Speckled Wood phenology

[peterc]

Two transects just 3 miles apart and similar altitude (about 100m) produced different peak flight times for the Speckled Wood over a 15-year period.

In transect 1, very few adults emerge in April and May and numbers reach a sustained peak through June and July. A new higher peak (double the numbers) by the middle of August before falling off in September.

In transect 2, which is typical for the region, a sizeable number usually emerge by late April, with other notable peaks in June, late July and again in late August and early September.

Overall, abundance in transect 2 is about double (average annual count c180) of that in transect 1 because there are more wooded areas in transect 2. Any thoughts anyone for the difference in the phenology for the two transects?

ATB

Peter

[petesmith]

Interesting...So, this suggests that the majority of Speckled Woods on transect 1 overwinter as 3rd instar larvae, whereas on transect 2 a substantial number overwinter as pupae. You mentioned there are more wooded areas in transect 2 - could this mean more shelter, therefore a warmer microclimate? Are there any other notable differences in habitat features between the two sites? Any potential significant differences in larval foodplant used?

[peterc]

Thanks for your comments, Pete. All valid points and the first one relating to the overwintering state is what struck me first but as with most things to do with butterfly behaviour it is probably a combination of factors.

What is interesting about transect 1 is that it is atypical in Hertfordshire for lateness of first emergence and low abundance of the first peak. Other species, single and multi-brooded, don't seem to be affected in the same way.

Has anyone else come across this flight pattern where numbers are very low in April and May but relatively high (say 8x) in August and September?

ATB

Peter

[NickMorgan]

The graphs below shows the average number of Speckled Woods recorded in East Lothian over the last five years, which shows a similar pattern to what you have described. This year's records are shown in red and they have bucked the trend slightly. I think the reason for that is because we had a lovely spring, but from mid-June we had lousy weather for about 8 weeks.

[peterc]

Thanks for your comments and graph, Nick.

It might be that the further north (and hence cooler conditions) you are in the UK, the more likely that, generally speaking, proportionately more Speckled Woods pass the winter in the larval stage and therefore later emergence of adults in the following spring.

Of course I don't think it is simple as that as my graphs below show for Stevenage and Knebworth Park. I suspect as Pete said, it may have something to do with the choice of hostplant and any interactions associated with the micro-ecosystem therein. So, for example, is there a higher mortality rate for larvae or pupae depending on the hostplant or habitat?

ATB

Peter

[NickMorgan]

Those are interesting graphs Peter. For many species here in SE Scotland we had very low numbers in July and August thanks to the very poor weather we experienced around then. It is interesting how this impacted on individual species depending on their life cycles. Speckled Woods were one of the few species to do well, with a spike in numbers in September. They have only been recorded in East Lothian over the last nine years and have spread remarkably to cover most of the county now. This year they were the second most commonly recorded butterfly (after the Red Admiral, which in itself is remarkable).
I have noticed quite a discrepancy between years when I have plotted the number of Speckled Woods recorded. The chart below shows 2015 and 2016.

I find that with relatively low numbers of records that we can get quite strange results. However, I have also noticed, for instance that Small Heaths regularly finish much earlier at one site in East Lothian than they do ten miles further along the coast. There must be a lot of factors involved.
I certainly notice that on average the first brood of the Speckled Wood is considerably smaller than the third (I am assuming we have three broods here). I put this down to higher mortality of caterpillars getting through the longer winter period. As you suggest, I suspect that they mostly overwinter as caterpillars here, rather then chrysalises.

[peterc]

Thanks, Nick. All very interesting.

Having thought about this subject for a while I am more inclined to think that the choice of hostplant (at least down in the south) plays a role in how the larvae will develop over the winter.

Does anyone here have any experience of rearing Speckled Wood larvae in the autumn/winter? As well as other factors like hostplant quality/quantity, predation and parasitism, ambient temperature may be an issue. The height above ground at which larvae spend most of their time feeding at this time of year may be important since ambient temperature varies at different heights and times of day. The hostplants on which female Speckled Woods select for egg-laying may vary in height (e.g. Common Couch up to 1m and Cocks-foot up to 50cm) but do the larvae feed at the same height on the these plants, e.g. very close to the ground? All conjecture of course but if anyone has any thoughts on this it will be good to know.

ATB

Peter