Bioactivity of 54 Essential Oil Extracts Topically Applied to Adult Azalea Lacebugs Stephanitis pyrioides (Scott) [Tingidae:Hemiptera]: A Rapid Bio-Pesticide Discovery Program

Abstract

Concern about genetic pest resistance and poisoning of non-target organisms are spurring the search for “softer” insecticides with greater selectivity and multiple modes of action. Essential oils are blends of secondary metabolites that serve plants as deterrents against insect herbivores, but remain relatively safe and even beneficial to vertebrates [1]. We used serial-time mortality bioassays to screen the essential oils from 54 representative plant species from 30 genera comprising 13 families of gymnosperms and angiosperms for bioactivity to laboratory-cultured azalea lace bugs, Stephanitis pyrioides (Scott). The principal developmental stages of bugs exposed to the essential oils were the adults-long-lived individuals that provide parental care to their leaf-infesting brood. Clevenger-type distillation extracted essential oils from dried plant material and lead components were purified and identified with gas chromatography-mass spectrometry (GC-MS). Oils were mixed with de-ionized water and a non-toxic emulsifier 0.9–9.0% dimethylsulfoxide (DMSO). All oil emulsions and sometimes their fractionated components were topically applied to adult bugs in randomized blocks at concentrations of 0, 650, 1300, 2500, 5000, and 10000 ppm. Overall bug mortality, as well as LD50, LD95 and LD99 values were calculated after 1, 2, 3, 4 and 5 hours of exposure. Mortality data were analyzed using multivariate probits [1] and preliminary data show that 1% emulsions derived from oil of Pelargonium (94.5% bug-mortality), Cinnamomum (91.4%), Hedychium (85.9%) and Tagetes (81.8%) were more efficacious than the malathion-DMSO emulsions (66.1%) and are four promising botanical sources from which to isolate compounds useful for developing new biorational crop protectants. Acknowledgements: We thank the many generous colleagues who supplied us with plant material and extracts: Ikhlas A. Khan (USA), K. Husnu Can Baser (Turkey), Betul Demirci (Turkey), Gulmira Ozek (Turkey), Temek Ozek (Turkey), Aruna Weerasooriya, (USA), Zengping Gao (China), Sui Zhang (China), Jian Zhang (China), Peng Nan (China), Zhijun Liu (USA), Hamidou Sakhanokho (USA), Cecil Pounders (USA), Sandra Gray (USA), Christine Murphy (USA), Eugene K. Blythe (USA).