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.

Streamsides in winter

some rewards of getting out and about

I take a walk on one of Juneau’s many trails almost every day, alone or with friends. Sometimes it’s a bit hard to get myself out the door, because there’s a deluge or big wind, or I’m just feeling lazy. So I remind myself that sitting inside my house practically guarantees that I won’t see much of interest—so get out there and look around, something may turn up. And something always does.

Here’s a sampling of small pleasures that turned up along Juneau streams in January:

–Fish Creek: Huge, thick plates of ice had washed far over low banks on the small, upstream floodplain and into the forest, and also into the meadows around the combat-fishing pond. It was fun to speculate what it would have been like to actually see the ice cakes pushed out of the creek and into the forest (from a safe distance, of course).

–Eagle River: A dipper was foraging along the edge of the river, occasionally disappearing under the ledges of ice that lined the shores. It searched diligently in the riffles and sometimes brought up something that required some work before swallowing—maybe extracting a caddisfly larva from its case.

–Mendenhall River: I checked out the ‘gooseneck’ peninsula where a breakthrough seems imminent. The narrow neck of land is thinner every time I look, and it seems as if one more good jökulhlaup might be enough to make an island of the peninsula tip. I have to guess that hydrologists have determined the large buildings just downstream to be safe from such events.

–Fish Creek: Winter-active beavers had dragged brush from recent cuttings over to their home pond, leaving trails in the old snow. These beavers, and others in Juneau, have obviously not read the books that report beavers holing up in their lodges for the winter.

–Mendenhall River: A pair of hooded mergansers, the snazzy, gorgeous male with a more-demurely -plumaged female, sailed sedately downstream. Hooded mergansers are the smallest of the mergansers; they eat a more varied diet that includes not only fish but also lots of invertebrates. Males and females commonly pair up in late fall and hang out together through the winter until nesting time in spring. Then the female choses a nesting cavity in a tree or nest box, usually not too far from water, lays her fertilized eggs, and incubates the clutch of eggs, while the male, having done his studly task, goes off and leaves her to do the work. When the eggs hatch, the tiny ducklings almost immediately jump out of the nest cavity, fluttering down to land with a little bounce, and follow mama to feeding areas. This species nests in some places in Southeast, but not commonly. In winter, it favors coastal waters such as shallow bays, estuaries, and tidal rivers, so we see it occasionally.

male-hooded-merganser-by-bob
Photo by Bob Armstrong

–Eagle River: Small insects were flying, possibly midges, looking rather like miniature mosquitoes. Some other insects, such as certain stoneflies, regularly fly in winter, but I’d like to know more about midges (if that’s what they were).

–Cowee Creek: A kingfisher winged upstream and perched over a pool. I hoped to see it catch a fish, but apparently it saw nothing worth pursuing. Kingfishers and many other birds have two small areas, called fovea, on the retina of the eye (humans have just one); foveas have a high density of visual cells and provide good acuity. One fovea, near the bill, is used for monocular, sideways vision; this fovea has especially numerous visual cells and is used for finding prey (as well as keeping track of other birds and predators). When a kingfisher dives and enters the water, its vision switches from that fovea to the other one, located away from the bill; using these lateral fovea in both eyes gives the kingfisher binocular vision and better depth perception as it gets close to an elusive small fish that may try to dart away. When the bird dives, its eyes are protected by a nictitating membrane. As some of us have found out when we try to grab something underwater, refraction often causes us to misjudge the depth of that object; kingfishers can avoid much of that problem by diving vertically after prey that is deeper than a couple of inches.

Kingfishers nest in burrows alongside or at least near streams. I have found nest burrows by several Juneau streams. The lower Mendenhall River and Cowee Creek, where it flows through the meadows, have cut steep mud banks that are perfect places for kingfisher nests. But there’s a problem: in both areas the streams are rapidly eroding those banks; what remains is still potentially suitable for nest burrows, but the stability of the banks is obviously uncertain, and high water in spring and summer could wipe out a kingfisher nest.

–Peterson Creek: A little light snow had fallen on top of old, crusty snow. Shrews had traveled far and wide over the top of the snow. One shrew had plunged, presumably deliberately, over a small mud cliff at the edge of the stream. A few feet downstream, its tracks continued on a flat shelf of shore-bound ice. It sure looked like the critter swam from the base of the cliff to the ice. There is a shrew that is adapted for swimming, but it is not as common here as ordinary shrews, which apparently can swim if necessary.

What did I get out of those little walks? Some really fresh air (Juneau is good for that!), mild exercise (followed by a comfy cup of tea), sometimes companionship, sometimes a solitary meditation, and some observations to think about. Not bad for a small investment.