## 4 Discussion
Additional research will be needed to further evaluate the potential of stemonine as a botanical insecticide against thrips. Nicotine, pyrethrins and rotenone were the most effective products against WFT in this study, an observation that is supported by previous research on WFT and other thrips species e.g., [11; 41; 42; 43; 44]. Pyrethrins and pyrethroids hyperactivate voltage-gated sodium channels, leading to paralysis and death of insects [45]. Additionally, pyrethrins also achieve repellency against insects through activation of olfactory receptor neurons [46]. The dual mode of activity, i.e., toxicity and repellency, makes pyrethrins a valuable tool in preventing infestation and damage and reducing pest abundance. Commercial products containing pyrethrins are currently widely available for management of insect pests in various commodities, use sites and countries, allowing easy access of this valuable pest management tool by many farmers. Rotenone and nicotine were the second and third most effective compounds in this study, respectively. Rotenone inhibits normal functions of complex I of the mitochondrial respiratory chain and stimulates the production of reactive oxygen species, leading to limitation of adenosine triphosphate (ATP) synthesis and damage of mitochondria (and thus apoptosis), respectively [47; 48]. Nicotine and neonicotinoids activate nicotinic acetylcholine subunits, leading to hyperactivity and eventually death of insects [49]. Although products containing nicotine and rotenone are largely unavailable or their pest management uses greatly limited in the USA, Europe, and other developed countries, nicotine and rotenone still represent effective, widely available, and economical pest management options in many developing countries. As demonstrated by this study, nicotine and rotenone are effective and viable botanical insecticides against WFT, particularly in countries where the use of these products is not restricted, or the availability of other effective products is limited. Due to their high mammalian toxicities, nicotine and rotenone must be used carefully to limit risks and impacts to human, non-target animals and the environment. We observed unacceptable levels of phytotoxicity on plants grown in greenhouse and sprayed with 0.1% nicotine, 0.05% pyrethrins, 0.1% rotenone and 0.5% stemonine. The damage appeared on the more tender flower tissues, instead of on hardened leaf tissues. In the laboratory, such phytotoxicity did not appear on flowers that were dipped in solutions of the extraction solution (ethanol), adjuvant (Tween 20) and plant extracts, suggesting that the cause of the phytotoxicity was not likely the chemicals used in this study. The appearance of phytotoxic reactions under sunlight (greenhouse) but not under artificial light (laboratory) suggested that the chemicals in the plant extract products might have interacted with ultraviolet light or other environmental factors to damage the flowers. Future insecticidal products containing pyrethrins, nicotine, rotenone, and stemonine will need to be formulated carefully to avoid such phytotoxicity. It would also be advisable to use the products for managing pests on leaves, instead of flowers or other tender tissues, to avoid phytotoxicity. Conceptualization, L.R. ; methodology, J.H.C. and L.R. ; software, J.H.C. ; validation, L.R. and J.H.C. ; formal analysis, J.H.C. ; investigation; L.R. ; resources, L.R. and J.H.C. ; writing--original draft preparation, J.H.C. ; writing--review and editing L.R. and J.H.C. ; supervision, J.H.C. ; project administration, L.R. and J.H.C. ; funding acquisition, L.R. and J.H.C. All authors have read and agreed to the published version of the manuscript. This research was supported by Guangxi Scholarship Fund of Guangxi Education Department [Gui Teacher Training 2017], National Natural Science Foundation of China [Project no. 31260436], and United States Department of Agriculture, National Institute of Food and Agriculture [Project no. SC-1700534]. This manuscript is Technical Contribution No.