When the sky is blue, the air is chilly and autumn leaves are falling, I simply must get outdoors. Recently, on such a day Leonard and I went to Ash Creek (Lassen County CA). It was gorgeous, however, the wildflowers were not in blossom, the dragonflies were not flying, it was too dry for fungi and the birds disappeared that day. So I set off in search of plant galls – a topic that has piqued my interest of late.
A tepharitid fruit fly (Aciurina bigeloviae) induces a fascinating gall on rabbitbrush (Chrysithamnus nauseosus). In the spring the adult cotton gall fruit fly emerges, mates and deposits an egg in an auxillary rabbitbrush bud. Via mechanisms that are not yet completely understood, when the egg is ovipsited chemical stimulants are also injected into the bud, which cause abnormal growth and development of the plant bud. Usually found on areas of newer growth, the rabbitbrush cotton gall is a compact cluster of white hairs surrounding a monothalamous (single) chamber where the larva develops once the egg hatches. The larva overwinters in the gall. White, round and resembling a cotton ball attached to the stem, rabbitbrush cotton galls occur singly or as pairs. Old galls persist on the rabbitbrush for years after the adult has emerged through a hole. Rabbitbrush cotton galls are slow growing with fresh galls being found into the late fall.
Rabbitbrush grows in areas with cold temperatures and deep snow in the winter. A. bigeloviae may produce cryptoprotectants, chemicals that help maintain internal gall temperatures and protect the resident larvae from external freezing conditions.
With plant galls, the relationship between insect (or fungi or bacteria) and plant species is specific. What fascinating associations!