What’s that on an oak leaf? Animal? Plant? Fungus? Nah, gall insect – Galls wasps, Cynipidae

This week we delve into the Bug of the Week mailbag to help solve a mystery of “fuzzy” somethings, at the base of white oak leaves found on a tree near the gentle Choptank River on Maryland’s eastern shore. These curious fuzzy somethings are the handiwork of gall wasps, one of the most diverse groups of gall-formers found on plants. Galls are abnormal growths on plants created by several species of insects, mites, and some microbes, that secrete potent chemicals into the plant’s undifferentiated tissues. These chemicals derail the normal developmental processes of the plant and create food and refuge for the insect or mite within the gall at the expense of the unwitting plant host. We met other gall-making insects including those distorting leaves of black gum, elms and hickory in previous episodes.

The gall wasp family, Cynipidae, is more than 1,300 species strong with most generating unusual growths on woody plants, but some actually parasitizing other species of wasps. Dozens of species of wasps in this family have evolved intimate relationships with different species of oak. The diversity of galls on the leaves, branches, and acorns of oaks is awesome. Each species of gall wasp creates its own distinct and unique gall. Some look like bullets, others appear to be clusters of wool, some look like apples, and still others are the visage of grotesque horned creatures attached to a branch. The video accompanying this episode provides a smidgeon of this diversity of galls discovered on oaks in Maryland.

Let’s visit some gall wasps on oak. Strange horns decorate the outside of the horned oak gall wasp while the pupa develops within. These small round leaf galls are home to developing wasp larvae. When we open these green round galls, we can see the larval cell with an exit hole used by the wasp to escape the gall. Rough bullet galls wear exit holes outside and inside you see where the wasp chewed through the larval chamber and gall to get out. Pretty midrib galls and a couple spangle galls decorate this oak leaf. Potato-like galls and pouch galls adorn small twigs. Green spongy oak galls in summer turn to brown papery balls in autumn beneath oak trees.

Let’s walk through the life cycle of one cool but dastardly trouble maker, the horned oak gall wasp, Callirhytis quercuscornigera, to learn a bit more about these fascinating creatures. The saga begins in spring shortly after bud-burst of oaks when female wasps escape from their nursery inside the gall through one of the strange looking horns. These wasps are all females and are part of generation produced asexually through a remarkable process called pathogenesis. Many species of insects reproduce parthenogenically, such as aphids and scale insects we met in previous episodes. You go girls!  Wasps emerging from leaves are poor flyers and move just a short distance to developing leaf-buds, where they lay eggs. Single eggs hatch and induce the formation of an inconspicuous leaf gall. Later that summer both male and female wasps emerge from the leaf gall, mate, and females deposit girls-only eggs in tender green twigs. These eggs hatch and induce the formation of a small woody gall that enlarges over the course of the summer. Some 30 months later, fully developed female wasps emerge from large galls to complete the life cycle.   

Although details of gall inducement are not fully known, the act of egg-laying and the growth-altering chemicals subsequently released by the larva of the wasp cause the multiplication of nutritive plant cells inside the gall and abnormal development of the infested plant tissue. Wasp larvae consume these cells while non-nutritive cells proliferate to form the bizarre and characteristic gall. Inside the relative safety of the gall the larva grows as the gall enlarges. As development nears completion, the plant forms a tissue layer which can be a relatively tough, seed-like cell around the larva. Within this small chamber the larva transforms into a pupa from which the adult wasp emerges. Using powerful jaws, the wasp cuts it way out of the chamber and the surrounding gall and flies off to find food and a mate. Development from egg to adult often takes place in the gall while it is attached to the plant. However, in some species like the jumping oak gall, Neuroterus saltatorius, the gall breaks from the plant and falls to the ground with the larva inside. While completing development in the gall on the ground, the movement of the larva within can make the gall jump in the air. That’s right Little Orphan Annie, not leapin’ lizards, but leaping galls instead!

Acknowledgements

The fascinating article “Biology of Callirhytis cornigera (Hymenoptera: Cynipidae) and the Arthropod Community Inhabiting Its Galls” by Eileen A. Eliason and Daniel A. Potter, and the Maryland Biodiversity Project were used to prepare this article. Special thanks to Sue Hauser for providing the image of fuzzy gall wasps on oak leaves and inspiration for this episode.

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