What killed my caterpillars?

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.

Gulf Fritillary caterpillar died from Nuclear Polyhedrosis Virus (NPV)

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.

Monarch with anal prolapse
Gulf Fritillary with anal prolapse

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.

A Monarch chrysalis failed to finish forming and sealing properly

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.

Monarch caterpillars quit eating due to Bt (Bacillus thuringiensis) bacteria ingestion

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

Dehydrated Red Admiral and Julia chrysalises

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.

A deadly fungus kills a moth caterpillar

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.

Bright green spit/vomit from pesticide exposure

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.

Monarch caterpillars abnormally colored

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.

Bacteria can cause abnormal coloration of pupae, such as with these three Monarch pupae

There are many signs that you can see from bacterial infection. An abnormal color of caterpillars or chrysalises can be caused by bacterial infection.

Improper molt of a Monarch caterpillar

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 infected Monarch butterfly

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.

Jonathan found a Gulf Fritillary caterpillar in the garden

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!

14 thoughts on “What killed my caterpillars?

  1. These were very interesting and helpful facts, ( but not fun facts). 😢

    On Thu, Feb 20, 2020 at 1:50 PM Butterfly Fun Facts wrote:

    > Edith Ellen Smith posted: ” 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. Natu” >


  2. Great information. I am always sad when raising monarchs that sometimes they just die before completion of all their different stages . Thank you for that detailed information! Very helpful.

    Liked by 1 person

  3. Hello Edith!
    I’m new to caterpillar rearing. Recently i had a few common jay caterpillars (each in different instars), and they all died squirming and oozing some brown liquid from their mouths 😦
    I’m not sure what caused this, they were fine for a week or two and then all of them simultaneously died the same way. The brown liquid left dark stains and the caterpillars were covered with it. I couldn’t find any such case on the Internet, so I would be extremely grateful for any information I could get.


    1. I suspect some type of pesticide that acts on nerves, like a cholinesterase inhibitor. Normally them spot/vomit green, so that part confuses me. Do you have pets with flea/tick medication? I’d so love to see a Common Jay. They aren’t here in the United States.


  4. This year all of our catepillars and even chrysalises keep disappearing. We think we have black swallowtail add they are on natural flourishing anise. Years before have always been successful. Could it be a new bird in the area actually taking them? What type of bird would like them?


    1. There are many species of birds that eat caterpillars and chrysalises. In addition to birds, there are wasps, mice, squirrels, spiders, and dozens of other predators that eat them. It sounds like this was just a year for predators in your yard. I’m sorry. Hopefully next year you’ll see more make it to become adults. Many people bring them indoors to raise them, simply to save them from predators.


      1. Thank you, Ms. Smith, for taking time to respond. I’ll check into maybe putting up a temporary screen over the anise next year. Bringing indoors won’t work due to our indoor cats.


  5. The milkweed that grew in my garden (spontaneously) was covered with a red and black bug (nothing like the lantern flies that are endangering our trees). I had no caterpillars and the milkweed looked eaten and dry with lots of white spots. I decided to cut it back And found one healthy-looking caterpillar and immediately brought it to the hatchery. I’m hoping for one monarch this summer. This was about the third year for that milkweed. The first year I had many butterflies; the second year a very few; this third year only the one. Is the wild milkweed not a good host in my garden?


    1. It is a great host. It is always a bit of hit-and-miss when it comes to caterpillars. Some years they will eat you out of house and home while other years they almost (or do) totally ignore you. Wild milkweed is great and so is milkweed that you buy at nurseries.


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