With spring busting out all over the place, rapidly dividing plant cells are forming flowers, leaves, and stems at an astonishing pace. Wouldn’t you know that in this time of luxuriant growth, there would be some diabolically clever insects that take advantage of these youthful plant tissues? Galls are abnormal growths on plants created by several species of insects and mites 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 at the expense of the unwitting plant host.
This week we visit hickory trees, pin oaks, and American elms that were victimized by these crafty gall makers. Hickories are delightful native trees, with beautiful habits and russet autumn colors, that produce bountiful nuts for native wildlife. On my way to work, I pass a mature mockernut hickory whose leaves are festooned with swollen green and reddish globes - the work of the hickory phylloxeran. The hickory phylloxeran is a small sucking insect, kin to an aphid, which survived the winter as an egg deposited on the bark of the tree or near an old gall from a previous year. About the time when leaf buds are breaking, these eggs hatch into tiny nymphs destined to become breeders called fundatrices. Each fundatrix hunkers down on the rapidly expanding leaf blade or its petiole and inserts its needle-like mouthparts into the leaf tissue. This feeding brings about remarkable transformations as the leaf develops. Chemicals secreted by the phylloxeran cause the hickory’s cells to differentiate and create a strange globular gall. Within the hollow gall, the fundatrix develops into a fully mature female that lays hundreds to more than a thousand eggs parthenogenetically, that is, without the assistance of a male.
After hatching, legions of tiny nymphs feed within the gall and eventually develop into winged forms. By late May, galls split open and the winged phylloxerans exit and move to the undersurface of leaves where they lay hundreds of eggs. These eggs hatch and produce nymphs destined to become males and females that will ultimately mate and lay eggs to endure the next winter. Talk about a complicated lifestyle, phylloxerans certainly have one.
Near the hickory tree grows a handsome young pin oak whose leaves now bear several small succulent galls about the size of a small marble. These galls are made by a member of the most diverse groups of gall-formers, tiny wasps in the family Cynipidae. 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 fruits of oak trees 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, and still others are the visage of grotesque horned creatures attached to a branch. In the case of the small spherical galls on the pin oaks, the gall was initiated earlier in the year when a female wasp “stung” the leaf with her egg-laying tube, called an ovipositor, and deposited an egg in the developing leaf tissue. Although the details are sketchy, the act of egg-laying and the chemicals subsequently released by the larva of the wasp cause the multiplication of plant cells that form the gall.
Inside the relative safety of the gall the larva grows and the gall enlarges. As development nears completion, the larva forms a small cell within the gall to serve as the pupal chamber. From the chamber and the gall itself, the beautiful adult wasp emerges to feed and repeat the process in another leaf or plant part. 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, 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. Leaping galls – how bizarre!
The last mischief-maker in this trilogy is an aphid that inhabits American elm where it produces a gall strangely reminiscent of a rooster’s comb, hence the name elm cockscombgall. Many aphids are produced in each gall during the spring. In summer, the gall splits open and the gall aphids eventually migrate to grasses where they spend the summer feeding on roots. Females return to the elm and lay overwintering eggs on the bark. In spring eggs hatch and the tiny aphids feed on expanding leaves creating the next generation of cockscomb galls. Fortunately, these galls usually do little or no harm to their hosts and no actions of remediation or retribution are necessary, just enjoy.
Two interesting references were used to prepare this episode – “The life history and description of Phylloxera caryaecaulis on shagbark hickory” by Doug Coldwell and Don Schuder and “Insects that feed on trees and shrubs" by Warren Johnson and Howard Lyon.