Caterpillar deaths

Asked August 28, 2018, 4:05 PM EDT

My wife and I raise caterpillars and we have had more than normal deaths before they become butterflies. Most of them eat fine and get big and chunky and when they get ready to go into “J”, they mostly end up dying hanging on top of our tomato cage that we keep them in. They usually do not make it into chrysalis. Some even fall and bleed out on the bottom of the cage. 1. Any possibility chemicals could play a role in this? We believe some chemicals have been used near our property but not extremely close. There aren’t any visible signs on our milkweed (yellow, brown or shriveled leaves) that they have been tainted. They look green, lucious and healthy. 2. Any possibility that bagged mulch from the store could be tainting the soil and poisoning our milkweed? All of our milkweed is in mulched flower beds. 3. Or does it sound like a disease or a parasite of some sort? Thanks for your input. We are trying to resolve this. It is so sad to see a caterpillar not make it to its goal, a butterfly. Jefff

Polk County Iowa

7 Responses


Thank you for your message and for contacting Iowa State University. I believe your item # 3 is probably the most likely explanation. There are many parasites and diseases of monarchs that frustrate enthusiasts. I spent Sunday afternoon at the Iowa City Monarch Festival. Failure of caged caterpillars to survive and emerge was a very common topic among the nearly 1,000 attendees.

This website lists 11 parasites and diseases that could be involved.

They listed pesticide-tainted milkweeds but that seems the least likely based on your description. Milkweeds would have to have fresh residues of insecticides (not herbicides or fungicides) to affect caterpillars and I would suspect small caterpillars would die fairly quickly if exposed to insecticides.

Monarch Watch mentions diseases as a common problem.

Xerces Society has an interesting article on diseases of monarchs in captivity.


Donald Lewis

Thanks for your response. As for as bagged colored mulch from like Menards or Lowe’s, is there any risk that the dye and whatever else might be on the wood chips from the manufacturers could taint or poison milkweed plants, which in turn could harm young caterpillars when consuming them or do you think mulch is safe? All of our milkweed is in mulched flower beds. Or is it better to just let the milkweed grow in soil beds without wood chips? The main reason we do wood chips is to control weeds. Thanks. Jeff

I don't know if colored mulches contain chemicals that would be water soluble and taken up by milkweeds. If there were, insects on other mulched plants would show similar mortality. This article from University of Massachusetts mentions potential issues but doesn't answer what impacts might result.


Donald Lewis

Hi Don. My wife and I are preparing to cold stratify our milkweed seeds for next Spring. We typically put out seeds in the freezer and start the seeds indoors first and then transplant outdoors later. This year we plan to make a mixture of milkweed seeds and spread them outside to stratify over the Winter. One of the seeds we have in question is tropical milkweed seeds. I have read that cold stratification is not required for tropical milkweed seeds but will these seeds be damaged or destroyed if they are cold stratified? Thanks.

Thanks for the followup. You are correct; "Seeds from the tropical milkweed, Asclepias curassavica (and other tropical milkweed species) do not require this treatment." per this source:

Greetings. So, even though cold stratification is not required for tropical milkweed seeds, if someone did cold stratify the seeds, would it hurt or damage the seeds (referencing tropical milkweed seeds only). Would those seeds grow if planted, having been exposed to cold temperatures. Thanks.

I have searched and searched for a research-based answer to this, without success. However, some sources indicate that too much water, leading to potential mold problems, can be a greater ‘threat’ than the cold. So, as long as it is a dry cold, it would appear that the temperature is not an issue. Try it, and see what results!