The battle against pests in Maine gardens is expected to feature no new enemies this year, but the way the state wages the war will be changing.
The winter moth caused major damage last spring to trees in coastal communities in York County, in Cape Elizabeth and Harpswell and on Vinalhaven. This summer, the damage may be less.
“This year, we might have a reprieve,” said Charlene Donahue, an entomologist with the Maine Department of Agriculture, Conservation and Forestry, during a talk this winter. “You can thank the snow and cold of December.”
December is when winter moths mate. The males fly to the flightless females, who wait for them on the trees. But males will fly only when the temperature is above freezing. Last December, there were just two days during the mating period warm enough for winter moth romance.
The moths first appeared in Maine in 2012 (they’ve been in Massachusetts for about a decade); that year, about 400 acres in Maine were affected. By last year, the number had jumped alarmingly to 5,000 acres.
Donahue said in a recent interview that the winter moth caterpillars will begin hatching about when this column appears. So she’ll soon be able to get a count on their numbers in Maine.
At the same time, Donahue will be releasing a parasitic fly as part of a state effort to keep the moths under control. The plan sounds like something straight out of a horror flick: the parasite lays its eggs where the winter moth larvae will ingest them, Donahue said. The eggs then hatch in the winter moth caterpillars, and the fly larvae eat the caterpillars from the inside out.
The winter moth caterpillar attacks hardwoods in late May and early June, and then makes a cocoon that stays on the ground until late November or December, when the bugs emerge as moths. The females climb the trees, the males fly – and the cycle begins anew.
The good news? After the caterpillars stop feeding in late spring, the leaves on the trees will start to grow, and they’ll last for the rest of the summer. The bad news, Donahue cautioned, is that trees that are defoliated for three or four years in succession could die.
Another big story last year was impatiens downy mildew, which was supposed to kill impatiens by June. Fortunately, the disease never arrived as expected.
“It only showed up in a couple of places in Maine,” said Sarah Scally, an assistant horticulturist with the Maine Department of Agriculture.
Some gardeners grew impatiens successfully last year, she said, and in the two places that impatiens downy mildew did strike, it didn’t do so until August or September.
Scally has heard several theories about why the disease did not hit: For one, fewer gardeners planted impatiens, which curbed its spread from plant to plant. Also, many nurseries treated the impatiens to prevent the disease. Finally, last year’s weather was not conducive to the disease.
But, she said, the disease could threaten the plants this year, especially if people go back to planting impatiens on the assumption that last year’s risk was overstated.
TRAPPING FRUIT FLIES
The spotted-wing Drosophila is still in Maine, but it’s not yet causing problems to Maine fruit. That could change by August. The Drosophila has a short life cycle, producing several generations each summer.
Most, but not all, die over the winter; with just a few to mate early in the spring, their numbers are small, thus they don’t eat much fruit. By August, however, several generations have bred, and the exponentially increasing population of flies can do serious damage to fruit.
The state has been setting up traps around Maine to figure out where the fruit flies are. Officials are also researching ways to set up traps that will kill significant numbers of them.
Two other dangerous pests are right on Maine’s border but have not yet entered the state – at least as far as we know.
The emerald ash borer has decimated ash trees in Michigan. It now has been found at two locations in New Hampshire. If it does reach Maine, it could eventually kill most of the state’s ash trees.
Also, the brown marmorated stinkbug, which attacks fruit including apples, has been found in Portsmouth, N.H., just a hop, skip and a jump (or a short stinkbug flight) from Maine. The pest originated in Japan or China, and was first spotted in the United States in Pennsylvania in 1998.
These pests travel to Maine in several ways. Donahue suspects the winter moth got to Maine in the soil when Massachusetts residents transferred plants from their year-round homes to summer homes here.
State officials believe the emerald ash borer traveled on firewood that visitors brought to Maine. For several years, the state has run an educational campaign against importing firewood; now such importation is banned.
Other pests, such as the stink-bug and Drosophila, slowly make their way into the state, flying under their own wing power into ever-widening regions.
Scally said some entomologists have speculated that the cold winter will slow down all of these pests. But no one will know for sure until the season truly gets rolling.
Tom Atwell has been writing the Maine Gardener column since 2004. He is a freelance writer gardening in Cape Elizabeth. Contact him at 767-2297 or at: