Will latest strategy help to tackle soft-fruit pest Drosophila suzukii?

Research into lure-and-infect strategy moving into field evaluations later this year.

Lure-and-infect strategies show potential to control the soft-fruit pest Drosophila suzukii - the spotted-wing drosophila or SWD - but only using a highly specific fungus, according to research by the EU-funded Dropsa consortium.

Researchers at Wageningen UR in the Netherlands have found that deploying a new isolate of the entomopathogenic (insect-harmful) fungus Metarhizium robertsii in such a strategy "reduces reproduction of virgin suzuki fruit flies very effectively", according to entomologist Dr Rob van Tol.

The flies were placed in cages with a small dish of a blueberry-fungus mix. After four weeks there was virtually no reproduction of the fruit fly compared with a control treatment. Two commercial products, by contrast, had less (Botanigard) or no effect (PreFeRal) on flies' reproduction, while the flies picked up five times more spores of M. robertsii than Beauveria bassiana (Botanigard).

However, entomopathogenic products already registered for commercial use "appear not effective enough to prevent economic damage by the fruit fly due to the slow mode of action" when sprayed directly on crops, says Van Tol, because fungi "need to prevent egg-laying within days". However, the lure-and-infect strategy ensures that the attracted insects pick up higher doses of the spores than via crop spraying, while avoiding any fungal residues on the plants and fruit.

But he adds: "There are still some hurdles to tackle before it becomes practicable. The bait with the entomopathogenic fungus has to be very attractive and able to compete with the fruit in the field. But the strategy could work if used in an early stage of the season before ripening of the fruit. What kind of bait is attractive enough in the field is still unknown and is part of the follow-up research. In 2017 we plan to move to the field evaluations.

"Some promising parasitic wasps are found in the area of origin (Japan/China) but it is unlikely that these non-native natural enemies will get a registration in Europe. Local parasitic wasps found in Europe and tested on the suzuki fruit fly are so far not effective enough to control the pest. Entomopathogenic fungi as a biocontrol option could be an alternative to the wasps."

Parasitoid evaluated

Despite these concerns, Dropsa partners are also evaluating the suitability of Asian parasitoid wasps for the biological control of D. suzukii in Europe. The most promising candidate is a small wasp provisionally identified as Ganaspis brasiliensis, a specialist and abundant parasitoid of the fruit fly in its native China and Japan.

"Considering the fact that D. suzukii develops many generations per year and attacks a wide range of fruits of wild and ornamental plants, long-term control solutions will have to involve area-wide control methods," says Swiss-based CABI head of risk analysis and invasion ecology Marc Kenis. "More work is required before a dossier for the introduction of the parasitoid can be submitted," he adds.

In Italy, Dropsa studies of the effect of surrounding vegetation on D. suzukii infestation of cherry orchards found "larger attack density and female adult density in orchards surrounded by higher forest cover". The University of Padua researchers note: "In forest habitats, the pest can probably find ideal microclimatic conditions and other biotic factors such as the presence of alternative host plants that can promote population growth." While removal of such habitats "does not seem to be a viable option", integrated pest management interventions "should be particularly timely in forested landscapes where high densities of D. suzukii can quickly attack the crop", they suggest.

A further Dropsa study of the routes by which D. suzukii has established itself worldwide, led by the French National Institute Agricultural Research in Montpellier and the Natural History Museum of Paris, found that European populations of the fly are genetically more homogeneous than those in North America and their most probable source is north-east China. The American populations appear to have originated in Japan via Hawaii and in south-east China.

Running from 2013 until the end of this year, Dropsa aims to develop targeted, cost-effective and practical integrated pest management of fruit pests and diseases of particular concern. The consortium consists of 26 international partners including the Food & Environment Research Agency, the University of Leeds, Imperial College London, CABI and Oxfordshire biotech company Oxitec. For details, see www.dropsaproject.eu.

BP100 Synthetic peptide compound shows potent bactericidal activity

In another Dropsa consortium study led by Emilio Montesinos of the University of Girona in Italy, the synthetic peptide compound BP100 has shown "potent bactericidal activity" against plant pathogens including Xanthomonas fragariae (angular leaf spot of strawberry) and X. arboricola pv. pruni (bacterial spot of stone fruit) in greenhouse and semi-field assays.

"The efficacy was comparable to antibiotics," says Montesinos. A trunk injection technique to deploy the peptide "has been validated and was effective" in controlling X. arboricola on glasshouse peach trees, he adds.

Meanwhile, "semi-industrial-scale production" of the biological controls Lactobacillus plantarum PM411 and Bacillus amyloliquefaciens EPS2017 have also shown promise, with a prototype formulation showing "significant efficacy" in the control of both Xanthomonas species.

The consortium has also completed first "in planta transcriptome" of the X. fragariae bacterium, which Joel Pothier of the Zurich University of Applied Sciences in Switzerland suggests "could be used to design custom antibacterial compounds".

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