Small things

…that live on leaves

Photo by Mary Willson

In early August, on the trail that starts near the end of the North Douglas Highway, I found a funny looking thing on the underside of an alder leaf. There was a long, thin, stick-like form underneath a transparent, plate-like covering. A photo was sent off for help with ID; the word came back that it is the typical cocoon of a moth in the genus Caloptilia. There are many species in this genus, and we don’t know which one this is. But, in general, the larvae are leaf miners at first, and later on they become leaf rollers, rolling up a leaf edge into a cone-shaped tube. The adult moths are very small, with a wingspan of about a centimeter. Judging from online images, the wings are very strange (at least in some species): the hindwing is totally fringed, and the rear border of the forewing is likewise fringed. Despite that unusual wing feature, apparently they can fly fairly well.

Photo by Mary Willson

Along that same short stretch of trail, many small spruces had obvious damage on the new growth. This turned out to be spruce needle rust (a fungus). It infects only the new needles and seldom kills the tree, although it may reduce growth of the tree because the new needles are a better source of photosynthesized carbohydrates than are old needles. Outbreaks of infection occur periodically, but seldom erupt in the same place for more than a year or two. Outbreaks are triggered by cool, wet weather in spring, when spore spread from the alternate host, Labrador tea, to spruce needles. Then in summer, different spores disperse from needles back to Labrador tea, and yet another kind of spore then spreads to more Labrador tea plants. The following spring, it’s back to spruce again. This part of the trail would seem to be ideally suited to this life cycle, because some of the area is muskeg, with Labrador tea shrubs, and spruces are colonizing the edges of the muskeg.

Some of the alders in the same area had small fuzzy, white caterpillars on the leaves. These were the larvae of the woolly alder sawfly. The fuzzy, woolly look is produced by a waxy secretion that is shed before the larva pupates. An adult female is about 7 millimeters long; she inserts eggs into alder leaves, usually those that are relatively young and low on the tree. The larvae feed first on the upper leaf surface, later moving to the underside. Their feeding activity can skeletonize leaves, leaving only the veins. When the larvae are fully developed, they drop to the ground and make a cocoon in the leaf litter. Thought to originate in Europe, this species may be largely parthenogenetic, since males are reported to be absent in North America and quite rare in Europe.

Photo by Mary Willson

Not far away, there were lots of galls on some willow leaves, mostly near the veins. Pointed underneath and reddish on top, they might be the temporary homes of larval gall midges (possibly an Iteomyia species). These seem to be better studied in Europe than in North America, but I found no details of the biology and life history. In general, the tiny female fly lays eggs on the leaf; when mature, the larva drops to the ground to pupate for the winter.

Thanks to Charley Eiseman for long-distance consultation and Elizabeth E. Graham of FSL for local consultation.

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Willows, midges, and moose

connections between tiny insects and big herbivores

The many species of willow are subject to chewing, nibbling, gnawing, and poking by a huge variety of consumers. Here are just of few of the complex interactions.

Most of us here have seen the ‘willow roses’ or rosettes that develop on the twigs or shoots of certain species of willows. The rosettes are galls, induced when a certain tiny fly called a midge lays eggs on the tip of the shoot. The normal elongation of the shoot is suppressed but leaves continue to develop and become crowded together, forming the rosette. The midge’s larva develops inside the rosette, feeding on the bases of the innermost leaves. The larva pupates inside the rosette gall, and the adult emerges the following spring, in time to lay eggs before leaves develop.

The rosette is formed of more leaves than would occur on normal shoots, perhaps forming a wall of defense against enemies of the midge larva (such as parasitoid wasps that would lay eggs on the larva). The inner portions of the rosette also have less photosynthetic capacity and more defensive compounds than the outer portions, which may deter parasitoids and pathogens. The midge larva is presumably is physiologically capable of dealing with the defensive compounds. However, I’ve not found out how well these deterrents work against such enemies. I’ve read that European titmice know how to open the rosettes to gobble up the larva; so of course I now wonder if our chickadees can do the same.

willow-gall-green-by-bob-armstrong
Photo by Bob Armstrong

Female midges are quite choosy about where to lay their eggs. Only some species of willow are susceptible to attack by this gall-forming midge; Barclay and Sitka willow are among them here. Experiments in other regions have shown that individual plants of the same species differ genetically in their susceptibility to these gall midges. And I have observed that rosette galls seem to be more common on shoots that are not shaded.

The rosette-bearing, stunted shoots cannot produce catkins, so the reproductive capacity of the willow plant is reduced in proportion to the number of rosettes. Eventually, the rosette kills the shoot, without apparently affecting neighboring twigs. The rosette, however, offers winter protection for spiders and beetles that shelter among the crowded leaves.

Willows are often heavily browsed by snowshoe hares, moose, and reindeer, and this activity can affect the abundance of various kinds of galling insects on the plant. Several studies have shown that some galls can be more abundant on heavily browsed stems. Unfortunately, I have found no such information for the rosette-forming midge specifically.

However, there is evidence for the reciprocal interaction: moose browsing is affected by the presence of rosette galls. Experiments with captive moose in British Columbia showed that moose clearly preferred to eat willow shoots that bore no rosettes. Although they sometimes bit a shoot with a gall, they soon spit out the rosette.

In the absence of rosette galls, browsing by mammalian herbivores, such as moose and hares, can have significant effects on willow growth and reproduction (by removing stems that would bear catkins). Some studies have shown that severe browsing, which leaves little more than a stump, leads to the production of so-called juvenile shoots and leaves. These often have a somewhat different shape from normal leaves and commonly have more defensive compounds, which reduce palatability and nutritional value; this protects the new shoot from further browsing, at least for a year or two. Moose and hares tend to avoid browsing twigs with lots of those defensive compounds.

However, moderate browsing may have very different effects: One study showed that winter browsing by hares on feltleaf willow twigs led to bigger, more nutritious leaves the following spring. In other cases, moderate browsing has elicited compensatory growth of the willow, but this is not feasible in habitats with low nutrient availability and poor growing conditions. The bottom line here is that the interaction between herbivorous mammals and willows varies a lot, depending on severity of browsing, growing conditions, the species of willow, and no doubt many other factors.

It is clear, at least, that herbivores selectively forage on different species of willow; even within a single species of willow, some plants are more palatable than others. Some such differences are genetic, while others have to do with growing conditions, such as the amount of shade. In either case, selective removal of favored kinds of leaves and twigs makes them unavailable for decomposers below the plants. Heavy browsing obviously reduces the amount of litter fall and can change the availability of soil nutrients that result from decomposition. So moose browsing can affect the soils, leading to changes in plant species composition and, potentially, the course of early plant succession below the browsed shrubs.

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!