"A Bad Summer Cuts Butterfly Numbers " Myth ?

[Mark Senior]

We are all saying that this month's lovely summer is helping the numbers of many butterfly species to recover fron the previous 2 bad summers but is that in fact the case ?
For many species for example White Admiral , Silver Washed Frit. , Dark Green Frit. which seem to be much more plentiful this year , they must have hatched from eggs laid in last year's awful summer .
Could it in fact be the case that the butterflies were in fact just as plentiful last year but because of bad weather on the wing for shorter periods of time and we softies did not see/record them because the rain kept us indoors ?

[Gruditch]

When you get a pretty bad week, weather wise, ( which happened a lot last year ), the transects still have to be done. On many occasions last year, even on the best day of a dismal week, not many butterflies took to the wing, hence they were missed of the transect reports. But were they still there, I think in most cases, they probably were.

Gruditch

[Padfield]

I imagine the population dynamics are quite complex. If I remember correctly, it was last year that birds were abandoning their nests because of their difficulty finding insect nutrition. I saw at least two pairs of black redstarts, for example, give up, leaving unhatched eggs, and saw far more discarded eggs than usual in the woods. If predator numbers in general were decimated, that might have had a knock-on effect on the percentage of butterfly eggs laid that reached maturity - mortality rate is as important as egg-laying rate in determining populations. This year, conditions have been perfect for caterpillars - warm, lush growth &c. Fewer predators, higher success rate?

Just a thought.

Guy

[NickB]

Rogerdodge was also speculating with me that "just because we don't count them (on transects) or see them, it doen't mean they are not there".
So, poor flying weather does not necessarily mean they are not doing what they need to do; we have all seen examples of females surrounded by males before they have even fully emerged or inflated their wings. If it is cool and wet like last year butterflies are still driven to reproduce like any other year - just out of our sight in the shelter of grass or bushes. I take Guy's point too, that predators also suffer in "poor" years, and followed by a harsh winter like last year, their numbers may fall, resulting in the good year we seem to be having this year.
Until someone manages to do more research into this area, I guess we are left with speculation; shows how little we still really know !
N

[Piers]

Certain species that have done well this year certainly benefited from the warm dry spring (Purple Emps as an example) that allowed larvae to develop quickly thus reducing the length of time that the larvae are vulnerable to predation from birds etc. The warm and (specifically) exceptionally dry weather early in the year, preceded by a hard winter in the South would also have benefited many species that suffer from mild damp winters and cool wet springs.

An example of this would be the Chalkhill Blue: This butterfly has suffered during several consecutive years from wet weather in May and Early June. Not only do cool springs retard the larval development leading to increased loss through predation, but the excessive rainfall gave raise to increased vegetative growth which may have lead to an adverse micro climate for larval development (too cool and damp) leading to high larval mortality; and in some cases utterly shaded-out the horseshoe vetch. Using the Chalkhill as an example the seasonal factors that would lead to a 'good year' would be: 1) warm sunny weather during late July / August for mating and egg laying. 2) an 'English' winter i.e. average rainfall, frosts in Autumn and Jan/Feb (mild wet winters do seem to have an adverse affect upon egg survival perhaps due to predators remaining active in mild damp weather as well as eggs simply failing to survive). 3) warm and most importantly dry weather during May - the critical stage for successful larval development. If all three of these criteria are met this species can have a bumper year even after a succession of poor ones.

The Silver Washed Fritillary has in all likelihood performed well this year because the warm dry spring lead to speedy larval development with a relatively high percentage of larvae making it through to pupation successfully.

I tentatively predict, that although on many sites Chalkhill Blues have had an appalling couple of years, numbers will be up this coming season. Given the right environmental conditions most butterflies can quickly make massive recoveries from successive poor years.

Felix.

[Gruditch]

I tentatively predict, that although on many sites Chalkhill Blues have had an appalling couple of years, numbers will be up this coming season.

I think your right, the Chalkhill will probably have a great year. At Danebury Hill today, their numbers seem to already be approaching, last years high. But, on a day when they would normally just sit there, we did have 30 people on a guided walk, that were inadvertently scaring them up. And had that been a transect walk, they would of been missed.
The weather for this week looks unsettled at best, I already think this week, is not going to give us the ideal conditions for butterfly recording. And if these conditions continue for a number of weeks, then the record books will show, that in 2009 the Chalkhill Blue had a rubbish year, when in fact the opposite may be true.

Gruditch

[NickB]

Re Chalkhill Blue: Indeed they already seem to have reached high numbers in Cambridgeshire's premiere site the Devils Dyke at Newmarket....
From Cambs&Essex BC Website , 12th July:

"I recorded 397 Chalkhill Blues on the Devil's Dyke (Grid ref: TL612619 ) transect today plus 30 in the Entry Level Stewardship field corner adjacent to the Dyke. Sharon Hearle"
(Highest numbers were around 600 to 700 I believe last year)

Last year a decent late Summer/early Autumn had me recording the highest numbers of CHB on the nearby Fleam Dyke in mid-August...

N