Tonya Smith, MN, told me about her recent experiences with monarch motherhood and Tachinid fly parasites and I asked her to share her story and photographs on the blog.
Tachinid Fly Parasites on Monarchs
Tonya Smith
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| Monarch chrysalis with signs of a parasitoid - Tonya Smith |
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| Success, free at last |
One of the many wonders of nature is the metamorphosis of the butterfly. I learned the basics in school, but now I was successfully raising one monarch caterpillar observing the process first hand. I was hooked ... on butterflies! From my training for being a docent at Bill Roston Butterfly House, I learned less than 5% of butterflies reach their adult stage. Of course, the good news is our planet would be defoliated if we didn’t have the checks and balances in nature to control the ravenous caterpillars in the Lepidoptera order which includes moths and butterflies.
Fast forward several weeks from my first experience raising a monarch caterpillar and it’s release. The swamp milkweed in my perennial garden had an abundance of the famous black, yellow and white striped caterpillars. My morning and evening “cat walks” became a regular occurrence, hoping to observe the ones that were leaving to pupate so I could watch over the chrysalids. I noticed many small caterpillars leaving the host plant possibly because it was over populated so I took some young instars in to finish raising them.
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| Cat with a strings attached |
Soon the horror show got underway. Caterpillars got into what appeared to be a healthy “J” position, but time would prove their bodies were parasitized. The caterpillars became limp and out came the tachinid fly larvae on gelatinous tendrils that look like white strings. In some cases, the caterpillar successfully formed the beautiful green chrysalis. But excitement turned to disappointment once the chrysalis became discolored and holes appeared for the tachinid fly larvae to exit. The success rate of the ones I was raising was zero percent.This scenario was replayed in my yard as I saw caterpillars and chrysalids hanging from the soffit or brick that had succumbed to the tachinid fly.
So here was the living proof of nature’s checks and balances. Tachinid flies belong to the Diptera order and are in the Tachinidae family. Their larvae
are internal parasites
of immature beetles, butterflies, moths, sawflies, earwigs, grasshoppers,
or true bugs. The adult fly may lay its egg on a host species or penetrate it to place the egg inside. The fly may even lay her eggs on plants where the maggots can be ingested by the future host after they emerge.
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| First look outside with more to come |
Once inside, the maggots begin to consume the hosts by eating non-essential tissue first which allows the host to continue to grow and feed normally. Only when this material is fully consumed, will the larvae turn to eating vital organs. It’s in the tachinid larvae’s best interest to allow their host to live as long as possible so they can grow fast. The larvae then pupate into adults inside or emerge to pupate outside the prey’s body as seen in my monarch's video. For a real world replay of
The Blob,
see my video.
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| Three in a chrysalis |
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| Leaving home |
Karen Oberhauser is an Associate Professor in the Dept. of Fisheries, Wildlife and Conservation Biology at the University of Minnesota. She and her student, Michelle Prysby, started the Monarch Larva Monitoring Project in 1996 which studied the reproductive ecology, host-parasite interactions, and factors affecting the distribution and abundance of immature monarch stages. Their research showed that the species
Lespesia archippivora is the most important monarch tachinid parasitoid. It is widespread throughout North and Central America, has been found in Brazil, and was purposely introduced into Hawaii for biocontrol in 1898. More details are at
Monarchlab.org.
Oberhauser's report titled,
Tachinid Flies and Monarch Butterflies: Citizen Scientists Document Parasitism Patterns over Broad Spatial and Temporal Scales (author Karen Oberhauser), cites an approximate 92% mortality during the egg and early larval stages from other natural enemies than the tachinid fly. In a study where volunteers collected and reared monarchs in all immature stages, they found from one to twelve flies emerged from individual monarchs. Later stages tended to produce more flies per monarch when comparing the 1st-3rd to the 4th-5th instars. So monarchs collected during the later stages were more likely to be parasitized and the daily risk of parasitism was higher during the middle three stages. This of course is why it is recommended to raise monarchs from the egg stage. So eventually my “cat” walks turned into a search for those that needed to be euthanized after observing the signs indicating larvae were using our beloved monarchs as their host body.
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| Larva on the move |
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| "Which way to a pupation station?" |
An article in
American Butterflies reports on a study of fly parasitism across North America which found up to 30% of monarchs die from parasitoid attack in some regions and years, with up to 10 fly maggots emerging from a single monarch pupa. These high rates of parasitism suggest that tachinid flies could be a major factor regulating wild monarch populations.
When Butterflies get Bugs: The ABCs of Lepidopteran Disease.
While many of us curse the tachinid fly, there are those who welcome and attract these parasitic insects and for good reason. On an agriculture level, they help control major crop pests including cabbage worms, Gypsy moth, Colorado potato beetles, corn ear worms, cucumber beetles, cutworms, earwigs, four lined plant bugs, Japanese beetles, Mexican bean beetles, sawfly larvae, squash bugs, and tobacco budworms. In fact, one Organic Farmstand and Education Center in Vermont states, “Never destroy caterpillars with white eggs on their backs as these will develop into more tachinid flies.” So for those of you with vegetable gardens, provide tachinid flies with a diversity of plants with small flowers including flowering herbs, and plants in the Aster family.
Cedarcirclefarm.org
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| Tachinid pupa |
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| Tachinid fly emerged |
There are several viral and bacterial pathogens that can infect monarchs, including a nuclear polyhedrosis virus and Pseudomonas bacteria (Brewer and Thomas 1966, Urquhart 1987). Protozoan parasites such as
Ophryocystis elektroscirrha and a microsporidian
Nosema species have also been identified in wild and captive monarchs. Much work and effort is invested in helping the monarchs. Besides the destruction of habitat and climate changes, I realize
it is a great tug-of-war between natural predators and human intervention to improve the monarch population.
Monarch Parasites and Natural Enemies
You can review all of my tachinid fly photographs in
this Flickr album.
Meanwhile, I prefer to end on a positive and happy note with a short video (
here on Youtube) of a monarch that beat the odds and hopefully is one of many hibernating in Mexico this winter."
