Monday, September 26, 2022

Larval Search Update

I am still more than struggling to find Emperor larvae this autumn, finding one for every 2 hours 30 mins of actual leaf searching - that's as bad a find rate as after a cold, wet summer. This is in the S Glos / N Wilts / SW Oxon region which was not drought affected and where very few sallows dropped leaves. Sallow foliage conditions there are Very Good.

I am also finding precious little else: hardly any of the moth larvae I normally find at this time of year (e.g. Buff Tip, Clouded Border, Pale Tussock, Pebble Prominent, Swallow Prominent). Yesterday, at last, I found the first batch of larvae of the sallow foliage-feeding sawflies normally encountered whilst searching sallows in autumn (this one is probably the common sallow-feeder Euura pavida) - 

It seems that the egg lay was very low, at least low down. I am concentrating on searching in the shadiest places, where all the larvae I have found have been located. 

One theory is that the egg lay was not poor, but that there was a high failure rate of baby L1 larvae - as can happen in heatwaves (I first noted this in 1976), coupled with hatched egg cases falling off the leaves. 

Such losses leave no tell-tale signs, whereas the vacant seat pad and old feeding leaves of predated L2 larvae are quite prominent (and have been diligently recorded by me each autumn).

This is tricky. I have no experience of eggs (hatched or unhatched) coming unstuck, but of the paltry nine larvae I have found so far this autumn the egg case base was present in only three instances - that's unusually low and may suggest that eggs (hatched or unhatched) might have come adrift in the hot weather.  

The show continues - I need a bigger sample, and I should eventually make some sense of the situation. It's not critical (at least away from the drought-struck South East, where it might be) but it is deeply concerning, and is totally outside my experience. As Fred Trueman used to bemoan on Test Match Special, usually when England were leaking runs through an untenanted third man boundary: "I don't know what's going off out there anymore!"

I have yet to find any sign of Emperor larvae or eggs in Hailey Wood, Cirencester Park Woods - my local wood, where I followed and chronicled the likes of 'Boris', 'Jacob', 'Rishi', 'Priti' etc. earlier this year. Nothing, and I'm keeping a keen eye open for egg case bases and vacant leaves from failed L2 larvae.

The results of my annual stratified random sampling for PE larvae in Savernake Forest will be particularly interesting. I've carried this out there each autumn since 2009.

Tuesday, September 20, 2022

Autumn Larvae are Very Scarce

 This early autumn I am finding Purple Emperor larvae at the rate of one every 2 hours 30 mins, searching as usual in Wilts, Oxon and Glos. That's extremely poor, and it looks as though something has gone very badly wrong. Larvae are as scarce as after wet and windy flight seasons, or scarcer.   

 In all previous autumns after hot weather flight seasons in which the butterflies were relatively numerous, I've found Emperor larvae at the rate of one every 15-20 minutes. That's the rate I expected when I started this season's autumn searches...

 And I'm not finding signs of failed larvae either (e.g. unhatched eggs, egg case bases with no sign of larvae, and vacant seat & feed leaves +/- the old egg case base where larvae have been predated). Normally, I find a fair few of these failed breeding attempts (which I dutifully record). 

It may well be that unhatched eggs are no longer around, having come unstuck in the heat (as suggested by the shortage of egg case bases where I do find larvae). Note that there is a known issue with eggs not hatching in heatwaves and Nick & Sam Brownley recorded a number of eggs failing to hatch in Sherwood Forest earlier. I have not recorded eggs coming unstuck before.

 Moreover, I'm finding very little else - and I also record moth and sawfly larvae whilst searching the autumn sallows. So, hardly any Pebble Prominent and Clouded Border larvae (just one of each), and no Buff Tips or larvae of the sallow sawflies. Worse, I'm finding no signs of Pebble Prominent larvae beyond, say, about 1/3rd grown - their larval feeding damage is diagnostic, this:-

 I am strongly suspecting a catastrophic event, or at least something severe... but whatever has happened is outside my experience. 

Caveat: My autumn searches are done from the ground. I haven't been able to climb sallows for several years due to severe osteoarthritis in both knees (I await at least one replacement kneecap). Crucially then, my writings are based on the assumption that (roughly) the same % of eggs is laid low down each year - and there's no data to suggest that this is the case, or isn't...

 Much depends on the Dangle Leaf season at leaf fall time (the easiest way to find Emperor larvae in most autumns).

 Watch this space...