Because of its ability to defoliate trees year after year, the spongy moth is a significant environmental, economic, and quality-of-life issue. Therefore, infestations and the treatments are taken very seriously. This insect pest is not native to North America and has no natural enemies to keep its population in check. Urban neighborhoods in the northeastern United States, much of the Atlantic Seaboard and Lower Michigan have been devastated by repeated defoliation by the spongy moth.
If sites are left untreated in Minnesota, here is what residents can expect within 5 to 10 years:
- In May/June, caterpillar numbers in any single backyard will reach into the thousands or even millions. Oak trees and other host trees and shrubs will be bare by the beginning of summer. Trees may grow a second set of leaves if they are healthy, but they will be weakened. If the tree is already stressed, as many urban trees are, they may die. If trees are defoliated for a second year in a row, they could die, falling victim to other pests because of their weakened state.
- As thousands of caterpillars feed, there is an audible sound of caterpillar droppings falling to the ground. Like any other excess nutrient, droppings get into runoff water and into the local watershed. Each caterpillar sheds its bristly skin four or five times as it grows, and the skins pile up. The bristles may become airborne and irritate human eyes, skin and respiratory systems. In fact, many people develop a rash if they come into contact with the bristles. It becomes extremely unpleasant to work or play outdoors; the caterpillars will also crawl on houses and may get inside.
- Caterpillars pupate in July, offering some respite to humans. The adult moths emerge shortly afterwards, and thousands of male moths begin fluttering around during the day, searching for females to mate with (female spongy moths don't fly). Females lay eggs, resulting in thousands of unsightly egg masses, visible and hidden, reachable and out of reach, waiting through the winter to start the cycle over. Eventually the moth population will crash, but will re-establish over several years and repeat the devastation.
Treatment at high infestation levels is more costly than the current low level of infestation and is typically funded in part or completely at the local level – often by individual property owners. Homeowners may have the option of not having their property treated once gypsy moth becomes established, but they will be unable to prevent neighbors from participating in local spray programs or using far more toxic pesticides than a public program might use. There is also considerable cost in removing dead trees and a decline in property value because of the loss of mature trees.
These are the effects of individuals living in a spongy moth-infested area, but what about the broader implications? When spongy moth becomes generally established, counties go into quarantine status, meaning that no outdoor household articles may be moved to non-quarantines areas without proof they have been inspected and found free of spongy moths. Individuals moving to new places, loggers transporting pulp wood, and nursery growers shipping their stock are all covered by the quarantine. Quarantines increase regulations, increasing costs to consumers. Minnesota has a responsibility to neighboring states to slow the spread of this pest. Not treating does not mean that we are letting nature take its course; the spongy moth is not a part of Minnesota's natural ecosystem, and is a serious threat to our environment.