Citricola scale (Homoptera: Coccidae) abundance on Chinese hackberry and scale control with spray oil or acephate trunk implants

Citricola scale, Coccus pseudomagnoliarum (Kuwana), infesting Chinese hackberry, Celtis sinensis Persoon, was studied during 1991-1994 in Davis, CA. Female scale and crawler densities on untreated trees increased each year in comparison with previous years; populations of this introduced subtropical species were recovering from a population decline possibly induced by cold temperatures in northern California during the winter of 1990-1991. In California, European fruit lecanium, Parthenolecanium corni (Bouche), P. pruinosum (Coquillett), and Calico scale, Eulecanium cerasorum (Cockerell), also infest Chinese hackberry. However, citricola scale was 5-25 times more abundant than the Parthenolecanium species combined, and no calico scale were found on sample branches. Citricola scale was more abundant in the east and north tree canopy in comparison with the sunnier south canopy; abiotic mortality from hot, dry conditions may help to reduce scale populations. Transparent sticky tape traps helped monitor scale crawler populations and were used to time foliar-season control actions. The number of crawlers trapped depended on scale density, not sticky trap size. Crawler density peaked at 353 DD (SD = 43) above 11 degrees C accumulated from 1 March, but laboratory development rate and threshold temperature data and sampling from other locations are needed to validate this empirical model. Three control methods were equally effective: a single foliar spray of 2% superior oil, acephate implants during the foliar season, or a single application of 3% supreme oil to bark during the dormant season. Each method reduced scale populations by approximately 50% during the season of treatment, but there was no difference among treatments 1 yr after control actions. Apparent parasitism of female citricola scale in Davis, CA, was only approximately 1%. Parasitism was significantly lower on treated trees.