This phloem-sucking soft scale (Coccidae) is a pest on citrus and hackberry in the Central Valley of California.
Identification
The tiny crawlers (mobile first-instar nymphs) are yellow with short legs. On leaves, the newly settled first-instar nymphs are flat, translucent, and barely visible with the naked eye. First instars become yellowish and plump by late summer. In the fall, nymphs become mottled brown second instars on leaves or twigs. In late winter (about March), the second instars on twigs enlarge and become mottled gray. Mature females are mottled dark brown to gray, convex, oval shaped, and about 1/4 inch long.
Brown soft scale, Coccus hesperidum, resembles citricola scale. The mature brown soft scale is smaller, about 1/6 inch or less, and yellow or dark brown, not gray. Because brown soft scale has several overlapping generations per year, different-sized life stages commonly occur at the same time. Citricola scale has only one generation per year, so most individuals are about the same size, except when the crawlers are emerging from beneath females.
Life cycle
Citricola scale occurs on leaves or twigs depending on the life stage and time of year. Females on twigs each lay several eggs per day beneath their body during a 1- to 2-month period in May through July. Eggs hatch after 2 to 3 days, and the mobile crawlers emerge. In northern California, citricola scale crawler density peaks at about 635 degree-days above 52°F accumulated from March 1.
Crawlers settle to feed mostly on the underside of leaves. In severe infestations, crawlers also settle on the upper leaf surface and on twigs. First instars grow slowly during the summer. In the fall, nymphs molt into second instars and migrate to twigs. In late winter, the second instars enlarge and mature into adult females by late April or early May. There are no males.
Damage
Citricola scales suck phloem juices from leaves and twigs and excrete honeydew on fruit and leaves. Dark sooty mold grows on the honeydew and along with the sticky honeydew can be annoying. High citricola scale populations over several years may kill twigs and reduce tree vigor, flowering, and fruit set.
Solutions
Citricola scale is well controlled by parasites in south coastal California, but not in California's Central Valley. Conserve natural enemies where they are present.
If applying horticultural oil, spray the underside of foliage during about late July through early September, after monitoring indicates that most crawlers have emerged and settled. Alternatively, hackberry can be sprayed with oil after leaves drop during the dormant season.
On large trees, the most practical control is to apply systemic insecticide according to label directions, preferably as a soil application or bark spray. See Pest Notes: Scales and The Scale Insects of California Part 1: The Soft Scales (PDF) for more information.
