scales, such as the oystershell scale (Lepidosaphesulmi), can infest
viburnum and cause branch dieback. If the infestation is severe enough, they
may kill the shrub.
DESCRIPTION OF THE PEST
oystershell scale can overwinter as full grown females that are attached to the
bark, or as eggs that are beneath the adult scale covering. The adult female is
1/8-inch long, brown or gray, and generally the shape of an oyster shell.
Crawler (immature stage) activity often coincides with the flush
of new plant growth in the spring. These crawlers are pale, smaller than a
pinhead and are the only mobile stage of the scale life cycle. Within a few
hours the crawlers will settle in a suitable spot to begin feeding and excreting
a waxy covering for protection.
Light infestations of scale can be scraped off by hand. Prune
out and dispose of any heavily infested branches. A 2% horticultural oil can be
sprayed in the early spring before new growth begins to kill overwintering
adults and eggs. Horticultural oil may be sprayed when temperatures are between
40 and 85 °F.
Monitor the crawler emergence in the spring with sticky cards,
double-faced tape wrapped around a branch, or by putting an infested shoot or
leaf into a baggie and watching for crawler movement. Spray with horticultural
oil in the spring after the plants begin growing and the danger of cold weather
has passed. Repeat this application after 10 days to better control the
crawlers, adults and eggs by smothering them.
Avoid using more toxic insecticides unless the plant is
seriously damaged from the scale infestation. These insecticides will often
kill the naturally occurring predators of scale. If insecticides are going to
be used, spray when crawlers are observed, as this is the only stage in the
life cycle that is controlled by contact insecticides. Insecticides labeled for
homeowner use against scale crawlers include acephate, malathion, cyfluthrin