It’s the height of summer, a great time to traipse through fields of goldenrod, hike in hardwood forests or wander through wild blueberry patches. If you have done any of those things lately you might have noticed some odd growths on the plants you passed by.
Papery brown orbs from oak leaves, swollen balls on goldenrod stems and bulging kidney-shaped bumps on blueberry branches are all the result of an interaction between insects and a host plant. Galls are objects that form from these interactions.
Mites, nematodes, bacteria, fungi or viruses can initiate gall formation, but insects are mostly to blame. When an insect lays an egg inside plant tissue and the larva emerges from that egg, it secretes a chemical substance that interferes with normal plant growth. The unusual growth that results creates a structure around the larva, both isolating it from the rest of the plant and providing a protective nursery in which the larva grows.
Galls are both species and location specific, taking on various shapes, depending on the instigating insect, the species of plant and the location of the disturbance on the host plant. For instance, goldenrod plants (of which there are over 100 species) attract many different gall-making insects. One example is the elliptical stem gall which looks pretty much like its name implies and is formed by a moth. Another, the goldenrod ball gall, also appearing on the stem, is prompted by a fly. A tiny gnat or midge creates a flower-like growth at the tip of the plant called the goldenrod bunch gall.
In each of these examples, the insect’s life cycle varies. In the case of the goldenrod ball gall, the female fly lays her eggs in the stem in late spring or early summer. Ten days later, a larva will hatch and begin eating. The gall forms, grows and provides nutritious food (and protection) for the larva. In the fall, the larva becomes dormant and remains so all winter. Pupation occurs in spring, and two weeks later, the adult emerges to start the process all over again.
Given this cycle, any hole that you observe on a goldenrod ball gall at this time of year is an indication of predation, rather than of emergence of the insect. Though galls provide protection from weather, and in some cases predators, birds such as chickadees and downy woodpeckers commonly peck into these galls to extract the protein rich larva. No protection is 100 percent successful.
By contrast, the oak apple galls commonly found at this time of year, will likely have a tiny hole that indicates their occupant has survived and moved out on its own. These green or brown (depending on their age) ping-pong ball shaped objects form when a tiny oak gall wasp lays her egg in a young oak leaf. The leaf mutates and forms a gall. In a few weeks, the larva completes its life cycle inside the gall and emerges as an adult in June or July.
Oak galls come in many forms. In fact, of the 2,000 insect species that stimulate gall formation, at least 800 of them use oaks as their host. In addition to the leaf-based galls, gouty oak galls are commonly seen on twigs. These gnarly formations take longer to develop and house the insect for up to two years.
Not all galls are single occupancy dwellings. The blueberry stem gall, a kidney-shaped lump, may contain about 12 larvae. These are more commonly seen on wild blueberries (farmers tend to prune them from cultivated bushes), but you can easily identify active ones versus abandoned ones. If there are tiny holes and the gall is brown, the adults probably came out in May or June. If the gall is reddish and whole, this year’s larvae are growing and eating within it. It will reach its full size by late August, the larvae will overwinter, pupate in spring and emerge as adult wasps next year.
The odd growths that are galls generally do not cause major damage to the host plants because they are kept in check by parasitic insects or other predators. So the next time you encounter unusual shapes on familiar plants, ponder the life cycle that may be taking place inside.