Housekeeping for Monarch Caterpillar Health

Monarch caterpillar pooping out frass

A little help can go a long way towards monarch caterpillar health. Supporting the caterpillar stage of life is a big part of monarch gardening. To help, we’ve expanded caterpillar segment of the mini-series to break out more detail on hosting caterpillars, including housekeeping for messy caterpillar guests.

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Monarch Caterpillars are Messy Garden Guests

Monarchs are milkweed eating machines. And they’re also pooping machines! This caterpillar poop, also called frass, is expelled as little dark green blobs. The exception (for healthy caterpillars) is right before a mature caterpillar pupates, when frass might have a slight reddish tint as the caterpillar empties it’s body for the final time.  Colours and any colour changes may be more noticeable for container raised caterpillars who are observed more closely. In our garden, I haven’t noticed anything other than the usual green frass, but we have a lot of active outdoor caterpillars and a lot of poop.

Yes, that’s an action shot of one of our caterpillars pooping above. All part of the magic and wonder of monarch gardening! Hahaha! 

Generally, the bigger the caterpillar, the bigger the poop. You might see frass on your milkweed that looks like tiny speck of dirt from baby caterpillars through to large frass blobs from late stage caterpillars. If you have a large growing group of caterpillars, there will be a whole lot of pooping going on. That makes for a very messy living area. 

Monarch caterpillar eating swan plant (milkweed)

Frass and Monarch Caterpillar Health

Cleaning Up After Messy Monarchs

Frass can catch in the foliage of plants and build up on the surfaces below. This increases the risks of bacteria and transferred diseases affecting exposed caterpillars’ health. If you are raising monarch caterpillars inside, you will need to ensure that their enclosure is kept clean and provide a safe fresh food supply, as you would with any other contained creature or critter. This can be a little trickier to do out in the garden.

Caterpillars in the Wild vs. Caterpillar Guests

If caterpillars were munching and pooping in the wild, housekeeping with be limited to wind, rain, and the natural decomposition of frass back into the surrounding soil. 

If you’re fortunate, your monarch garden may be hosting and helping a higher concentration of caterpillars than in a wild scenario. Giving nature a helping hand to maintain balance can help support a healthier caterpillar population, especially if you have successive guests taking up residence on your milkweed plants over the season. 

Fortunately, we’ve not yet had any known issues with illness or disease with our monarch guests; however, we still try to foster a safe and healthy environment, where we can. Here are some of the housekeeping measures we take to try and reduce risks for monarch caterpillar in our garden. See the note at the end of this post for health-related resources.

Monarch caterpillars on swan plant with seed pods

Maintaining a Healthy Environment for Caterpillars

A safe and stable food supply of healthy swan plants (milkweed) is essential. In addition to providing food, there are a other environmental health and safety considerations when inviting groups of eating, pooping, and crawling caterpillar guests into your garden. 

Spreading Out Swan Plant (Milkweed) Plants

Our milkweed plants are grouped for ease of management and supply control, and these groups are dispersed (plus our potted milkweed). This also helps to reduce the likelihood that all of our milkweed plants (and/or the caterpillars on them) would be hard hit with the same issue at once. I also have the luxury of being able to redistribute caterpillars and/or rotate my tented caterpillar feeding ares between different plantings, if needed.

Environmental Conditions and Weather

Moist environments allow fungal diseases to set in on your plants and also creates conditions for bacteria to thrive. Some of these could harm your caterpillars or disrupt their food supply, so maintaining healthy growing conditions is a must. Our garden gets great airflow (sometimes a little too much!) and the netting used to tent the caterpillars for predator protection still allows for good air movement and moisture transfer. 

I usually avoid overhead watering, but I make a special exception for our milkweed in prolonged periods of dry weather. If there is no dew or rain, caterpillars and chrysalids get a light morning misting to maintain hydration. In these conditions, there are no issues with moisture lingering around to foster health issues for the plants or our monarch guests. I also do periodic plant washes, when needed. See cleaning below.

Keeping Caterpillar Areas (Somewhat) Clean

On occasional fine mornings, after a careful check for caterpillars, our milkweed plants are rinsed with water to try and clean up some of the mess. Special attention goes to flushing out stem joints and junctions where caterpillar frass tends to catch and collect. 

For further protection and hygeine, the top of the soil on potted plants can be covered to reduce contact or the post can be periodically rinsed. In ground plants can be covered at the base or buffered with a safe mulching material. Water for any cuttings needs regular changing, but all the moreso if caterpillars are munching and pooping above.

Deep Cleaning and Pruning

Deep cleaning can be helpful if you detect or suspect illness. It’s also good practice to deep clean and/or sterilise things like containers or enclosures between use. 

We use a large mesh dome as a  protective enclosure over the caterpillar nursery to try and keep wasps and other predators away from our caterpillars. The mesh occasionally gets removed for  cleaning or other maintenance. This is also a great opportunity to do a thorough clean-up and pruning (when needed) of the covered milkweed.

Pruning milkweed helps to reduce the risk of disease buildup. It also helps to keep the plants healthy, bushy, and control their overall size so as not to overcrowd the enclosure.

Our caterpillars are usually on one of the milkweed patches or in the dome, but they do tend to wander off sometimes, especially when they’re getting ready to pupate. Any else that we notice being used used to support a chrysalis is tidied up after the butterfly departs, including removing any remnants of chrysalis casings.

Swan plant (milkweed) growing under insect netting

Monarch Health Help

Worried about your caterpillars? Your butterflies? Both? Need help with a specific issue or concern? Monarch Butterfly Garden and Save Our Monarchs have helpful articles, including photos, for identifying and trying to prevent common monarch diseases and parasites.  If you have a monarch caterpillar (or caterpillars) that you are concerned may be unwell, you might want to temporarily relocate them to a caterpillar quarantine area using cuttings or potted milkweed that can be isolated from the rest of your caterpillars while you investigate potential issues and/or allow some time to monitor.

Housekeeping for Monarch Caterpillar Health

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