Control opportunities exist now for controlling Lecanium scales. The following is more information.
Two species of Lecanium scales are problematic in our landscapes and the next few weeks offer good management opportunities. Fletcher scale (Parthenolecanium fletcheri) affects a wide variety of coniferous plants such as Taxus, arborvitae, junipers, and even Taxodium spp.. European fruit lecanium (Parthenolecanium corni) affects a broad range of broadleaved woody plants such as redbud, hawthorn, dogwood, and oak.
Lecanium scale adults appear like little brown lumps grouped along branches. They feed on plant sap and leave a clear sticky substance (honeydew the scales' excrement) that gets on leaves, cars, etc. A black fungus called sooty mold often grows on it. European fruit lecanium has eggs (1000-3500/female) by May 9 and production occurs from 319-1328 GDD (median=654).
For Fletcher scale, eggs were noted by April 16 and continued from 171-1438 GDD (median=592). There is one generation per year. The crawler (hatchling nymph) stages look like tiny yellow-orange dots when tapped onto a white sheet of paper. Crawlers were seen in our area this past week and are later this year than the normal median crawler observations (between 850-900 GDD). The crawlers will migrate to the undersides of leaves/needles and spend the summer months there then migrate back to the branches to spend the fall and winter. They will resume growth, mate, and lay eggs the following spring.
The settled crawler stage is the best time to control lecanium scales. Many insecticidal products are registered for this purpose. Horticultural oil (1-2% v:v) applications work quite well. Occasionally, a re-application of oil is needed about 3 weeks after the initial application to ensure all crawlers have emerged and settled. Many naturally occurring parasites also control lecanium scales look for tiny holes in the adult scale's cover. Many predators like lady beetles also feed on lecanium scales. Insecticides sprayed well after crawlers have settled provide only limited effectiveness. Soil systemic insecticide applications of neonicotinoids such as imidacloprid (Merit) and Dinotefuran (Safari) work well against lecanium scales if applied in advance.
Information from Casey Sclar, IPM Coordinator, Longwood Gardens