Saturday, April 2, could have been disappointing for an insect photographer. True, an early overcast gave way to a strong, early spring sun, and the day looked warm enough. But a cold, persistent northwest wind marred the effect.
Still, there was cause for optimism. Insects live at much smaller scales than humans do. Cold winds are sharply attenuated at ground level, and even a pebble or thin branch is an effective windbreak if you’re only a quarter-inch long. The human observer regrets leaving the jacket in the car. But in a crevice in tree bark on the sunny leeward side of a tree, a spider or fly enjoys a bubble of microclimate 20° warmer than the ambient temperature.
Exploiting this circumstance, a small but significant suite of plants and insects has evolved a commitment to life in the first stages of spring. To be sure, the strategy poses challenges: Temperatures can still plunge far below freezing in March or April, few resources are available, and a late snowfall can smother what resources there are. But on the plus side, there is little competition for whatever food is present. And many potential predators are not yet active or not yet arrived on their migration, vastly reducing the chances of getting eaten. From an evolutionary perspective, the conditions of early spring represent an opportunity waiting to be exploited.
Mid-morning on April 2, then, I bucked the wind expressly to see what was happening at a patch of willows I know of in Correllus State Forest. Leaving the jacket in the car was a mistake! But I was confident I’d find interesting activity when I arrived. Willows — in the genus Salix — are among the plants that have embraced an early-season reproductive cycle. Leafing out and, especially, flowering early, willows are a surprisingly important feature of the early spring landscape. (Their value extends later into the season, as well; for example, willow leaves frequently host caterpillars of the mourning cloak butterfly.)
These small, often shrublike trees, characterized by brittle wood and a fondness for moist settings, are not very prevalent on the Vineyard. Our half-dozen or so species, some native, some introduced, are widespread but inconspicuous, and never very common. My State Forest stand of willows has had some rough years: A few trees have been uprooted by storms, and many branches have broken. But the surviving plants produce plentiful catkins, and some of the male ones had matured to produce yellowish, pollen-laden stamens.
Most of the mature catkins proved to be liberally sprinkled with small, dark flies. Photographing them proved challenging. The catkins were in constant motion due to the wind; the flies were in constant motion as they fed on the flowers; and my camera was in constant motion as I “free-handed” photographs of subjects at about my eye level. But the decent images I managed show flies that are probably members of the family Anthomyiidae, and the date and context offer useful clues toward a more specific identification.
Members of one Anthomyiid genus, Egle, are commonly called “willow catkin flies” for their early-season flight period and their very close association with willow flowers. Given the paucity of fly species active in the first week of April, most or all of the flies I observed were likely in this genus.
Perhaps more interesting were the bees that I found visiting these catkins. Again, identification from photographs proved challenging. But on reviewing my photos, it was clear I had images of at least two species of bees, both apparently in the large genus Andrena. They are surely among a number of species in this genus that have committed to an early spring life cycle — a commitment that almost inevitably leads these bees to a fondness for willows.
The relationship between these insects and willows is a mutually beneficial one. Willows, at least some species, apparently can pollinate one another successfully relying only on the wind to carry pollen. But pollen transport works much more reliably for these plants if insects are involved. With their early-season bloom period coinciding with the flight periods of various early spring insects, willows readily attract assistance for their pollination. And the availability of willow nectar and pollen is a priceless resource for many of our earliest insects, both willow specialists like Egle and more generalist species that are waiting for a greater variety of plants to start to bloom.
Our landscape today probably features more types of early-season flowers than it did before the arrival of European people, plants, and animals. Most of the early spring flowers you see in settled areas — dandelions, mustards, and bulbs such as crocuses and daffodils — are imports, and were not present when our native insects were evolving their ecological relationships. Aboriginally, willows were about the only game in town in the first few weeks of spring. The flies and bees I found on April 2, that is to say, were carrying on an age-old tradition of visiting willow catkins, a highly evolved relationship that allows these plants and animals to flourish when the growing season has just begun.