Emperor "Assembling"

I have on several occasions had adult females of the Emperor Moth Saturnia pavonia come to moth traps and once or twice have even seen the day-flying males, but by far the easiest way to record the species is to rear through the early stages and use the newly-emerged females to attract a mate using their own pheromones.  This has been an annual event for me now since May 2006 when a batch of 40 larvae was found feeding on bramble near Bicester in Oxon and 14 of them were brought home.

Emperor pair at Westcott

The moths are on the wing during April and May.  In captivity the females tend to lay the majority of their eggs (up to 200) in one batch the night after they've mated.  The same may not be true in the wild because I have had females come to light who have laid perhaps 50 or more eggs in the moth trap, so it is more realistic to expect that they lay several smaller batches.  The early-instar (black) larvae which I've managed to find in the wild have always been in smaller batches too and never more than about 40-50 in one go.

The Emperor is widely thought of as a heathland moth, with heathers being the main larval food-plant, but in Buckinghamshire it is a widespread if thinly-spread species across a variety of habitat types and I've found the caterpillars on blackthorn and bramble as well as on heather.  In captivity they will certainly accept sallows, while other food-plants mentioned in the literature include buckthorn, hawthorn and birch.  When rearing them it should be remembered that the final instar larvae are voracious feeders so it is worth choosing a food-plant of which there is a plentiful supply locally!  Goat Willow Salix caprea is my food-plant of choice because I have it in the garden, it has larger leaves than blackthorn or hawthorn and is easier to handle than bramble (which would be my second choice).  Others have suggested that changing the type of food-plant mid-way through rearing is not a good idea but I've never found that to be a problem. 

The eggs hatch after approximately two weeks.  For the newly-emerged larvae (which all tend to appear together overnight) it is important to ensure that young leaves are included in the food supply, especially if using bramble, because the older leaves may initially be too tough for them.  The larvae pass through several instars, each skin-change involving a change of colours from the initial all black through black and orange to black, orange and green and then to the mostly green final instar.   

Newly emerged larvae

Early instar larvae

Later instar larvae

Final instar larva

        
If reared indoors the larval stage should take about six or seven weeks from emergence through to pupation, so all the work is over well before the end of June.  I then move them to the garage and, apart from making sure that the container they're in is relatively clean of frass and decaying vegetation as well as being protected from the prying eyes of any small rodents, I disturb the cocoons as little as possible until into the new year.  They are then moved to the garden shed which gets them used to daylight again (the garage is dark) and a more normal range of temperature fluctuations so that, hopefully, their emergence will co-incide with those in the wild.  At this time I attempt to separate the males from the females based on the size of their cocoons (the larger ones being female) and place them on egg-trays within two different net cages. 

The cocoons should be monitored daily from late-March onwards (necessary because I once had a female to a light trap locally on 31st March) but the second or third week of April is the most likely period for emergence.  They tend to make their appearances late morning or over lunchtime and it is usually early afternoon before their wings are capable of flight, although only the males are interested in flying during the daytime.  It should be remembered that females can be mated as soon as they're out of the pupal case which is why it is important to try and separate them from the males if you have "assembling" in mind.  It is also worth mentioning that sometimes a small percentage of pupae will decide to lay over for a second winter before emerging, so don't assume that non-emergence means that they are no longer alive.

If males emerge first I take them some considerable distance away that evening (usually out of the county), the direction being dependent on the prevailing wind direction and whereabouts I have in mind to try "assembling" with any females.  However, more often than not it is a female who makes the first appearance and the net cage with her inside is hung out in the garden to obtain that all-important garden record for the year.  Once she has drawn in a "wild" male I let her pair up to provide a supply of eggs for the following year.  That also allows me to release into the garden any further males that emerge.  Subsequent virgin females are taken on a pre-determined tour of the county, trying to find males within 10km squares which don't yet have records of the species. 

Females emerging from their cocoons

Newly-emerged female starting to expand her wings

Net cage with females "calling"

Males successfully attracted to the pheromones of the females

The same principle as used in the garden is adopted for obtaining records away from home.  Assuming the weather is reasonable, an afternoon road trip is undertaken to some likely spots chosen in advance, taking into account any decent local habitat (particularly scrub or woodland) and the wind direction.  The cage containing the female(s) is suspended reasonably low down on a convenient branch or fence-post and the waiting game starts.  Sometimes males appear within five minutes, sometimes it takes longer, but if nothing has happened within an hour then I give up and move on.  Although dependent upon how many different sites I hope to visit and how many females I have available, the first male arrival at any given spot is usually allowed to pair up and I then release the happy couple straight away.  In 2015 I had a dozen females emerge (five on 17th April, six on 20th April and one on 21st April).  In addition to the garden at Westcott, they successfully attracted males at eight out of nine sites visited.  That was better than average but was very welcome after the experience of 2014 when I had no success at all with "assembling" - luck can sometimes be quite an important factor!    

To conclude, on the map of VC24 Buckinghamshire which appears below my own Emperor Moth records are marked with a black disc, of which those containing a white 'x' are successes with "assembling" (red outline for 2015, green outline for earlier years).  The small red crosses are post-millennium sightings by other recorders.  More effort will need to be applied to obtaining records in the southern half of the county next year but it is clear that the moth can be found almost anywhere.