Identification of kairomones for the biological control of Tuta absoluta (Meyrick) and Trialeurodes vaporariorum (Westwood), two major pests of tomato Solanum lycopersicum L.

Abstract

The tomato pinworm, Tuta absoluta and the phloem sap sucking greenhouse whitefly, Trialeurodes vaporariorum (genus Trialeurodes thereafter abbreviated as Tr) cause economically important tomato yield losses in many parts of the world, especially in sub-Saharan Africa (SSA). Sprays of synthetic chemical insecticides with broad-spectrum active ingredients have led to negative impacts on the environment, and development of resistance in these insect pests, thus failing to control them. Biological control of these pests could be improved through the application of kairomone lures that could attract and increase populations of native and exotic natural enemies in field crops. Kairomones are chemical molecules that mediate interspecific interactions beneficial to organisms, hereafter natural enemies, that detect the cues. In SSA, Nesidiocoris tenuis, a predator of T. absoluta and Tr vaporariorum, and the whitefly parasitoid Encarsia formosa are native, while Dolichogenidea gelechiidivoris, a parasitoid of T. absoluta, is exotic. However, little is known about the kairomones that attract these natural enemies to the host/prey and tomato. In this PhD research the kairomones were investigated taking in accout three tritrophic systems as follows: tomato-Tr vaporariorum-E. formosa (Chapter 2), tomato-T. absoluta-D. gelechiidivoris (Chapter 3), and tomato-Tr vaporariorum and T. absoluta-N. tenuis (Chapter 4). Behavioural assays were conducted to assess the repsonses of the natural enemies to herbivore-induced plant volatiles (HIPVs), pheromones and to frass and honeydew volatiles. The findings revealed that attraction of the specialist parasitoid D. gelechiidivoris to HIPVs was positively correlated with the infestation density of T. absoluta, unlike the specialist parasitoid E. formosa attraction to HIPVs induced by Tr vaporariorum. The generalist predator N. tenuis displayed both types of responses. Encarsia formosa was attracted to Tr vaporariorum honeydew volatiles, N. tenuis to T. absoluta larval frass volatiles, and D. gelechiidivoris to larval frass volatiles and sex pheromones. Chemical analyses revealed both quantitative and qualitative differences between volatiles of healthy and herbivore-infested tomato plants. Volatile emission increased at a moderate infestation level of Tr vaporariorum adults and declined at high infestation level, whereas volatile emission increased as the infestation levels of T. absoluta larvae are increased. Tomato plants infested with T. absoluta larvae emitted higher amount of HIPVs than plants infested with Tr vaporariorum adults. Most of the honeydew and frass volatile compounds were found in the plant headspace volatiles. The natural enemies were attracted, mainly, to terpenes. When the attractive compounds are mixed in ratios found in the volatile extracts of the attractive plants, the specialist whitefly parasitoid E. formosa was strongly attracted to a blend of β-ocimene, β-myrcene, 3-carene and α-phellandrene, and D. gelechiidivoris was also highly attracted to a blend of methyl salicylate, β-ocimene, β-myrcene, α-terpinene, (E)-β-caryophyllene, α-pinene and α-phellandrene. Whereas the generalist predator N. tenuis displayed a relative attraction to the blend of 3-carene, β-ocimene, α-pinene, α-phellandrene and β-phellandrene.

Overall, the specialist parasitoids showed stronger attraction and detected the variations in compositions of the kairomones than the generalist predator. Molecules that are attractive to three natural enemies of Tr vaporariorum and T. absoluta were subsequently identified. The results open new avenues of research for formulating kairomone lures for their application to recruit these natural enemies into tomato crops for biological control of these pests.