Goat Moth
[1]  butterfly-conservation.org/3538/Moth-Factsheets.html
      > Goat Moth Priority Species Factsheet

Moths index
Notes on Larvae Rearing in Captivity:

As I had not been able to photograph a Goat Moth in the wild, when I saw that Devon Butterflies were advertising larvae for sale in September 2016, I decided to order some and attempt to rear them.  In the wild, Goat Moth larvae can take 3 - 4 years to reach full size, but in captivity this can be reduced considerably to 1 year, and is just about possible in 6 months.  This was my goal, as we were due to go on holiday at the end of March 2017 and I needed the larvae to pupate before we left.

They arrived promptly, but were being fed on apple.  I had read that swede was readily accepted by the larvae and, being denser, lasted longer, so I immediately transferred them and they soon burrowed into the swede.

For a while, frass was being produced by all of them, but gradually some tunnels appeared to become inactive.  After a fortnight of no frass production from all but one tunnel, I transferred the one active larva to a fresh piece of swede and discarded the rest.

It was difficult to monitor the health and growth of the larva, as I only saw it when I moved it to fresh food.  This was a tricky process, since using a knife to cut the piece of swede was too risky as it was impossible to know where the tiny larva was, so I had to tear the swede apart until I could find it. 

After 55 days, the larva moulted to L2.  Until then, I had kept it in an unheated room, and in order to try and speed up growth, I moved the container into a much warmer room.  However, although the container had holes in the lid, it was small, ventilation was poor, and the swede developed moulds and softened rapidly in the warm environment.  As this may have been harmful to the larva, I transferred it to a small vivarium where there was much better air circulation and, although the small piece of swede became somewhat leathery, it did not rot so quickly. 

As swede frass seemed to be much wetter than wood frass and may have caused clogging, I cleared the tunnel daily as much as possible to aid ventilation and drainage.  It occurred to me much later that lack of air circulation and/or accumulation of liquid may have been why I lost most of the L1 larvae (the survivor had chosen a very small, rather dry, remnant of swede and frass was more readily pushed clear).

You are maybe thinking at this point that I was fussing unnecessarily but, having only the one larva, I wanted to give it the best chance of survival that I could.


The 4th instar last longer than before, and it did not moult to L5 until the 33rd day. The inactive moult period was also longer, at over 3 days this time.  I had been gradually increasing the size of the pieces of swede as the larva grew, and now started to use a wood drill bit (easily turned by hand) to create the initial tunnel in the fresh food.  This was more efficient than the apple corer I had been using up until then and I wish I had thought of it much sooner, as it made a clean, deep, hole and the larva quickly made the changeover. 

When we returned 11 days later in early April, to my surprise I found that, not only had the larva not pupated, but it had moulted to L7 (old head found in side tunnel).  At this point it was 200 days since I had received the larvae, so well-passed my 6 month goal!

April turned into May and the larva was still working its way through the swedes - I continued giving it whole ones, which I left on the substrate ready for eventual pupation.


By the time we were due to go away, the larva was 28 days from its last moult and, unfortunately, still showed no signs of pupating.  In the wild, pupation takes place late-April to June [1], but I did not know if this would be the case with a captive-reared one.  So, I prepared a whole swede, in the hope that it would last until the larva was ready to pupate or we returned from holiday - whichever came sooner.  I placed the swede on top of a deep layer of damp compost mixed with chopped, dead leaves, and, with some misgivings, we went away.


After a further 20 days of this regime, the larva moulted to L3.  It was now large enough that I noticed the head was white after moulting, but it gradually darkened down.  Once the head was black again, the larva resumed eating.

Unlike some recommendations, I did not place the swede on a damp substrate of compost and/or sawdust, but used a sheet of kitchen paper.  This turned out to be an advantage since, in the early stages, the larva came out of the swede to moult on several occasions and I was able to see the skin and old head clearly on the paper and, therefore, knew when a moult had been successfully completed.  Lack of substrate did not seem to be detrimental.


During the second week in May, when the larva was 233 days old, it stopped eating, created a chamber at the end of a tunnel and span a fine web of silk around itself, sealing off the tunnel to the outside.  This behaviour seemed to indicate that yet another moult might be imminent - although I had still not been able to find out how many instars to expect - but after several days with no frass production, I began to have doubts and wondered if pupation was taking place inside the swede (in the wild larvae will sometimes remain in the host tree to pupate [1]).  However, about a week later, the silk seal had been broken and frass pushed out.  On investigation, the moult chamber was full of frass, therefore it must have been eating for a few days, and, when clearing it out, I found the old head.  So the larva was now L8 and noticeably darker, fatter and longer (5.5 - 6.00 cm).

28 days after it moulted to L5 it became inactive and did not eat, which was unexpected, as, despite its small size, I thought it should have started wandering prior to pupation - 5 instars being common among other moths in my (admittedly limited) experience.  Then, on the 31st day it moulted again.  I double-checked my notes to make sure I had counted correctly but they confirmed that the larva was now in its 6th instar - I had seen the discarded head each time so I knew I had not lost track.  It was almost three days after the moult before it resumed eating, and, meanwhile, it span a thick web over the entrance hole to its tunnel, although I peeled this back after a while to make it easier to clear the frass once it started being produced again.

I continued changing the swede frequently when I thought it was becoming used up or it started to deteriorate badly.  With each new swede there was a lot of debris ejected at first and I assumed that, rather than this being wholely the result of increased appetite for the fresh food, it was also spoil from tunnel excavation.


33 days later, in the second half of June, it moulted to L9 (day 274), then to L10 after a further 25 days.  With the weather being warmer and the damp atmosphere in the vivarium from the substrate, the swedes were decaying quickly around the damaged areas and I was replacing them more often, so, in an attempt to slow down the decay, I moved the swede away from the substrate and back on to a piece of kitchen paper.

Another 25 days on, the larva showed signs of starting another moult, in that it stopped eating and plugged the entrance hole with silk and swede.  Although, this time, I was unable to find the old head three days later as proof-positive, I thought it highly likely that it was now L11, as the timing was right and its behaviour followed the same pattern as with all other moults.

September arrived and I wondered if the larva would hibernate although, as I was keeping it warm, "hibernate" was not really an accurate description.  Despite being unable to find data confirming that this diapause is an autonomic response in Goat Moth larvae, triggered at a certain stage of their development, I assumed this was the case, since, had it been due to environmental changes like cooler temperatures or reduced sap, it would logically have taken place each Autumn during the larval phase.  Then again, maybe it does, but inside the host tree so it has never been observed...?  Whatever the case, in readiness for this stage, I placed the swede back on to the substrate.  Then, during the second half of September, 32 days after the last moult and 361 days since I received it, the larva came out of the swede and promptly burrowed into the substrate. 

[to be continued]
 
Prepared whole swede, cut into 3 for easy access to the larva
Goat Moth L6
Goat Moth L8
Goat Moth L4
Goat Moth L3 - old head bottom left

After another 20 days, it moulted to L4.  It seemed to take longer with this moult, though, and it was nearly 3 days before it started eating again.