For what purpose are you rearing monarchs?

If you are rearing caterpillars for MonarchHealth, please click here for detailed instructions. (Please do not use tips below).

If you are trying to maintain monarchs that are infection-free (not for Monarch Health monitoring), please follow the tips below.

Have a question? Go to our Frequently Asked Questions section.



Rearing monarchs without OE (not for Monarch Health monitoring)

In our lab we often rear hundreds of monarchs for our research on OE (pictured below). Through the years, we've learned what to do to maintain healthy (non-infected) populations. Here are some tips for maintaining parasite-free monarchs.


Tip # 1: Sterilize all materials that contacted larval and adult monarchs, including rearing containers, flight cages, and countertops, with 20% bleach solution. Soak plastic ware and fabric for a minimum of 4 hrs in a basin filled with bleach solution, and allow to soak overnight if possible. Bleach surfaces that contacted monarchs several times daily.

Sterilize butterfly nets that may have contacted infected adults.

Tip # 2: Always wear disposable gloves when handling milkweed, tubs, larvae and adults. Change your gloves frequently while working! This will prevent spreading infected spores to healthy monarchs.


Tip # 3: Use only fresh milkweed to rear larvae. If you have a greenhouse, keep the plants indoors or away from potentially infected adults. Soak wild milkweed cuttings in 10% bleach solution for 20 minutes, then rinse thoroughly with tap water before giving it to the larvae.


Tip # 4: Rear larvae in washable (and bleachable) tubs, at the lowest densities possible (ideally, 1 larva per container, or no more than 10 per container). When finished, bleach all tubs before using again.


Tip # 5: If any pupae turn brown, black or grey before the adults eclose, they're probably infected with OE (or another pathogen) and must be removed (and frozen) before they eclose and spread spores to the other individuals. Click here to see other possible signs of OE.


Tip # 6: When the adults eclose, they should be removed from the container as soon as their wings have hardened (within 8 hrs), placed in individual glassine envelopes, and checked for OE (see instructions here). Freeze any infected individuals. Everything that touched that infected adult must then be sterilized (bleached) thoroughly.

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 neccessarily 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 and find your state. Unless you live in a tropical area, we discourage using Asclepias curassavica, tropical milkweed, which is not native to North America.

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

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 durring migrations. Keep 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 fins 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 to 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 Q-tip on the abdomen to get a sufficient amount for a sample. You should see black coloration on the Q-tip after sampling.

6. What do I do with infected monarchs?

If you our 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 prevention 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