Viburnum is one of the most ubiquitous shrubs in American landscapes, and it’s No. 1 on the menu for a nasty little bug called the viburnum leaf beetle. First spotted in the U.S. in Maine in 1994, viburnum leaf beetle (VLB; Pyrrhalta viburni) has established itself in many areas of the East Coast and the Southeast, and it appears to be moving steadily west. Areas of the Midwest, including Illinois, Michigan, Minnesota and Ohio, have been affected by VLB, with relatively new confirmations in Cook and DuPage counties in Illinois. It’s more than likely that humans are aiding and abetting its spread, even if they’re doing so unwittingly. Although the pest is not a good flyer, it gets around by hitchhiking on plants that are being transported for sale or for transplantation.
Viburnum leaf beetle is a rather nondescript little pest; the adults are brown and about one-quarter-inch long (females are slightly larger than males). They often can be found on leaves, but will fly if disturbed.
Eggs, although tiny, may be easier to identify, as they’re deposited in straight lines along the underside of young branches. Rows of small bumps (comparable in size to the head of a pin to the head of a match) protect the eggs, which the females have laid in cavities and have covered with a “cement” composed of excrement and chewed bark.
Larvae emerge in late spring or early summer. First instar larvae are greenish yellow to off white, and about 1 mm long. Second and third instars appear yellowish brown with black spots along their backs, and may grow to about one-half-inch long. Pupae are nearly impossible to find; during late June through early July, larvae crawl down plants to pupate in the soil.
Both larvae and adults cause damage to leaves. In late April to summer, larvae feed almost exclusively between leaf veins, leaving foliage skeletonized. Damage caused by adults occurs from late June through the first killing frosts; irregularly shaped holes sometimes cross the vein while leaving it intact.
The damage can appear primarily aesthetic, although a significant infestation can cause defoliation. If plants are defoliated two or three years in a row, it is likely they’ll die.
Tom Tiddens, supervisor of plant health care for the Chicago Botanic Garden in Glencoe, Illinois, says he has received reports from a neighboring community, where a few properties have experienced complete defoliation of viburnum. “You can’t look at a home in this area that doesn’t have viburnum,” he states, adding that the Chicago Botanic Garden is home to nearly 3,200 of the shrubs.
Economic value of Viburnum
There currently are more than 150 species of Viburnum in production in the U.S. According to a report from Clemson University (citing 2009 statistics from the USDA), more than 4.5 million evergreen and deciduous viburnum plants were produced by nearly 1,600 U.S. growing operations. Wholesale revenue from viburnum sales in 2007 reached nearly $41 million. The shrub is the quintessential bread-and-butter plant for U.S. landscapes.
Economic impact of the loss of viburnum could be catastrophic. However, not all species are susceptible to damage from the viburnum leaf beetle. According to Dr. Paul Weston, woody plant entomologist at Cornell University, species can be “ranked” according to their vulnerability. Highly susceptible plants are the first to be affected, and are likely to be destroyed within the first two to three years following infestation. Susceptible species are the second-stringers; they’re not necessarily the targets until the highly susceptible species have been devastated, but they’ll eventually fall victim. Other species are considered either moderately susceptible or resistant.
Following are the plants considered most likely to attract viburnum leaf beetle:
- V. dentatum
- V. nudum
- V. opulus
- V. propinquum
- V. rafinesquianum
- V. acerifolium
- V. lantana
- V. rufidulum
- V. sargentii
- V. wrightii
It’s fortunate, however, that there are at least 13 species that are considered moderately susceptible, meaning that infestation by VLB is unlikely unless all other options are unavailable. These include V. alnifolium, V. burkwoodii, V. × caricephalum, V. cassinoides, V. dilatatum, V. farreri, V. lantanoides, V. lentago, V. macrocephalum, V. × pragense, V. × rhytidophylloides and V. tinus.
Those viburnum considered most resistant to VLB include V. bodnantense, V. carlesii, V. davidii, V. × juddii, V. plicatum, V. plicatum var. tomentosum, V. rhytidophyllum, V. setigerum and V. sieboldii.
“Scouting and monitoring are the cornerstone to management,” says Tiddens. “Distinctive eggs appear on the stems of viburnum; adults will lay eggs in a straight line on stems that are just bigger than the diameter of a pencil. Pruning out those branches can help a lot.”
VLB is easier to kill at the larval stage, when they’re easier to spot. “Biorationals such as spinosad or pyrethrin can be used on larvae,” Tiddens explains. “Or give beneficials a chance; they’ll go after the larva.”
Once viburnum leaf beetle is established in an area, “designers should be looking at some of the least susceptible species,” Tiddens concludes.
Photos courtesy of Paul Weston, Cornell University; Bugwood.org