confused Brimstone?

[petesmith]

Hi Folks, Just wanted to pick some brains regarding Brimstone butterfly foodplants. As far as I am aware, the only two known foodplants for our Brimstone in the UK are Alder Buckthorn and Purging Buckthorn. Other species of Rhamnus are apparently used elsewhere in Europe. In my garden in central Lincolnshire I have two Alder Buckthorns which are used to varying degrees each year by egg-laying female Brimstones. I also have a large Russian Vine, Fallopia baldschuanica, which despite my best efforts at eradication continues to be a permanent presence!
A few years ago I noticed female Brimstones taking quite an interest in the vine, and occasionally alighting and curling their abdomens around as if about to lay eggs on this plant, before realising the error of their ways and eventually moving on.
This year, for the first time, I observed a female Brimstone actually depositing 2 eggs on the Russian Vine on 25th May. Two days later I found 4 more eggs on the vine.

This was quite a revelation to me, and I felt the need to investigate this further.
To this end, I tagged the eggs so that I could monitor their progress. At the same time, I removed two young Brimstone larvae from one of the Alder Buckthorns in my garden and boxed one on Russian Vine and one on Buckthorn to observe their behaviour.
On 28th May, 4 of the 6 eggs laid on the Vine had disappeared completely. We had experienced some heavy winds, and the vine had been thrashing about wildly for at least 24 hours - I wonder if the eggs had actually become detached...
The two remaining eggs were sleeved in situ.
By 30th May, the larva that was boxed on Russian Vine had perished, having completely refused to eat this plant. By comparison, the boxed "control" larva on Buckthorn was healthy, growing and feeding well.
Examination of the Russian Vine sleeve in mid-June showed no sign of feeding damage, and no sign of young larvae.

It seems that Russian Vine is not suitable as a foodplant for our Brimstone, so why did the female choose to lay precious eggs on this plant?
I can only guess that she had been confused by some similar "chemical fingerprint" common to both Russian Vine and Buckthorn. I am thinking that there are two possible outlooks from this -firstly, Russian Vine may act as a sink-hole, and if other Brimstone females get similarly confused, they may potentially deposit many of their valuable progeny onto an inhospitable plant. In areas where Russian Vine is common this may adversely affect population size - secondly, perhaps this is the first phase of adaptation to a new potential foodplant, and this could have a potentially positive benefit to the species.

Ideally I would like to look at the chemical composition of Alder Buckthorn, Purging Buckthorn and Russian Vine, perhaps via solvent extraction and mass spectrometry, to see if there is/are common chemical constituent(s), to further investigate this phenomenom.

I would be very interested to hear of anyone else who has experience of this, or indeed to hear of anyone's comments on this.

[bugboy]

Interesting.

A couple of years ago I watched a female Brimstone on Denbies Hillside become fixated on a sprig of wild Privet. She never laid an egg as far as I could tell but continuously returned to the same sprig and tested for egg laying potential. Given that, as I understand it, female butterflies are looking for certain chemical markers to make sure they have the right plant, I made an assumption that Privet may share one (or have one with a similar chemical makeup) with buckthorn that was enough to confuse this female.

Another theory that crossed my mind was that since there was a buckthorn very close by and perhaps the wind was blowing in the right direction to waft some Buckthorn scent over, enough to confuse my Brimstone?

Is your Russian Vine close enough to your Buckthorns to perhaps be blown against it and maybe transferring some of these chemical markers over?

[petesmith]

The Russian Vine is about 8 metres away from the Buckthorn, so not close enough for physical contact. Airborne transfer of chemical markers would theoretically be possible but there are other closer shrubs to the buckthorn such as dogwood, and I never see Brimstones showing an interest in that.
Bugboy - your observations on wild Privet are interesting...

Any analytical chemists out there with access to a mass spec??

[KeithS]

I wonder if it airborne pollen from nearby flowering buckthorn tainting leaves of other nearby plants and confusing the brimstones? I have found one or two Brimstone eggs on both honeysuckle and privet in my garden, quite near a flowering purging buckthorn. The eggs hatch but the larvae do not seem to survive as I never see any feeding damage.