Growing resistance to the organophosphate chlorpyrifos, huge populations and no natural parasites have elevated citricola scale to the No. 1 citrus pest in the San Joaquin Valley.

Its dubious distinction is no doubt why a larger-than-expected crowd showed up at the Lindcove Research and Extension Center in Exeter, Calif., to hear University of California citrus IPM specialist and research entomologist Beth Grafton-Cardwell not only talk about the pest, but show growers, PCAs and others what to look for in scouting for citricola scale.

Citricola scale is a voracious pest that in high numbers can dramatically reduce yield. A typical Valencia orange tree can produce 325 oranges per tree. If there are 10 citricola scale per twig, yields can drop by 70 fruit per tree, a yield loss of more than 20 percent.

The yield loss may be even more severe with Navel oranges and lemons, Grafton-Cardwell says.

The tiny scale also secrete honeydew that can create sooty mold on citrus. This can be washed off in the packinghouse.

Grafton-Cardwell and her crew of research technicians set up a mobile teaching lab for field day attendees to examine scales under a microscope. They also sent growers and PCAs into nearby groves in search of the not so elusive citricola scale. The research technician crew also collected scale infested leaves and distributed hand lenses to help growers identify dead and live scale on leaves.

The field day was held in mid-September, a month or so after what Grafton-Cardwell says is the prime time (late July/early August) to control the scale. This is when the first instar nymphs are on leaves and most susceptible to insecticides.

The more dead scales found, the less chance of a big outbreak next year.

The only effective control method in the valley is insecticide treatments. The parasitic wasp that is an effective biological control in Southern California citrus does not control the pest in the valley because of the high populations of scale produced under hotter valley conditions.

Citricola scales reproduce asexually allowing rapid population growth, according to the UC IPM website. It has only a single generation per year. Adults are found in the spring and early summer on twigs, and immatures are typically found underneath leaves in late summer and fall. Females produce approximately 1,000 eggs during their lifetime. Eggs hatch into mobile first instar nymphs, also known as crawlers. Crawlers relocate to find a suitable location, become fixed, then molt into second instar nymphs that produce large quantities of honeydew and are often tended by ants.