The Crafty Chemists of Tryon Creek
Gall Insects: The Crafty Chemists of Tryon Creek
By Bruce Rottink, Volunteer Nature Guide and retired Research Forester
On those evenings when you just don’t want to make supper, most often you’ll go to a restaurant for dinner out. But sometimes you pick up the phone, and order food delivered to your home. Most forest organisms use the “dinner out” option as they fly, crawl, swim or slither to their next meal. However, a few insects have learned how to get food delivered to them. These crafty critters are the gall insects.
What is a gall exactly?
Galls are odd-shaped lumps or blobs of abnormal plant tissue, much like a tumor in a human. Galls can form on leaves, stems, flowers, roots or buds. Galls can be caused by certain kinds of bacteria, fungi, nematodes, mites or insects. For now, let’s focus on the insects.
More than 1,000 different species of insects can cause plant galls.
Each of these insect species attacks a certain species of plant. In most cases, an adult gall-forming insect lays one or more eggs in soft plant tissue that is still growing. The eggs quickly hatch into larvae that stay inside the plant. Now here’s where it gets interesting.
It’s All About Auxin
Plants control their own growth with chemicals the plant produces which are called “plant growth hormones.” One of these hormones is auxin. Auxin stimulates the growth of some plant tissues, for example the shoot tips. Auxin also signals the plant to send nutrients like nitrogen, phosphorus and sugars to those parts of the plants which are producing the auxin. This process is called “hormone directed transport.”
Some gall insect larvae have become tiny chemists who have the ability to produce auxin (or in some cases, auxin-like chemicals) in their salivary glands. These chemicals signal the plant to send lots of nutrients to the place where the larvae are living. This causes the plant cells around the insect to start growing very rapidly. As a result, the plant tissues produce the blob of plant tissue we call a gall. The gall provides the insect larvae with both shelter and food. The larvae don’t have to go any place for food, the food comes to them.
The gall insect larvae eat a lot of the nutritious plant tissue inside the gall. After the larvae are full size, they stop eating. In many cases the larvae use the gall as a nice safe home for the winter. In other cases, they tunnel out of the gall, and pupate on the ground.
At Tryon Creek, I’ve seen three different types of galls.
The first was on the stem of a thimbleberry (Rubus parviflorus) plant near Old Main Trail. The thimbleberry forms this type of gall when it is attacked by the thimbleberry gall wasp (Diastrophus kincaidii). In this particular case, the wasp lays several eggs inside the stem, not just one.
Thimbleberry (Rubus parviflorus)
Next I Found…
Another type of gall I spotted was on a twig of black cottonwood (Populus balsamifera spp. trichocarpa) tree growing near the main entrance to the park. The picture below shows a one-year-old twig. Experts at Oregon State University and the Oregon Department of Forestry said the insect that formed this gall was the Poplar twig gall fly (Hexomyza schineri). From the hole in the side of the gall, it looks like the insect has successfully overwintered inside the gall as a pupae. The adult emerged from the pupae, and bored its way out of the gall.
Black cottonwood (Populus balsamifera spp. trichocarpa)
The third type of gall is really weird.
It was on the snowberry bush (Symphorocarpos albus). It is formed when a sawfly lays an egg in a young bud of the snowberry bush. As the egg hatches, the nutritious plant tissues that make up the gall grow around the insect larva, as shown in the picture below.
Snowberry bush (Symphorocarpos albus)
When you cut open the snowberry gall in the middle of May surprise, surprise, inside is the larva of the sawfly, as shown in the picture below. The gall is now hollow because the larva has eaten out the center of the gall. The only bad news for the larva is that all the little dark balls you see inside the gall are insect poop (if you want to get fancy, scientists call it “frass.”)
While having the plant deliver food to you seems like a wonderful scheme, there is a downside! Once an insect has formed a gall, the insect is not only trapped inside until it matures, but the gall is advertising the larvae’s location to the smarter predatory animals.
And the Upside
One such animal is, well, ME! I grew up in Minnesota. In the winter my Dad and I would pick the galls we found on goldenrod (Solidago sp.) plants before we went ice fishing. Out on the frozen lake, we would cut open the galls, pluck out the juicy larva and use it for fish bait. The sunfish loved the goldenrod gall larvae. Dad and I loved the sunfish.