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Larvae, Larvae Leaf Damage and Pupae (Late April to late June)
Q: Viburnum leaf beetle larvae fed extensively on my viburnums this spring. Most of the leaves are brown and shriveled, with just veins left. Will the plants put out new leaves?
A: Viburnums, like most woody deciduous plants, generally produce a second flush of leaves if they are defoliated in the spring. But the plant must have good food reserves stored in the roots to fuel this growth. If this is the first or second year of extensive defoliation, the plants will probably produce a second flush of leaves.
At this point, there is no need to cut back the plants. They may look ugly, but normally this is not a good time of the year to prune the plants. Just allow them to leaf out again. The best time to prune is when the viburnums are dormant during winter and early spring. If you cut back plants then, you'll also be removing egg-laying sites and reducing the number of larvae that will hatch out that season.
If the beetles have defoliated your viburnums several years in a row, however, your shrub may be in trouble. Three or four years of uncontrolled defoliation by viburnum leaf beetle can easily kill susceptible viburnums.
Q: It's late June, and the viburnum leaf beetle larvae are now gone. Are my plants safe for the rest of the year?
A: No! Adult viburnum leaf beetles often feed on the same plants that the larvae feed on. After larvae finish their development, they crawl down the plant and into the soil to pupate. (The pupa is a resting stage between the larva and the adult.) After several weeks, adult beetles will emerge from the pupal cases and crawl out of the soil. (This typically occurs in early July in Upstate New York.) Because both the larvae and adults feed on the leaves of the same plants, it is difficult for the shrub to replenish its nutrient reserves. This is why repeated infestation by viburnum leaf beetle is so devastating to susceptible viburnums.
Q: Is there any way to kill the pupae when they're in the soil, or prevent the adults from emerging?
A: Currently, there is no known way to kill viburnum leaf beetle in the pupal stage. Digging up the soil around infested plants is likely to be more damaging to the roots of the infested plant than to the pupae. Besides, even if you could kill the pupae under your own plants, the adults are good fliers and can infest your shrubs from adjoining properties. It is far more effective to target control against the larvae or adults.
Researchers recently learned that the larvae don't drop from the leaves to the soil to pupate. Instead, they crawl down the tree. While it hasn't yet been tested, it is likely that putting a sticky barrier such as Tanglefoot tree pest barrier around the base of the shrub could keep larvae from reaching the soil to pupate.
Adults and Adult Leaf Damage (Late June to leaf drop in fall)
Q: It's now July, and I see small beetles feeding on my viburnums that were eaten by viburnum leaf beetle larva this spring. Will killing the adult beetles help protect my shrub?
A: Yes. Reducing the population of adults might improve your viburnum's chances of surviving. First, defoliation by adults can impair the ability of the shrub to replenish nutrient reserves in the roots. (See first question above.) Second, controlling adults helps prevent the spread of this pest. Larvae aren't very mobile, but the adults can fly away to other susceptible viburnums where they can lay eggs that will hatch next season.
Q: I've read that pesticides such as acephate, carbaryl, cyfluthrin/imidocloprid, or malathion can be used against viburnum leaf beetle adults. Do these insecticides need to be sprayed directly on the adults to kill them?
A: First, note the most effective means of control for viburnum leaf beetle is pruning and destroying infested twigs after egg laying has ceased in the fall -- anytime from October to April -- or spraying one of these chemicals when larvae first appear in early May. A second application in mid- to late summer on feeding adults may be helpful if damage is excessive. Spraying adults alone is not the most effective management strategy.
Most of these insecticides have some degree of residual activity. (Acephate and imidacloprid have the highest residual activity because they can move into the leaf, remaining active for a longer period of time.) While direct contact is most effective, these pesticides can still control beetles through indirect exposure.
Q: I'd rather not spray pesticides all over my viburnums to kill the adult beetles. Is there another way to effectively protect my shrubs from the beetle?
A: Again, note the most effective means of control for viburnum leaf beetle is pruning and destroying infested twigs after egg laying has ceased in the fall -- anytime from October to April. Research does suggest that soil application of a systemic insecticide such as imidacloprid is highly effective against viburnum leaf beetle. (Systemic insecticides are taken up by the plant and transported to the leaves where the beetles feed.) In fact, we have found this to be the most effective, longest-lived insecticide for control of the beetle.
Please check with local Cooperative Extension office for local restrictions on the application of imidacloprid. In all of New York State, imidacloprid products, including Merit 75WP, are now restricted use pesticides, which means that only certified pesticide applicators can purchase and apply these products. Further, all consumer products containing imidacloprid are banned from sale and use on Long Island (Counties of Nassau, Suffolk, Queens and Kings).
Imidacloprid is applied as a solution of the insecticide in water to the soil around the plant, either by "drenching" (pouring the solution on the soil) or by injection (using a specialized pesticide injector). It is important that the application be made well before the adult beetles emerge because it takes time for the product to be taken up by the plant. Also, soil moisture must be adequate to ensure uptake.
Imidacloprid has a very long residual life in the plant when applied in this manner. It should prevent adults from laying eggs during the year of application (when applied before significant adult activity occurs) and the following year. It should also prevent larval infestation for two years.
Q: I can't find Merit at my local garden center. Is there anywhere else to get this product?
A: A limited number of garden centers carry Merit. If you cannot locate one, you can contract with a commercial pesticide applicator to apply Merit for you. For help finding qualified local applicators, contact your county office of Cornell Cooperative Extension.
Q: I'd rather not use any pesticides to protect my viburnums from viburnum leaf beetle. Are there any non-chemical control options?
A: Again, note the most effective means of control for viburnum leaf beetle is pruning and destroying (by burning or composting) infested twigs after egg laying has ceased in the fall -- anytime from October to April. It's best to wait until after frost when the last adult beetles have died and finished their egg laying so that you minimize the amount of plant material that needs to be removed. Because the eggs that are laid in the summer do not hatch until the following spring, there's no rush.
Several predaceous insects will eat larvae of viburnum leaf beetle. Adults and larvae of multicolored Asian lady beetle and larvae of lacewings will eat considerable numbers of viburnum leaf beetle larvae, and spined soldier bugs will eat both larvae and adults of viburnum leaf beetle. These beneficial insects can be purchased from commercial sources. For more information on these and other beneficial insects, see Biological Control: A Guide to Natural Enemies in North America.
If you have the time, additional control of the adults viburnum leaf beetle could be achieved by knocking adults into a basin of soapy water; hold the basin over a branch, and gently tap. When disturbed, the adults tend to drop straight down rather than fly off. Because they will move in from neighboring areas, however, you will need to perform this ritual often throughout the summer.
Dr. Paul Weston has been testing "soft" pesticides, including soap-based products and horticultural oil. Check out the most recent details at Managing VLB.
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