If you find your arborvitae looking sickly and covered in black sticky scale, chances are you have Fletcher Scale, Parthenolecanium fletcheri. This soft scale attacks a variety of hosts cedars, arborvitae, yews, hemlocks, and occasionally junipers.
The most commonly seen stage of this insect’s life cycle is the female, who forms an almost hemispherical tan/brown protective scale where she is attached to the stem of the host tree or shrub. It is from this protective cover that she inserts her moth parts in to the phloem of the plant and dines on carbohydrate rich sap. This sugar rich diet results in the scale secreting the excess carbohydrate as sticky honey dew that coats the foliage below the scales. This sugar is then attacked by a sooty mold that turns the foliage black. If the infestation is severe the plant will turn yellow and will lose foliage.
Females are the only known sex that these scales exhibit. Reproduction is by parthenogenesis, without sex.
The female lays as many as 600 eggs in May and these will hatch in June, into crawlers that move a short distance to a feeding site. They remain in this feeding site as a second instar nymph, (developmental stage) until next spring when they rapidly develop into mature females.
Control of this pest is not difficult if your timing is perfect. Treating early in the year with appropriate dormant oil reduces the number of overwintering second instar nymphs. Verdant oil or registered chemical control must be applied when the crawlers are active and exposed. Typically this occurs early June to early July.
Because timing varies due to climate, when Japanese tree lilac, or little leaf linden are blooming, you are a week ahead of crawler hatch. Another simple way to determine crawler emergence is to take several infested branches from your plant, put them in a well-sealed zipper bag and put it on your desk or shelf out of the sun. Check the bag daily and when the foliage appears to have fine orange dust or speck on it they have hatched. Again you will be a week ahead of the hatch in the outdoors as your office should be warmer than the woods.
I.S.A Certified Arborists are industry recognized experts in plant insect and diseases control. Contact your I.S.A. Certified Arborist to see if you have this damaging plant pest.