viburnum leaf beetle

(Question)

Thank for answering my last question so swiftly.

My viburnum had the viburnum leaf beetle this spring so I cut off all the branches and left a tree truck, basically. I tried to squish as many beetles as I could before they had a chance to go underground. Now I notice that some leaves are growing back. I was prepared to dispose of this tree in the fall and replace it with one that is not susceptible to this disease. What do you recommend I should do?

(Answer)

The eggs of the Viburnum leaf beetles overwinter in twigs, and hatch in May as larvae.  Given the timing, it is the larval stage of this insect that you have observed.  The larvae feed on the leaves of the plant, and in June, move into the soil to pupate as you have observed, with adult beetles emerging in late July.  Female beetles lay their eggs from late summer until first frost by depositing them in holes they chew in small twigs on the plant.

Ideally the life cycle of this insect should be stopped, or slowed, in the late fall or early spring, before the larvae hatch, by closely examining the small twigs from last year’s growth, checking for egg sites, and pruning out the affected wood.  The appearance of these egg sites is described by the Ontario Ministry of Agriculture here.  On your Viburnum, this season’s larvae are now making their way into the soil around the plant.  The Ontario Ministry of Agriculture also notes that two to three years of defoliation may kill an affected Viburnum.  A previous Toronto Master Gardeners post on this subject,  http://www.torontomastergardeners.ca/askagardener/killer-bugs/ provides more detail on the life cycle of this insect pest, and also notes some of the insect predators of the Viburnum leaf beetle.

Certain species of Viburnum are more resistant to the leaf beetle than others.  Here is a website that lists these: http://www.hort.cornell.edu/vlb/suscept.html  You don’t say which species of Viburnum you have.  If yours is susceptible, you may want to consider replacing it with another plant entirely, or with a Viburnum from the list of resistant varieties.  In either case, it is advisable to clean up by removing all dead wood or leaves and putting these into the City’s pickup rather than into your home compost which will not heat up sufficiently to destroy any pupae or emerging adult beetles.