In my neighborhood, there are biennial outbreaks of plant-eating apparitions. (No, I have not been smoking anything illegal.) Let me explain. “Apparition” in Greek is phasma: we see biannual outbreaks of phasmids, the leaf eating insects commonly known as walkingsticks.
There are approximately 2,500 described walkingstick species worldwide; most of those are tropical, including some that can grow to 12 inches long. Only eight of those 2,500 different walkingstick species are found in North America. Where I live in southern Vermont, only one of those eight is commonly found; the northern walkingstick (Diapheromera femorata). And in my neighborhood, I see it only on even-numbered years: 2002, 2004, and 2006 were walkingstick years. The years 2003, 2005, and 2007 were not. Years of searching on my property have yet to turn up even one walkingstick in an odd-numbered year.
The biannual appearance of the local walkingstick population is a result of their biology: eggs that are laid in a summer often do not hatch the following spring, but rather hatch the year after, especially in more northerly places. The two-year life cycle often results in large numbers of walkingsticks – and sometimes relatively high levels of damage to plants from walkingsticks – in alternate years.
Early researchers were interested in the potential economic impact of walkingstick infestations. Walkingsticks eat leaves. Young walkingstick nymphs feed on the leaves of low, shrubby plants such as blueberry, hazelnut, and wild rose. Adults feed in the treetops on oak, wild cherry, basswood, paper birch, aspen, hickory, black locust, and apple, among others. They generally shun maple and box elder, as well as the conifers.
Entire leaves are consumed, and a heavy outbreak of these large (up to four-inches-long) insects can denude sizable stands of trees. When the trees put out replacement leaves, those are also eaten. At their worst, walkingsticks can seem as destructive as gypsy moth caterpillars, though they are much less mobile, moving too slowly to ever travel far. Walkingstick populations do not usually reach destructive levels, but major defoliations have been reported in Wisconsin and Minnesota. Outbreaks there in the late 1800s resulted in trees and underbrush stripped of leaves. Great clusters of walkingsticks hung from bare twigs and branches.
My walkingstick population was in a young black locust stand, and there was never a heavy enough infestation to seriously harm the trees, although it’s not unusual for a casual, ground-level, late summer or early fall survey to turn up 20 or 25 of these twig-like creatures.
They are easy to see in the early fall because they descend nearly to the ground to mate. The females show up first, clinging motionless to the trunk of the tree with the tips of their abdomens curled out. They are, according to one source, “probably releasing pheromones to attract males.” Soon, males arrive and without ceremony mount the females.
And there they stay, coupled and motionless from dawn to dusk. Pairs of walkingsticks found at 8 A.M. will often still be frozen in place at 8 P.M. Their lengthy matings are said to be the male’s way of guaranteeing his paternity. He simply stays coupled with the female long enough to ensure that it’s his sperm that fertilizes her eggs. Walkingstick males are usually much smaller than the females, and some species carry this mating strategy to the extreme of the male actually riding the female, as a jockey rides a horse, for days or even weeks. However, staying coupled for 10 or 12 hours seems sufficient for the northern walkingstick.
Furthermore, the male northern walkingstick may be altogether unnecessary. Many of the Phasmatodea can reproduce parthenogenetically: the female can lay unfertilized eggs that will develop into an all-female generation, which in turn can produce other all-female generations until eventually (in most cases) a male resumes his duties.
When her eggs are fertilized, the female goes back to feasting in the treetop. From then until frost kills her, she lays about three eggs every day, dropping them casually to whatever fate might befall them on the forest floor. Thus, her job is done.
The eggs rain down into the leaf litter, and would seem vulnerable to a wide array of parasites and just more fodder for a host of hungry scavengers. Miraculously, there is a species of ant that collects walkingstick eggs, carries them underground, and consumes only a non-vital bit of the egg casing called the capitulum. The egg itself is unharmed and will hatch normally.
The walkingstick nymphs that hatch from those eggs look like tiny, pale-green versions of the adults. And they act like the adults. They either sit motionless or gently sway in the breeze during daylight hours, and they feed all night. They look so much like a part of their environment that they usually go unnoticed by predators. Should one be discovered by a hungry bird, however, it will readily sacrifice a leg to escape.
That sacrifice is only temporary. Walkingstick nymphs can regenerate lost legs, so after its next molt, the nymph will again have six legs, though the regenerated limb may be slightly shorter than the rest. To reach full adult size, a northern walkingstick molts its exoskeleton (which becomes restrictive as the insect grows) five times, and at each molt, lost limbs can be regenerated. If a leg is lost by a full-grown adult, however, the loss will be permanent.
Northern walkingsticks have one further trick that helps keep them from being detected. They change colors. As the season progresses and the fresh green twigs and leaf petioles fade to shades of gray and brown, walkingsticks also fade. At each successive molt, the nymphs – pale green when hatched – get progressively browner until the adult females are almost all brown, while the males may still have some green, especially on their legs.
Walkingsticks are by nature hard to see. I once found one atop a bouquet of fall wildflowers that had graced my dining table for two full days. The four-inch-long insect was in plain sight – and yet unseen. Many unsuspecting people had admired the flowers. A few even bent down, nose buried in the blossoms, to better enjoy their fragrance. For phasmids, mimicry works.
I see walkingsticks in late summer and early fall. They are at or below eye level on the trunks of a certain small stand of young black locust trees. They come down from the treetops to mate.
There is a simple elegance to this. The insects move from twig to branch to trunk, going down the tree until there is but one path to follow. There they find each other. It seems so simple and straightforward to our minds. Walkingsticks appear common and easily observed for this brief period…and then like apparitions they disappear – right before our eyes.