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Crop management guidelines for minimising wheat yield losses from wheat bulb fly

Project Report No. 598

Crop management guidelines for minimising wheat yield losses from wheat bulb fly

Kate Storer, Steve Ellis and Pete Berry ADAS High Mowthorpe, Duggleby, Malton, North Yorkshire YO17 8BP 

Abstract

Wheat bulb fly (WBF) is one of the most serious pests of winter wheat and is particularly prevalent in the east of England and Scotland. WBF lays eggs in bare ground during the summer and its larvae hatch during winter and can reduce the yield of wheat by killing shoots and reducing final ear number. The potential yield loss depends on the shoot population in winter, the size of pest population and how much damage an individual larvae can cause.  The aim of this project is to develop a pest threshold scheme to predict the minimum plant population, latest sowing date and need for an insecticide treatment (seed or foliar) to minimise the risk of yield losses to WBF. 

A WBF threshold scheme was developed that used information from the autumn survey of WBF incidence in September, egg viability, the maximum shoot number the crop could achieve by late winter, and the number of shoots that a single WBF larvae could destroy. This model showed that typical variation in the maximum shoot number had a large effect on the chance of yield loss because well grown crops produce excess shoots which can be sacrificed without affecting yield. A model of shoot production was developed based on thermal time and plant population that was embedded within the WBF threshold scheme.  This was done to allow a prediction of yield loss from WBF to be made in time for decisions about sowing date and seed rate. A review of literature showed that most WBF mortality occurs in the larval stage between egg hatch and plant invasion. The lowest level of mortality recorded was 56% and this value was used to help calculate the numbers of shoots likely to be lost to the pest. The literature also suggested that the number of shoots destroyed by an individual WBF larva was typically four.

Five winter wheat field experiments with combinations of sowing date, seed rate, variety, seed or foliar insecticide treatments were set up in the 2015-16 and 2016-17 seasons to calibrate and test the WBF threshold scheme and the shoot production model. Independent tests showed the shoot production model performed reasonably well, but it should be recognised that it does not deal with site specific factors that may limit tillering (e.g. soil capping). Some field experiments were deliberately done at sites which historically have been at high risk of WBF damage, however there was insufficient pest pressure against which to effectively test the threshold scheme due to nationally low levels of pest oviposition. The potential to estimate WBF prevalence using water trapping, rather than laborious egg counts from soil samples, was assessed by reviewing literature and testing at 12 sites. The literature on this topic demonstrated that this approach should work, however the field tests were inconclusive due to low WBF egg levels in the seasons of testing.

The project has developed prototype guidelines summarising how sowing date and plant population should be adjusted, and insecticide seed treatments targeted, for different WBF risk situations. Further work is required to field test the WBF threshold scheme in situations of high WBF pressure, and to develop it to deal with varietal differences in shoot number and site specific factors. 

 

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