Activity #3: Estimating Parasitism Rates

Download the datasheets and directions (PDF)

We are interested in learning about the natural enemies that may affect monarchs. Some of these enemies are parasitoids, organisms whose young develop inside the monarch larvae, eventually killing them. This activity will help us learn how common this kind of parasitism is at your site.

Objective: Obtain an estimate of parasitism rates in monarch larvae collected at your site. These data will help us measure the importance of this mortality factor in populations of different densities and at different times and locations.

Method: Collect any 4th or 5th instars found at the site each week as you complete Activity #1. If you would like to collect earlier instars as well, you can do this; just be sure to note that you are collecting earlier instars on the site information form. Rear these larvae indoors and record whether they survive to adulthood, and, if not, what caused their death (parasitized by flies, parasitized by wasps, etc.). Release butterflies back at the site. You can release the parasites too, but you should put them back as pupae, so that they are not given an advantage in surviving. You can also send samples of the parasites to us to identify.

Instructions for Rearing Larvae to Estimate Parasitism Rates:

1. Larvae (caterpillars) can be kept in an aquarium, large jar, ice cream bucket, bug cage, or another relatively large cage. The container should be easy to open, since you need to clean it every day. It should have a screen covering or holes for air flow and should allow you to see the larva inside. Unless you plan to move the pupae, the cage should be large enough for the adult to expand its wings when it emerges. Keep the cage out of the sun or other hot places (like car trunks in summer). High temperatures can kill the larvae.
2. Cages must be cleaned daily. Simply empty out the caterpillar frass and old milkweed. Wash out your container periodically, using a 20% bleach-water solution.
3. Larvae must be provided with fresh milkweed daily. You can pick several days worth of caterpillar food, and keep it in a plastic bag in a refrigerator. It stays fresher in the cage if you keep the end moist by wrapping it in a wet paper towel and covering the towel with aluminum foil, or use the plastic containers in which single cut flowers are sold by florists.
4. The 4th and 5th instars that you collect will likely pupate within a week. When they are ready to pupate, the larvae will crawl to the top of their cage, attach themselves with silken thread, and form a prepupal “J” before shedding their skin for the last time. This process is fun to watch, but it happens quickly. You can tell that they will shed their larval skin soon (within minutes) when their tentacles hang very limply and their bodies straighten out a little. Be careful to not jostle the container while the larvae are pupating.
5. The pupa stage lasts nine to fourteen days. Pupae turn darker the day before butterflies emerge, and look black on the day they emerge. At this point, the wings are visible. The butterflies usually emerge in the morning, and watching this happen is also fun, but hard to catch. Their wings will be soft, flexible, and wet when they emerge. If they fall, carefully pick them up by holding the thorax, and hold their legs next to the top or side of the cage. They need to hang with their wings pointed down. A pupa that has been very dark for more than a few days is almost always dead.
6. Larvae that have been parasitized by flies will not pupate successfully, but will hang limply and die (see pictures below). Fly maggots will come out of the larval body and pupate on the bottom of the container. Larvae that have been parasitized by wasps may pupate successfully, but wasps will emerge from the chrysalis instead of butterflies. In both cases, be sure to remove the wasps or fly pupae from your container. Ideally, you should return them to your monitoring site in the stage they emerge from the monarch (larva, pupa or adult). You can also send specimens to us to identify.
7. Remove diseased larvae from any container with other larvae to avoid spreading the disease.
8. Butterflies should not be handled for the first four or five hours after they emerge, and can be kept in the cage until the next day, when they should be released. They can also be released the day they emerge, especially if it is warm and sunny. Hold the butterflies carefully with their wings closed when you release them, or simply open their cage to let them fly free.
9. Record your data on Data Sheet #4: Estimating Parasitism Rates. Make sure you write the date of collection, the stage of the larva at collection, and the outcome (parasitized by fly – how many flies emerge; parasitized by wasp – how many wasps emerge; healthy monarch – gender of monarch; dead from another cause – is the cause accidental or disease).
   

	The tachinid fly life stages you might observe include the larva (maggot), the pupa, and the adult fly. The pupa is hard, brown, oval, and about one centimeter long. The adult fly looks similar to a house fly.
	When the tachinid larva emerges from the monarch larva or chrysalis, it drops down on a white thread. The remaining thread is one indicator that the monarch was parasitized by a tachinid.