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Rearing Monarchs to Collect Monarch Health Data

If you are planning on rearing monarchs from larvae for the purpose of sampling them for Project Monarch Health, here are some suggestions on how you go about this. 

Note: If you are trying to maintain monarchs that are infection-free, (not for Monarch Health monitoring) please click here for detailed instructions (do not use tips below).  If you are rearing monarchs for the purposes of augmenting wild populations, please read the statement on the release of mass-reared monarchs posted here.

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. 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. 

  • 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.


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 butterfly was infected and the area was not properly cleaned, you could potentially infect a healthy caterpillar using infected containers.

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. Click here for information on what to do with infected monarchs. 

Common Issues with Rearing


Some issues may arise that are not necessarily from 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, 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 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.


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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