(Click here for an audio version of this blog.)
Riding my bike down on Roaring Run back in June, I had to keep my eyes on the path immediately beneath my tires rather than on the much more interesting woods around me. Crawling across the gravely surface of the trail was a mixed parade of forest millipedes and gypsy moth caterpillars, and it took all of my attention to avoid running them over.
Gypsy moths (Lymantra dispar) were/are one of the great invasive scourges of our eastern forests. A brief review of their history: they were brought to North America (Medford, Massachusetts to be precise) in 1869 by Etienne Leopold Trouvelot who intended to breed them with Asian silk moths so that he could develop a domestic silk industry. The gypsy moths escaped from Trouvelot’s home and quickly became a recognized pest in the oak forests of New England. In 1906 the U.S Department of Agriculture released an exotic, European, parasitic, tachinid fly (Compsilura concinnata) to try to get the exploding gypsy moth populations under control. Over the next eighty years the USDA repeatedly released more and more of these tachinid flies throughout affected forests in a vain attempt to control the spreading gypsy moths. As pesticides were invented, they were thrown at gypsy moths, too. Pathogenic fungi were also developed and used to weaken and reduce the gypsy moth masses.
Those of us of who remember the gypsy moth outbreaks here in Western Pennsylvania in the early 1990’s can recall trees covered with writhing masses of caterpillars, and sidewalks, streets, and driveways coated with crushed caterpillars. We also remember that we had to wear hats when walking in the woods because of the constant raining down of tiny fecal pellets from the swarms of gypsy moth caterpillars feeding on leaves up in the tree canopies. Many oak trees were completely defoliated. Many oak trees were killed. But, then the explosive numbers died back and for most us “went away.”
There are still significant areas of Pennsylvania where gypsy moths are an overwhelming pestilence, but many more areas of forest where they have become a baseline part of a tolerable equilibrium. Maybe the pathogenic fungus was the key weapon for control. Maybe letting the population become so dense triggered a crash from which the species has not yet recovered. Either way, biking along Roaring Run Trail and dodging a few crossing gypsy moth caterpillars is so much better than the slipping and sliding over thousands of their crushed carcasses just 25 years ago!
Last year (2018) the Pennsylvania Department of Conservation and Natural Resources sprayed forests in eight counties in northeastern and central parts of the state in an effort to control gypsy moth outbreaks. The war against the gypsy moth, then, is far from over!
Unfortunately, though, there is a great deal of collateral damage from many of the biological control systems that have been unleashed on the gypsy moths. Those tachinid flies in particular that were released during eighty years of fruitless attempts at biocontrol became established in North America and are doing a great deal of harm.
Let’s think about these parasitic flies and how they interact with their host caterpillars. First, many parasites of moth caterpillars lay their eggs on the surface of the skin of the larvae. Since larvae go through a number of growth and skin shedding stages (their “instars”) many of these surface eggs are in fact shed with each instar molt. Also, many of the moth parasites are very specifically matched to a species of moth caterpillars. Consequently, the parasite becomes active only during the seasonal activity time of the host caterpillar and has a very focused and direct impact on a specific moth species. These two features of the host/parasite interactions enable both species to reach equilibrium populations in which persistence of both species without explosive growth are achieved.
Compsilura concinnata, our introduced tachinid fly, however, exhibits none of these focusing or restraining parasitic features. This tachinid fly inserts its eggs into the body of a host caterpillar. No skin molting will shed the lethal parasite egg load once they have been delivered to the caterpillar. Also, this tachinid fly displays the antithesis of host specificity. It parasitizes nearly two hundred species of Lepidoptera (moths and butterflies) and Coleoptera (beetles) and Symphyta (sawflies) in North America alone. Further, C. concinnata instead of having its life cycle timed to the seasonal cycle of a particular host is able to have up to four generations in a single year. Each generation will encounter different butterfly, moth, beetle or sawfly species and have deleterious impacts on each of them.
There was, then, no targeting of C. concinnata on gypsy moths! Almost every native moth and butterfly species in North America was exposed to this aggressive, generalist parasite! Monarch caterpillars are killed by C. concinnata, as are luna moths, cecropia moths, polyphemus moths and promethea moths. The decline of these “giant silk moths” in particular has been observed throughout their North American ranges, and some experts feel that C. concinnata is responsible for over 80% of their population loss!
Looking up from the gypsy moth caterpillars on the bike trail path I notice that the masses of tent caterpillars (Malacosoma americanum) are growing especially thickly on the terminal branches of the cherry trees all along the trail. These caterpillars are an inevitable sign of late spring, and, while they are not as beautiful as some of the other aspects of the season we have mentioned in the past, they are the principal food of one of the glorious birds in our area: The Baltimore Oriole. As I wrote in a blog several years ago:
“The oriole males are vying with each other for prime breeding territories and are getting ready for the anticipated arrival of the females. Baltimore orioles (and this species is distinct from Bullock’s oriole so recent attempts to lump both species together as the “northern oriole” are not valid) spend their winters in southern Mexico and Central America and then in the spring spread themselves out across their breeding territories in the United States from North Dakota to Maine and Oklahoma to the Carolinas.”
The orioles time their mating and egg laying and nestling emergence to the abundance of the eastern tent caterpillars! Fast food for fast growing nestlings! As John Irving once wrote in his novel “The Cider House Rules,” “be of use!” He could have been describing these tent caterpillars!
So, caterpillars are all around us! Some are vestiges of a colossal, human-generated disaster and others are enmeshed into natural trophic networks and serve as the primary fuel for some glorious baby birds. Sunlight to leaves to caterpillars to majestic orioles: it sounds so simple.