Bug Lady: Galls only have exit holes
One of the most interesting things that some insects can do is to make plants grow special food structures for them. These structures are called galls and come in all sorts of sizes and shapes. They grow on all sorts of plants, from trees to herbaceous plants. They usually do not harm the plants, although they might look odd or unsightly to humans. They can be on leaves, buds, stems and even roots.
Gall-makers include certain types of flies, wasps, moths, psyllids, aphids and mites. Chemicals in the gall-maker’s saliva serve as plant growth regulators and cause the plant to grow a structure that feeds and protects the creature that lives inside. The gall is very high in nutrients.
Some plants, especially goldenrod and oak, are home to many kinds of galls. They are so common that early botanists thought they were normal parts of the plants and showed them in illustrations. Probably the very commonest gall is the goldenrod stem gall, a round structure that looks like a ball in the middle of a stem. They are very easy to find, especially in fall, and if you wander through a field full of dead goldenrod, you are likely to find them without searching.
It occurred to me as I write this that many of you have probably never walked in a field of goldenrod and probably won’t have a chance. Fields of wild flowers are rapidly being mowed down to grow subdivisions, and homeowner associations frown on “weeds” being allowed to grow along fences. Wild flowers are home to all sorts of insects, which in turn provide food for our birds. It’s no wonder that we see fewer and fewer songbirds. But that’s a subject too depressing to pursue!
The goldenrod stem gall is produced by the goldenrod gall fly, Eurosta solidaginis. Eggs are laid in spring on the stems, and the newly-hatched larvae chew their way into the stems and the galls grow. There is one gall per stem and one larva in a gall (although some other creatures may have more than one per gall).
The larva feeds until fall, then chews an exit tunnel that almost reaches the outside but leaves a little protective covering. It then returns to the gall center to pupate and sleep until spring. The adult fly does not have chewing mouthparts and could not chew its way out of the gall, but it can push its way through the thin cover of the tunnel in spring.
If you feed birds, you no doubt know our smallest woodpecker, the little black-and-white downy woodpecker that frequents feeders. I feed the birds all year round, so I had the opportunity to see just how clever these little birds are.
I have had several hummingbird feeders for many years and never had any other birds pay any attention to them. However, a couple of years ago, a female downy woodpecker figured out how to hang on the hummer feeder and drink nectar. She must have told her mate, because before long the male was doing it, too. Since then, the downies have been at every hummingbird feeder every year.
The downies have also figured out that a goldenrod gall can contain a big, juicy larva. Studies showed that the birds know how to find the larvae. If they take a few pecks on a gall and don’t find an exit tunnel, they quit. If they hit a tunnel, they peck into the middle of the gall and get the larvae.
If you see a gall with any sort of hole in it, the resident is gone. There are no entry holes, only exits. Either the creature has grown up and departed or a predatory insect or bird has eaten it.
Send your insect questions to Claire Stuart by e-mail at firstname.lastname@example.org or write her (with self-addressed stamped envelope) in care of Living Section, The Journal, 207 W. King St., Martinsburg, WV 25401.