Insects in August

nesting bees and willow galls

An observant friend was hiking up Mt Juneau one day in mid August. He was apparently the only one of the hiking group to notice a bumblebee that was digging a hole in the dirt at the side of the trail. A little farther along, he spotted another one, doing the same thing. I am envious, because this is something I’ve never seen here.

These bumblebees are queen bees that will hibernate in such holes over the winter. It seemed early to be thinking of hibernating, but perhaps our cold summer is sending them to bed before fall really arrives. Or perhaps they need to scout around for a while to find a suitable site. The queens have already been fertilized by the males, and these newly-fertilized queens are the only ones to live to next year: the males, the worker bees, and their queen-mother all die.

bumblebee-digging-nest
Bee digging nest

Next spring, the queens will emerge and forage on flower nectar and pollen. Each one will build a small nest of plant fibers and lay a few eggs, usually less than ten or so. Sometimes the mothers-to-be take over old mouse nests for rearing their broods. The queens provision the nest with pollen, on which the larvae feed. After roughly three weeks, the larvae become worker bees (sterile females). The queen makes several broods during the summer, each batch of short-lived workers helping to feed the next brood. Bumblebees live in much smaller colonies than honeybees; most nests are only two or three inches in diameter.

Both kinds of bees, however, are seriously declining, apparently because of virulent pathogens to which they have little resistance. The decline of the bee populations becomes a serious problem for humans, because so many of our fruits and vegetables are pollinated by bumblebees and honeybees. Think tomatoes and squashes, peas and beans; think apples and cherries, blueberries and blackberries and strawberries…the full list is very long indeed. If a solution to bee declines is not found, our diets will be much impoverished.

Out near the glacier, watchful rangers noticed a bear, which had just feasted on sockeye salmon, nipping off certain willow leaves. Each of the selected leaves had at least one and sometimes six or eight spherical lumps near the midrib. A few of the lumps were reddish on top, but most were pale green. Each one was about the size of the end of my finger.

These round lumps are galls, produced when an insect lays eggs on the plant. The insect’s activity, and that of the developing larva, manipulates the plant’s hormones in a way that induces the plant to divert some energy and materials to making the gall.

The galls are hollow, each inhabited by an insect larva that feeds on plant tissue inside the protective sphere. Dissection of a few galls by a helpful researcher at the Forestry Sciences Lab showed that the larvae are Hymenoptera—the order that includes bees and wasps. A little further research identified the gall as belonging to a kind of sawfly that specializes on willows. They are called sawflies because each female has a long ovipositor (egg-placer) with which she saws a hole in plant tissue to house the egg.

From the larva’s perspective, the gall provides not only a degree of protection from many (but not all) enemies, but also nutrition. The lining of the gall contains lower concentrations of several defensive chemicals than the outer part of the gall or the rest of the leaf.

From the willow’s perspective, the gall does relatively little harm to the plant. But male willows may be attacked more heavily than females by the galling insect—in at least one willow species, males provide more nutrients because they have more nitrogen, phosphorus, and potassium in the leaves than females. In some cases, the gallers prefer to use willows that grow most vigorously and have the longest shoots.

What might the bear be getting from its selective foraging? No one knows!

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