Wednesday, January 13, 2010

Taking A Closer Look: Blueberry Stem Gall

One of the plants that make the Adirondacks special is the blueberry, which likes to grow in, or alongside, a variety of wetlands. I recall one of the highlights of summer camp was when the nature counselor made her blueberry fritters. Campers and counselors alike would flock to her nature room as the rumor of fritters spread like wildfire. Her “Live off the Land” camping trips were never complete without blueberry fritters for breakfast.

But blueberries aren’t just special to people; lots of wildlife benefit from the fingertip-sized fruits, not least among them birds and bears. Not all blueberry fanciers are after the fruits, though. The blueberry stem gall wasp (Hemadas nubilipennis) is more interested in the stems of the plant. Highbush, lowbush, the variety probably doesn’t matter, not when reproduction is on the line.

I encountered my first blueberry stem gall this summer while paddling the Hudson River with a friend down near Glens Falls. Because plants are one of her favorite things, she was constantly parking her boat and hopping out to ascertain what was in bloom at various spots along the shore. As we cruised through the wooded banks, she pointed out this kidney-bean-like thing on a blueberry plant. “That’s blueberry stem gall,” she said. I took its photograph and we moved on.

It turns out that blueberry stem galls are made by a very small (less than 1/8 inch) shiny black wasp. Come summer, this tiny insect seeks out tender blueberry shoots and lays her eggs inside. Once she has completed this task, she climbs to the tip of the shoot and repeatedly stabs it, causing severe damage to the plant. In other words, it won’t grow anymore at this spot. To make matters worse, the egg-laying itself has damaged the plant cells within the shoot, resulting in abnormal growth that creates a nice snug chamber around each egg.

Within two weeks, the eggs hatch into grubby white larvae, about twelve, on average, in the group. Each larva starts to chow down on the walls of its birth chamber, which results in more abnormal cell growth on the part of the plant. The final result is this kidney-bean-shaped gall. When the gall first forms, it is a rather spongy greenish thing. It will continue to grow throughout the summer, reaching its peak (0.75-1.25” long) sometime in August. By now it has turned bright red and hard; come fall it will be brown.

Winter arrives, and the larvae are all snuggly resting in their protective home. Here they wait in relative comfort for the warm days of spring, which signal the time to puapte. When late-May and early-June roll around, the new adults emerge, and the cycle begins all over again. If you find a blueberry stem gall that’s all hard and grey and riddled with holes, you’ll know you’ve found one that successfully hatched its waspy cargo.

One has to wonder, though, if this process has any negative impacts on the blueberry plant. Obviously, it doesn’t do that particular shoot much good, but a blueberry plant with only one or two galls is probably not bothered too much by the invasion. If, on the other hand, the plant is covered with tens or hundreds of galls, then yes, this can be problematic, especially if the plant is part of a commercial blueberry operation (or even a hobbyist’s garden). This is because a stem “infected” with a blueberry stem gall will not produce fruit buds (remember that the female stabbed the daylights out of the shoot’s tip). No fruit buds, no fruit.

Still, I imagine that in the wild, where monocultures of blueberries rarely exist, this isn’t usually too much of a problem. A gall here, a gall there – no harm done and there are still plenty of blueberries to feed the ravenous hoards, be they birds, bears or boys. But now that I know what to look for, you can be sure that I will start to monitor the presence of blueberry stem galls in the blueberry patches near my home.

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Ellen Rathbone is by her own admission a "certified nature nut." She began contributing to the Adirondack Almanack while living in Newcomb, when she was an environmental educator for the Adirondack Park Agency's Visitor Interpretive Centers for nearly ten years.

Ellen graduated from SUNY ESF in 1988 with a BS in forestry and biology and has worked as a naturalist in New York, New Jersey, and Vermont.

In 2010 her work took her to Michigan, where she currently resides and serves as Education Director of the Dahlem Conservancy just outside Jackson, Michigan.

She also writes her own blog about her Michigan adventures.





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