Welcome to The STEM Sessions Podcast. I am your host, Jarl Cody.
This year’s quest to raise monarch butterflies has gotten off to a slow start compared to the data I had collected by this time last year. Though in the last week or two, we’ve had a sharp increase in visits from adult monarchs, including lots of egg-laying. So I’m expecting the number of caterpillars to increase shortly.
Unfortunately, another difference from last year is an increased number of caterpillar deaths, including some causes I’ve not seen before. The caterpillars that have successfully formed a chrysalis have all emerged as healthy adults, but fewer are making it that far.
Unsure if I was doing something wrong, I took a deep dive into at-home anecdotes and observations as well as the controlled studies regarding causes of monarch caterpillar deaths. While morbid, it was satisfying to solve one cause without question, and several more with a high level of confidence. Hopefully, I’ve learned this information in time to adjust my processes before the predicted spike in eggs hatch.
This is The STEM Sessions Podcast – Episode 13. The Trouble with Monarchs
A monarch butterfly female can lay up to 1000 eggs in her 2-5 weeks lifetime given optimal conditions. On average, though, the total is less than half that number, between 300 and 500. Once laid on the underside of a milkweed leaf, the egg will become an adult butterfly in 20-35 days depending on factors such as temperature, precipitation, and availability of food. This development cycle includes the caterpillar undergoing five instars in which it sheds its skin to allow additional growth, and concludes by forming a chrysalis and pupating into an adult butterfly.
The likelihood any given egg becomes an adult varies from study to study and anecdote to anecdote, but the upper limit seems to be 10% and the lower limit around 2%. So of those 400 eggs a female monarch will produce, only 18-40 will become adults.
This high mortality rate is primarily due to predation during the egg and early instars. One study showed 78% mortality of monarch eggs and 59% mortality of first instars. This equals an overall mortality rate of 91% before reaching the second instar. Ants and spiders are the primary predators in the early part of the lifecycle.
After making it to the second instar, predation drops dramatically as the caterpillars are too large to be preyed upon by most other insect or spider. And for the remaining stages of the lifecycle, parasitical species, microbes, and chemicals are the primary causes of death.
Every year I’ve raised monarchs, I’ve observed a handful of caterpillars die during the J-stage right before pupating. The dead caterpillars looked deflated with white threads hanging from their body. And even before death, I noted they looked skinny, like they hadn’t eaten enough. In fact, that’s what I what I believed the cause of death to be; they tried to pupate before having enough mass to be successful. It turns out that assumption was incorrect.
While researching a different incident, I learned the deflated body and white threads are indicative of the caterpillar being parasitized by the tachinid fly. The tachinid fly lays its eggs on the caterpillar, and upon hatching, the tachinid larva burrow into the caterpillar and begin feeding on it. The caterpillar doesn’t grow as it should. It will be smaller and skinnier than its non-parasitized peers. And when the tachinid larva have eaten enough, usually when the caterpillar is ready to pupate, the larva exit the caterpillars body and repel to the ground on white strands. There, they turn into red bean-like pupae.
I’ve often seen red bean-like pupae in the bottom of the enclosure when cleaning. I never made the connection between the dead caterpillars and these red bean-like pupae. I just assumed something else was on the milkweed when I cut it. But after pondering a bit, I’m quite certain every time I saw a deflated monarch with white threads dangling from it, there were also red bean-like pupae in the enclosure.
Can anything be done to prevent or minimize tachinid parasitization? According to studies and home observations, the tachinid fly seems to lay its eggs on caterpillars third instar and larger, with earlier instars less often victimized. So the sooner you can bring a caterpillar into the enclosure, the better; and bringing them in as eggs is best. As for ridding your garden of the adult tachinid flies? That’s a losing proposition. When seen up close, they can be distinguished by their large red eyes, but without magnification, they look like all other flies buzzing around. And any chemical you might use to kill them will also likely be harmful to the monarchs. Speaking of chemicals…
The deaths that started this round of research turned out to be caused by pesticides. One day, three or four caterpillars, all in the fourth or fifth instar, spit out a green liquid; something I’d never seen them do before. Those same caterpillars all failed to pupate, as if they aborted the process about a third of the way through. It was a bit unsettling to witness.
