Single brooded Holly blue populations?

[ChrisStamp]

I've read in some places that Holly blue is single-brooded in the north. However here in Scotland on the very northern edge of the range it has been clearly double brooded (even a very small number of third brood individuals). I initiated a 2023 postcard and Facebook-based survey for the residents of Wormit & Newport on the south bank of the Tay (where the Holly Blue arrived in 2021/2022) with the following results:

(The abnormal peak at the start was probably just a peak of interest at the launch of the survey)

So I'm curious about the 'single brood' theory - does anybody have direct experience of this or know of any reference info? What would be the timing and foodplant be for a single brood colony, given that they normally prefer holly in spring, and ivy in late summer?

[bugboy]

I am aware of a population in Essex where I believe the first brood is very late due to the only available spring food source seeming to be bramble. There's no shortage of ivy for the second brood, but giving the lateness of the first brood there's likely no time to fit in a third like other sites down here, so I can see how a single brooded population could exist based on available foodplant choice. Having said that, the only requirement for a plant to be considered as a foodplant for a female Holly Blue seems to be the presence of a flower bud!

[ChrisStamp]

Interesting, thanks bugboy. Because they overwinter as pupae,a first brood being late must mean they have locally adapted to emerge from pupae later? Or maybe more likely they just disperse on normal emergence, and only re-colonise the site when the foodplant is suitable? So the larvae there end up behind other sites because it only receives the latter end of the first brood egg laying activities?

At the moment I am a sceptic about the existence of single brooded populations, but interested to learn of any evidence.

[bugboy]

I would be tempted to go with the first, that emergence is delayed but I will also admit to having no evidence of this. However I will say, based on personal obs, that the population on my local patch, Walthamstow Marshes, does seem to have altered its phenology to adapt to a new spring foodplant. When I first moved here around 15 years ago I'd see lots active in early spring and using Dogwood as a foodplant. But in recent years there seems to have been a shift to using Goat's-rue, a non native legume, in the spring. This flowers later and I now see very few early in the year but a big glut of activity later when it's flowering in May and June. I have reared them on this and when collecting fresh food I would invariably end up collecting more eggs and larva, they seem to obsessively favour this over native plants.

[bugboy]

I would be tempted to go with the first, that emergence is delayed but I will also admit to having no evidence of this. However I will say, based on personal obs, that the population on my local patch, Walthamstow Marshes, does seem to have altered its phenology to adapt to a new spring foodplant. When I first moved here around 15 years ago I'd see lots active in early spring and using Dogwood as a foodplant. But in recent years there seems to have been a shift to using Goat's-rue, a non native legume, in the spring. This flowers later and I now see very few early in the year but a big glut of activity later when it's flowering in May and June. I have reared them on this and when collecting fresh food I would invariably end up collecting more eggs and larva, they seem to obsessively favour this over native plants.

[Padfield]

If emergence is delayed, it should nevertheless be a full emergence at the later time. If early emergers disperse, the later bramble-feeders should represent only a fraction of the population. This could be a way of telling which of the two things is happening. Is the apparently late first brood particularly small?

Guy

[bugboy]

If emergence is delayed, it should nevertheless be a full emergence at the later time. If early emergers disperse, the later bramble-feeders should represent only a fraction of the population. This could be a way of telling which of the two things is happening. Is the apparently late first brood particularly small?

Guy

Unfortunately I don't visit the site regularly enough to get a good handle on the true emergence pattern, it's my WLH site and I regularly see worn individuals flying with them, and have also witnessed ovipositing on Bramble. I've never found any Holly, dogwood or any other commonly used spring foodplant in the area. All circumstantial I'll admit but...

[millerd]

Some more observational stuff:

On my local site, first brood butterflies peak in mid-May, and second brood between mid-July and mid-August. There is a regular small third brood peaking in mid-October. The first brood females lay (as Paul mentioned earlier) on almost anything with a flower bud, including holly, dogwood, vetches, trefoils and goats' rue. The latter plant is certainly favoured and the butterflies thrive on it judging by the second brood numbers seen in the areas where it grows.

Second (and third) brood females here lay almost exclusively on ivy, though I did see one laying on the second flush of goats' rue in August this year.

I have visited the same WLH site in Essex that Paul refers to, during the last week of June or first week of July when at home Holly Blues are very much between broods (there can be days when I see none at all). However, at the Essex site (both at the WLH "hotspot" and more widely in that area), they can be numerous and still quite fresh; the females are invariably laying on brambles. I have also observed a female laying on trefoil in one of the nearby Heath Fritillary woods at the same time of year.

I'm guessing that local populations evolve to adapt to local conditions and locally available foodplants. Holly Blues appear to be a successful species, with numbers and range increasing in recent years. It therefore seems likely that they are capable of adapting very quickly to changes and to new areas, and their flexibility in choice of foodplant in the first brood assists this in no small way.

Cheers,

Dave

[ChrisStamp]

Very interesting Dave. Do you see Holly Blue as a species that forms discrete, isolated colonies and could therefore form very local adaptations? I generally had an idea that they wander quite freely and sometimes even travel large distances (eg they turned up in Aberdeen this year, 50 miles north of any other record), in which case colonies would be unlikely to be isolated enough to develop adaptations over time?

My suspicion is (but could be completely wrong) that the butterflies seen at a particular site might be ones within a wider population that are occupying the site at at a particular time rather than a locally adapted colony - ie butterflies will be present when conditions are good and will be elsewhere when they aren't. This could give an impression that they are emerging at different times, but actually they are just occupying the site at different times?

I could be convinced otherwise though - just idle speculation really.

[Jack Harrison]

How long will they take to reach up here near the Moray Firth?

An earlier ambition (1001 thing to do before you die) was to see Comma up here. Fulfilled a few years ago.
As I'll be 85 in a month's time, I cannot reasonably hold out much hope of seeing Holly Blues here. But you never know!

Jack

[ChrisStamp]

Jack, if they have another year in 2024 like they did in 2023, you might not have to wait long. They appear to have jumped from Fife to Aberdeen in 2023, although of course there might have been previous undetected movement. There’s even a possibility that they arrived in Aberdeen from across the sea. Who knows if there are other populations in the north awaiting discovery - definitely worth checking Holly in the spring and any ivy-covered churchyards or old estates in late summer.