Development of effective biorationals for sustainable management of the invasive South American tomato leafminer Phthorimaea absoluta Meyrick (Lepidoptera: Gelechiidae)

Abstract

The tomato leafminer, Phthorimaea absoluta Meyrick (Lepidoptera: Gelechiidae) formerly known as Tuta absoluta is an invasive pest of Solanaceous crops with preferences for cultivated tomato (Solanum lycopersicum) where it is a major pest. Herbivory by P. absoluta larvae causes 80-100% yield losses in unprotected fields, with an estimated mean seasonal production loss of at least 114,000 tonnes, translating to about US$ 59.3 million in economic losses in Kenya. Synthetic chemical control has been the mainstay in managing the moth, with limited success because of the cryptic nature of the feeding larvae, the rapid evolution of resistant populations of the moth, and more importantly the negative effect of synthetic chemicals on the environment and non-target organisms. Hence, there is a global demand for more eco-friendly alternatives such as using biorationals to control the moth. Using behavioural assays, chemical analysis, and field trials, behaviour-modifying compounds that mediate the interaction of host, non-host plants, P. absoluta, and two native natural enemies associated with the pest including the generalist mirid predator Nesidiocoris tenuis (Reuter) (Hemiptera: Miridae) and the larval parasitoid Stenomesius japonicus (Ashmead) (Hymenoptera: Eulophidae) were identified. The mirid predator N. tenuis is zoophytophagous and may become a pest in the absence of prey by feeding on the tomato plant. The results from these studies revealed that terpenes from constitutive volatiles of host plants such as tomato and nightshade and some selected non-host Asteraceae plants including marigold (Tagetes minuta) and blackjack (Bidens pilosa) elicit significant behavioural responses in the moth and the associated natural enemies. Additionally, terpenes and green leaf volatiles (GLVs) from the predator N. tenuis-induced host plant defense volatiles influence the tritrophic interaction between host plants, P. absoluta, N. tenuis, and S. japonicus. Field trials also showed that combining the sex pheromones (1-octanol and hexyl hexanoate) identified from the Kenyan population of N. tenuis with specific visual cues such as red and white delta traps can be used to suppress both P. absoluta and N. tenuis populations, respectively in a tomato field. Overall, these studies identified different behaviour-modifying chemicals that can be developed into effective biorationals for the sustainable management of P. absoluta and the predator N. tenuis when it becomes a pest.