Wednesday, May 22, 2019

Rearing butterflies and moths

With the rearing projects I've done over the years, I've never written out everything I do or problems I've learned from. The following instructions are based on my personal experiences rearing various species, mostly in the Saturniidae family (giant silk moths). These notes are aimed towards the casual rearer, someone who may have found a caterpillar or wishes to raise a few eggs through to adults.

Large-scale rearing operations require established host plants in order to support numbers of caterpillars in the fifties or hundreds. In order to contain the larvae and protect them from predation and parasitism, people usually place large mesh sleeves over branches that are tied off at both ends, or occasionally they may have large walk-in mesh cages with potted bushes.

My rearing operation is what is considered a small-scale operation, with less than 40 larvae of any one species, mostly fed with cuttings of their host plants. If the plants are small and something I can buy at a nursery, like kinnickinnick or other forbs, I'll keep the whole plants in my rearing cages, which are in three sizes: 12" cubes, 14x14x24" and 24x24x36".

Place a paper towel in the bottom of a plastic or glass container (I often cut to fit) and then place the eggs on the towel. Keep the lid of the container on, opening it once every day or two when you check the eggs. Keep in mind these critters are tiny and need relatively little oxygen, so you won't suffocate them in an airtight container. My preferred container size is a square plastic Rubbermaid sandwich container, although I've used a variety of containers, including small glass canning jars.

In some species, eggs change color or develop a pattern a day or two before the larvae hatch. Sometimes you can even see the caterpillar through the eggshell within 24 hours of them hatching. Eggs of some species overwinter (these are usually laid in the fall), but eggs of most species will hatch in 7-14 days, with most in my experience hatching around 10 days after being laid.

Rubbermaid sandwich container used for hatching eggs and feeding 1-3 day old larvae

Newly-hatched caterpillars
Within a couple hours of the larvae hatching, place one or two small sprigs (or leaves if it's something big like madrone) of their host plant on the paper towel in the container. If it is an airtight container, the cutting will usually remain fresh for 24-48 hours without any additional moisture. The paper towel helps absorb excess moisture that could lead to fungal growth which can kill the caterpillars.

Reasons I don't put the newly-hatched larvae immediately into a large cage on branches of host plant:
  • If I'm not sure they'll eat a particular host, it gives me the chance to test a few options in small amounts and be able to see if they're eating one or the other.
  • New larvae don't stay on the host plant very well during the first 24-48 hours, they often wander. In the wild, they would typically be on a large plant or tree so would have lots of room to crawl around and still be on the host. If I put them in a large cage right away, they sometimes fall off the cut branches or crawl off one and onto the cage walls, then crawl all over the cage trying to find the food again.
  • The airtight container keeps the food fresh for at least 24 hours without requiring it to be wrapped in a wet paper towel or placed in a vase with water. Larvae are very susceptible to drowning, especially when they're so tiny that it doesn't take much water.
All of the above gives the larvae the best chance at survival. Once they are eating well, they're less likely to wander off of the plant cuttings. This is when you should move them to the main rearing cage. They should not be kept in the airtight hatching container for more than 2-3 days or the humidity may build up too much and cause bacterial or viral infections, or the food can become moldy.

Rearing caterpillars in mesh cages
Rearing cages come in a variety of shapes and sizes. I use the collapsable (aka popup) style with fine mesh on all sides except one side that is made of clear vinyl for easy viewing. These cages allow good air flow while keeping even the tiniest caterpillars contained and also protected from parasites and predators. Small 10-12 inch cube cages are excellent for portability (I always keep one in my truck in case I find something in the woods I want to take home!) and for rearing small batches of larvae. I use medium cages (14x14x24) for most rearing and two large cages (24x24x36) when I have particularly large cut branches and/or caterpillars, like when I reared the Ceanothus Silk Moths on Douglas-fir and Pacific Madrone last year.

Some of these pop-up cages don't sit flat very well because of the curved wire frame. This can be a problem if your plant cuttings are top heavy or in a vase that is easily tipped over. I get around this by using large, flat-bottomed containers such as quart or half gallon canning jars and being careful to place the cage on a hard surface like a table or supported by walls in a corner of my porch or living area. If you or someone you know is handy with wood-working, you might try cutting a piece of plywood to fit the inside of the bottom of your cage to stabilize it. You would need to curve or otherwise blunt and sand the corners to fit properly so the board doesn't wear holes in the mesh.

Place an old towel or layers of paper towels on the bottom of the cage to collect the frass, this way you can easily roll it up and shake it out in the garbage to keep the cage clean. You should do this every few days or at least whenever you change the plant cuttings, to keep the caterpillars' environment clean.

Don't overcrowd the caterpillars, this can lead to viruses or other problems killing them or resulting in otherwise unhealthy larvae.

Keep the cage in a well-ventilated area, outside exposed to the natural temperatures is best if you have a covered space, but I usually keep them inside next to a bright window.

The most critical part of rearing caterpillars is keeping the food fresh.

Live plants
If at all possible, feed your caterpillars on potted plants inside a medium to large rearing cage. Many host plants are available at nurseries, even if they aren't the exact native species your butterfly or moth is used to. There are a few native plant nurseries in the northwest that are also good sources.
Be mindful of how the plants have been grown and cared for. I've read that most plants aren't sprayed at the nursery where you're buying them, only where they were grown, which would wear off by the time you buy them (I bought some plants recently that specifically said they were only grown with biological control methods, like ladybugs to control aphids). However, milkweed plants being sold by some big box stores a couple years ago were discovered to have tags indicating they had been grown with systemic neonicotinoids, a very deadly pesticide to insects that can remain in the plant for months. I don't know if this is still being done, but just be careful to read all the tags on the plants prior to purchase.

