GardenLine | Common Saskatchewan Pests | Plant Galls

A gall is an overgrowth or swelling in plant tissues usually resulting from insects or diseases. Generally speaking, insect galls are unsightly but do not kill plants. However, galls caused by disease organisms such as bacterial or fungi may present a more serious problem, requiring intervention on the part of the plant owner.

Galls may be found on any part of a plant, including buds, stems, flowers, leaves, petioles, and roots. However, usually a specific disease or insect confines itself to one part of a specific plant. In other words, the poplar bud gall mite only causes galls on poplar trees which form only at buds, never on roots or leaves and not on other trees. These galls will usually have a shape and form specific to that mite.

Most galls form when the invading organism causes hormonal changes in the plant which stimulate rapid abnormal plant growth. These abnormal growths divert food away from normal tissues, weakening the plant. Moreover, the fast rate of growth may also crush normal tissues, cutting off sap flow or otherwise interfering with plant function.

Perhaps the most commonly encountered galls are those caused by Eriophyid mites. These microscopic mites cause flower galls on ash and poplar trees, leaf galls on elm, maple, linden, basswood, birch, poplar and cherries. Each species of tree has it is own particular species of mite, with the trees and mites having evolved together. The mites are highly specialized and usually not reason for major concern. The galls can be very unsightly, but typically do not do any serious damage. Eriophyid mite galls on flowers and buds can remain on a tree for several years before falling off. Mite galls that form on leaves may discolour or distort foliage, and even though the leaves drop in the fall, the mite infestation may return.

Description

Eriophyid mites are very small, and are only visible by means of a microscope. The mites are worm like, and have only two pairs of legs. Males and females appear identical. They are poor crawlers and the primary method of population spread is by wind.

It is much easier to identify a gall mite infestation by the symptoms of distorted growth than it is to attempt to identify the actual mite. The galls that form on flower buds of ash and poplar are woody and gnarled in appearance. The galls on leaves are green, yellow, red or purple in colour. Leaf galls appear as bladder, blister or spindle-like fleshy projections. The galls are found mainly on the topside of leaves. There is a yellowing directly underneath the galls with a pore in the centre of the discolouration. Although a large number of these very small galls may be present, leaf function is often only slightly reduced.

Life Cycle

Eriophyid mites overwinter as specialized winter hardy females which crawl into bark crevices or buds (depending on the mite species). Females emerge in the spring and travel to the flowers or leaves. The males deposit a packet of sperm on the leaves. The females find the packets and fertilize themselves. They then lay eggs which hatch and go through two larval stages. Adults live for approximately one month, with several generations occurring each summer. When the leaves start to harden, late in the year, the specialized winter hardy females are produced. These females fertilize themselves and move to a place on the bark which offers winter shelter.

Damage

Mites produce galls by injecting or altering the normal growth hormones of the plant, by their feeding. These chemicals result in abnormal growths and swellings. Typically the galls are most plentiful early in the year on new growth and foliage, and primarily found in close proximity to the trunk. The bud galls will stop further twig and leaf development.

Control

Since mite galls cause no serious damage, no treatment is necessary except for aesthetic reasons. Pruning out infested twigs and destroying infested twigs in the late spring is a good means of control as it removes the adults. If the infestation is severe, a dormant oil mixture or a miticide such as Kelthane, or Cygon may be sprayed early in the spring.

Several hybrid poplar appear to be immune to flower gall mite. Selection of these hybrids should be considered in order to avoid gall mite infestations. `Northwest' poplar is particularly susceptible to gall mite.

© 1995 Celine Burns

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Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.