Disease is all over in nature. Butterflies and moths die from various diseases right in front of us. Because predators and the very nature of the diseases themselves are at work, we rarely notice the diseases that are affecting caterpillars in our own yard. From viruses to fungi, from bacteria to protozoa, there are many types of diseases that kill butterfly caterpillars.
Most people refer to OE as a disease. It is a protozoan parasite. Ophryocystis elektroscirrha (OE) affects butterflies that host on milkweed. In the US, Monarchs (Danaus plexippus), Queens (Danaus gilippus), and Soldiers (Danaus eresimus), are affected by OE.
Pesticides or insecticides can be disease pathogen based or may be simply poison. If a caterpillar is vomiting or spitting bright green fluid, most likely the problem is poison, not disease. Some species of caterpillars will spit a bright green fluid when they are touched. Most will not unless they are exposed to pesticides. Caterpillars that die from pesticide/insecticide normally will die in a C shape, on the bottom of the rearing container (or ground), and are pretty much normal colored. Most that die from disease turn dark and many will change in consistency (liquid or harden)
To confuse the issue, butterfly hemolymph (blood) is bright green. Where hemolymph dries black within a few minutes, the spit/vomit stays green. Oddly enough, a chrysalis like this one, that fell when it was pupating, is considered ‘disease’. We won’t talk about that type of ‘disease’. The literal meaning of the word is ‘dis-ease’ and covers almost anything that isn’t normal. Yes, you read that right. Literally, if you accidentally squish a caterpillar, it is considered ‘disease’.