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I was watching some female Adonis Blues (and possibly late Chalkhill Blues) laying eggs on Therfield Heath yesterday.
As well as the odd egg on Horseshoe Vetch, I saw quite a few other eggs of exactly the same size and shape on Salad Burnet and Mouse-Ear Hawkweed...

Are either species of Blue known to use these plants as larval foodplants or do you think they are just laying their eggs randomly in the expectation that their larvae will find their way to the correct foodplant (which will be nearby) by the time they hatch.

Why would they do this - seemingly in preference to using their normal foodplant (I saw more eggs on the "wrong" foodplant than the "correct" one)?... Is it possible that the environment has been so dry that the butterflies are having trouble identifying the correct plant with the appropriate nitrogen content (by testing with their feet) and laying on the other plants which may "feel" a closer match to their usual egg-laying site?

Hi Trev,

Many years ago I used to spend a lot of time with Chalkhill Blues on the downs of the Isle of Wight. The females would frequently lay their eggs on dead bits of grass or general debris at ground level, always close to Horseshoe Vetch but often not actually on the host-plant itself. I believe Adonis females behave similarly.
Cheers

Pete

They certainly do lay on nearby vegetation, although I’m not sure if anyone has come to any conclusion of the benefit of doing this.

Cheers,

- Pete

My thinking, as far as Chalkhills is concerned at least, is that since they hibernate as eggs, it makes little difference where the eggs are laid so long as it's in a places where Horseshoe Vetch grows. There's no point trying to lay the egg on a leaf that by the time the caterpillar hatches will no longer be edible.

This off course doesn't explain species which hatch in the same season but since horseshoe Vetch grows widely on its favoured sites, then the resulting caterpillar doesn't have to look far for food. All the female needs to know is that she's in the close vicinity to the LHP. If the female has to lay as many eggs as possible, then this method is a perfectly viable shortcut to achieve this.

It's far from unique, it's well known the violet feeding Fritillaries don't necessarily lay on the foodplant, I've watched PBF lay on leaf litter numerous times and DGF on dead grass stems in meadows. Presumably the same shortcut works for these species too.