Plum and prune-Scale

European fruit lecanium scale (Parthenolecanium corni
San Jose scale (Quadraspidiotus perniciosus)

Pest description and crop damage Lecanium scales are up to 0.2 inch across, reddish brown, and rounded, resembling small helmets or bumps on branches, stems, and the underside of leaves. The crawlers are flat, oval, and pinkish brown. San Jose scale has a tiny white knob in the center with a series of grooves or rings around it. Severe infestations can kill twigs. Large quantities of honeydew are produced, which makes leaves and fruit shiny and sticky. Sooty mold fungus may grow on this, giving the plants a dirty, sooty appearance.

Biology and life history Lecanium scale overwinters as an immature scale on twigs and branches. They resume feeding in the spring, and eggs are laid underneath the scales in May to June. The eggs remain under the scales until hatching in early summer. The young scales, called "crawlers," migrate to the undersides of leaves to feed. Young scales also can be dispersed by wind, rain, irrigation, or by the movement of people and machinery. After 4 to 6 weeks on the leaves, the young return to the stems and twigs to feed, mate, and overwinter. There is one generation per year.

San Jose scale overwinters in an immature state and is black in color. In spring, the tiny winged males emerge and mate with wingless females. "Crawlers" are very small, flattened, and yellow, and move around on bark and foliage before settling down to feed. Young scales also can be dispersed by wind, rain, irrigation, or by the movement of people and machinery. A few days later, they secrete a waxy coating over their body that protects them from pesticides, therefore time pesticide applications when crawler populations are present. From this point, female scales do not move. Crawlers appear during June and July and again in August to September, so there are two generations per year.

Pest monitoring Inspect twigs during the dormant season for scales. Pay attention to weak plants. The crawlers are best observed during May to July with a 10X magnifying glass. Crawlers can be monitored by wrapping a piece of black sticky tape around an infested branch with the sticky side out.

Management-biological control

Larvae of green lacewings, lady beetles, and other insects are aggressive predators of scale.

Management-cultural control

Home Orchardists: Scale can be rubbed off plants by hand with a glove or toothbrush. Major infestations can be pruned off. Tanglefoot, "stickem," or a similar adhesive can be applied around infestations of adult scales to catch the crawler stage. As with aphids, avoid excessive nitrogen fertilizer or water applications, as this favors increases in scale populations.

Management-chemical control: HOME USE

Dormant-season spray

Apply only during dormant or delayed-dormant period. Use enough water to cover all the tree thoroughly including small limbs and shoots.

  • horticultural mineral oil-Some formulations are OMRI-listed for organic use..

Growing-season spray

  • acetamiprid
  • azadirachtin (neem oil)-Some formulations are OMRI-listed for organic use.
  • carbaryl
  • gamma-cyhalothrin
  • horticultural mineral oil-Some formulations OMRI-listed for organic use.
  • imidacloprid-Plum only. Soil drenches may have residual activity in woody plants lasting for 12 or more months. If short-term management is the goal, consider other approaches.
  • insecticidal soap-Some formulations OMRI-listed for organic use.
  • lambda-cyhalothrin
  • plant-derived essential oils-Some have shown efficacy against scale. Some formulations are OMRI-listed for organic use.
  • pyrethrins (often as a mix with other ingredients)-Some formulations are OMRI-listed for organic use.
  • spinosad-Some formulations are OMRI-listed for organic use.
  • zeta-cypermethrin

Management-chemical control: COMMERCIAL USE

Dormant to delayed-dormant spray

If lecanium scale is the main problem, dormant oil sprays are most effective. If San Jose scale is the main problem, use oil plus diazinon.

  • chlorpyrifos (Lorsban 4E) at 1.5 to 4 pints/a + horticultural mineral oil (rates vary; check product label). Apply at dormant or delayed-dormant, but do not exceed one application per year. REI 4 days. Extremely toxic to fish and aquatic invertebrates; avoid spray drift and runoff to surface waters.
  • diazinon (Diazinon 50W) at 3 to 4 lb/a. REI 4 days. Do not exceed one dormant application of diazinon per growing season.
  • diazinon (Diazinon AG500) at 4 pints/a + horticultural mineral oil (rates vary; check product label). REI 4 days. Do not exceed one dormant application of diazinon per growing season. Flowable formulations are preferred when used in combination with oil.
  • horticultural mineral oil (rates vary; check product label)-Use oil at the highest concentration only in the dormant period. REI varies; check product label. Some formulations are OMRI-listed for organic use.
  • methidathion (Supracide 2E) at 3 to 12 pt/a. REI 3 days. Use for scale only during the delayed-dormant period. Do not apply to Red Beaut plums during the delayed dormant stage.
  • pyriproxyfen (Esteem 35 WP) at 4 to 5 oz/a + horticultural mineral oil (rates vary; check product label). REI 12 hr.

Spring and summer spray

If delayed-dormant spray has not been applied, then a summer spray will effectively control scale in the crawler stage. Summer spray is directed at those crawlers that appear in late June, mid-July, and September.

  • buprofezin (Centaur WDG) at 34.5 oz/a. REI 12 hr. PHI 14 days. Do not exceed two applications per season.
  • diazinon (Diazinon 50W) at 1 lb/100 gal water (4 lb/a). REI 4 days. PHI 21 days. Do not exceed one in-season application per growing season.
  • phosmet (Imidan 70W) at 2.13 to 4.25 lbs/a. REI 7 days. PHI 7 days.
  • pyriproxyfen (Esteem 35WP) at 4 to 5 oz/a. REI 12 hr. PHI 14 days.
  • spirotetramat (Movento, Ultor)-Product formulations vary, check label for rates. REI 24 hr. PHI 7 days. Under heavy scale infestation pressure or where difficult control conditions exist, a second application may be necessary.

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