Monitoring Results
Tachinid Flies
Many MLMP volunteers track mortality due to tachinid fly parasitoids. Parasitoids are insects that feed on hosts from the inside and eventually kill them; tachinid fly females lay their eggs on the surface of monarch larvae, the fly larvae burrow into the monarchs and eat their tissue. The monarchs die during the late caterpillar or early pupa stages.
Tachinid fly larva
emerging from a
monarch larva
Tachinid fly puparium, after fly has emerged
Adult tachinid fly. Note the bristles on its abdomen,
giving it the name "bristle butt."
Many MLMP monitors collect 4th and 5th instar larvae and record whether they survive to adulthood, and if not, what causes their death. This information helps us learn what percentage of monarch larvae are parasitized by flies, and how that number varies across geographical regions and time. Tachinid parasitism rates vary from 0% to over 50% in different milkweed patches, but until 2003 we had never observed significant differences between years. At the U of M Monarch Lab, we geared up for a large research project on environmental factors that affected tachinid parasitism rates. Our research team collected over 400 monarch larvae from sites throughout the Twin Cities metro area and beyond, and with the exception of six monarchs from one site, none of them produced tachinid flies. The exceptional site is an interesting one. It is a garden in Roseville, MN, monitored by MLMP director Karen Oberhauser and her daughter Amy. In 2002, 85% of the monarch larvae collected in this garden were parasitized. However, in 2003 only 6 of 59 larvae (about 10%) produced tachinid flies. This pattern of low rates of tachinid parasitism was not just a Twin Cities phenomenon; the graph below compares the rate of parasitism by tachinid flies in all of our Upper Midwestern sites (those in WI, MN, MI and IA). Clearly, something was unusual about 2003.
Comparison of rates of tachinid fly parasitism in monarch larvae collected
by several MLMP volunteers in the Midwestersn states of WI, MN, and MI
over 4 years.
We’re still trying to figure out what happened to the tachinid flies in 2003. These flies overwinter as pupae in leaf litter, and it is possible that low snow cover in much of the Midwest resulted in high winter mortality. It is also possible that low monarch densities in 2002 after the Mexican storm, or low densities of another host of this generalist parasitoid, resulted in low numbers in 2003. While one bad year for tachinids can’t provide a definitive answer to this question, long-term data, combined with experiments, probably can. Interestingly, the low rates of parasitism that we observed in the Upper Midwest do not seem to be a population-wide phenomenon. The graph below compares 2003 parasitism rates in the Upper Midwest to those in OH and PA, where parasitism rates were over 5 times higher.
Comparison of rates of tachinid fly parasitism in monarch larvae collected
by several MLMP volunteers in two different regions in 2003 only. 2003 had
exceptionally low rates of parasitism accross the Upper Midwest, but not
in the central states of OH and PA.
We encourage more MLMP volunteers to study parasitism rates. While we hope that this is part of regular monitoring, it can be done as a stand-alone activity; simply choose a milkweed patch and collect all of 4th and 5th instar larvae that you find, rearing them to adulthood (or until they die). Directions for this activity are on our website at http://www.mlmp.org/monitoring/directions.html (scroll down to activity #3). Please contact us with questions!