Cut plants
In most cases for small-scale rearing, you will be dealing with cut branches. I manage these in two ways. The critical part is ensuring that your caterpillars can't access the water source and drown! This is a common problem, so you are not alone if you lose a couple caterpillars this way, but it is nonetheless devastating.

Method 1: wrap the cut end with soaked paper towels and place in a plastic baggie tied off tightly with a rubberband or twist tie. I do this for small to medium twigs, such as when I'm starting off some young caterpillars or rearing a species that has very small larvae even when they are full grown.

Method 2: use a vase or other container with water, plugging the opening very tightly with paper towels or plastic wrap. This can make it difficult to change out the stems, so I usually do this when I have a lot of caterpillars requiring large branches that take up a lot of water, or if I am using host plants that do very well as cut stems for long periods of time, such as willows or evergreen trees and shrubs. The easiest way to do this is with a narrow-necked vase that you can put 1-3 stems in it and then stuff paper towels around any remaining gaps in the vase to keep the larvae from crawling down the stems and drowning in the water. If you have larger branches and need a bigger container, I like to use quart or half gallon glass canning jars, but these can be more difficult to seal up. If you put so many branches in the jar that it only needs a few gaps closed off, it's not hard to use pieces of paper towel. Sometimes I cover the top of the jar with 2-3 layers of plastic wrap and a rubberband, then pierce the plastic layer with the stems, then seal up any loose gaps or rips with pieces of paper towel.
If you stuff a lot of paper towel into the gaps to the point that it starts absorbing water from the jar, it can open up some of the holes, so keep an eye on that and stuff more paper towels in it.

No matter what, check your caterpillars frequently and watch for any that may slip through a gap you missed. If you find one early enough, you may be able to revive it if you put it on a dry paper towel and very gently blot it.

Plants that do well as cuttings can be kept fresh for up to two weeks. If the cuttings start dropping leaves or become brittle, replace them as soon as possible to keep your caterpillars healthy. Most cuttings can be kept in the refrigerator for a week or two and stay very fresh, just wrap the cut ends in wet paper towels and place the entire cutting into a gallon ziploc bag or even a small trash bag, seal it most of the way, then blow into it to inflate the bag and quickly seal the rest of the way. I've had great success with this method! I'll also keep extra cuttings in a bucket with water to change out the cuttings in the rearing cage through the week for larger projects, such as the Ceanothus Silk Moths I reared on Douglas-fir and Madrone last year.
If you can obtain fresh cuttings more often than once a week, that is of course ideal, but if you can only gather the cuttings on weekends, just make sure to plan ahead and gather a little more than you think you'll need for the week.

Moving the caterpillars
Use a small paintbrush, like a watercolor brush, for moving the larvae to fresh food. Be gentle and slide it under them from the side, kind of rolling it a little and gently pry them off the stem they're on. You may need a larger brush if the larvae are larger than an inch long and have stinging spines. Otherwise with most species, once they're about an inch or bigger, they become easier to move by hand or with a small stick. Sometimes I just clip off the section of stem they are on and prop it on the new cuttings, often they'll immediately start to crawl onto the fresh leaves.
Count your caterpillars! This ensures you've accounted for all of them when you move them to fresh cuttings. They can be easy to miss, so usually I remove the old cuttings and clip off all the leaves or pieces of stems they are attached to and place them in a temporary "holding pen", then once the fresh cuttings are ready, I transfer all the larvae to the new branches.

Preparing for pupation
Once your caterpillars have finished eating they will start crawling around the cage looking for a place to pupate. Butterflies will usually hang from or otherwise attach their chrysalis to stems of the host plant or the inside of the rearing cage. Moth species that spin cocoons will spin up in the branches of the host plant cuttings. When they start doing this, do not replace the cuttings or disturb the caterpillars, keep them in a spot where they won't get jostled around. Once the cocoons are formed, wait at least a week before moving them, to allow time for the pupa to form. If you know the cocoons or butterfly chrysalids will overwinter, you may carefully cut the twigs they are attached to and prepare them for overwintering (I'll save that for another blog post). Otherwise, just dump out the water in the jar but keep the twigs in the cage and wait for the adults to emerge.

If you are rearing a moth species that pupates underground without spinning a cocoon, watch out! Those caterpillars will chew right through your cage and hide in all corners of your house! If you have earth-pupaters (like sphinx moths, a few Saturniids and others), keep a close eye on the larvae as they reach maturity and as soon as they start crawling around the cage for a day without eating, immediately move them into a plastic or glass container filled with crumpled paper towels. A five gallon bucket with a lid works well for this. You can provide them a pot of soil at least 6 inches deep, but you'd need to make sure the soil was free from fungus and potential parasites. It will take a few days for the caterpillar to develop into the pupa, so leave them in a quiet dark place for at least a week. If they are a species that overwinters as the pupa, place them in a container for the winter (I'll discuss this more in a future post on overwintering), otherwise, place the pupae back into your rearing cage to await the adults in a couple weeks.

I'm open to other rearing tips, so please share your experiences!

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