This is the second of three articles about the gypsy moth, one of the most destructive pests ever to arrive in North America. In the previous article, we introduced Étienne Léopold Trouvelot, who deliberately introduced gypsy moths in Massachusetts, then carelessly allowed them to escape. Trouvelot’s folly was immediately followed by a folly of a different sort – an attempt to exterminate the moth instead of managing its spread. Today’s story is about current management of gypsy moth. It is, to a large degree, as success story.
The gypsy moth front has continued to move from its origin in Massachusetts. The animated map of its spread since 1900 shows a gradual expansion, especially to the south and west. It has also spread into the Atlantic provinces of Canada, but its northward expansion is limited by lack of the favorite food plant of the moth – oaks.
The spread of the gypsy moth has definitely slowed down, and it only occupies about 1/3 of the range that it could. The abundant oak-hickory forests of the Midwest have so far not been much affected.
There are multiple factors that are slowing the spread of the moth, and that may provide some comfort for understanding other invasive species. Every invasive species is different. The factors that limit the spread of gypsy moth may not apply to emerald ash borer except in a general way.
Let’s look at the spread of gypsy moth in recent decades. There are several key factors that are slowing the moth and a few that accelerate its spread. The principle accelerator of gypsy moth spread is the motor vehicle. Female moths commonly lay eggs in any overhang, whether tree bark, shingles or the underside of cars and trailers. Vacationers who camp in infested areas during the breeding season are likely to take the moths home.
Spread of the gypsy moth is slowed by several features of its biology and of control strategies. They include
- The female moth cannot fly. Female caterpillars spread by ballooning – climbing to the top of a structure, attaching a silk line to the top of the structure and jumping into the wind. Males can fly.
- Prevailing winds are in the wrong direction. Our winds are primarily from the west. This slows the rate of spread of female moths, though not of males. Since laying eggs is how gypsy moths gain a new foothold, the males don’t really count.
- Pests and Pathogens. beginning in the 1970s, several biological control methods introduced: a virus called NPV, the bacterium Bacillus thuringiensis and a fungus,Entomophaga maimaiga . The fungus was introduced from Japan but has been spreading naturally. NPV along with a parasitic wasp are effective but only limited supplies are available.
- The Slow The Spread program (STS), a concerted effort by State and Federal agencies to work at the front of the expansion zone to reduce the rate of spread. Slow The Spread uses gypsy moth sex pheromones to bait traps to monitor for males, and to spray in carefully targeted areas to confuse mate-finding and prevent reproduction. STS also uses a small group of insecticides that are highly selective and have low toxicity. Slow the Spread uses traps in the Uninfested Zone to scout for male moths, uses a combination of traps and spraying of biological and chemical agents in the Transition Zone, and provides limited support for suppression in the infested zone.
There are also important environmental and biological factors that slow the spread of gypsy moths. Like many forest insects, gypsy moths go through boom and bust cycles in their populations caused by complex, and not completely understood, factors.
The lesson from the relative success of control methods is that control can be effective in slowing the spread of an invasive species, but probably not of eliminating the pest completely. The other lesson is of more concern: the success of Slow the Spread clearly depends on funding. In the annual reports of the program, it is clear that as funding has declined, its success in slowing the gypsy moth has declined. Nearly every Federal natural resource management program suffers from declining funding, but the failure to fully fund STS is having a dramatic effect on the spread of gypsy moth.
From a historical perspective, it is interesting to speculate about whether the gypsy moth could have been stopped or slowed down much earlier by taking a control approach rather than wasting time and money on unsuccessful eradication programs. We will never know.
In the last installment we will discuss the newer threat, the Asian gypsy moths. Although they are the same species as the European gypsy moth we have been discussing, they have one big advantage: the females can fly. As we will see, the Asian gypsy moth is having a large economic impact on international shipping.