Massed Apiomorpha munita tereticornuta galls on Eucalyptus ovata
Galls - Intriguing Plant GrowthsPlant galls are lumps, bumps or malformations that form on leaves, stems or flowerbuds of plants. Plants form galls in response to attack by many different kinds of organisms (e.g. fungi, nematode worms, bacteria, viruses) or to physical damage - but the majority of plant galls are induced by insects. In Australia tiny wasps, midges, psyllids, thrips and scale insects are the main insect groups that induce galls but a few beetles, moths and whiteflies are also gall-inducers. Eucalypts and acacias are the most gall-prone Australian plant groups.
No-one knows how insects make plants produce galls. In many cases the stimulus appears to come from the saliva of the larva - while in others it seems to be chemicals injected when the adult insect lays its eggs.
Like effective parasites, gall insects do not usually kill their host plants but they do draw off resources that the plants would otherwise use for themselves. When present in high abundance galls can be considered unsightly on ornamental plants. If the galls are on flowerbuds the plants may not be able to set seed. This last fact, combined with the high specificity of each gall insect species for only one or a few closely related host species, has been used to advantage in the biological control of Australian native plants that have become invasive weeds (e.g. the paperbark tree (Melaleuca quinquenervia), coastal tea tree (Leptospermum laevigatum), long leaved wattle (Acacia longifolia) and golden wattle (Acacia pycnantha)
Recently members of the Australian Plant Society Tasmania have been fascinated by an ‘outbreak’ of spectacular eucalypt galls induced by the activity of a scale insect, Apiomorpha munita tereticornuta. Scale insects induce galls on a variety of plants but all the species in the genus Apiomorpha induce galls only on eucalypts.
Apiomorpha galls caused by male insects look like small, woody, open-ended tubes and are no more than one cm long and three mm wide. Those of females are usually relatively large (one to several cm long) and have shapes that are unique to the particular inducing species.
In the case of A. m. tereticornuta the base of the female gall (the part that contains the female and offspring) is about one and a half cm long and one cm wide. Cylindrical or narrow blade-like appendages, several cm long, spread out from around a hole at the top of the gall. Female galls form on eucalypt stems. The small reddish male galls form on the greenish outside surfaces of the female galls.
Apiomorpha munita tereticornuta female galls
Male and female Apiomorpha insects are also different from each other in other ways. (sexual dimorphism). Adult males are fragile, tiny insects that have antennae, legs, simple eyes and wings. Adult females appear undeveloped even though they are sexually mature (termed neotony). They are ‘grub-like’ and larger and more robust than males. Their antennae and legs are reduced and they have no eyes or wings. They remain in the gall they have induced for their entire lives. Baby scale insects, called nymphs, feed within the female gall until mature enough to leave and induce their own galls.
Female Apiomorpha insects excrete honeydew through the hole at the top of the gall. They also secrete a white waxy substance that helps protect their bodies from contamination with honeydew. This white secretion can often be seen around around the hole at the top of the gall. In addition, ants regularly remove honeydew that is excreted by the gall insect. This is an example of a mutualistic relationship – the ants obtain food and the scale insect has its waste matter taken away