In this blog post Joelle Lemmen, Alix Whitener, Boyd Mori and Peter Witzgall discuss the recent paper by Boyd Mori and colleagues ‘Enhanced yeast feeding following mating facilitates control of the invasive fruit pest Drosophila suzukii

Spotted Wing Drosophila (SWD) is currently the most economically important insect in Europe and North America. SWD damages a wide range of our favourite berries and soft fruit, including blueberries, strawberries and cherries. Unfortunately, very few insecticides or biological methods are available to control this insect. SWD is a sign of our times. In the age of accelerating global trade, SWD was likely imported from Eastern Asia, to Europe and the USA, where the invasions have rapidly spread.

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SWD – Drosophila suzukii – pupae and larvae on cherry, raspberry and blueberry (Photo credit: Joelle Lemmen).

Five decades after “Silent Spring” by Rachel Carson, the most toxic insecticides have been deregulated in the USA and to a greater extent in Europe to protect consumers, beneficial animals and the environment. The drawback is that it has become more difficult for growers to control insect pests. Crop protection against insects is even more critical in a changing climate, since elevated temperatures and altered rainfall patterns facilitate insect outbreaks.

On the bright side – we are challenged to bring our knowledge of insects to practical application. An elegant approach is to use natural, non-toxic olfactory signals to manipulate insect behaviour. The “chemical ecology” research field investigates how insects use odours to find mates and host plants. Such odours or “semiochemicals” – mediators of insect behaviour – are widely used to control insects in orchards, forests and to some extent in agricultural crops.

Mori and colleagues neatly applied chemical ecology tools, showing that a yeast symbiont of SWD, acting as a natural producer and source of semiochemicals, can be used to control SWD. This work became possible through an exchange of researchers between the Chemical Ecology Unit, SLU Alnarp, Sweden and the Department of Entomology, WSU at Wenatchee, WA, USA, combining expertise in laboratory and real-world field studies of SWD chemical ecology and population control.

Many insects contain and carry symbiotic yeasts. These yeasts provide essential nutrients and protection against detrimental fungi, while insects enhance yeast growth in plants and ensure yeast dispersal.

Yeasts facilitate this collaboration by producing aroma compounds that attract insects. Mori and colleagues show how a yeast that is specifically associated with SWD attracts adult flies and elicits feeding. This yeast can be blended with a killing agent, an insecticide derived from a bacterium, which is not toxic for vertebrates. Application of the insecticide is restricted to bait drops, rather than spraying the entire crop, which eliminates contamination of the fruit. The result is an environmentally-friendly and sustainable “attract and kill” method. It may also become possible in the future to replace the insecticide component with an insect pathogen.

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SWD adult fly attracted to feed on yeast paste with a colourant. A biological insecticide could be added to this formulation for SWD control (Photo credit: Peter Witzgall).

A central idea behind this technique is that adult females are attracted to the yeast, before laying eggs in fruit. In comparison, insecticide sprays covering the entire crop mainly aim at control of larvae, the life stage producing the damage. It may be advantageous to instead target adult female flies, before they lay eggs, before the damage is done.

Research on biological techniques to control SWD is a current main effort in Europe and North America. The yeast-based attract-and-kill technique designed by Mori and colleagues is compatible with other emerging techniques, which can be combined for enhanced efficacy against this destructive pest.

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