Natural Decline of a Pine Needle Scale (Chionaspis pinifoliae [Fitch]), Outbreak at South Lake Tahoe, California following Cessation of Adult Mosquito Control with Malathion

The population dynamics of a pine needle scale infestation Chionaspis (Phenacaspis) pinifoliae (Fitch) were studied at South Lake Tahoe, California from 1969 to 1971. A uniparental population on lodgepole pine, Pinus contorta Dougl., and a biparental population on Jeffrey pine, P. jeffreyi Grev. and Balf., declined dramatically during the study period. The dominant mortality factors for the lodgepole pine population were crawlers which failed to settle, parasitization by Prospaltella bella Gahan, predation by Chilocorus orbus Casey var. monticolus Drea and Cryptoweisia atronitens (Casey), and an unknown mortality of adult female scale. In the Jeffrey pine population, the dominant factors were crawlers which failed to settle, unknown mortality of immature scale, and predation by the same cocinellid species. An intrageneration, intertip analysis of scale populations on lodgepole pine revealed that predation destroyed a constant proportion of the scale in all study areas. Prospaltella bella responded similarly in one lodgepole pine site. Futhermore, parasitization by P. bella showed a within—tip response that increased (proportionately) with decreasing needle age. In the Jeffrey pine site, intertip analysis indicated predation upon ;a constant proportion of the scale population. Achrysocharis phenacapsia Yoshimoto, like P. bella, increased its percent parasitism with decreasing needle age. Laboratory bioassays in which adult diaspid scale parasitoids were exposed to foliage collected from areas fogged for mosquito control indicated insecticide residues were present. The results of the bioassys, the location of the infested area, the field observations on arthropod associates as well as the decline of the C. pinifoliae population following cessation of malathion fogging (for adult mosquito control), all implicated the mosquito control program as causal to the scale outbreak at South Lake Tahoe.