If you’re noticing lots of holes in the leaves of your viburnums from late spring through summer, chances are good that you have viburnum leaf beetle (VLB). Native to Europe, VLB was accidentally introduced to North America sometime in the last century. Since then, it has become a significant landscape pest and has discouraged the planting of susceptible viburnum species.
VLB larva hatch in mid to late spring and immediately start feeding on their host plants. Larva are initially greenish-yellow, turning yellow-brown with black spots as they develop, reaching a total length of about 10 mm. Larva usually feed on the undersides of leaves, making them difficult to spot, especially when they are small. Groups of larva will often feed on the same leaf, exacerbating damage further. The tell-tale mark of VLB feeding is “skeletonized” leaves, or in other words, leaves that have been eaten between the major leaf veins, leaving the leaf mid-rib intact.
In late June through early July, VLB larva crawl down the trunk of the shrub to the ground to pupate in the soil. In a very short amount of time adult beetles emerge and begin feeding on viburnum foliage once again, creating distinctive irregular oblong holes in the leaves. In fact, it takes as little as 8-10 weeks to go from hatching to adult. When they aren’t damaging plants, VLB adults are mating and laying eggs on the new growth of viburnum shrubs. Females chew small holes in twigs and lay around eight eggs inside each, sealing up the cavities with a cap made of excrement and chewed bark. Rows of 1-2mm brownish-black bumps on the undersides of new growth are the evidence of VLB egg laying. Adult females can lay hundreds of eggs before the first killing frost.
The most important management strategy is to remove and destroy egg-infested twigs, which are easiest to locate once the plant has dropped its leaves between October and April. Encouraging beneficial insects is also key. Lady beetles, lacewings, spined soldier bugs, and assassin bugs are all known to prey upon VLB and provide a good measure of control. Boost their populations by avoiding the use of broad-spectrum insecticides and maintaining or increasing plant diversity in the yard and garden. Chemical control options should be viewed as a last resort. Some insecticides are registered for use against VLB and are generally most effective if applied when larva are active. Spinosad and insecticidal soap are slightly friendlier options in terms of protecting beneficial insects, but they must come in direct contact with larva to be effective.
Know that some viburnums are more susceptible to VLB than others. Arrowwood (Viburnum dentatum), European cranberrybush (V. opulus), American cranberrybush (V. opulus var. americana), and downy arrowwood (V. rafinesquianum) are especially attractive to the pest and frequently suffer considerable damage. Avoid VLB problems from the start by planting resistant species such as Koreanspice viburnum (Viburnum carlesii), siebold viburnum (V. sieboldii), witherod viburnum (V. cassinoides), and blackhaw viburnum (V. prunifolium).
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