. It goes to fresh milkweed, crawls on it as if it is going to eat, but doesn’t eat. Three days later it will die. If you have caterpillars that will not eat, you’re sure that the plant you’ve given them is milkweed, and it hasn’t been treated with anything to change its taste, most like Bt is the culprit. Once they have taken a few bites, they cannot be saved. After eating Bt, their gut lining will rupture and bacteria will enter their blood/hemolymph. The caterpillar slowly dies. Their bodies become flaccid as they begin to starve while the bacteria in their bodies is killing them.

I walked through the field of Hairy Indigo weeds one day, years ago. I took photos of quite a few moth caterpillars that had died from a fungus infection. I went straight to the farm from the field and fed Monarch caterpillars. Before too many days, A Monarch caterpillar in the lab was covered with the fungus.

Several years later, a Monarch caterpillar had died from a fungus and was stiff as a board. Fungus can be seen growing from its body. Fungus can grow on a caterpillar that has already died or can kill one that is healthy and alive. There are several types of fungus that kills caterpillars.

The lesson here is to WASH well before touching your caterpillars, their habitat/cage/popup, or their food. You can transfer problems from your yard to your caterpillars indoors. Although you may not have touched a plant that you thought would be contaminated, few people would have noticed the moth caterpillars in the field as I walked. I noticed them because that was the purpose of my walk, to look for and photograph caterpillars.

Green stains are either from spit/vomit or blood. If the liquid stays green for more than five minutes, it is vomit or spit. Blood begins to turn black very quickly.

There are two basic causes for caterpillar spit or vomit. One is being pestered too much. Caterpillars will sometimes vomit or spit when they believe they are threatened. Another is pesticide. Pesticide comes in many forms. From pet flea/tick medications to ant spray to termite treatments to pesticide/insecticide treated host plants to … you name it. Just when we think we have ALL sources of pesticide/insecticide eliminated, another one sticks its head in the door and surprises us.

This is another reason hands should be washed well before touching a caterpillar habitat/popup/cage. Many things we handle during the day may have pesticide on them. As we walk through a store or down a sidewalk, things like stair rails and door handles may have pesticide on them. Although normal pesticide exposure comes through the plants or the homeowner, every now and then it enters through an unusual vector.

Pets are a source of pesticide. Flea/tick medications, both the topical and the oral treatments, can be deadly to caterpillars. Oral flea/tick medications can ooze out of a dog or cats skin in its oils. As we pet our dogs and cats, we can pick up enough medication to kill our caterpillars. Again, we cannot stress how important it is to wash BEFORE touching a caterpillar, its rearing cage, or its food.

Sometimes caterpillars and/or chrysalises die from parasitoids. Although they look much as if they are diseased, their problem is that there are tiny insects inside them, eating them from the inside out.

Chalcid Wasps lay eggs in soft chrysalises. They smell a caterpillar when it gets close to time to find a place to pupate. They will follow the caterpillar until it attaches itself. They then either sit on or by the J’ing caterpillar until it pupates. Once it pupates, it lays eggs in the soft chrysalis. The little wasp larvae eat the insides of the developing chrysalis. Once they are mature, they eat a small hole in the side of the chrysalis, emerge, and fly away. These wasps are the size of tiny gnats. I’ve counted over 400 that emerged from one chrysalis shell. Visit our Chalcid Wasp page to learn how to prevent these wasps from laying eggs in your soft chrysalises.

Tachinid Flies lay eggs on the skin/cuticle of caterpillars. They will often lay eggs on young caterpillars. The eggs hatch into fly maggots/larvae that eat the caterpillar’s hemolymph/blood. When it hangs in a J or after it pupates, it leaves the caterpillar or chrysalis and forms a little fly pupa in leaf matter. In a rearing container, the maggot finds a little corner or fold in a paper towel and changes into a pupa. In the US, Tachinid Fly maggots leave a string when they leave the caterpillar or chrysalis. In other species, they may cause the death of the caterpillar before it J’s and emerge from the dead caterpillar without leaving a string. There are several species of Tachinid Flies.

Quite often, when wild caterpillars are brought indoors, they are already infected with fly maggots. When you bring them indoors, you cannot stop the maggots from killing your caterpillars. You can prevent the maggots from becoming adult flies by flushing or freezing the maggots or fly pupae.

Some diseases cause a Monarch caterpillar to turn dark. That being said, there are many ways a caterpillar can appear and be called ‘dark’. In the photo to the left, the entire caterpillar is a smokey dark color. This indicates bacteria in its blood/hemolymph. The hemolymph of a caterpillar/chrysalis/butterfly is yellow/green. Bacteria turns it dark. This is different than dark bands from being raised in low light conditions. Compare the two caterpillars to see the different in the white color between the dark bands. We discuss wide messy black bands on another webpage here on our website. Some people see wider dark bands when temperatures are cool.

Cannibalism is often encouraged by overcrowding caterpillars or allowing them to run out of food. If you see cannibalism in your habitat, make note of how many caterpillars are together. Overcrowding causes chrysalis damage from caterpillars crawling about to find a place to pupate. They will crawl over other caterpillars.

When a caterpillar pupates, its cuticle/skin always breaks open right on the back of its thorax unless it has been broken elsewhere. When a caterpillar crawls on a J’ing caterpillar, the little hooks on its legs will make tiny (or large) tears in its cuticle. When it begins to pupate, instead of breaking on the back of its thorax, it will break at the weakest point. When it breaks at a spot other than the back of its thorax, the result is a deformed chrysalis and, if it lives to emerging time, it normally cannot emerge right and will die in chrysalis.

‘Black Death’ is a generic term and does not refer to any one specific disease. It basically is saying, “my caterpillar/chrysalis turned dark and died.” It can be caused by a virus, bacteria, microsporidia, or other cause. We encourage people to avoid the term ‘Black Death’ and use specific signs and symptoms instead of a generic term.