To test the hypothesis that pest species diversity enhances biological pest control with generalist predators, we studied the dynamics of three major pest species on greenhouse cucumber: Western flower thrips, Frankliniella occidentalis (Pergande), greenhouse whitefly, Trialeurodes vaporariorum (Westwood), and two-spotted spider mites, Tetranychus urticae Koch in combination with the predator species Amblyseius swirskii Athias-Henriot. When spider mites infested plants prior to predator release, predatory mites were not capable of controlling spider mite populations in the absence of other pest species. A laboratory experiment showed that predators were hindered by the webbing of spider mites. In a greenhouse experiment, spider mite leaf damage was lower in the presence of thrips and predators than in the presence of whiteflies and predators, but damage was lowest in the presence of thrips, whiteflies and predators. Whitefly control was also improved in the presence of thrips. The lower levels of spider mite leaf damage probably resulted from (1) a strong numerical response of the predator (up to 50 times higher densities) when a second and third pest species were present in addition to spider mites, and (2) from A. swirskii attacking mobile spider mite stages outside or near the edges of the spider mite webbing. Interactions of spider mites with thrips and whiteflies might also result in suppression of spider mites. However, when predators were released prior to spider mite infestations in the absence of other pest species, but with pollen as food for the predators, we found increased suppression of spider mites with increased numbers of predators released, confirming the role of predators in spider mite control. Thus, our study provides evidence that diversity of pest species can enhance biological control through increased predator densities.

1. Indirect interactions between populations of different prey species mediated by a shared predator population are known to affect prey dynamics.2. Depending on the temporal and spatial scale, these indirect interactions may result in positive (apparent mutualism), neutral or negative effects (apparent competition) of the prey on each other's densities. Although there is ample evidence for the latter, evidence for apparent mutualism is scarce.3. The effectiveness of using one species of predator for biological control of more than one pest species depends on the occurrence of such positive or negative effects. 4. We used an experimental system consisting of the two prey species Western flower thrips (Franklineilla occidentalis Pergande) and greenhouse whitefly (Trialeurodes vaporariorum Westwood) and a shared predator, the phytoseiid mite Amblyseius swirskii Athias-Henriot. We released all three species on the same plant and studied their dynamics and distribution along rows of plants.5. We expected that the more mobile prey species (thrips) would escape temporarily in the presence of the other prey species (whitefly) by dispersing from plants with the predator. The predator was expected to disperse slower in the presence of two prey species because of the higher availability of food.6. Evidence was found for slower dispersal of predators and short-term escape of thrips from predation when whiteflies were present, thus confirming the occurrence of short-term apparent mutualism.7. The apparent mutualism resulted in a cascade to the first trophic level: a higher proportion of fruits was damaged by thrips in the presence of whiteflies.8. We conclude that apparent mutualism can be an important phenomenon in population dynamics, and can significantly affect biological control of pest species that share a natural enemy.

De Californische trips, Frankliniella occidentalis, is een groot probleem in bloemisterijgewassen onder glas. De tripsen geven snel schade aan bloemen en blad en kunnen virussen overbrengen. Voor een effectieve bestrijding van trips is een systeemaanpak van groot belang waarbij maatregelen die de weerbaarheid van de plant verhogen worden gecombineerd met biologische bestrijding, vangtechnieken en integreerbare (bio)pesticiden. Het doel van dit project met de titel "Masterplan tripsbestrijding in bloemisterijgewassen (KV1406 087)" was om tot betere bestrijdingsstrategieën van trips te komen door te werken aan: 1) een weerbaarder gewas met endofyten, 2) preventieve inzet van natuurlijke vijanden (standing army) en 3) gedragsmanipulatie van volwassen tripsen. Dit rapport presenteert de resultaten van dit project dat is uitgevoerd in de periode 2015-2019.

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