Field evaluation of insecticides and application timing on natural enemies of selected armored and soft scales

Abstract

Scale insects (Hemiptera: Coccoidea) are among the most problematic pests of woody plants in protected culture and urban landscapes. When outbreaks of scale insects occur, insecticides are often applied to reduce host damage. To select reduced risk insecticides that can complement the mortality contributed by natural enemies, the selectivity and residual toxicity of each insecticide must be considered within the context of the seasonal biology of both the pest and its natural enemies. We studied how timing of applying reduced risk insecticides affected the natural enemies of scales in relation to their life histories and seasonal abundance. This study was conducted on trees that were naturally infested with either the soft scale, calico scale Eulecanium cerasorum (Cockerell) or armored scales, striped pine scale Toumeyella pini (King) and pine needle scale Chionaspis pinifoliae (Fitch). Insecticides were applied to target the most susceptible life stages of each scale insect. We found that patterns of the seasonal abundance of natural enemies were synchronized with the life history of each scale insect and took the shape of a rising and falling wave. Overall, impacts of insecticides on parasitoids and predators were influenced by the timing of their applications in relation to when natural enemy abundance began to rise at the cusp of their wave of abundance. Application one month prior to this cusp did not reduce seasonal totals of natural enemy abundance, regardless of insecticide selectivity or residual toxicity. In contrast, when applications were made at the cusp of the natural enemy wave, foliar applications of bifenthrin had greater adverse side effects on natural enemy populations compared to foliar applications of pyriproxifen, spirotetramat, spiromesifin, and chlorantraniliprole or a soil application of dinotefuran. We present evidence to suggest that restricting applications during the cusp of the natural enemy activity wave can result in a better integration of chemical and biological control.