Small black caterpillars on viburnum – Viburnum leaf beetle

(Question)

Hello, I have a viburnum shrub that is about 30 yrs old. Has always done so beautifully with lots of blooms. This year after 3 days all the leaves were destroyed by these very small black caterpillar-like insects. They are about 5mm in length ( like size of rice). They don’t appear to be “hairy” nor do I see any signs of them clustering together on the trunk or branches. Only when I cut a branch, a lot of them clustered to the cut wood. Can the shrub be saved or should I cut it down. Will it affect my other plants?

(Answer)

This is the dreaded viburnum leaf beetle.

The eggs of the beetles overwinter in twigs on the shrub, emerging as larvae in May (today is May 31 2020).  In June, the larvae will migrate down the stems of the plant to pupate in the soil, and adult beetles will emerge in late July. Both the larvae and adult beetles feed on the leaves, and can damage the viburnum extensively during a single growing season, although it may take 2-3 years to kill the plant.

The adult beetles lay eggs starting in late summer in the little holes they make in the plant stems.  The best time to try and interrupt the life cycle of these critters is in the late fall or early spring, by pruning out any of the twigs that are infested with eggs – this is easiest to do once the leaves have fallen.  Put these in a bag, into the City’s yardwaste pickup as opposed to in your home compost, which may not destroy the eggs.  See also Cornell University’s Managing viburnum leaf beetles.

We’ve been asked this question a few times before.  See the following detailed earlier posts:

  • Ask a Master Gardener. Viburnum Mohican “pests”, which provides links to excellent resources that provide details about the beetle’s life cycle and how best to control it.  As well, if you ultimately decide to remove your shrub and want to plant another viburnum, the University of Maryland Extension link suggests several beetle-resistant varieties from which you can choose.
  • Ask a Master Gardener. Viburnum leaf beetle.

Your viburnum has performed so well for you over the last many years, it would be a shame to remove it if you didn’t have to! I suggest that you try and get rid of this pest or minimize its impact on your shrub, by going after egg-infested twigs come this fall and/or early spring.  By late next spring and summer, you should know if this strategy works – and if it does (e.g., if a lot fewer beetles are evident), follow the same steps to control the eggs in the coming years.  However, if during any spring/summer season there is a major larvae/beetle infestation that significantly weakens the viburnum, it may be time to destroy the plant.   As noted above, you could select a beetle-resistant viburnum variety to replace your beloved plant.

Finally, the beetle is not a threat to your other plants – it just munches on viburnum!