Factors Influencing Insecticide Efficacy against Armored and Soft Scales

Abstract

Scale insects (Hemiptera: Coccoidea) are among the most economically important pests of ornamental plants. Soft scales (Coccidae) are phloem-feeding insects that produce large amounts of honeydew. By contrast, armored scales (Diaspididae) feed on the contents of plant cells and produce a waxy test that covers their bodies. We studied two species of armored scales [pine needle scale (Chionaspis pinifoliae) and oleander scale (Aspidiotus nerii)] and two species of soft scales [calico scale (Eulecanium cerasorum) and striped pine scale (Toumeyella pini)] to compare efficacy of selected insecticides. In addition, we assessed how the duration of first instar emergence might influence insecticide efficacy. Several reduced-risk insecticides (chlorantraniliprole, pyriproxyfen, spiromesifen, and spirotetramat), horticulture oil, and two broad-spectrum insecticide standards (bifenthrin and dinotefuran) were evaluated. Efficacy of insecticides was consistent within each scale family. Bifenthrin and pyriproxyfen were the only insecticides that killed soft scale insects. By contrast, all insecticides killed armored scales when the crawler stage was the target of application. Armored and soft scales may differ in susceptibility to pesticides because of likely differences in the chemical composition of their integuments and covers. Finally, we found that the effectiveness of a single application of insecticide declined by >15% when the duration of the crawling juvenile period was increased from 1 to 4 weeks. Increases in duration of a scale crawling period decreased the efficacy of a pesticide application.