Lecanium scales—Parthenolecanium spp.
At least five Parthenolecanium spp. occur in California. These and several similar species of soft scales (Coccidae) were formerly named Lecanium spp., hence their common names. The species are now separated into the genera Eulecanium and Parthenolecanium.
Identification
Lecanium scale nymphs occur on leaves and twigs and are oval-shaped and orangish to yellow. Mature females have a dark brown, hump-shaped, oval body and sometimes several ridges along the back. Mature females are 1/12 to 2/5 inch in diameter and usually occur on young twigs. On evergreen hosts mature females can occur on both leaves and twigs.
Males of most species are uncommon or not known to occur. Male nymphs form elongate, waxy puparia (hardened skins) within which they mature into adults. Adult males have a reddish body, one pair of translucent to whitish wings, and two filaments projecting from their rear end.
European fruit lecanium. Parthenolecanium corni is the most common lecanium scale in California. It may be a complex of more than one species. European fruit lecanium feeds on a few evergreen species and many deciduous hosts including alder, cottonwood, coyote brush, elm, fruit and nut trees, poplar, and toyon.
Oak lecanium. Parthenolecanium quercifex occurs on oak and other Fagaceae (e.g., chinquapin, Chrysolepis). It is most common on coast live oak. The look-alike European fruit lecanium is generally not found on oak.
Frosted scale. Parthenolecanium pruinosum, sometimes called globose scale, is usually a pest only on walnut. Populations that commonly are innocuous occur on many other deciduous trees. Unlike the other lecanium scales, frosted scale females are covered by a white, waxy powder in spring that weathers away by early summer.
Life cycle
Lecanium scales develop through three life stages—egg, nymph, and adult. Nymphs mature into adults in the spring. Each female then begins laying up to several hundred eggs. Eggs hatch into crawlers (mobile, first instars), which emerge from beneath the female's body during May to July. Nymphs feed mostly on leaves, then in the fall molt into second instars and move to overwinter on twigs. Lecanium scales have one generation per year.
Damage
Lecanium scales suck phloem sap and excrete copious honeydew, which attracts ants, makes a sticky mess, and promotes the growth of blackish sooty mold. Lecanium scales commonly are not abundant, but feeding by high populations over several years can weaken trees and kill twigs and small branches.
Solutions
Prune out heavily infested branches if few in number. Do not prune woody species so extensively that they are damaged.
Conserve natural enemies by controlling ants and dust and avoiding the application of residual (persistent), broad-spectrum insecticides. Many natural enemies help to control lecanium scales. Common predators include lady beetles, such as Hyperaspis spp., Rhyzobius lophanthae, and twicestabbed lady beetle (Chilocorus orbus); brown lacewings and green lacewings; a predaceous sap beetle (Cybocephalus californicus); and predatory seed bugs (Phytocoris spp.). Frequently lecanium scales are heavily parasitized by tiny wasps including Aphytis, Coccophagus, Encarsia, and Metaphycus spp.
If warranted, apply horticultural oil during the early dormant season (before mid-January) or delayed-dormant season (as buds begin to swell). Oil sprays are not recommended on plums and walnuts during the dormant season. If treatment of plum or walnut is warranted and soil moisture during the growing season has been adequate for good growth, apply oil precisely timed during the delayed-dormant period, after buds begin to swell but before shoots begin to elongate. Alternatively, wait until leaves completely open and spray the first-instar scales on foliage.
If spraying during the foliage season, monitor scale crawlers with several traps of double-sided tape wrapped around twigs. Apply oil when monitoring beginning in the spring indicates that most scales are early instars or when crawler abundance declines after it has peaked.
If scale-infested trees will be sprayed and are large or tall, hiring a pest control company with the equipment and experience to effectively spray trees can be a good idea. Discuss in advance with the company how they plan to control your pest problem.
See Pest Notes: Scales and The Scale Insects of California: Part 1: The Soft Scales for more information.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM). |
Oak lecanium females.
Eggs exposed beneath female lecanium scale.
Relative size of adult female lecanium scale, side view.
Crawler-stage scales in sticky tape trap.
Nymphs of lecanium scale, two parasitized (left).
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