The spotted wing drosophila (Drosophila suzukii) (SWD) is a vinegar fly native to Asia which has recently been introduced into North America and Europe (EPPO A2 list) (Calabria et al., 2012). The pest is a highly damaging pest of a wide range of soft and stone fruits and has established and is seriously damaging in a very wide range of climates from lowland sub-tropical to high mountainous areas in northern temperate climates. SWD infests thin-skinned fruits and poses a significant risk to soft fruits, stone fruits as well as grapes. This is in marked contrast to most other species of vinegar flies, which only infest overripe, fallen and rotting fruits. Thanks to their serrated ovipositor, SWD females are able to lay eggs in healthy, ripening fruits. Larvae develop in fruits which become soft, may eventually collapse, and are unmarketable (Rota-Stabelli et al., 2013a). The impact of SWD on fruit production is therefore enormous and exacerbated by the high number of generations per year (10-15), and the high fecundity of females – a single female can lay up to 500 eggs over its lifetime (Goodhue et al., 2011). SWD develop fast, from egg to adult taking around 8 days at 25°C, with three larval instars. Damage is initially caused by larvae feeding on the flesh of the fruit, but secondary damage by other insects and fungal and bacterial infections may contribute further to fruit decay (Walsh et al., 2011).
SWD was first found at East Malling Research (EMR) in Kent, UK on 29 August 2012 a few days after adult monitoring traps were first deployed in fruit crops at the institute. Research and experience in the USA, Canada and Europe indicate that temperature and humidity conditions in UK soft fruit tunnels are probably ideal for SWD. Soft and stone fruit growing have been badly affected by SWD in other countries and experience indicates that monitoring for adults and for larval infestation in fruit through the season in all crops at risk is vital. Control is by programmes of sprays of organophosphate, synthetic pyrethroid and spinosyn insecticides, targeted against adults. Once egg laying females have been recorded, a 7 day programme of sprays of suitable insecticides is applied, from when the fruit starts to colour to the end of harvest, rotating insecticide groups to reduce the risk of resistance.
Home produced soft and stone fruit is graded before packing or processing and imported fruit is often repacked; processes which generate a proportion of waste. These packing facilities are usually close to areas of UK production and present an easy opportunity for SWD to multiply. Waste fruit during picking is usually disposed of in the field and is not currently collected. However, European evidence demonstrates that this waste must be removed from the plantation and not discarded in open heaps (Poldervaart & Ziegler, 2013). Strategies for addressing the problem of soft and stone fruit waste have not yet been developed and currently the UK fruit industry invests little in safe disposal of waste fruit and does not attempt to disinfest it from crop pests. Waste disinfestation methods will be essential for management of SWD through crop hygiene. Effective technologies for disinfesting waste soft and stone fruit from SWD are needed at all stages of production including harvesting, grading, packing, storage, and processing. Untreated and exposed fruit waste at each of these stages is attractive to SWD and a potential source of contamination.
Current practices of dealing with waste are ineffectual. Open heaps or containers of raw and decomposing waste fruit allow the adults to escape and will not prevent the eggs or larvae from maturing. Such heaps of untreated decomposing waste fruit are known to attract SWD and are thought to be a potential overwintering site (Poldervaart & Zelger, 2013). Without effective disposal methods, infested fruit waste threatens to further increase the proportion of fruit that is wasted, both in the crop and at each of the subsequent production stages.
Disposal of organic food waste by deep burial costs up to £130/tonne, results in the formation of methane and other greenhouse gases and is not permitted in Scotland or Wales. Treatment of high moisture fruit and vegetable wastes through in-vessel composting or anaerobic digestion has been shown to be safe in terms of pest and pathogen destruction (Noble et al., 2009; Henry et al., 2013). Larvae of several flies have been shown to die rapidly under controlled composting conditions (Crohn et al., 2007). Fruit wastes have been used successfully as cattle and pig feeds (Angulo et al., 2012; Marquez & Ramos, 2007) and for ensiling (Tanaka et al., 2010). Depending on the size and nature of the business, the quantity of waste fruit on a farm can range from <1 to >10 tonnes per week. Now that SWD has invaded the UK, investment in purpose built waste recycling facilities is warranted for larger waste producers. Investment in temporary and mobile waste storage facilities and in joint investment in waste recycling facilities is potentially viable for smaller waste producers and groups of growers. This project will develop storage and treatment facilities for high moisture, rapidly putrefying fruit wastes that are attractive to SWD and are potentially infested. Economically and environmentally sustainable methods of safely converting soft and stone fruit wastes into usable by-products such as composts, digestates, animal feed, food or beverage ingredients and bioenergy will be developed.
SWD is attracted to a variety of fermented foodstuffs, and traps baited with a mixture of wine and vinegar are currently used for detecting this pest (Landolt et al., 2012a, b). However, mixtures of ethanol and acetic acid, major components of wine and vinegar respectively, were not as attractive as wine and vinegar (Landolt et al., 2012b), indicating other components are also important for attraction. Cha et al. (2012) identified 13 and 7 components in volatiles from wine and vinegar respectively that elicited electroantennographic (EAG) responses from SWD in addition to ehtnaol and acetic acid, with all 7 components from the vinegar present in the wine volatiles. In subsequent field tests, Cha et al. (pers comm from Peter Landolt to Jerry Cross) showed that mixtures of 4 of these compounds with ethanol and acetic acid were as attractive as the “standard” wine and vinegar mixture.