Over the years, North America has received some excruciatingly horrible pests courtesy of the Old World. In ornamental horticulture, few are as voracious as the viburnum leaf beetle. Abbreviated to VLB or, entomologically Pyrrhalta viburni, unlike most pests these small brown beetles feed in both as larvae and adults.
April and May are the months when the eggs the female beetle inserted beneath the bark of twigs hatch. Since the female is a fertile little brute, she could have laid up to 500 eggs to hatch – and she is seldom alone. The emerging tiny greenish-yellow larvae commence to devour fresh young foliage at a speed that belies their size. In their wake, the leaves are tattered and skeletonized.
Having created havoc, the young crawl down branches into the surrounding soil and pupate. This stage lasts but 10 days and then they are ready to emerge as adult beetles. They will remain around until killed off by frosts, feasting on foliage and mating. Just as the wretched viburnums are commencing to recover, they are attacked again, left with tiny holes chewed in the leaves. The damage this time may not be as severe as the initial attack but, after a couple of years of such indignity, the shrub is unlikely to survive. Their nemesis, however, have flown onto to pastures – or at least shrubs – anew.
Native to parts of Europe and Asia, it was first identified here in 1947 on Ontario’s Niagara Peninsular. According to the University of Vermont, breeding populations were discovered near Ottawa in 1978 and by 1996 VLB had reached upstate New York. It is now known throughout the upper New England states along with Pennsylvania and Ohio with every sign that it will spread further afield.
Viburnums, both as native and introduced ornamentals, are highly valued in the garden. Their profuse white to pink blooms, many deliciously scented are followed by brightly coloured berries. The foliage, if not decimated by the beetles, may turn red in the fall, further decorating the garden. Native forms are also valued in natural landscapes. Many cultivars have been widely planted in parks and institutional installations.
Several predators prey upon VLB larvae, notes Cornell University. These include lady beetle adults and larvae, lacewing larvae and spined soldier bug nymphs, while adult lady beetles and adult spined soldier bugs eat the adults.
These are not enough to control the pest though. In the United States and Britain, systemic insecticides have been suggested. Over all the VLB range in Canada, these are unavailable to home gardeners. Most other pesticides, both chemical and natural, also eliminate the predators. Fine grade mineral oil, known as dormant oil and considered a ‘natural’ product when sprayed on egg sites reduced egg hatch 75-80% applied before leaf emergence, researchers report from Cornell University. They also suggest trapping the larvae crawling down to pupate by spreading a sticky barrier such as ‘Tanglefoot’ around the base of the shrub.
Other natural controls such as neem oil and insecticidal soap have been reported as less devastating to predacious insects but must be applied every few days following egg hatch. In Britain, which has been plagued by the pest since 1943 and perhaps causing more damage than the Luftwaffe, the Royal Horticultural Society has added pyrethrum to the list of organic solutions. VLB, it should be noted, leads the RHS top ten garden pests list, beating out even slugs and snails.
Pruning out the egg-infested twigs is considered perhaps the best control. Close examination will reveal tiny black-brown bumps where females laid their eggs the previous fall and covered them with a mixture of chewed bark and excrement. Do not compost these trimming but dispose of them in tightly tied plastic bags.
The problem with such pruning is it removes the flower buds and subsequent berries. An alterative is to replace the demolished species with more resistant forms. Those with harder, less succulent leaves perhaps further offputting by being fuzzy or hairy. Highly recommended in this group are the Burkwood viburnum
V. burkwoodii, Koreanspice
V. carlesii, Judd viburnum
V. judii, Japanese snowball V. plicatum, and Siebold viburnum V. sieboldii.
Unfortunately, at the other end of the spectrum those particularly susceptible to VLB attack include some old and well-established favourites. These include the mapleleaf viburnum
V. acerifolium, arrowhead viburnum V. dentatum, wayfaring tree V. lantana, nannyberry
V. lentago, European highbush cranberry V. opulus, American highbush cranberry V. trilobum, and Sargent viburnum V. sargentii. The past few years have seen many highly promoted new cultivars hit the market. Until proven resistant, they should be treated with some suspicion.
