When we’re raising caterpillars in our homes, we are fighting nature. Nature has a goal of killing 98% of them before they become adult butterflies. The odds are stacked against us before we even bring a caterpillar from our garden to our home.
Nature uses disease as one method to keep a butterfly species alive while killing most caterpillars.
NPV – Nuclear Polyhedrosis Virus – is one of the worst diseases. One caterpillar can have a billion virus particles in its body. The virus causes caterpillars to crawl upward before they die. The caterpillar or chrysalis turns to liquid and the virus particles drip and splash, covering a large area as it rains or we water or gardens. Another caterpillar needs only to eat a leaf with invisible virus particles on it and it will soon die of the disease too.
Anal prolapse may happen naturally. It can also be caused by exposure to some pesticides. The digestive tract begins to come out of the caterpillar’s anus. When this happens, the caterpillar will die.
It is natural for a chrysalis to be deformed or fail to finish forming properly. It shouldn’t happen often. If it happens to more than a couple at a time, chances are that they have been exposed to a pesticide. Some pesticides cause failure to pupate.
Bt is a natural soil-dwelling bacteria. When caterpillars take a bite of food that includes Bt, its gut lining begins to break down and it stops eating. Three days later, the caterpillars die. One of the first signs of Bt ingestion is what you see in the photo above. Caterpillars had been given fresh food 18 hours earlier and didn’t eat more than a couple of bites
Chrysalises can dehydrate if they are in a dry area. In nature, they usually have plenty of humidity from the plants around it. In captivity, they sometimes die from lack of humidity. The butterflies fully form yet die before emerging. The tell-tell sign is that chrysalises are light as a cotton ball and when broken open, they are dry inside.
Fungi kills caterpillars, chrysalises, and adult moths and butterflies. When you touch caterpillars or their food, always wash well first. It is easy to track/carry spores with you as you touch a plant in nature with the spores before picking food for your caterpillars.
Caterpillars that are exposed to certain pesticides will spit or vomit green liquid. Some spit green as a defensive mechanism. In time, you’ll learn which naturally spit green when touched and which are reacting to pesticide exposure. Spit/vomit stays green. Hemolymph (like blood) that drips from a caterpillar or chrysalis will turn black or gray within five minutes.
One of the signs of disease is abnormal coloration of caterpillars. Sometimes the abnormal color is natural, from cooler weather or genetics, but sometimes it is from disease.
There are many signs that you can see from bacterial infection. An abnormal color of caterpillars or chrysalises can be caused by bacterial infection.
Caterpillars should do a complete molt, their cuticles coming off in one piece. When a molt looks like this, it is a sign of other problems, disease or even a too-dry environment.
OE (Ophryocystis elektroscirrha) is a protozoan parasite that infects Monarch, Queen, and other species that host on milkweed. In the photo above, we see the discolored abdomen and crumpled wings, classic signs of OE. The adult butterfly is often sticky with a darker than normal abdomen. Quite often, in the worst cases, adult butterflies cannot get out of their chrysalis shells.
What can you do? Use caution. Stay clean. Clean the rearing area well. Disinfect rearing containers between every generation of caterpillars. Check every milkweed-eating butterfly for OE spores. Avoid pesticides. When purchasing host plants, ask if they have been treated for plant pests and be aware that the retail nursery may not have been told if the wholesale nursery has treated the plants earlier. Remember that organic pesticides are as deadly as inorganic pesticides – both are created to kill plant pests.
Be aware that when you bring in a caterpillar, it may already have had a death sentence and you can do nothing about it. Focus on the butterflies and moths that you raise successfully.
Thank you all for what YOU do for butterflies!