Walnut-Scale insects


Frosted scale (Parthenolecanium pruinosum)
European fruit lecanium scale (Parthenolecanium corni)

Pest description and crop damage The frosted scale is the most serious soft scale pest of walnuts. Mature scales are brownish, convex, and covered with frost-like wax. Fine waxy filaments may protrude from the base of the scale body. European fruit lecanium scale is similar to frosted scale but lacks the frosty wax coating. These insects suck plant juices, causing loss of vigor and potentially reducing nut yield and quality. They also produce copious amounts of honeydew, which can result in sooty mold buildup and decreased photosynthetic activity in trees. Low to moderate populations may be tolerated, but high populations can be damaging. In Oregon, insecticides are rarely used against scale but if needed would typically target the crawler stage, which tends to emerge in late-spring. Conditions permitting, a dormant or delayed-dormant treatment can also be used.

Biology and life history The frosted scale overwinters as a nymph on twigs and small branches. In spring it grows rapidly, becomes convex, forms a frostlike waxy cover, and secretes large amounts of honeydew. In late spring females lay many eggs, which fill the entire space beneath their cover, and die after egg production. The white waxy substance weathers away, leaving oval, dark brown covers that may be present for a year or more. Newly hatched nymphs or crawlers emerge from beneath the scale cover in late spring and settle mostly on the underside of leaves. Here they feed for the rest of the summer. In fall, the nymphs molt and move back to twigs.

Management-biological control

Parasitoid wasps typically keep populations under control and they attack all life stages. Avoid broad spectrum insecticides that may impact scale natural enemies and flare populations. Monitor parasitoid activity and time treatments to avoid interfering with them. Look for small emergence holes from nymphs and mature females and uncharacteristically dark nymphs that indicate that wasps are developing inside.

Management-cultural control

On small trees it may be possible to prune off some of the scale colonies. Control ants if they are abundant, as ants will protect scale from biological control in order to 'farm' scale for honeydew.

Management-chemical control: HOME USE

Control has not been necessary in the PNW for home orchards. This pest is held in check by natural factors. If control is needed, 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.

  • dormant oil

Growing-season spray

  • azadirachtin (neem oil)-Some formulations are OMRI-listed for organic use.
  • bifenthrin
  • carbaryl
  • horticultural oil-Some formulations are OMRI-listed for organic use.
  • insecticidal soap-May require several applications to be effective. Some formulations are OMRI-listed for organic use.
  • lambda-cyhalothrin (often as a mix with other ingredients)
  • plant-derived essential oils (clove, garlic, peppermint, rosemary oil, etc.)-Some formulations are OMRI-listed and have shown efficacy against scale.
  • permethrin
  • pyrethrins (often as a mix with other ingredients)-Some formulations are OMRI-listed for organic use.
  • zeta-cypermethrin

Management-chemical control: COMMERCIAL USE

Control has rarely been necessary in the PNW. This pest is held in check by natural factors. If control is needed, use:

  • buprofezin (Centaur) at 34.5 to 46 oz/A. One application per season. Target crawler stage. PHI 60 days. REI 12 hr.
  • clothianidin (Belay) at 0.05 to 0.1 lb ai/A. Maximum 0.2 lb ai/A. Retreatment 10 days. PHI 21 days.
  • methidathion (Supracide 25W) at 11 to 12 lb/A. PHI 7 days. REI 2 to 14 days, depending on rate. Do not tank-mix with oils, which can injure trees. Do not graze. Liquid formulations are also available.
  • methoxyfenozide (Intrepid 2F) at 0.12 to 0.25 lb ai/A. PHI 14 days. REI 4 hr. Apply at first sign of larval infestation. Do not exceed 24 fl oz/A per application or 64 fl oz/A (1 lb ai/A) per season. Do not apply within 25 ft of an aquatic habitat, 150 ft if applied by air.
  • pyriproxyfen (Esteem 35WP) at 4 to 5 oz/A. PHI 21 days. REI 12 hr. Do not exceed two applications per season. Note ai available under additional labels.
  • spirotetramat (Movento) at 6 to 9 fl oz. PHI 7 days.