Efficacy of Horticultural Oil and Insecticidal Soap against Selected Armored and Soft Scales

Abstract

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.