Green vomit and incomplete pupation or mal-formed chrysalides are the tell-tale signs of pesticide exposure. The question I faced is where the pesticide came from. While I use pesticides in certain areas around the house, I don’t apply it in the garden beds or near any milkweeds or plants that adult monarchs might take nectar from. Flea and tick control medications used on pets are also pesticides, and I have three cats, each receiving a monthly application of a topical flea medication. But they don’t go outside, and I thoroughly wash my hands after applying the medication. So that wasn’t the source either.
The only other possible source was the milkweeds I bought from a local nursery to supplement the supply I was growing. The nursery employee and tags on the plants claimed the plants were pesticide free, but who knows if they really were. Upon this conclusion, I gave the plants a very thorough rinsing with water in case the pesticide was recently applied and pruned them back to promote new growth which should be pesticide free.
I’m about 75% certain the nursery milkweeds were the culprit, because I haven’t seen similar deaths since I stopped feeding it to the caterpillars. This is why its important to grow as much milkweed as you can. Only you can ensure zero pesticides are used. And incidentally, pesticides labeled “safe for beneficial insects” often do not count caterpillars among beneficial insects. So while bees or butterflies may not be adversely affected, caterpillars will be, because they directly ingest the poison. This goes for organic pesticides as well. Neem oil and diatomaceous earth are both detrimental to caterpillars.
If you see a caterpillar vomit, gently rinse it in clean water and then place it on fresh milkweed that you know has no pesticide. Eating clean milkweed may be enough to detox the caterpillar, allowing it to successfully pupate. I’m happy to have that bit of emergency medical knowledge on hand, though I’ll do everything I can not to need to use it.
The threats to monarch development do not end at predation, tachinid fly parasites, and pesticides. There is black death in which the Nuclear Polyhedrosis Virus (NPV) or Pseudomonas bacteria cause the caterpillars and chrysalides to deflate, turn black, and liquefy. There are Trichogramma Wasps which lay their eggs inside monarch eggs, killing the monarch embryo. Chalcid Wasps lay their eggs in newly formed, but still soft, chrysalides; parasitizing the monarch and resulting in hundreds of baby wasps. Even a fellow caterpillar brushing against a peer in the j-stage may damage its skin preventing it from shedding correctly which results in a mal-formed chrysalis.
So with the odds stacked against successful monarch development, why do I invest the time in daily feedings and cleanings and counts, and risk seeing these somewhat horrific deaths? Well, personally, I find the backyard science fascinating, and watching a newly emerged adult monarch fly away is incredibly rewarding and makes me happy. For the monarchs, a population in severe decline, it may help their species survive. A study of the Eastern monarch population published in 2019 predicted a mere 4% increase in survival of breeding monarchs could be enough for the overwintering populations to recover. Just 4%. Those odds are good enough for me to do what I can to help.
Thank you for listening to The STEM Sessions Podcast.
This episode was researched, written, and produced by me, Jarl Cody.
While I strive for completeness and accuracy, I encourage you to do your own research on the topic we discussed, and confirm what I’ve presented. Corrections and additional information are always welcome.
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Until the next one, stay curious.
U.S. Forest Service
“Monarch Butterfly Frequently Asked Questions (FAQs)”
Western Monarch Milkweed Mapper
“Western Monarch Biology: The Monarch Lifecycle”
“Fecundity, lifespan and egg mass in butterflies: effectsof male-derived nutrients and female size”
Sara L. Hermann, Carissa Blackledge, Nathan L. Haan, Andrew T. Myers, & Douglas A. Landis
“Predators of monarch butterfly eggs and neonate larvae are more diverse than previously recognized”
“12 Monarch Diseases, Parasites, and Caterpillar Killers”
“Common Monarch Ailments 101”
“Reducing Pesticide Use & Impacts”
Butterfly Fun Facts
“Monarch Butterfly Diseases
Karen Oberhauser, Ruscena Wiederholt, Jay E. Diffendorfer, Darius Semmens, Leslie Ries, Wayne E. Thogmartin, Laura Lopez-Hoffman, Brice Semmens
“A trans-national monarch butterfly population model and implications for regional conservation priorities”