Small Tortoiseshells suffering from the summer heat?

Orange, yellow, brown and blue butterfly on grass
Small Tortoiseshell. Photo: Brian Arnold.

Brian Arnold sent us in this great photo of a Small Tortoiseshell recently and said from his observations “They seemed to be quite scarce last year, but … they have virtually disappeared this year”. We asked our Records Officer, Bill Shreeves to comment.

Bill replies as below, though he stresses this is all very hypothetical:

We have seen rather more Small Tortoiseshells in North Dorset [than South Dorset, where Brian lives] but I also am anxious about their situation. As far as I can see their numbers in 2019 will depend on the size of the mid-summer brood and in north Dorset at least there seem to be problems. That brood came out early and then fizzled away very quickly. The count on our combined North Dorset walks peaked at 26 in week 12 [mid-June] and dwindled to only one in week 16 [mid-July]. There is a theory that the hot weather may have caused them to ‘aestivate’ [i.e. spend a hot or dry period in a prolonged state of torpor or dormancy] as some were seen taking refuge in a garden shed. Even if this did happen, it would mean a delay in egg laying and thus fewer late summer butterflies feeding on buddleias before hibernation. These Small Tortoiseshells are programmed not to mate until the following spring.

An alarming quote from Jeremy Thomas in his “Butterflies of Britain and Ireland” book spells out the danger of this hot and dry summer. Working with Ernie Pollard (who invented Transect Walking]) they found that ‘the butterfly’s numbers generally crashed after a hot, dry summer, presumably because few nettles contained the lush nitrogen-rich leaves preferred by egg-laying females. I suspect the droughts of 2005-2007 played a part in recent declines’. Let’s hope it does not turn out as badly as that.

view of a Peacock nectaring with wings open wide on a pink flower

Peacock. Photo: Gordon Cryer

 

Peacocks have also disappeared quickly this year and they are the ones which are supposed to be feeding up ready for hibernation. Nigel Spring says that when his children were young they built an outdoor shed and put a rolled carpet in it. Around August they unrolled the carpet and found many Peacock butterflies. It could be that they were ‘aestivating’ as Meadow Browns do in the south of France and so disappearing until the weather is cooler. The nymphalidae family all over the world is often knownto aestivate and both Peacocks and Small Tortoiseshells belong in this family.

Just to explain the yearly cycle of the Small Tortoiseshell, we thank Jeremy Thomas for this quote from his book “Butterflies of Britain and Ireland” (new revised edition 2010):

“There are usually two distinct broods… one emerging in June and July, with their more numerous offspring appearing at any time from August to mid-October. The second brood goes into hibernation quite quickly, reappearing in the first warm days of spring to mate and breed, and often surviving well into May. However, after a cold, late spring, many first-brood (mid summer) adults also enter hibernation, saving their eggs for the following year. The trigger for this switch is the changing day-length: early-emerging adult butterflies respond to the lengthening hours of daylight by producing a second brood, but only in the south… There is only one generation a year in Scotland”.

Looking at the sightings sent into this website, which are interesting, though not scientifically valid, August is the month in which we expect to see the most Small Tortoiseshells,so please keep a good eye out for them and report any you see via our sightings page.

Graph showing numbers of Small Tortoiseshells seen in summer months for last three years

One thought on “Small Tortoiseshells suffering from the summer heat?

  1. Malcolm Hull

    Bill, I’ve been monitoring Small Tortoiseshells hibernation habits for several years through a weekly transect carried out in my garden shed. Last year (2017) I had 23 go into hibernation in the shed between mid and late July. They all stayed there until emerging in April this year. This year so far only 9 Small Tortoiseshells have gone into hibernation in the shed, all before rhe end of July.
    In 2015 and 2016 about half went in in July and the second half in August. I think what’s happening is hibernation is occurring earlier and some/all of the individuals are skipping a second brood. Less sure why this is happening, but it could be a way of avoiding the parasitic wasp which has decimated local populations. I’m in St Albans Herts, but the same may well be happening in Dorset. I’m planning to write a more detailed note on both this and Peacocks who are also hibernating in the same place this autumn – Malcolm Hull

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