Industry representative: Ross Mitchell
HDC project cost: £50,000
Gall midges as agricultural and horticultural pests in the UK
Plant feeding gall midges (Cecidomyiidae) are important pests of agricultural and horticultural crops in the UK and worldwide, often causing injury and serious crop losses. In the UK, wheat blossom midges, swede midge, brassica pod midge, pea midge, raspberry cane midge, pear midge and the leaf curling midges that attack apple, blackberry, blueberry, cherry, raspberry and pear are all serious pests which growers attempt to control with pesticides, with varying degrees of success. They are a significant cause of pesticide usage.
Gall midge sex pheromones and their exploitation in monitoring and control
Gall midges have powerful female-produced sex pheromones. The chemical structures of 17 species have been identified to date, including 6 of the most important pests of fruit crops in the UK identified by us (Hall et al., 2012). Many of these sex pheromones are successfully exploited for pest monitoring in commercial practice. However, there is also great potential to exploit them for control.
In HortLINK project HL0175 (raspberry Integrated Pest and Disease Management), preliminary work was done by us to develop methods for controlling the raspberry cane midge using its sex pheromone (Cross et al., 2011). The efficacies of several Mating Disruption (MD), Attract and Kill (A&K) and a Mass Trapping (MT) treatment comprising a wide range of dispenser/device types and dose rates of pheromone were evaluated. These trials indicated that MD or A&K with a high density of low dose sources was the most promising approach. One of the main problems encountered was sustaining an adequate release of pheromone through the season. A proprietary wax emulsion formulation (SPLAT) was the best for ease of application and steady release rate, and the most promising for further development.
Recently, work in Sweden and Switzerland has demonstrated sex pheromone mating disruption control of the swede midge (Ylva Hilbur and Jorg Samietz, pers. comm.). However, the high doses of pheromone were released from impractical dispensers and further work is needed to develop a suitable formulation for economic and practical use.
Host volatile female gall midge attractants
Female-produced sex pheromones attract only conspecific males. Attractants for the females, particularly mated females, would potentially be far more valuable for both monitoring and control of the pests. Traps baited with the attractants would give a better prediction of the laying of eggs and the appearance of larvae. Traps or other devices could be used to lure mated females away from the host crop or the attractant chemicals could be used to disrupt the ability of the females to find the host plant and lay their eggs. There is good evidence for attraction of females of at least four species of midge – apple leaf midge, blackcurrant leaf midge, raspberry cane midge and wheat blossom midge – to volatiles from their host plants for oviposition. Unlike many other plant-based attractants for insects, the attraction seems to be remarkably specific.
In previous work in LINK project HL0175 (Cross et al., 2011) and a part-completed PhD studentship funded by the HDC, significant progress has been made in identification of chemicals released when raspberry canes split and become attractive to females of raspberry cane midge (Hall et al., 2011). This work has involved the use of novel techniques for trapping volatile compounds, analysis by gas chromatography (GC) linked to electroantennography to detect biologically-active compounds and by GC-mass spectrometry (MS) to identify them. Eleven compounds [(Z)-3-hexenol, 6-methyl-5-hepten-2-ol, linalool, myrtenal, geranial, citronellol, methyl salicylate, myrtenol, nerol, geraniol, benzyl alcohol] were identified as being produced in larger amounts from splits in raspberry canes in comparison with unsplit canes. Three of the compounds [6-methyl-5-hepten-2-ol, myrtenal and myrtenol] gave strong electroantennogram responses. However, as yet we have been unable to develop a synthetic lure attractive to females. We believe the blend and release rate need careful adjustment. In addition, a suitable trap design for trapping females has yet to be identified.
As far as we are aware, there have been no successful attempts to exploit host plant volatiles for control of gall midge pests, but there is clearly great potential. In the Netherlands, non-host volatiles are used commercially to protect grafting wounds from attack by the red bud borer (Resseliella oculiperda) on newly grafted apple rootstocks by incorporating them into the grafting tape used to secure and protect the graft wood, which repels females (van Tol et al., 2007).
Raspberry cane midge as a model species for development of semiochemical-based control methods
Raspberry cane midge is an ideal choice of model species for development of semiochemical based control methods, for the following reasons:
•It is an important pest of raspberry, an important and valuable crop in the UK, and it can only be controlled currently with chlorpyrifos. Therefore, developing an alternative control method is important.
•We have identified the midge’s sex pheromone (Hall et al., 2009), determined the optimum release rate for competitive attraction, and have already demonstrated that it is possible to use it for control of the midge, though we have not yet perfected a formulation and method of application. This was the first time control of a midge pest with a sex pheromone was demonstrated and is a very significant scientific breakthrough.
•Some midge sex pheromones have relatively complicated chemical structures and would be difficult and very costly to produce but those of the raspberry cane midge could conceivably be produced on a large scale at a comparatively low cost. The A&K approach also has the advantages that comparatively small amounts of pheromone and pesticide are likely to be required and that the pheromone is regarded as a co-formulant of the pesticide employed, considerably simplifying registration procedures.
•We have identified the key host plant volatile compounds produced by cane splits that are used for female attraction and these are not produced continuously by the plant in large amounts, but only when and where cane splits occur.
Raspberry cane midge is therefore a good model midge species and we will be able to apply the results generally to other species.
Opportunities to develop semiochemical based control methods
As outlined above, there are clearly promising opportunities to exploit the raspberry cane midge’s sex pheromone and the host volatiles produced by cane splits for control, both individually and in combination, and this is the overall aim of this proposed project. In our previous work, after preliminary laboratory work to develop formulations and determine release rates, we tested doses, formulations and methods of application in large scale field trials. Trials had to be done on a massive scale (we used 1 ha plots, conducting 32 comparisons of a treated with and untreated control over a 5 year period). For this reason the experiments could not be properly replicated and we were unable to control the level of midge infestation. In this project, we propose to use large field cages and potted uninfested plants for development work so that we can make properly replicated comparisons using known artificially introduced populations of midges.
Aims and objectives:
- To develop the first effective semiochemically-based control method for a gall midge pest that will be suitable for use in commercial practice.
- To develop a competitive attract and kill (A&K) control method for males to determine whether cane split finding by females can be disrupted by artificially provided host volatiles. If the host volatile approach is feasible, we will optimise the blend, develop a method for practical application and determine whether A&K and host volatile disruption (HVD) can beneficially be combined;
- To test different control methods in replicated experiments at different pest population densities;
- To maintain a proactive engagement between researchers, the UK fruit industry and CRD to ensure that a commercial formulation is developed and registered in the UK.