Out of sight, but not to be kept out of mind; Invasive Gypsy moths will return

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Rhode Islanders’ memories of last summer’s invasive gypsy moth outbreak would probably make for a great horror movie. My memories of the destruction are vivid from my walks through the usually pristine woods in my hometown of Foster. I remember creepy caterpillar silhouettes against the backdrop of the sky crawling over one another, making the bare tree canopy squirm and quiver. To my horror, I realized what sounded like light rain was actually a chorus of thousands, perhaps millions, of minuscule caterpillar mouths munching away and tiny fecal pellets cascading down through the leaves the voracious insects hadn’t gotten to just yet.

The public’s concern for intermittent gypsy moth outbreaks in Rhode Island also resembles an audience reaction to a horror movie. You’re initially shocked, the shock persists for some time afterward, and then it is forgotten until the movie (or outbreak) appears several years later. Future outbreaks are inevitable, but lack of awareness during periods of low gypsy moth population is a missed opportunity to reduce the severity of future outbreaks and protect Rhode Island’s forest ecosystems.

Unlike many other invasive species (nonnative species that harm the environment, economy, or human health), which seem to spread uncontrollably, the gypsy moths alternate between years of very low and high population called outbreaks. The Minnesota Department of Natural Resources suggests that outbreaks last 1-3 years and occur on 8-12 year cycles. During the 8-12 year inter-outbreak period, the moths are actually well controlled by two pathogens: NPV virus and Entomophaga maimaiga fungus. Gypsy moth populations can drop so low that they are nearly undetectable; however, the controls are imperfect. The pathogens’ spread is highly weather-dependent. In fact, the Rhode Island Department of Environmental Management (RIDEM) has blamed last spring’s drier-than-usual conditions for rendering the gypsy moth pathogens ineffective. When a combination of favorable factors is present, outbreaks can develop.

These controls have certainly been reducing outbreak frequency, but they aren’t reliable and the reduction isn’t sufficient. A severe defoliation every 8-12 years may seem like a long time from a human perspective, but considering the long lifespan of trees and slow development of forest ecosystems, this frequency is still taxing. A gypsy moth caterpillar’s favorite tree is oak; but as we’ve witnessed, they’ll feed on other species during outbreaks. Mature broadleaf species can tolerate several years of defoliation. The same cannot be said though for young trees, diseased and/or stressed trees, and conifers (like pine and hemlock), which are often killed after just one year of defoliation. The US Forest Service (USFS) predicts that forest composition will change due to gypsy moth defoliation, even at current outbreak frequencies, as oak trees are preferentially targeted. In fact, the 2002 RIDEM report “The Forests of Rhode Island” notes that in a matter of 13 years, between 1985 and 1998, the area of timberland in the state classified as oak/hickory (the state’s most dominant type) declined by 10 percent. In the same period, the amount of maple/birch and pine forest in the state increased, which RIDEM attributed in part to severe gypsy moth defoliations during the 1980s.

According to RIDEM, the caterpillars defoliated nearly half of the state’s 400,000 acres of forest cover last spring/summer. The amount of damage this year mirrors that of the last significant defoliation in 1986219,000 acres. Even more defoliation is expected next year, especially if there is another dry spring. Rhode Island’s oak trees, which are irrefutably one of the most ecologically important species in Rhode Island’s forests, are particularly threatened. Oak acorns are an incredibly important food source for several mammals and birds. They supply energy that most plants cannot provide right before wintera time of year when animals most need it. Despite current gypsy moth controls and reduced outbreak frequency, Rhode Island’s forests are still threatened. Since it’s difficult for scientists to predict where and when populations will expand, it’s important that we support and encourage agencies like RIDEM and USFS to closely monitor populations and develop management strategies. There is strength in numbers, so individual homeowners around the state can help fight the invasion during years with and without outbreaks by putting sticky barrier bands around susceptible trees (especially oak) in their yards, spraying the tan egg masses with a 50/50 solution of water and soybean oil, and alerting RIDEM of local outbreaks. By not working to further suppress gypsy moths even when they aren’t as abundant, we risk losing the diversity and function of RI’s future forests, which would be a real horror.

Joseph A. Loffredo of Foster is a junior at URI where he is majoring in Environmental Science and Management.

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