Zachary Frederick from Kildeer inquired about unusual white spots on his pachysandra bed. Upon examining the photo, it appears the pachysandra is infested with euonymus scale, marking the first instance of this pest on such plants. Euonymus scale damages the host by feeding on plant sap with its piercing-sucking mouthparts. The pest is challenging to control because adults form protective coverings that render insecticides ineffective.
Control efforts should commence when the infestation becomes large; however, immediate action may not be necessary. Female scale insects overwinter on plants and lay eggs in late April and May. Eggs hatch into crawlers over a period of two to three weeks. Males feed under a narrow white coating and mate with females in the spring. The crawlers wander briefly before feeding and developing a waxy cover. A second crawler generation emerges from late July to August.
Prioritize pest control during the crawler stage. In March, prune the infested areas to eliminate overwintering females. Dispose of plant debris through garbage or landscape waste facilities where compost piles heat sufficiently to kill insects. Crawler activity in late May to early June and late July to August provides opportunities for applying control measures. Removing overwintering adults can significantly diminish pest populations.
Applying a summer-weight dormant oil to pachysandra during crawler stages further controls the pests. For expert plant advice, contact the Plant Information Service at the Chicago Botanic Garden via [email protected]. Tim Johnson serves as the senior director of horticulture at the Chicago Botanic Garden.