Developing Sterile Insect Technique for SWD control - STOP-SPOT

Title: New sustainable solution to save healthy fruit from spotted wing drosophila: STOP-SPOT
Funder: Innovate UK
Industry partners: Big Sis (Lead), Berry Gardens Growers Ltd
Term: October 2021 to March 2023

Since the arrival of SWD in the UK in 2012, commercial soft and stone fruit growers have been heavily dependent on the use of conventional plant protection products to control the pest. Early research helped us to understand how best to monitor for the pest and manage its control but lately, Niab has been engaged in developing novel management and control techniques that rely less on conventional chemical control. In this project, Niab collaborated with BigSis, a start-up company, and Berry Gardens Growers Ltd, to exploit a new approach called the Sterile Insect Technique (SIT).

Sterile males are produced and introduced regularly by BigSis staff, to the semi-natural areas surrounding crops and within the crops themselves. These sterile males compete with wild males to mate with wild female SWD, which subsequently fail to produce any offspring. Such an approach is sustainable, non-toxic as the sterile males can’t establish in the environment, and is species specific, so has no effect on beneficial insects or other fauna.  As native wild SWD are used to create the sterile males, there are no barriers to introducing the control system in the UK once the technique has been proven to be effective.

The project

Early trials on strawberry, now published, showed very encouraging results with SWD levels remaining very low throughout the season compared to the SWD populations in adjacent crops with no SIT release which received plant protection products only. Further studies in 2022 assessed the SIT technique in the laboratory, in further field trials, and in ‘semi-field’ trials. The laboratory work has been testing, for example, how well the sterile males compete with wild female SWD and what ratios of sterile males to wild females are required for effective control.

Results

The research found that small plot experiments are vulnerable to border effects, where wild males can migrate across adjacent fields, so future commercial experiments need to be done on a minimum field size of 7 ha. Work on blackberry provided season-long suppression compared to an untreated control. However, different crops have different dynamics and this has implications for release rates and tactics to control wild populations. It has also been found that if no or low releases are made for one to two weeks or more, wild populations increase rapidly and it is impossible to regain control using SIT.

In small scale cage studies to assess the optimum ratio of sterile males to wild males, a ratio of 5:1 was efficacious; however, in commercial practice, BigSis is aiming for a ratio of 10:1. In the early stages of this work, BigSis were rearing sterile males by hand, but this is time consuming and cannot produce sufficient numbers to provide a commercially reliable service to growers. BigSis has since been developing a fully automated system for rearing larger numbers of SWD males using several micro-production units. They plan to produce millions of sterile males per week, which will be released in commercial soft fruit crops over very large areas. Producing such high numbers will avoid the problem of having insufficient numbers to release for a week or two in the middle of the season, which can lead to loss of control.