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By Lisa Moore
Hometown Weekly Correspondent
New England forests are being defoliated at alarming rates this summer, with the damage visible from space. Satellite photos taken of parts of New England taken one month apart in May and June of this year show advancing brown swaths across forested areas of the region. The destruction to deciduous trees is due to the voracious eating habits of the leaf munching gypsy moth caterpillars.
The Latin name for the gypsy moth is Lymantria dispar. “Lymantria” translates to destroyer, and “dispar” means unequal. The gypsy moth lives up to its name, consuming the leaves of over 500 species of shrubs and plants. The oak tree is a particular favorite of the gypsy moth caterpillar, and while a single caterpillar can’t do much damage to an individual tree, when the number of caterpillars is in the millions, they are a force to be reckoned with. According to the Department of Agriculture, insects and disease damage 45 times more forest than wildfires each year and this year we are in the midst of a gypsy moth population explosion.
The gypsy moth was first introduced to the region in 1869 by Etienne Leopold Trouvelot who imported the gypsy moth from Europe in hopes of developing a better alternative to silk caterpillars. The moths were accidentally released from his residence in Medford MA and within ten years of their release were considered a nuisance, with the first major outbreak occurring in 1889. In 1890, an eradication program began, but the caterpillar continues its slow natural spread across the country, averaging about 13 miles per year without intervention, according to the Department of Agriculture.
The gypsy moth undergoes a complete metamorphosis in its life cycle. In the summer, the female moth will lay eggs in masses on branches or trunks of trees, in cracks between rocks, under vehicles, in foliage, or any other sheltered area. The eggs are covered by a coating of hairs that deter predation and protect the eggs as they over winter for eight to nine months. The larvae develop inside the egg within the first month and then development stops in winter as eggs adjust to winter climate changes by expelling water to prevent freezing. In spring, warmer temperatures stimulate growth again, the eggs rehydrate, and the larvae continue to develop until they emerge as tiny black hairy caterpillar larvae. The hatchings coincide with the budding of most hardwood trees, and a single egg mass can contain between 500 and 1000 eggs. The tiny hatchlings spin silk threads and take to the breezes as means of dispersal.
The larvae will spend the next seven weeks eating and growing. As the caterpillar grows, it must shed its exoskeleton to accommodate its increasing size. Each time a caterpillar molts it gets larger, the time between molts is called an instar. A male gypsy moth will molt 4 times and a female will molt 5 times, growing larger than the males. When the larval stage ends, the caterpillar stops eating and surrounds itself with silk, forming a pupa. The pupa will undergo a complete metamorphosis, changing form within its silken case until it emerges 7- 14 days later as an adult moth.
The adult moth has one goal: to find a mate. They will not eat as adults and live for approximately seven days. The female moths are white, larger than the males and are flightless despite fully formed wings. They climb up any vertical object and release pheromones to attract a male. Males are brown, smaller than females and have feathery antennae. They emerge two days or so earlier than females. Males can be seen flying up and down any vertical object in search of a female, seeking out her pheromone scent trail. A male will mate with multiple females before it dies. Once the female has mated, it will lay her egg mass on a branch or other sheltered spot where they will overwinter before starting the cycle again.
One of the most destructive pests of hardwood trees in the US, this foreign invader has few natural predators. The gypsy moth caterpillar is covered in spiny hairs that deter predation. The gypsy moth population historically shows periods of low population followed by several years of population explosions, receding within a few generations to lower levels again. The population of the white-footed mouse has a direct correlation to gypsy moth populations. The white-footed mouse is a major predator of the larvae and as populations of mice increase, populations of caterpillars decrease. Ecologists partly attribute this year’s increase in the gypsy moth population to a decrease in white-footed mice, which has shrunk in recent years as a result of low acorn production by oak trees.
The main reason for the caterpillar population explosion, however, is the drought conditions currently seen across the region. Dry weather decreases the production of bacteria that produce toxins that kill the caterpillars and keep the population in check.
From 1970 to 2010, over 80.4 million acres of trees have been defoliated as a result of the gypsy moth invasion causing millions of dollars in damages. While most trees can withstand some degree of defoliation, repeated seasons of defoliation will decrease seed production like acorns in the oak trees, and can eventually kill the tree. Forest defoliation can also impact the population and reproductive success of forest dwelling birds, whose exposed nests fall prey to predation. Clearly, this pest can wreak havoc on the local ecosystem. One way to help prevent the spread of gypsy moths from region to region is to not transport firewood. Moths typically lay eggs on wood and bringing invested wood from one area to another can have devastating effects and increase the spread of this harmful pest.
