Gall-inducing insects

Have you ever noticed strange lumps and bumps on the leaves and stems of native plants? Galls are especially common on gum trees and wattles, and they are abnormal plant growths that form in response to invasion of plant tissue by a variety of organisms. Galls can be caused by certain species of wasps, flies, beetles, psyllids, coccids, thrips, moths and aphids, as well as by nematodes, mites, bacteria or fungi. This post concentrates on some common gall-inducing insects found in Australia.

Galls form as a reaction to insects feeding on, or their eggs being laid in, plant tissue. When a female gall-inducing insect deposits her egg in the leaf or stem of a host plant, a biochemical reaction between the egg and the host may produce the gall, inside of which a hatching larva will feed. Or the larva may modify the plant’s normal response to injury with salivary secretions, which stimulates the plant to grow the gall (rather than scar tissue), resulting in food and shelter for itself. Gall-inducing insects (adults and juveniles) are usually very small, rarely more than a few millimeters in length, but the galls they induce are much more obvious.

citrus gall waspGall wasps are found throughout the world and the most well known gall wasp in Australia is the citrus gall wasp (Bruchophagus fellis) which forms grotesque stem galls on all varieties of citrus (above). Citrus gall wasp is actually an Australian native insect whose native host is the Australian finger lime (Citrus australasica). It has grown to love our cultivated lemons though! Galls formed by other species of wasps are found on eucalypts, acacias, banksias and some cut flower natives like Geraldton wax and Thryptomene.

Wattle apple gallGalls on wattles are often called “apple” galls (above) and are induced by wasps of the genus Trichilogaster. Depending on the species of wasp and the size of the gall, hundreds of larvae may feed inside a gall. Some species of wasps form galls on leaves which are characteristically spherical and are bisected by the leaf margin, and often cause leaves to twist. Galls formed by other insects are located on only one side of a leaf.

gall midgeFlies which form galls are usually midges in the family Cecidomyiidae. The larvae of some gall midges cause a thickening and shortening of leaflets, or pimple-like blisters (as above) on leaves (especially on eucalypts). Eucalyptus leaf galls usually contain a single orange or red larva which distinguishes it from most other larvae found in galls which are typically cream or white in colour.

apiomorpha gallCoccids are related to scale insects and cause hard woody galls, some of which have distinctive shapes. Galls caused by male coccids are usually a different size and shape to those caused by female coccids. The gall-former shown (above) is a species of Apiomorpha in the family Eriococcidae. Female Apiomorpha induce the large stem galls while  males induce the small leaf galls.

psyllid gallPsyllids in the family Triozidae cause galls on native trees. Schedotrioza spp. form distinctive spherical “apple” galls on eucalypt leaves (above). The galls of the notorious lilly pilly psyllid (Trioza eugeniae) are quite different (below). The nymphs of this pest form pit galls on flush growth usually on the underside of a leaf with a corresponding lump on the upper surface. Unlike most other gall insects the nymphs are not fully enclosed by the gall. The nymphs are the small flattened yellowish creatures in the image below.

lilly pilly psyllidSome varieties of lilly pilly like Syzygium luehmannii and Acmena smithii are less susceptible to attack than others. But lilly pilly shrubs of susceptible species (such as Syzygium panicultaum and Waterhousea floribunda) can be devastated by psyllid attack. Most other gall-inducing insects are not so serious though, an infestation won’t kill a tree or shrub, but can be unsightly and may affect plant vigour.

Identification of gall-forming insects is not easy. You could keep a gall in a jar to see what emerges, but that may not help because what emerges might be a parasite of the insect which caused the gall, or a hyperparasite (parasite of a parasite). Sometimes an egg may be laid into a gall already containing the larva of a different species, the new egg hatches and the emerging larva will feed in the gall alongside the original larva. When they both hatch out as adults you won’t know which insect caused the gall. Insects which adopt this ‘cuckoo’ style of living are known as inquilines. Insects never cease to amaze me!

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