Latest results of UK industry funded project which ended in March 2017
In this section:
- Pest population dynamics and habitat survey
- Sampling and extraction methods
- Waste disposal
- Crop protection
- Bait sprays and repellents
- Developing a new project to continue the research
In the first two years of the project, significant findings were made. Populations of SWD have been found in all fruit growing areas of the UK and Ireland and the numbers of adults being caught in traps has increased year on year. Populations each year have risen steeply from July/August, reaching a peak in December. Populations in fruiting crops have been found to decline at the end of harvest when the foliage has gone from the crop and it is believed that SWD migrates into hedgerows and woodlands for protection against the elements. SWD resides in these habitats in the winter months and can find shelter from winter weather until the following spring. SWD moves into cherry orchards long before it is able to lay eggs in fruits and it was observed in 2016 that the adults can feed on the nectar from extra-floral nectaries at the base of the newly opened leaves. A similar finding has been found on plum, although these leaves do not produce as much nectar. Other research has also demonstrated feeding on the nectaries of cherry flowers.
In the third year of the project, similar trends in population increases were found. In a habitat survey conducted using traps deployed in a range of crops and in neighbouring wild areas and woodlands, SWD was recorded in the traps every week, in contrast to the previous year, when for five weeks in the spring, none were recorded. Trap catches showed a similar pattern to previous years with activity increasing from 14 April to 8 June, with more pronounced trap catches towards the end of the summer from 27 July until the winter. Numbers peaked in late autumn and winter with considerably higher catches than the previous year. However, it should be noted that adult trap catches become less reliable estimates of population density during the fruiting period because the fruits compete with the traps for SWD ‘attention’.
Several methods to extract SWD from ripening and ripe fruits were assessed. Flotation was found to be the optimum way of detecting SWD larvae in fruit, with a sugar solution working best, although this does not provide 100% recovery of larvae. The use of emergence testing, where fruit is incubated and adult SWD allowed to emerge over a period of weeks will provide additional information on fruit infestation early in the season and is generally more sensitive than flotation testing, but takes 2-3 weeks. New training videos on how to do flotation and emergence testing are available to view on the AHDB Horticulture SWD web pages.
Overseas work has demonstrated the value of regular picking (every 2 days) and following good hygiene practices in soft and stone fruit plantations by removing all old, overripe, diseased or damaged fruit from plantations, to reduce further damage. All fruit waste is attractive to SWD, as a source of egg laying and feeding and must be disposed of in a hygienic way.
Work in the first two years of this project identified that anaerobic treatment of fruit waste in sealed plastic pallet bins with a capacity of between 500-670 litres, effectively kills all life stages of SWD in the waste. A combination of depleted O2 and high CO2 concentration is needed to kill the SWD. Holding the waste in the sealed bins for two days at waste temperatures of at least 18oC will ensure eradication of SWD. If waste temperatures are below 18oC, at least three days are required.
Work in the third year showed that there can be a low level of survival of SWD in stone fruit waste treated for three days, particularly if the waste temperature is below 16oC, so a four-day treatment should be used for stone fruit.
The treated waste is still attractive to SWD adults, so requires to be disposed of in a way which no longer attracts the insect. Rotavation of treated waste into soil to a depth of 20cm is a suitable disposal route. The rate of application to land should not exceed 125 tonnes/ha.
Mixing the treated waste with other organic waste (at least 90% w/w) is an alternative suitable disposal route.
As a result of this work, Factsheet 19/16 ‘Disposing of fruit waste affected by spotted wing drosophila’ was published by AHDB which offers comprehensive guidance to growers.
There are numerous commercial traps and their design is constantly evolving as we learn more about the biology of the pest. A comparison was made of the most promising of the commercially available traps and baits with results suggesting that the most practical for growers to use is the 2014 Biobest trap combined with Gasser lure liquid bait. However, it was shown that because of the larger entry hole sizes in this trap, a significantly greater number of >4 mm insects are captured making identification of adult SWD more time consuming for monitoring. A dry bait produced by NRI containing the same 4 components as the Cha-Landolt bait was trialled in a cherry crop and shown to be attractive to SWD and more selective for this species, with less by-catch of other insects. Growers should keep abreast of the latest trap developments for SWD specificity for monitoring and consider using traps with high SWD catches for precision monitoring around crops.
An ‘attract and kill’ device is being developed in conjunction with the NRI. Miniature, dry versions of the Cha Landolt lures have been shown to be effective in attracting D. suzukii. Progress has been made on the design of the device to aid the entry of SWD.
