By Charley Eiseman Gazette Contributing Writer
The monarch butterflies have left us for the winter and are well on their way to Mexico. Most people know that as caterpillars these remarkable creatures feed exclusively on milkweed leaves. A lot less attention is paid to the food choices of other insects, so you might think it’s unusual for a species to be this finicky.
In fact, if you visit your local milkweed patch, you’ll find all sorts of other milkweed-specific insects, many of them similarly brightly colored to warn predators of their toxicity. Other insects munching the leaves include milkweed tussock moth caterpillars, milkweed longhorn beetles, milkweed weevils and swamp milkweed leaf beetles. Each of these has its own particular style of feeding.
Perhaps the most distinctive is the milkweed longhorn, which bites the leaf’s midrib to stop the flow of sticky latex, then is able to eat the tip of the leaf without getting its mouthparts gummed up. The larva of each type of beetle has its own niche: leaf beetle larvae feed openly on the leaves, the weevil larvae develop inside the stem, and those of the longhorn feed on the roots.
In addition to these insects with chewing mouthparts, various beaked insects suck milkweed juices, including large and small milkweed bugs (two distinct species) and at least three species of aphids. There are also two midge species that develop in galls (insect-induced deformities) on milkweed; one causes stem swellings, and another distorts the leaves. And then there are leaf-mining fly larvae that create large, discolored blotches as they feed inside the leaves.
This type of host specificity is by no means unique to milkweed-feeding insects. In fact, the majority of herbivorous insects are very particular about what plants they feed on, and it seems that just about every kind of plant has a whole array of interesting creatures that are particular to it.
For example, consider oaks, the dominant trees in many local forests. There are hundreds of different insects that are found on them and nothing else. Some of the most intriguing are the gall wasps, each species of which lays its eggs in a particular part of a particular type of oak, producing a characteristic gall in which the larvae develop. One species forms big white pom-poms with red polka dots on white oak twigs, another distorts red oak leaves to produce hollow galls resembling ping-pong balls, while a third produces hard, apple-like galls on the sides of acorns. Many of these galls are deciduous: they drop to the ground before the adult wasps emerge. The larvae in some galls have other interesting tricks like secreting honeydew, a sweet, sticky substance that attracts ants, which in turn defend them against parasites.
Opportunities to observe these plant-insect interactions are all around us. Last spring, at the Norwottuck Rail Trail parking lot on Damon Road in Northampton, I peeked under some sycamore leaves and found a tiny caterpillar that clears the fuzz off the leaf underside and makes a big pile of it to hide in. Walking down a sidewalk in Florence, I found a colony of ragweed-specific beetles (isn’t it nice to know that something eats ragweed?) in various stages of development; many were pupating in lacy cocoons, which is very unusual for a beetle. Later that day, at the edge of a nearby forest, I discovered a beautiful, rectangular mesh cocoon belonging to a moth whose caterpillar feeds only on aspen. It turned out this species had never before been documented in Massachusetts.
There is a larger significance to all this specificity. Each kind of plant has a set of organisms that have evolved with it and depend on it for survival. Plants that have been in North America for a long time tend to have far more associated organisms than those that were introduced recently. In fact, a big part of why some plants become “invasive” is the relative scarcity of host-specific herbivores and pathogens in their new habitat.
If you look mainly at birds and mammals, it might not be clear why the widespread replacement of native plants by exotic ones would be an issue: the dense thickets formed by such non-native species as multiflora rose and Eurasian honeysuckles in New England can be great habitat for cottontails, bobcats and veeries (a type of thrush), because these animals are responding mainly to habitat structure rather than to species composition. And many exotic plants spread precisely because vertebrate animals eat the fruits and then excrete the seeds elsewhere.
But the spread of exotic species with few specific feeders has consequences for vertebrates as well. The whole reason migratory birds come north every spring is to take advantage of all the insect food that is suddenly abundant during the growing season. Not all insects eat plants, of course, but it seems likely that having a larger proportion of the landscape occupied by recently introduced plants leads to a smaller insect population—reducing the food supply not just for birds but for the insectivorous mammals, reptiles and amphibians that are themselves food for larger animals.
Charley Eiseman is a naturalist and author. His most recent book, co-authored with Noah Charney, is Tracks and Sign of Insects and Other Invertebrates.
Earth Matters, written by staff and associates of the Hitchcock Center for the Environment at 525 South Pleasant St., Amherst, appears every other week. For more information, call 413-256-6006, or write to us.