Holly Blue larvae & Black Garden Ants

Holly Blue larvae and Black Garden Ants

by Douglas Hammersley

Fig. 1

Black garden ant stimulating Newcomer's gland

In 1990 I planted two small holly bushes in my garden with a view to attracting Holly Blue butterflies (Celestrina argiolus). One bush, Ilex aquifolium "Golden King", is a female holly (despite its name) and the other is Ilex aquifolium "Silver Queen", a male plant. Both bushes are about one metre tall and they have proved to be excellent for observing and photographing the development of Holly Blue larvae and their association with black garden ants, Lasius niger. I soon came to realise that, in the male plant, the larvae feed on the young leaves but in the case of the female plant, the emerged larva feeds on the immature berries by burrowing its head into the berry and eating its contents from the inside. When the larvae are very small they appear to be disregarded by the patrolling ants but as they develop, the larvae are regularly attended by ants. The ants merely attend a larva and make no attempt to diven it or influence its progress when it moves around its host plant. Usually, one or two ants act as outriders and one or two others ride on the larva, often showing particular interest in the caterpillar's seventh segment where Newcomer's gland is situated. This gland is seen as a small, transverse invagination which can be stimulated to secrete a kind of "honey-dew" on which they feed. Ants are seen to use their antennae to drum around the gland and cause it to open and secrete (see Fig. I).

Site of right tubercle -- Head of larva biting holly leaf

"Honey-dew" may also be exuded from secretary pores scattered about the surface of the larval abdomen, because sometimes one can observe individual ants showing interest in other parts of the larval surface besides Newcomer's gland on the seventh segment. Two dorso-lateral organs, or tubercles, on each side of the eighth segment of the larva also have some special significance for ants. On one occasion I observed these two tubercles being suddenly evened as little green pillars, causing immediate excitement on the part of two ants, although the tubercles were retracted seconds later. I have no idea what this meant. The most exciting observation was made on 17th June 1993, when I had a clear illustration of the way in which ants protect threatened Holly Blue larvae. A Holly Blue caterpillar was on "walkabout" looking for a new source of immature holly berries on whicli to feed. As it ascended the stem of the plant escorted by three ants, a spider made a sudden attack. In a split second it was thwarted by the rapid action of one of the escorting ants whicli caused the spider suddenly to stop in its tracks. It appeared to be momentarily paralysed for about one second, before scuttling off to hide among the leaves. It was almost as if the spider had been subjected to an electric shock. l can only assume that it had been sprayed with formic acid ejected by a nearby defending ant, or been bitten by the ant. The drawings in Fig.2 (A and B) were made from photographs taken at the time.

These observations merely confirm what has already been reported in the literature, but it is pleasant to recall that they were made whilst sunning oneself on the back lawn!