In the first year of the research work, protected strawberry fruits treated with crop protection products whilst still on the plant were assessed by picking at intervals post spraying, exposing to SWD adults and then incubating for three weeks to determine the numbers of SWD produced. Fruits were assessed up to two weeks after insecticide application to determine any effect of residue decay. Spinosad (Tracer), cyantraniliprole (Exirel) and chlorpyrifos (Equity) gave control of SWD for up to two weeks after spraying. Spinosad and chlorpyrifos (no longer approved) in particular have direct effects on adult SWD. Lambda-cyhalothrin (Hallmark) gave very short and variable control of SWD – up to two days. Spruzit (pyrethrin + rapeseed oil) gave up to 50% efficacy against adult SWD and persists for approximately 2-3 days, depending on environmental conditions. Neither chlorantraniliprole (Coragen) nor deltamethrin (Decis) were effective at controlling SWD in this strawberry trial.
Further trials in the next year on unprotected raspberry provided similar results. However spinosad (Tracer) efficacy declined after the first week (from 29% emergence compared to controls at 7 days to 67% at 14 days), potentially because on unprotected raspberry rainfall reduced the longevity of the product.
In the third year, a further replicated trial was conducted, this time in a protected cherry crop at NIAB EMR to assess the effectiveness of a range of products which were selected in consultation with the UK cherry industry. Spinosad (Tracer), lambda-cyhalothrin (Hallmark), cyantraniliprole (Exirel) and a coded product gave good control over the duration of the trial. Deltamethrin (Bandu), acetamiprid (Gazelle) and another coded product gave only moderate control for less than 7 days. One application of lime and a pyrethrins mixture gave relatively poor control. However, laboratory tests at NIAB EMR and research in Switzerland has shown that sprays of the lime and the micronutirent formulation (ProLime) can deter egg laying and may be useful as part of Integrated Pest Management. It should be noted that the use of ProLime is not currently approved for use in this way.
In the final year of the project (2016), work was done to investigate the use of baits to attract SWD with a view to further developing attract and kill strategies. A number of laboratory techniques were developed to assess the attractiveness of a range of baits. In a large arena choice test, strawberry juice, Gasser and molasses were significantly more attractive than other baits (some of which are commercially available in other parts of Europe) over a period of 12 hours and no differences were found in their attraction to both males and females.
These results were achieved under laboratory conditions and further work is required in the field over longer time periods to establish if these baits can compete with the attraction of fruits in the crop. Their efficacy also needs to be tested in combination with control products to ascertain if they improve the death rate of SWD.
Work was also done using repellent compounds to determine whether we can prevent SWD detecting fruit or repel the pest from fruit to prevent egg laying. The work was done in a cherry orchard assessing different compounds in sachets or rubber septa hung in delta traps above a petri dish of fruit (strawberries and raspberries). The trials were conducted in July and August. Four compounds were found to have potential but when sachets were distributed throughout the tree, egg laying was generally not deterred, although it should be noted that SWD was already present in the crop in high numbers. Further work is required to consider the optimum timing of use, the formulation of the application of the repellents and to assess their use in the spring before SWD enters the crop.
The current project (SF 145) has taught us a great deal about the pest’s behaviour in UK cropping conditions, how and where it survives, how best to monitor for its presence, how to dispose of fruit affected, how to manage crops to reduce the threat of damage and which products offer best control of the adults. AHDB, through crop protection managers Vivian Powell and Bolette Palle-Neve, continue to work constructively with agrochemical manufacturers and CRD to gain new approvals to increase our armoury of control products and during the past year, emergency approvals have been secured for use in cherry and outdoor strawberry to provide essential control options. By gaining approvals for products with different modes of action we aim to allay the onset of insecticide resistance of SWD.
However, we can’t rely solely on traditional control methods, particularly given the paucity of products available to the industry, and no single control method will provide sufficient control on its own. It is widely agreed by scientists working on SWD all across the world that commercial control measures will need to rely upon an integrated approach using several different strategies to reduce pest populations in the vicinity of the crop, prevent the pest from entering the crop, deterring it from feeding on developing fruits, controlling the pest before it alights on the fruit and seeking biological and other novel control methods. All of these will need to be integrated within a production system which maintains the highest levels of crop hygiene throughout.
Spotted Wing Drosophila: A guide (click on the headings below to explore further information)