KRM Branch,18 September 2023

The speaker at the September meeting was Daniel Heald, who delivered a presentation entitled ‘A Madness Most Discrete – Gall Biodiversity’.

Daniel explained that galls are the lumps found on branches or leaves of trees or shrubs and that there is an enormous variety. He then showed some slides that gave examples of galls found around Perth. However, not all abnormal growths are galls. For example, Witches’ Brooms are an abnormal proliferation of twigs and branches, which form in the crown of trees, shrubs or herbaceous plants, generally growing from a central source and resembling a broom; lignotubers are not galls, they are woody swelling of the root crown; and Fasciations are not galls but are contorted growing tissue that may be caused by hormonal, genetic, bacterial fungal, viral and environmental causes (Daniel said that he has seen this in Dandelions at Munster).

A gall is an abnormal swelling on the outside of plants, fungi, and animals. Plant galls are caused by viruses, bacteria, other plants, nematodes, mites, and insects (examples were shown). The gall hijacks the growth mechanism of the host plant to form galls that are nothing like the normal growth of the host. There is a long list of gall-forming insects aiming to create a safe place for the insect larvae to grow safely, e.g., the gall-forming Jewel Beetle. The life cycle of the gall-forming insect is often complicated, especially in Gall Wasps. Daniel discussed an overseas example of Oak Apples, where mated female wasps lay all-female eggs on the root of the host oak. The offspring then winter underground, mostly dormant until the following summer when they ascend the tree to lay male and female eggs. Many of the gall-forming insects are garden pests, such as the Pittosporum Leaf Miner (Phyobia pittosporphylli), Trioza eugenia that attack Lilly Pillies and the Citrus Gall Wasp (Bruchophagus fellis).

Daniel then explained that there are over 13,000 known gall-forming insect species, with 500 on Oak trees alone. Details were provided for some of these, including the Honeydew Gall (Disholcaspis eldoralensis) that attracts ants that then provide protection for the gall. Examples of many local gall-forming species were then also presented. The audience thanked Daniel for a most informative presentation.

Colin Prickett