THE BRIMSTONE (Gonepteryx rhamni): OVA TO PUPA, SEARCHING, FINDING AND REARING.
The Brimstone is, for me, one of the most interesting of our native butterflies. Probably the longest living of the butterflies, with a life span (including hibernation) of up to ten months, it is quite unique in many ways. Of our native butterflies that hibernate as adults, it stands alone from the Nymphalids. The males are easily seen and the females are easily overlooked. Unlike the Peacocks and Tortoiseshells, which in the spring, usually look like they have been run through a mincer, post-hibernation Brimstones often look as fresh as the day they emerged.
The foodplants chosen by egg laying females in May, are the shrubs, purging buckthorn (Rhamnus cathartacus) and alder buckthorn (Frangula alnus). Purging buckthorn has a preference for chalky or lime soils and alder buckthorn likes heavier and wetter soils. For those who are not familiar with these shrubs, a good identification book should be carried in the field or much time will be wasted searching dogwood, spindle or privet.
The egg is very much like that of the Large white, skittle shaped, about 2.5mm high and a greenish creamy white colour. The eggs are laid usually on the underside of a fresh terminal leaf, although I have often found them on the edge or upperside of a leaf. Many books advise searching well up on the bush, but my own experience leads me to the medium and lower hanging leaves, often those hanging down into the grass. The sunny side of the bush is usually the best, but not always, and the smaller bushes often reveal the most eggs. Young, thin and scraggy buckthorn in a recently planted hedgerow, were found to have up to 30 eggs on each last year. Sadly, this was only discovered because the buckthorn were defoliated and by this time most of the larvae had starved to death, although the empty egg cases could still be seen, on odd remaining terminal leaves.
An easy way of collecting eggs is to grow a buckthorn in a bucket and place this, in May, in a hedgerow or woodland edge where the male Brimstones have been seen flying up and down. You may never have seen a female Brimstone, but she will be there and she will find your bush and cover it with eggs. Whether one Brimstone lays all the eggs or several, I am not sure, but I would suspect several, as a female confined on a netted bush is usually reluctant to lay more than a couple of eggs.
Once you are sure of the foodplant, no mistake can be made if an egg is found, as nothing else lays a similar egg, singly, on the buckthorn. For rearing, the shoot can be cut from the bush and placed in a small airtight container until the egg hatches, usually around ten days if the egg is freshly laid. New leaves should be prepared in clean containers every two days and the caterpillar should be left on its present leaf and this dropped onto the new ones. Up to 12 larvae can be reared together in a container the size of a 16 oz. margarine tub, although as they near pupation, it is better to thin them out to five or six per container. They can also be sleeved on a growing bush in the garden but I find that this results in more losses. Plastic containers should have at least the lid of transparent plastic to admit light, although direct sunlight should be avoided. Low light levels and a temperature of 65 to 750F seem acceptable. I have had no troubles by changing the diet from alder to purging buckthorn at various instars, although it is usually a good rule to stick to the same foodplant when rearing any species.
Caterpillars are as easy to find as eggs (once you have found the correct foodplant), and medium to low branches should be searched. The caterpillar will be found on its own, and has remarkable camouflage being the same green as the leaves. It will usually be found lying along the midrib of a leaf, on the upperside and a few leaves with chunks eaten out of them betrays its presence. If you find a bush of about a cubic yard in size, do not be surprised to find twenty or so larvae. Of larvae collected at various instars from the wild, only around 1% have been found to be parasitised and even these manage to pupate, the parasite emerging from the chrysalis, a few days after the pupation. The chrysalis is of the same form as the large white, with the tail attached by silk and a silken thread as a support girdle. These can be carefully cut with a scalpel and the chrysalis placed on corrugated cardboard in the emerging cage. The pupal stage lasts around 14 days and the sex of the adult can clearly be seen by the colour of the chrysalis prior to emerging. A friend of mine regularly releases 20 or 30 freshly emerged adults at his favourite woodland edge. A couple of dabs of various colours of nail varnish ruins the wings for a potential set specimen and show up as "one of his" when caught the following spring. [P.W.B.].
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