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.
THE GOOD NEWS
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.