Control of tomato whiteflies using the confusion effect of plant odours

Colin R. Tosh Barry Brogan Glasshouse tomato production is reduced by the whitefly Trialeurodes vaporariorum, a major worldwide pest of glasshouse crops. Whitefly control is achieved using pesticides or parasitoids, which have drawbacks. There is thus a need for other control methods. For instance, the use of volatile compounds from plant odours could profoundly change the behaviour of plant-eating insects. This practice should be safe because plant odours are usually harmless to humans. Therefore, we have studied the effect of plant volatiles on whiteflies feeding on tomato plants. The rationale was to use the confusion effect by supplying whiteflies with a super-abundance of volatiles. We removed plant headspace volatiles from multiple whitefly host plants. Then, we presented these volatiles to whiteflies feeding on tomato plants. At the same time, whitefly stylet penetration is monitored using electrical penetration graphs (EPG). Plant colonisation, egg laying and honeydew production are analysed in separate experiments. Data are compared to controls in which extracted volatiles from tomato and air alone are presented to the whiteflies. The only significant effect produced by exposure to multiple-host volatiles is a pronounced reduction in the incidence of phloem-related waveforms during the 15-h EPG recording. This represents a delay in, rather than a cessation of, phloem-related activities as there is no related reduction in long-term performance. The confusion effect thus does not appear to exert strong effects on whitefly behaviour here. - The glasshouse whitefly, Trialeurodes vaporariorum, is a major worldwide pest of glasshouse crops with control in the glasshouse usually achieved by chemical or biological (typically parasitoids) means (Fig. 1). Both control methods have their drawbacks, and here, we begin to investigate the possibility of supplementary control using harmless (to humans) plant volatiles that may nevertheless fundamentally alter the behaviour of insect plant pests. Humans and other animals undertaking a task but supplied with an abundance of distracting information often become task-inefficient or confused. In the field of predator-prey interactions, this has been termed the confusion effect and has been used to explain why many prey animals live in groups (Milinski 1977; Krause and Ruxton 2002). In psychology, the positive relationship between reaction time to find a focal object and the number of items in the set containing the focal object has been used to derive influential models of visual search such as the Feature Integration Theory (Treisman and Gelade 1980; Wolfe et al. 2010). In entomology, this idea is embodied in the Neural Limitations Hypothesis (Dall and Cuthill 1997; Bernays 2001), stating that plant-eating insects specialise to increase the accuracy of plant location and selection behaviour. So far, the confusion effect and related ideas have been applied mainly to issues in fundamental biology, but what of its utility in applied biology? A few studies have investigated the relationship between plant species complexity and whitefly plant utilisation efficiency (Bernays 1999; Smith et al. 2001; Bird and Krger 2007; Roff et al. 2012; Mansour et al. 2012), and most indicate that, at least for some performance parameters and host plant mixtures, plant diversity negatively impacts whiteflies. This conclusion reflects the wider relationship between plant species diversity and herbivore plant utilisation efficiency (reviewed in Risch 1983; Fig. 1 Whiteflies and tomatoes: the nature of the problem. The commercial production of tomatoes can be a large-scale industrial process (a). Infestation of tomatoes growing in a glasshouse with a pest insect that eats tomato plants, and whose populations develop rapidly, can be disastrous. The glasshouse whitefly, Trialeurodes vaporariorum (b), is a major worldwide pest of tomato. It has long, thin mouthparts which it sticks into the plant tissue (b), and the mouthparts move through the tissue until they locate the phloem tissue from which they ingest. An infestation of T. vaporariorum usually consists of adults (b and c) and stationary plaque-like larvae (c). Both adults and larvae ingest from the phloem and eject partially digested phloem sap from their rear end. This sweet, sticky honeydew coats surrounding vegetation and acts as a medium on which dark, sooty mould fungus can grow (d). The sooty mould problem is principally aesthetic, but if enough of it accumulates, it can reduce photosynthesis and affect yield. T. vaporariorum and other whitefly species can also spread plant viruses Andow 1991; Stamps and Linit 1998; Schlapfer and Schmid 1999; Hooks and Johnson 2003; Barbosa et al. 2009; Letourneau et al. 2011; Cardinale et al. 2011; Agrawal et al. 2006). Most relevant to the present article is the study of Bernays (1999) because her study was specifically designed to investigate sensory confusion of whiteflies in relation to plant diversity (see also Bird and Krger 2007). She studied the movement and fecundity of the whitefly Bemisia tabaci on two sets of three host plants. Insects were observed on multiple plants of each of the three species and in a mixture of each. It was demonstrated that insects were significantly more restless on plant mixtures than on any of the same-species plant groups. Effects on fecundity were less pronounced. Here, we repeat elements of this study with the glasshouse whitefly, T. vaporariorum, including various refinements, adopted both to increase understanding of the precise mechanisms underlying confusion effects and with a long-term view to exploitation of observed effects within modern, large-scale horticultural systems. Firstly, by extracting plant volatiles from mixed species and moving them through an apparatus to feeding whiteflies, we concentrate on a single sense of the whitefly, olfaction, which is likely to be a key determinant of plant diversity effects on whiteflies. Secondly, as well as considering standard behavioural and life history metrics of performance (specifically here, short- and long-term settling rate, long-term honeydew production and long-term fecundity), we also subject volatile-exposed whiteflies simultaneously to an electrical penetration graph (EPG) recording of tissue penetration activities so the whole process of plant exploitation from settling through to long-term phloem utilisation can be studied in relation to plant volatile diversity. EPG (Tjallingii 1978) is a method in which a sap-sucking insect is attached to an extremely fine gold wire and a current passed through it. When the insect pierces the tissue, a circuit is completed, and the various subsequent behaviours of the mouthparts in the plant tissue can be recorded as distinct electrical waveforms. Lastly, while previous studies have generally studied the effects of plant mixtures on whitefly behaviour within these mixtures, if the confusion effect is to be used (...truncated)