Gall makers—Various organisms
The many causes of galls include nematodes, parasitic mistletoe plants, pathogens e.g., crown gall bacteria, and certain plant-feeding insects and mites.
Gall-causing invertebrates include ceanothus stem gall moth, ficus gall wasp, gall midges e.g., honeylocust pod gall midge, oak gall wasps e.g., twohorned oak gall wasp, willow gall sawflies, and certain species of adelgids, aphids, and gall mites, or eriophyids e.g., fuchsia gall mite.
Many galls caused by insects harbor a single, legless larva.
Other galls may harbor several larvae, some of which may be different species that are predators or parasites of the gall maker.
Secretions of invertebrates apparently induce the growth of abnormal plant tissue. The invertebrates feed inside this galled plant growth.
Galls are distorted, sometimes colorful swellings in branches, flowers, leaves, trunks, twigs, or roots.
Most gall-making insects and mites are not known to harm trees and shrubs. Prune and dispose of galls if they are annoying. This may provide control of some invertebrate species if pruning is done when the immatures are in plant tissue and before the adults begin to emerge.
Pink-spined turban galls (left)
Reddish oak cone galls (right)
Oak stem gall
Poplar gall aphid swellings in leaves