Over-wintering Brimstone Butterflies

[PhilBJohnson]

After finding a hibernating position in late summer or early autumn, it was thought (by me) that native ivy was given a preference by this species, to hibernate in.
It was thought that 'evergreen' foliage was given preference by Brimstones, and if so, how might they determine a leaf they are hibernating under, isn't just going to fall off in the fall (autumn), if the leaves have not started to turn (change colour)?
13C micro-climate
It was thought (by me), that female Brimstones were more often seen later in the year than male Brimstones. Is emergence from hibernation of the female in the Spring, determined by a slightly higher temperature, if they are not disturbed?

As thought by me, will the male Brimstones seek out the females and mate with them while the females are not so 'flighty'?

[David M]

Not sure if females emerge later than males, Phil. I think it's just a case of them being seen more infrequently. At least 80% of all spring Brimstones I see are males, but given that they are constantly flying around looking for females that makes them more conspicuous.

Whenever I've seen females, they have tended to fly much smaller distances and I rarely see them high up in the air like the males.

[Essex Bertie]

I read somewhere that they have a preference for overwintering in conifers but I have never seen activity around these trees. I have seen them go to roost in bramble thickets and their camouflage certainly imitates the underside of a bramble leaf. But this doesn't seem like the best overwintering option and perhaps they tuck themselves away so well within dense evergreen foliage that their camouflage is irrelevant in the dormant state.

[PhilBJohnson]

Thank you, David.
My Alder Buckthorn isn't in leaf yet, so i guess a female Brimstone lays its eggs from approx. mid April onwards.
Specifically, for an individual butterfly, camouflage might be irrelevant in a given situation, but with evolution in mind, evergreen foliage in a native broadleaf English woodland, does not leave many plant species to choose from, for a Brimstone to hibernate in or under.

I observed a Red Admiral repeatedly roosting under Ash leaves where the tree branch intersected a Cypress but my preference would have been to cut the Cypress down as that hybrid supports very little native wildlife and leaves (or leafs ), little growing beneath it.
Screens and Shelter can be provided in other ways that don't grow fast for approximately 300 years.

[PhilBJohnson]

How does A female Brimstone recognise a Buckthorn to lay it's eggs on?
Is it the smell of chemicals released by young leaves sprouting?
Does it look at the spacing between leaf buds?
Does it remember the latin name?
Or something else

[David M]

I would venture to suggest that olfactory reasons are the main ones. Female butterflies have a particularly heightened sense of smell. It can scarcely be visual given that the shrubs/trees are not even in bud during March when Brimstones first emerge.

One only has to watch female Brown Hairstreaks to understand that butterflies are instinctively drawn to their foodplant. These insects will seldom settle on any other plant and when they DO end up on blackthorn, they inspect it and survey it in a way quite unlike their behaviour on any other plant they might visit.

[PhilBJohnson]

It certainly makes sense to me, for the male Brimstones to be flighty first in March, to find a female, and for the females to be flighty later in April, looking for Buckthorn which is in the stage of starting to leaf, to lay eggs on.