Exploring the Use of Plant Volatiles as a Sustainable Control Method for Glasshouse Whiteflies on Glasshouse-Grown Tomato

Abstract

In the UK, tomato (Solanum lycopersicum L.) is cultivated in bespoke glasshouse systems which facilitate the use of sustainable pest control techniques such as integrated pest management (IPM). IPM focuses on the use of naturally occurring control agents and is globally endorsed as the future paradigm for crop protection. Despite this, important tomato pests such as glasshouse whiteflies (Trialeurodes vaporariorum Westwood) are often still managed with insecticides. These synthetic chemical sprays, to which whiteflies have shown resistance, are extremely detrimental to the environment and human health. Plant volatile organic compounds (VOCs) offer an environmentally benign alternative to insecticides and have great potential for incorporation into IPM systems. This thesis explored the use of VOCs to control glasshouse whiteflies on tomato in a commercial glasshouse setting. In a series of intercropping experiments with whitefly non-host plants, we found French marigold to effectively ‘push’ whiteflies from a tomato crop through emission of airborne limonene. We then used repellent limonene dispensers in place of companion plants, and also in conjunction with methyl salicylate, a potent inducer of plant defence. This combined VOC-based protection method performed no better than the standalone limonene treatment, meaning there was no synergy between the two compounds. These experiments highlighted the defensive inadequacies of cultivated tomato and prompted us to analyze whitefly resistance mechanisms in Solanum pimpenillefolium, a wild tomato species more resistant to whiteflies. Superior defences in S. pimpenillefolium were characterized by greater quantity and diversity of VOC emissions than cultivated tomato. In olfactory preference experiments, S. pimpenillefolium VOCs were less preferred by whiteflies and more attractive to the whitefly parasitoid Encarsia formosa. Host plant resistance is a key component of IPM and these results illustrate the importance of tomato VOCs for whitefly resistance. The methods and concepts explored in this thesis could be used to support current IPM systems and help achieve sustainable whitefly control.