ITHACA, N.Y. -- The eggs of the viburnum leaf beetle have hatched, and the larvae are beginning to chow down once again on viburnum shrubs in New York state and New England. Without the use of pesticides to protect susceptible varieties, the bushes are doomed, says an entomologist at Cornell University, New York state's land-grant institution.
"This pest is going to be with us from now on, and it will definitely kill those varieties of viburnum that are susceptible," says Paul Weston, a Cornell senior research associate who specializes in pests that attack woody ornamentals.
The beetle devastated large swaths of native and imported viburnum species last year, and it is expected to do so again this year. It feeds during its larval stage until early or mid-June and then returns in July as an adult for a second helping.
Weston says he has found that the most effective pesticide against the pest is a soil application of Merit 75 WP, which this year became a restricted-use-only pesticide in New York. That means that only certified pesticide firms can legally purchase or apply the product. Merit has a long residual life in the shrub and will protect the ornamental bush through the entire season and probably the following year as well when applied as a soil drench.
"Consumers can apply pesticides, such as malathion and Sevin, themselves, and while these products are quite effective, they will only provide short-term protection, and other applications would be necessary later in the season," says Weston. He is conducting several studies of biological control and other minimally toxic control methods this growing season to see which are most effective for future infestations.
Not all viburnum bushes are susceptible to the beetle, Weston says. A number of species are nearly immune to the pest. A susceptible viburnum, however, can survive only up to three years of double-whammy defoliation, but after that, the bush is likely to die, he says.
To determine if particular species of viburnum are vulnerable, the Cornell Web site http://www.hort.cornell.edu/VLB provides information to help consumers determine which varieties they have and their susceptibilities to the beetle. It includes detailed images of the beetle and of the bush it loves to eat, as well as extensive information on updated sightings, the life cycle of the beetle and tips on how to control it. It also provides information on the Viburnum Leaf Beetle Citizen-Scientist Project, an outreach effort that asks citizens to report sightings and other observations.
The leaf beetle, which already has invaded 39 of New York's 62 counties as well as Maine, Vermont, New Hampshire, Pennsylvania, Massachusetts, Connecticut and northeastern Ohio, will eventually spread to the mid-Atlantic states and to many states across the northern half of the country, Weston says.
Young larvae begin as pale greenish-yellow, which gets patterned with black dots as they grow. Both larvae and adults are devastating to plants. An adult female can lay up to 500 eggs, and when the larvae hatch in late April or early May, they feed on the viburnum leaves throughout the larval period, which lasts four to five weeks. After a subterranean stint, they reappear as adults, usually in mid-July, and consume the second growth leaves. They then mate and lay eggs on the shrub's twigs. Adult beetles (Pyrrhalta viburni) are hard to see, resembling small, dark-brown blotches about the size of the head of a large matchstick.
"I will be doing survey work for viburnum leaf beetles in some of the eastern counties to determine if this invasive pest has yet invaded these areas," says Cornell entomologist E. Richard Hoebeke, who has been tracking the beetle since its first public appearance in New York in July 1996 at Fair Haven Beach State Park in northern Cayuga County.
The economic impact of the beetle on the ornamental agriculture business is unknown, but a continuing infestation will severely affect the native woodland species, such as arrowwood (Viburnum dentatum), Hoebeke says. The fruit of viburnums, such as the cranberry bush viburnum (V. trilobum), is an important food source for birds. Viburnum also plays an important role in woodland ecology, providing a niche in the understory where mammals forage and birds nest.