Shared phytochemicals predict efficacy of essential oils against western flower thrips (Frankliniella occidentalis) in the greenhouse

Abstract

Western flower thrips (Frankliniella occidentalis; Thysanoptera: Thripidae), or WFT, are a global pest of commercial crops, particularly those grown in greenhouses. Current management recommendations often involve judicious use of pesticides to which WFT have evolved multiple resistance phenotypes. Essential oils (EOs) have shown promise as a less toxic alternative for WFT greenhouse management. However, challenges remain in predicting which EOs are most likely to be insecticidal to WFT and ensuring that the efficacy of EOs under bioassay conditions reflect performance in whole-plant application scenarios. To address these challenges, 9 EOs were tested for contact toxicity against WFT in small container assays, then gas chromatography–mass spectroscopy (GC–MS) profiles of each EO were used to quantify concentrations of 22 chemicals shared by at least 5 or more of the plant species. Of these, 13 compounds were positively correlated with thrips mortality. Effective compounds were a mixture of sesquiterpenes, cyclic monoterpenes, and noncyclic monoterpenes. Interestingly, no bicyclic monoterpenes shared among the essential oils tested correlated with thrips mortality. Whole-plant assays of the four best EOs from the container assay showed significant reduction in the number of thrips per plant, although mortality in EO treatments in the whole plant assay was lower than in the container assay. In addition, all four EOs were as efficacious as the conventional insecticide flonicamid. Identifying other EOs with high concentrations of the efficacious compounds that were identified in this study and using container assays to screen these oils for WFT thrip mortality and phytotoxicity could help integrated pest management (IPM) practitioners and greenhouse staff to more rapidly accumulate a suite of EOs as low toxicity alternatives for management of WFT in greenhouse settings. Graphical Abstract

Efficacy of Oil and Photosensitizer against Frankliniella occidentalis in Greenhouse Sweet Pepper

“…Indeed, PSG was effective in controlling WFT in the ingestion, contact and greenhouse assays in this study. More recently, Durr et al [ 10 ] observed that essential oils of peppermint, fir, arborvitae and thyme were effective in reducing a greenhouse WFT population relative to a water control. We hypothesize that mineral oil causes a variety of effects, including “arrested activity” in WFT with a repellent effect that discourages egg disposition and feeding, consequently making it effective even at low concentration, as found in this study.…”

“…Cyantraniliprole is a registered pesticide in Canada and the US, with no WFT resistance reported to date, although Wang et al [ 24 ] have reported WFT resistance to cyantraniliprole in China. WFT use detoxification enzymes including cytochrome P-450 monooxygenases, and P450s have been confirmed as a key factor in the initial development of cyantraniliprole resistance by Aphis gossypii Glover and field-evolved resistance in Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) [ 10 , 25 , 26 ]. It is therefore possible that thrips’ resistance to cyantraniliprole will continue to increase and spread geographically, intensifying the need for alternatives.…”

“…WFT are notoriously difficult to control due to their rapid population increase, cryptic behaviour, high level of vagility and polyphagous nature [ 2 , 3 , 9 ]. Additionally, WFT have a propensity for developing insecticide resistance, primarily associated with four biological parameters: (a) short generation time, (b) high female reproductive capacity (fecundity), (c) a haplo-diploid breeding system and (d) their diverse detoxification enzymes, including cytochrome P-450 monooxygenases, esterases, and glutathione S-transferases [ 5 , 10 ]. The Arthropod Pesticide Resistance Database ( ) lists at least 175 documented cases of insecticide resistance in WFT to at least 30 active ingredients.…”

“…Horticultural (mineral) oils are compatible with modern, sustainable agriculture practices, including IPM, and pose several advantages over conventional pesticides such as very low mammalian toxicity, low residual activity and significantly decreased disruption to biological control programs compared to broad-spectrum insecticides [ 10 , 13 ]. Although petroleum-based mineral oils have been used for insect pest control for over a century, they have only gained popularity for regular use as fewer options have become available in recent years, with continuing pest resistance to conventional products [ 10 , 14 ].…”