Galls are distorted swellings of plant parts caused by the feeding or infection of various types of organisms. The many causes of galls include insects and mites (discussed below), root knot nematodes and certain other nematodes, plant-parasitic mistletoes, plant pathogens such as those that cause crown gall, olive knot and oleander gall, and western gall rust, and the various causes of witches' brooms.
Identification
The appearance of galls is as varied as the types of organisms that cause the galls. Gall-causing insects and mites include
- adelgids
- aphids such as ash leafcurl aphid, manzanita leafgall aphid, woolly apple aphid, and certain other woolly aphids
- ceanothus stem gall moth
- eriophyid (family Eriophyidae) mites including blister, bud, erineum, gall, and rust mites such as fuchsia gall mite, grape erineum mite, and live oak erineum mite
- ficus leaf-rolling psyllid
- gall midges including baccharis gall fly, boxwood leafminer, and honeylocust pod gall midge
- gall wasps including ficus gall wasp, oak gall wasps such as twohorned oak gall wasp, and willow gall sawflies
- grape phylloxera
- poplar and willow borer, a snout beetle or weevil
- thrips such as Cuban laurel thrips and weeping fig thrips
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 parasitoids (parasites) of the gall maker. Galling eriophyid mites are microscopic, pale colored, and carrot-shaped or wormlike with two pairs of legs coming out of their widest (head) end.
Life cycle
Certain infectious organisms (e.g., pathogenic microbes) and secretions of arthropods (mostly insects and mites) induce the growth of abnormal plant tissue. With arthropods the immatures (larvae or nymphs) commonly feed inside the galled growth sucking and feeding on phloem sap (e.g., galling aphids and psyllids) or chewing and feeding on the abnormal plant tissue (beetle, fly, and moth larvae). Arthropods such as gall wasps commonly have only one or two generations per year while others such as gall aphids, midges, and mites have at least several generations per year.
Damage
Galls are distorted, swollen plant growths in branches, flowers, leaves, trunks, twigs, or roots. Some galls are colorful swellings while other galls are the typical color(s) of plant growth. Galls can be interesting curiosities or annoying causes of reduced aesthetic quality of plants. Some types of galls cause dieback of leaves, shoot terminals, or other plant parts, but galls caused by insects and mites usually do not threaten plant health or survival.
Solutions
Most gall-making insects and mites are not known to seriously 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 inside plant tissue and before the adults begin to emerge.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
