My snowball bushes have been attacked for the second year in a row. The leaves are almost all gone. They only had a couple blooms befroe the leaves and the blooms were eaten. I have identified it as Virburnum leaf beetle from all the larvae on the underside of all the leaves. Brownish black to start but larger ones have the subtle stripes on them that I have seen from images. I have not actually seen a beetle.
The leaves only came out two weeks ago and there is virtually nothing left. I did not spray with horticultural oil in the early spring before the tree started to leaf so instead, I have sprayed twice with vegetable oil/dish soap/water combination and it appears to do nothing to the larvae. I am considering digging them up and getting rid of them before it spreads to something else like my vegetable garden nearby. The bushes are small, 3 feet high, but they look awful already. I may consider compost and mulch and watering to help the bush if I could get rid of the beetles.
Should I spray with Horticultural oil now? If so, do I spray the tree and the surrounding soil? How do I know if I have resolved the issue?
Snowball bush is a common name for a number of plants, including several species of Viburnum. Many species of Viburnum are very susceptible to Viburnum leaf beetle, including Viburnum opulus which is sometimes know by the common name of snowball bush. On susceptible species, the beetle can kill a specimen within two to three years if left unchecked.
The larvae of the beetle emerge from eggs laid on twigs in May, do their damage, and then migrate into the soil in early to mid-June to pupate. The adults will emerge about 10 days later and continue to feed on leaves. The females will lay eggs throughout the late summer into the fall.
Horticultural oil (or dormant oil) works on most pests by smothering overwintering insects or their eggs. It is too late in the beetle’s lifecycle now to be effective. Horticultural oil can burn leaves so its best not to apply after bud break. If you decide to try to keep these plants, you should cut out and destroy any twigs with eggs in the fall, and then apply horticultural oil in early spring once the temperature is consistently above freezing. You can find more information and ideas about controlling this pest from previous responses.
You may find also wish to replace your vulnerable species with a less susceptible or even resistant species of Viburnum. You can find a list of resistant species here.
Viburnum leaf beetle is a specific feeder and will not spread to other plants outside the Viburnum family, so your vegetable garden is safe from these pests.