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Rearing Monarchs to Collect Monarch Health Data

Note: We do NOT encourage mass-rearing monarchs with the goal of bolstering their population.

Captive rearing and releasing of monarchs in large numbers highly increases the risk of spreading OE infections to wild monarch populations, which threatens their ability to complete their annual migration.

For more information on the risks that captive rearing poses to monarch populations, please read this essay written by Dr. Sonia Altizer, Dr. Lincoln Brower, Elizabeth Howard, Dr. Karen Oberhauser, and other monarch experts.

However, rearing small numbers of monarchs from late instar larvae for the purpose of sampling them for our project can be a helpful way to contribute to our knowledge of how OE infections are changing over time and across monarchs' range. Below, we include instructions on how to responsibly rear monarchs to monitor OE prevalence in your area without spreading OE infections through the rearing process.

Note: If you are an educator trying to maintain monarchs for instructional purposes that are infection-free, (NOT for Project Monarch Health monitoring) please click here for detailed instructions. 

Rearing caterpillars to adult butterflies


Step 1: Collect larvae


Collect 4th or 5th instar larvae (greater than 1.5cm in length) from wild milkweed plants using a pint-sized plastic container. This is important because 4th and 5th instars will no longer be susceptible to OE infections, so your data should accurately reflect OE prevalence in your area. 

Here's what the 1st through 5th instars of monarch larvae look like:

Instars of Monarchs

1st - 5th instars of monarch larvae

Step 2: Set up containers for larvae


  • Put on gloves to prevent contamination.

  • Prepare a small, resealable container (like Tupperware) with small holes or fine mesh on the lid. If you have reared monarchs in this container before, make sure you have sanitized it with 20% bleach solution.

  • Put a damp paper towel (moist to touch but no standing water) in the bottom of the container. 

  • 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.

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. Once the monarch has pupated, remove the plant material and frass, and put a clean, dry paper towel in the bottom.

recently emerged monarchs

Step 4: Sample the adult butterflies for parasites


Between 4-12 hours after emergence, sample the butterfly for parasites using the testing procedures. Then, release your butterfly into the wild. Do not keep your butterflies as "pets," especially if they are infected! 

Step 5: Sterilize the containers and rearing area.


This is very important! Thoroughly sterilize container with 20% bleach solution and clean all supplies and tools with bleach wipes before rearing another wild monarch. Bleach is the only reliable way to kill OE spores. If the butterfly was infected and rearing materials were not properly cleaned, the contamination could infect future healthy caterpillars.

OE spores can persist for 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. Click here for information on what to do with infected monarchs. 

Common Issues with Rearing


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


1. Caterpillar Death

OE does not show symptoms during the larval stage, but larvae can die for many other reasons, including infection with bacteria, viruses, fungi, parasitoids, pesticides, 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 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.

If larvae or pupae turn to "black goo," this is likely due to a bacterial, fungal, or viral infection. These occur naturally, but they are much more likely if conditions are too moist. Make sure there is no standing water in your rearing containers and that there is plenty of airflow.


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 may want to treat for pests if you feel they are making your milkweed inedible to caterpillars. However, never use insecticides on milkweeds you plan to feed to larvae. Always use non-toxic, mild soap solutions such as MPEDE or dish soap mixed with water.


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.

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