Post last updated: October 9th, 2019

Monarch butterflies have a mortality rate of over 90% due to parasites, predators and diseases. A Monarch butterfly flitting through your garden is a miracle of survival. It victoriously made it through the entire life cycle — egg, caterpillar, chrysalis, and adult. After all that striving, it will live a mere 2 to 6 weeks as an adult butterfly.

This blackened Monarch caterpillar showed up in a small patch of milkweed I provide as a host plant for monarchs.


I consulted my friend, Cee, the Butterfly Lady, to find out what happened and she provided me with several possibilities.



Monarchs in tropical areas have the highest rate of infection from Ophryocystis Elektroscirrha, mostly referred to as OE probably because no one can pronounce it. OE is a parasitic single cell organism infecting only milkweed butterflies in the Lepidoptera order. The most well-known of these are Monarchs, Queens and the Luna Moth.

The Tachinid Fly, another parasite, attacks anywhere from 10% to 40% of the Monarch population by laying eggs on the caterpillar. The egg hatches and the fly larva bores into the butterfly caterpillar where it becomes a maggot that feeds on the caterpillar from the inside out. Doesn’t that sound awful?



The list of predators is long. Ants, mites, spiders, Chalcid and Trichogramma wasps, plus larval forms of other insects can attack the Monarch eggs. Birds and rodents can also attack caterpillars. As an adult, the Monarch may not make it to old age because of birds, dragonflies, mantids, parasites, wasps, rodents, and insecticides.



The two most common infections that result in “Black Death” are Pseudomonas and NPV. In both cases, the caterpillar shrivels up, turns black, and goo seeps from both ends.

Pseudomonas is a bacterial infection that likes warm, moist environments. It is found all over the world in soil, water and plants. It’s a good idea to allow your plants and soil to dry after watering to discourage development.

NPV, or Nuclear polyhedrosis virus, is most evident in areas with short or mild winters. The longer growing season allows time for NPV to spread. It causes caterpillars to crawl toward the top of the plant and then hang in an inverted V when it dies. Each caterpillar can have a billion virus particles. All that goo seeping from both ends spreads the virus.

A freeze might kill the plants on which virus particles from dying caterpillars exist but it does not kill the virus. Six hours of direct sun will kill the virus but, of course, the sun does not reach shady areas.

It’s hard to tell whether Pseudomonas, NPV or a predator killed a caterpillar because the caterpillar death often looks the same.

This is not a natural progression of a chrysalis. It became spotty, discolored and stayed that way for several days. The change to dark happens more evenly and rapidly in a healthy chrysalis. Photo by Cee.


More examples of infected chrysalis. Photo by Cee


Despite the discouragement of parasites, predators, and diseases, the female Monarch can produce 200 to 600 eggs in her short lifetime which means the 90% mortality rate still leaves almost 10% of caterpillars becoming butterflies. When you see one of those 10% flitting about in your garden, recognize the miracle you are witnessing.

You can help the caterpillars become butterflies by making room in your yard for native plants. According to Jim McCormac, in the Columbus (Ohio) Dispatch, “Nearly all caterpillar species shun non-native flora.”

The University of Florida offers a list of host and food plants for Florida butterflies. You can’t go wrong with a few varieties of milkweed plants which are host plants for the Monarch, Queen and Soldier/Tropical Queen butterflies.


  1. I have seen a wasp dining on “my” caterpillar on Milkweed. I have seen that goo coming from a Monarch..
    I did not know about all the predators etc. that attack the Monarchs. It is an eye-opener. I think I might have encouraged all the viruses etc. by watering too much.
    This summer I counted about 80 Caterpillars total, but saw very few Chrysalis. But there were always Monarchs flying around.
    Thank you for this very helpful article, Linda. I will print it out as a reference.

    1. Meta – I’m glad you found the post useful. I worried it would be boring but I write about what I seein my garden. Estimates on survival rates varied widely — some sources said as little as 1% survive to the butterfly stage — but I chose to use a less depressing rate.

  2. Linda, I spent some of this afternoon re-reading your various butterfly blogs from last year. I am glad I did. I had forgotten most of them and am back up to par now.

    1. Meta – I need to do the same thing because I can’t remember the butterfly names except Monarch and Queen! Did you notice that I now have Butterfly on the Main Menu? Makes it easier to get to all the butterfly posts.

  3. You are right. It is much easier. I was accessing it through the menu on the bottom of the page. Much easier now.

    1. Meta – I am constantly working on improving the blog, updating older articles, etc. even if I’m not posting something new. Moving to the self-hosted side of WordPress was a good move for me. I started going to WordPress Meetups and Camps which increased my knowledge. Neither of those is really suited for the side.

  4. I believe I lost several last year to tachinid flies. I had a container outside with caterpillars and one day noticed a fly inside and many had died. I’m just guessing, though. They truly are a miracle!

  5. This is the first year we have had a monarch caterpillar. Yes…one. He got to full size and the next day, he was gone. I hope it was to make a chrysalis! They are so pretty.

    1. Hello MsDoolittle! It’s been a long time since I heard from you. Did you look under any of the leaves? A caterpillar usually forms a “J” to spin a chrysalis which means it will often attach itself under a leaf out of sight of predators.

  6. Very interesting. I always enjoy seeing the butterflies. We have more bees than butterflies, I enjoy watching both flit from plant to plant.

    1. Miss Priss – When you don’t have very many butterflies, it means you are not planting the nectar and host plants they are hunting. In all likelihood, they are just passing through your yard.

      1. My butterfly bush was showing strong and just needed some cleaning up. I did not know they don’t naturally drop last year’s blooms.

  7. This morning I noticed two brown spots on one of the chrysalis that was formed several days ago. None of the others have had this. Any advice?

    1. Rosemarie — My butterfly expert, Cee, suggests googling “diseased monarch chrysalis.” She says it could be anything from Black Death, parasite or viral but she can’t tell without a photo. She has one she’s watching, too, and says it has been a bad season for raising butterflies. She has even had problems with ants in her butterfly house.

  8. Yep, even the ants were attacking a swallowtail chrysalis in my butterfly house! I still have it but I’m not sure if it’s viable. The ants are gone now that I found the source – a bag of mulch sitting outside.

    Mother Nature has been taking care of most of the caterpillars and butterflies in my garden this season. I currently have a giant swallowtail chrysalis, a gulf fritillary chrysalis, a monarch chrysalis (that doesn’t look good) and 2 caterpillars (eastern black swallowtail and giant swallowtail). The swallowtails may overwinter. -Cee

    1. Cee — I don’t know whether or not Mother Nature has been taking care of my caterpillars. I hadn’t even SEEN any until this week when I found monarch caterpillars on milkweeds in two different spots in the yard. This is really kinda late!

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