Pit scales—Asterolecaniidae family
Pit scales are named for the roundish depression that commonly forms in plant tissue where each scale nymph feeds. At least eight species occur in California. Most damaging are Asterodiaspis (=Asterolecanium) species of oak pit scales that feed on and can severely damage Quercus species. Pittosporum pit scale, Planchonia (=Asterolecanium) arabidis, damages and prefers ceanothus and Pittosporum tobira, but has a long host list.
These scales are circular to oval and immobile except for briefly when they are recently hatched first instars (crawlers). Mature pit scales of most species are 1/10 to 1/20 inch in diameter. Pittosporum pit scale including its marginal wax fringe can be up to 1/5 inch long.
The pit scales' color can be brown, gold, green, tan, yellow, or whitish. Generally the most distinctive feature is their damage, the round depressions (pits).
The particular species can commonly be discerned by the hosts on which they occur. Agave pit scale, Asterolecanium agavis, in California has been reported only on Joshua tree, Yucca brevifolia. The large pit scale, Asterolecanium grandiculum, feeds on various Agave and Yucca species. Bamboo pit scale, Bambusaspis bambusae, most commonly infests bamboos (including Bambusa, Dendrocalamus, and Phyllostachys species) and is generally innocuous on its various other hosts.
Euphorbia pit scale, Planchonia (=Asterolecanium) stentae, infests numerous plants in the Asclepiadaceae and Euphorbiaceae families. Euphorbia pit scale and the bamboo pit scale closely resemble the pittosporum pit scale, but lack the lengthwise fringe of white wax on top the body and at maturity are less than one-half as large as pittosporum pit scale. Euphorbia pit scale in California has been reported only in commercial nurseries.
Pittosporum pit scale prefers ceanothus and pittosporum. Bush anemone, chaste tree, hebe, privet, and other plants can also be infested. See the U.S. Department of Agriculture's ScaleNet database for a complete list of the hosts of these scales.
Pit scales develop through three life stages: egg, nymph, and adult. Nymphs generally resemble adults but are smaller and have less wax in species that excrete wax. Once crawlers settle to feed, the scales remain fixed in the same spot the rest of their lives. For the species with well-known biology, they have one generation per year.
Agave pit scale and large pit scale cause discoloration and pitting of Joshua tree foliage. These scales generally are not abundant and not significant pests.
Bamboo pit scale may occur in large groups on leaves and stems but does not make pits. Their translucent green to yellowish body takes on color of host leaves or stems where they feed, so this species is easily overlooked. Late instars (older nymphs) excrete a waxy fringe around their body, causing them to resemble certain species of whiteflies.
Oak pit scales can severely damage oaks. Distortion caused by their feeding is most noticeable on the bark of younger twigs. Surrounding the feeding pit is formed a doughnut-shaped swelling with the scale in the center. If there are large numbers of scales, the pits coalesce, making the twig surface appear dimpled and roughened. Feeding by oak pit scales can kill twigs, and the dead leaves remain on infested twigs through the winter. Abundant scales feeding year after year can kill young oaks and stunt the growth of older trees.
Pittosporum pit scale leaves ring-shaped depressions in bark and causes twigs to become distorted and swollen. The scale commonly congregates on the growing tips where plant distortion can be so severe their feeding obliterates the individual pits. Infested leaves may be smaller than normal and twigs may die back.
Agave pit scale, bamboo pit scale, and large pit scale are generally innocuous and no control is warranted. Oak pit scales and pit-making pittosporum scale can be controlled by thoroughly spraying terminals with horticultural oil in late winter or early spring, before buds open or new leaves are produced. Where infestations are severe, annual application of oil or adding a persistent insecticide such as bifenthrin or other pyrethroid may be needed to sufficiently reduce the abundance of scales and severity of their damage. See Pest Notes: Oak Pit Scales and The Scale Insects of California Part 2: The Minor Families for more information.
Adapted from the publications above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).