Reproduced with permission from the University of Georgia Monarch Parasites website: IBBA is not responsible for external links on this page.

Some volunteers find it easiest to rear monarch caterpillars until they become adult butterflies to sample for parasites. Here are some tips for rearing caterpillars to adult butterflies:

Rearing Caterpillars to Adult Butterflies

Step 1: Collect 4th or 5th instar larvae (greater than 1.5cm in length) from wild milkweed plants using a pint-sized plastic container. Here’s what the 1st-5th instars of monarch larvae look like:

1st-5th instars of monarch larvae.
1st-5th instars of monarch larvae.

A 4th instar larvae munching on milkweed leaf.
A 4th instar larvae munching on milkweed leaf.

Step 2: Set up containers for larvae.

Wear gloves to prevent contamination. Set up plastic containers with damp paper towel—moist to touch but no standing water.  Add a sprig of milkweed, then add the larva in and close the lid.  Label container with collection date, collection site, and the estimated size of caterpillar at time of collection. Milkweed must be from outdoors (and not from inside a mesh enclosure). Do not wash milkweed. This ensures that we are sampling the natural level of OE present on milkweed plants exposed to the open environment.  

Put only one caterpillar in each container.

Put only one caterpillar in each container.  Put only one caterpillar in each container.

Step 3: Monitor caterpillars.

Each day, empty frass (caterpillar feces) from bottom of container, replace paper towel if soiled, and add a fresh milkweed stalk. 

Monitor caterpillars

Step 4: Caterpillars usually spin silk pad on the lid within 5-6 days and hang from this.  After the green chrysalis forms, discard the plant material and wait for butterfly to emerge (10-12 days). 

wait for butterfly to emerge

Step 5: Sample the adult butterfly for parasites.

Between 4-12 hours after emergence, sample the butterfly for parasites using the testing procedures.

sample the butterfly for parasites

Step 6: Sterilize the containers and rearing area.

Thoroughly sterilize container with 20% bleach solution and clean all supplies and tools with bleach wipes before rearing another wild monarch. THIS IS VERY IMPORTANT! If the caterpillar was infected and the area was not properly cleaned, you could potentially infect a healthy caterpillar using infected containers.

Infected butterfly

It is important to remember that OE spores can persist for many years and tolerate a wide range of temperatures and external conditions. Therefore, careful examination of monarchs and surface sterilization with bleach is necessary to prevent continued transmission.

Common Problems

Some issues may arise that are not necessarily from infection by OE.

1. Caterpillar Death

Larvae can die from many causes besides OE, including infection with bacteria, viruses, parasitoids, and temperature extremes. What you may see:

  • Larvae turn black and fall to the bottom of the tub
  • Larvae stop feeding and wither away
  • Larvae will turn to mush when trying to form pupa
  • Parasitic flies form inside larvae/pupae (if larvae came from outside)

To maintain healthy populations, you must remove any dead larvae immediately and replace all milkweed in the tub with fresh plants. Always wear gloves when doing so and sterilize all equipment after.

2. Milkweeds pests

Milkweeds are susceptible to thrips, aphids, spider mites, fungal gnats, and powdery mildew. Spider mites can cause damage to your milkweed supply and reduce its nutritional value to monarch larvae. Thrips can actually eat monarch eggs. For these reasons, you need to ensure that your milkweed is pest-free. However, never use insecticides on milkweeds you plan to feed to larvae. Always use non-toxic, mild soap solutions. Also, always ensure that milkweeds are pest-free before using it for ovipositioning- thrips eat monarch eggs.

 3. Adult Death

Adults can die, particularly if you keep the adults under conditions that are too hot and dry, or if they are refrigerated for too long below 10-12 degrees celsius. If they are not fed regularly, adults will starve. Check their abdomens- if they are plump, they are ok, if they appear thin, they need to be fed.

Frequently Asked Questions

Some issues may arise that are not necessarily from infection by OE.

1. Where can I get milkweed plants?

Finding milkweed in the wild can be difficult, but it’s the easiest way to find whole plants. Otherwise, check your local nursery for native milkweed species for your area. To see which species are native to your area, click here External Linkand find your state. Unless you live in a tropical area, we discourage using Asclepias curassavica, External Linktropical milkweed, which is not native to North America.

You can also order milkweed seeds from online native plants suppliers such as Prairie Moon Nursery,External Link Butterfly Encounters,External Link or Easywildflowers.External Link

2. Where can I find caterpillars to rear or adult butterflies to sample?

The best places to find caterpillars in the wild are areas with milkweed plants. Successfully finding caterpillars and adult butterflies will depend on where you live and the time of year. In many areas, butterflies are not often found in the summer months. Sightings can be more common in the spring and fall periods during migrations. Be persistent! If you have kits and are not seeing butterflies, don’t give up–migrations may be just around the corner!

The easiest places to find caterpillars and adult butterflies are state parks, gardens, and other natural areas. Please always make sure when removing caterpillars or catching adult butterflies on private property or in botanical gardens to ask permission and/or notify the proper authorities.

3. Why do I need to wear gloves and try and maintain sterile conditions?

The parasite spores are very small and very easy to accidentally spread around. It’s important to wear gloves to minimize the spread of spores from one sample to another. Always change gloves between handling different butterflies. One monarch may be infected, while another may not. It’s important to try and keep a clean work area when handling and sampling the butterflies for parasites. Bleaching any containers, tools, and work surfaces the butterflies touched prevent cross-contamination of different samples and help avoid touching a non-infected sample with infected spores from another. We suggest a 20% bleach solution be used between sampling individuals and also between different sampling periods.

4. Why are my caterpillars sick?

Larvae can die from many causes besides OE, including infection with bacteria, viruses, parasitoids, and temperature extremes.

  • If you observe group deaths or individual caterpillars showing any of the following symptoms: vomiting, writing, diarrhea, then it is likely they have been exposed to chemical poisoning. This can be a result of any pesticides used on the milkweed plants.
  • If caterpillars are not eating, lethargic, and turn brown or black, then bacterial or viral infections are possible.

In any case where you observe caterpillar illness or death, immediately remove the affected individuals from the rearing containers, bleach the containers, and move remaining healthy individuals to new, clean containers with fresh plants.

5. Will sampling for parasites hurt a butterfly?

No. Monarchs are very sturdy and it is difficult for scales to be removed. It is important to apply enough pressure with the sticker on the abdomen to get a sufficient amount for a sample. You should see black coloration on the sticker when placed on the notecard. .

6. What do I do with infected monarchs?

If your results indicate any of the individuals you have sampled are infected with OE, then we suggest you do not release it back into the wild. This will prevent further spread of OE to healthy butterflies. Infected monarchs can be euthanized by placing in an envelope and setting in a freezer for at least an hour.

For other questions and concerns, please contact our MonarchHealth coordinators at