Pests from both the plant and animal kingdoms are continuing to invade our local gardens, and professionals in the industry have been getting reports at their winter meetings. All of the news is not bad, however. Some problems have eased over time, and some are not as bad as people initially thought.
One big problem is Boxwood Blight, which has not been found in Maine yet but has made it to Massachusetts and is likely to arrive in Maine soon.
Bruce Watt, a plant pathologist with the University of Maine Cooperative Extension in Orono, told professional landscapers meeting at O’Donal’s Nursery in Gorham that the blight is a fungus that first shows up as brown cankers on boxwood leaves and defoliates the plant. The blight also infects the stems, causing dark brown or black lesions.
So far, the blight has been found to damage all varieties of boxwood, and there is no known control.
“You can’t eradicate it,” Watt said. “I would be hesitant to recommend that people plant boxwood in the future.”
The blight was first discovered in the United Kingdom in the mid-1990s. It is not known how it came to the United States.
I wrote in January about the spotted wing drosophila, European crane fly and marmorated stink bug, and they continue to cause concern.
But some pests that were problems in the past are beginning to come under control, said Richard Casagrande, a professor of entomology who specializes in biological controls.
Casagrande told a class last month at New England Grows in Boston that he has been having some success releasing wasps that kill the lily leaf beetle, and that two populations of them have established in Maine. The Tetrastichus setifer has colonized around Orono, and Diaparsis jucunda has been established in southern Maine.
But some gardeners might have to change their usual gardening practices. It still will take time for those colonies to expand their range to the rest of the state.
“Mulching the lilies is not good for the beneficial wasps,” Casagrande said.
If you don’t think biological controls work, think back to the 1970s and 1980s, when the gypsy moth caterpillar was decimating Maine’s softwood forests.
“Then in 1989, a fungus showed up in the population which controlled the gypsy moth and spread over the entire range,” Casagrande said. “When we have a drought, we will have localized outbreaks of gypsy moth” because the fungus does not do well in dry conditions.
Another pest that has made it to New Hampshire but not yet Maine is the mile-a-minute plant, Persicaria perfoliata, a barbed trailing vine. This vine grows in disturbed ground and in ditches next to roads, and will climb and smother all vegetation in its way.
“A weevil native to China has been quite effective where it was released in Delaware,” Casagrande said.
He noted that biocontrols don’t always work, but when they do, they are more cost-effective and less harmful to the environment than using chemical weed killers.
The biocontrols have to go through a long period of testing before they can be released. The scientists want to know that the predator insect or fungus will control the pest, and that it will damage only the pest. The worst thing that could happen would be for something to be released that would hurt native species.
Swallowwort — an invasive vine that looks a bit like morning glories, is related to milkweed and develops pods that look like milkweed pods — has been around for years, but has more problems than I thought. Yes, it will entwine among plants that you like, but it is also harmful to Monarch butterflies, which require milkweed in the reproduction cycle.
“Butterflies lay their eggs on swallowwort, but they don’t survive,” Casagrande said.
Lois Berg Stack, an ornamental horticulture specialist with the University of Maine Cooperative Extension in Orono, reported at New England Grows that Japanese Stilt Grass is another invasive that has reached southern New England. It looks like many native grasses, but is more aggressive, survives in full sun to deep shade, and grows in disturbed soil and along stream beds.
The best way to control it, she said, would be to find it wherever it first reaches the state, watching closely at areas where it is likely to grow, and remove it before it can spread.
“Eradication is less effective than prevention,” she said. “We try to practice early detection and rapid response. We have to be diligent.”
Berg Stack did outline ways to remove invasives when they arrive, including chemicals, weed wrenches and smothering with plastic, mulch and sheet vinyl. But it is easier to keep pests out in the first place.
Jeff O’Donal, speaking at the same meeting as Watt, said he is confident that the hemlock woolly adelgid will not decimate Maine’s hemlocks, although it has decimated the hemlock populations in Connecticut and other places farther south.
“If you looked at those trees even before the adelgid, they didn’t look healthy,” he said.
And while the adelgid has been found in southern Maine, most of the infected trees are healthy enough to fight it off.
Another bit of hope, O’Donal said, is that the viburnum leaf beetle that hit with a vengeance about 15 years ago is not doing as much damage as it has in the past, even on varieties of viburnum that seem to be susceptible to it. Either something has arrived to keep the beetle under control, or the beetle is just less prevalent.
Tom Atwell has been writing the Maine Gardener column since 2004. He is a freelance writer who gardens in Cape Elizabeth, and can be contacted at 767-2297 or at: