Plant Galls: Where Do They Come From?

Creosote gall.

This morning while hiking I came across a few odd masses of leaves growing on Creosote Bush.  These masses of leaves are technically galls caused by the Creosote Gall Midge.  This insect lays an egg or eggs along with fungal spores in part of a Creosote Bush.  There are 15 species of the Creosote Gall Midge, each specializes in laying their eggs in a specific location on the bush.  Some specialize in laying the egg and forming galls in the leaves, others on stems, others buds or flowers and so on.  Each species also does this only at specific times of the year.  When the fungus begins to grow it forms a gall on the Creosote.  Later, the midge eggs hatch and the larva begin eating the fungus inside of the gall.  Once mature, the larva eat their way out of the gall and morph into adult insects.  Once adults, the whole process is of laying eggs with fungal spores in the Creosote begins again. 

Galls are not unique to Creosote.  In-fact they are quite common, especially among oak species.  Gall wasps actually specialize in forming galls, most often on oak leaves.  Gall formation almost always is part of the reproductive life cycle of these wasps.  There are well over 1000 species of Gall Wasps, each individual species specializing in forming galls on one specific plant species.  These galls are often quite unusual looking, sometimes looking even Christmas ornament like.  Below are a few pictures of galls.