Lymantria, Tramps, and Thieves: The Moth Wreaking Havoc on Northeastern Forests

LD moths* (Lymantria dispar) first came to North America in the late 1800s in Boston Massachusetts. Etienne Leopold Trouvelot, a french astronomer and naturalist, imported them from Europe in an attempt to find a new source for silk (1). Some eventually escaped his home and only ten years after their release, LD moths were already causing major problems. They have subsequently made their way across the northeast US and southeast Canada via the movement of firewood, vehicles, and other natural and anthropomorphic means. Occurring New England to South Carolina and as far west as Minnesota, L. dispar has been shown to spread on average 5 miles per year (2). Cyclical outbreaks occur every 8 to 12 years and can last for several years (3). LD caterpillars are generalist herbivores but favor oaks and poplars among other trees. They can cause major defoliation events, averaging 3 million acres of defoliation per year (4).

Caterpillars of LD moths are black with stiff bristle-like hairs characteristic of subfamily Lymantriinae, also known as the tussock moths. Because of their thick hairs, LD moths are unpalatable to most birds. Blue jays, Orioles, Cuckoos, and a few others have been known to prey on LD caterpillars. Chickadees feed on LD moth egg masses. Many small mammals including chipmunks, skunks, and raccoons also eat LD caterpillars and pupae (5). Mice are one of the main predators of gypsy moth caterpillars. Decreases in mouse populations have been shown to correlate with LD moth outbreaks (6).

Male and female LD moths display sexual dimorphism, meaning they look different. Males are dusty brown with feathered antennae, females are flightless and white. After pupating, females emerge from their cucoons, climb up high, and begin releasing pheromones to attract males. Female LD moths typically mate once per year and can lay hundreds of eggs. Their eggs are covered in a dense brown layer of fuzz which protect their eggs from predation.

There are two subspecies of LD moths, one from Europe and the one from Asia. Outbreaks in North America are mainly the European subspecies. The Asian subspecies, with the help of management efforts, has yet to establish in the US. The major difference between European and Asian subspecies is that females of the asian variety are capable of limited flight. If the asian subspecies establishes in the US, the yearly spread of LD moths could increase greatly as females could travel miles away from their place of birth to lay a clutch of eggs (7).

Efforts to manage and prevent the devastation caused by LD outbreaks have had varied success. Mating disruption and pesticides are the most common forms of management. Organizations implementing these techniques have to take into account effectiveness, costs, and potential effects on non-target organisms.

Male LD moths locate females through pheromones. Because of this, mating can be prevented through use of chemicals which mimic female pheromones. Males are attracted to the synthetic pheromones, trapped, and killed, or simply become too disoriented to locate mates. Mating disruption is typically more successful at low densities as males can also locate females by sight or chance at high densities. No non-target impacts have been observed using this method as only LD males are attracted to the pheromones (8). Traps containing synthetic LD pheromones are available for purchase or can be made easily at home (see this video).

Pesticides used against gypsy moths may be chemical or biopesticides. Many are applied aerially and effects are closely monitored by the USDA or other government organizations. The effectiveness of pesticide applications can be effected many factors such as temperature, precipitation, population density, and proximity to other populations. For this reason, different techniques are used depending on situational factors.

One of the more common chemical pesticides used to manage LD moth infestations is Diflubenzuron, an insecticide that effects insects ability to molt. It has been shown to be highly effective in reducing LD moth populations however, the risks of negative effects to non-target organisms including a variety of other insects and aquatic invertebrates are fairly high (8).

Bacillus thuringiensis (Bt) is a bacteria which is commonly used against lepidopteran pests. Bt forms crystalline protein structures in the digestive tracts of insects when ingested, damaging the gut and ultimately killing the insect (9). While effective against LD moths, Bt has been shown to negatively impact non-target caterpillars, resulting in secondary impacts for birds and other organisms that feed on caterpillars (10). Several pesticides containing Bt are commercially available.

Entomophaga maimaiga is a fungal pathogen originally native to Japan. It can now be found throughout the range of LD moths. E. maimaiga is largely specific to LD moths with little non-target effects. The fungus can spread through populations, particularly in areas with high moisture or rain levels. Fungal spores release from carcasses of infected caterpillars and may last overwinter in soil (11).

Gypchek is a pesticide containing a virus that causes “wilt” in LD moths. The virus is highly specific to LD moths and therefore unlikely to effect other organisms. Gypchek is recommended for use in sensitive areas, such as those home to threatened or endangered lepidopteran species. It is also useful for managing outbreaks as the virus can pass from moth to moth in high density situations (10). As long as the moths don’t start wearing tiny masks or develop tiny Gypchek vaccinations, it is an effective management technique.

Many of these pesticides are not commercially available or may require a pesticide applicators license in order to use. Many are most effective when applied aerially and are therefore not practical for the average homeowner trying to manage LD moths on their property. However, there are a few things homeowners can do to minimize LD moth impacts.

Egg masses can be located on tree bark and structures such as picnic tables and siding. In late summer or fall, scrape the egg masses off into a container using a knife. Burn the egg masses to prevent the larvae from emerging in the spring. In spring, duct tape can be applied sticky side out to the bases of trees. The sticky tape will stop caterpillars from climbing up trees to munch on the leaves. Tape may need to be reapplied periodically, and may not be practical on large properties. Oaks, Poplars and other favorites of LD moths should be prioritized. For smaller trees, there are many commercially available pesticides that can be applied to prevent LD moth damage. Also, you can apparently just vacuum them off?? (See this video.)

The spread of LD moths is cause for concern. Defoliation of trees has major economic and ecological impacts for communities across their distribution. Defoliation impacts native caterpillar species as it reduces the amount of available food resources. This further impacts organisms that depend on the caterpillars as a food source, or as pollinators later in their development. Moths also serve as vectors to spread tree pathogens like oak wilt, further impacting forests. With the effects of global warming and changes in climate, outbreaks of LD moths may be more difficult to control and predict. For more information on what can be done to help stop the impact and spread of these moths, visit the USDA website.

*LD moths are more commonly known by another name (Cher fans, peep the title of this post) however recently that name, like so many other common names, has been re-thought and deemed not kind nor politically correct. I will therefor follow iNaturalist’s lead in referring to these moths as LD moths, rather than the more frequently used common name.

Source: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11