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Insecticidal soap and horticultural oil have the potential to kill individuals within populations of soft-bodied insect pests by suffocation. However, scientific literature is inconsistent about the efficacy of insecticidal soaps and petroleum-based oils against armored scale (Hemiptera: Diaspidae) and soft scale (Hemiptera: Coccidae). We examined the efficacy of horticultural oil and insecticidal soap against armored and soft scales at different developmental life stages. Studies were conducted in the laboratory and field with two species of armored scale [pine needle scale (Chionaspis pinifoliae) and oleander scale (Aspidiotus nerii)] and two species of soft scale [calico scale (Eulecanium cerasorium) and striped pine scale (Toumeylla pini)]. All insecticide applications were made at a rate of 2 gal per 100 gal water. Our laboratory results suggested that horticultural oil and insecticidal soap killed both calico scale (73% and 93%, respectively) and oleander scale (67% and 78%, respectively) when insecticides targeted 1-day-old scales. Scale insects built up tolerance to both materials over time after they settled. However, our field data indicated that horticultural oil had high control of settled armored scale [oleander scale (90%) and pine needle scale (83%)], but failed to control settled soft scale [stripe pine scale (5%)]. Insecticidal soap reduced armored scale [oleander scale (54%)]. Neither horticultural oil nor insecticidal soap significantly reduced populations of adult armored or soft scales compared with a control. Overall, horticultural oil killed a greater percentage of armored scales than soft scales, whereas insecticidal soap gave greater control against soft scales. We suggest that differences were driven by chemical properties of both insect integuments and insecticides. The waxy cover of an armored scale might reduce penetration of polar insecticidal soap whereas polar integument of a soft scale might impede infiltration of the lipophilic horticultural oil. Consequently, timing application to crawler stage is important for effective management of armored or soft scale with horticultural oils and insecticidal soaps.

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.