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Improving the efficacy of chemical control of wheat bulb fly

HGCA PROJECT REPORT 226

Improving the efficacy of chemical control of wheat bulb fly

by
J E B YOUNG AND S A ELLIS

MAY 2000

Abstract

Wheat bulb fly (Delia coarctata) is a frequent and serious pest of cereals in eastern and north eastern areas of the UK. Wheat and barley are often attacked when sown after crops such as potatoes, sugar beet and vining peas or on land which was bare fallow during the summer months. The chemical control of wheat bulb fly has relied heavily on the use of organophosphorus insecticides. In recent years, many of these have been withdrawn, leaving a much diminished range of products to combat the pest. The aim of this project was to investigate ways of optimising the use and performance of some of the currently available wheat bulb fly insecticides. There were two themes of research in the project. Firstly, the use of the systemic insecticide dimethoate was investigated to establish if the efficacy of this chemical could be improved by manipulating a range of factors including; spray timing in relation to the developmental stages of the crop and pest; spray application rates and spray volumes; sequential applications and the use of adjuvants. Secondly, a recently approved synthetic pyrethroid seed treatment, tefluthrin (Evict), was evaluated to determine its efficacy and value in wheat bulb fly control strategies. Variable levels of pest control were obtained with dimethoate. In general, targetting different larval instars or crop growth stages had little impact on insecticide efficacy. However, there was some indication that dimethoate sprays applied to coincide with peak invasion by first instar (stage) larvae was more effective in preventing yield loss than later applied sprays. Spray volume, at 100, 200 and 300 litres/ha, did not to influence the level of pest control. A two-spray programme of dimethoate applied at the full label-recommended rate, timed to coincide with peak invasion by first instar (stage) larvae, with the second spray applied 10-14 days after the first, gave better control of wheat bulb fly larvae and crop damage than single sprays. The use of the adjuvants LI-700, Arma, Slippa and Actipron with one or two sprays of dimethoate did not improve the level of pest control or reduce crop damage. There were no significant increases in yield in response to any of the dimethoate treatments, owing primarily to compensatory crop growth and favourable weather. Tefluthrin seed treatment was compared with chlorpyrifos egg-hatch sprays (timed to coincide with the start of hatch of wheat bulb fly eggs) and an untreated control. In addition, the need for one or two follow-up sprays of dimethoate was also studied. Tefluthrin seed treatment reduced wheat bulb fly larval numbers and crop damage and improved crop yield by only 3% in the two December-sown crops studied. Chlorpyrifos sprays gave a lower level of control compared with tefluthrin, but their performance was disadvantaged by the organic soil at one site and a late-germinating crop at the other. There was no benefit in yield from following-up the seed treatment or egg hatch sprays with additional sprays of dimethoate, although dimethoate did give some control of larvae and crop damage. The results highlighted the ability of wheat to compensate for attack and a continuing need to develop systems to predict in which crops the control of wheat bulb fly will be cost-effective.

SUMMARY

Wheat bulb fly (Delia coarctata) is a frequent and serious pest of cereals in eastern and north eastern areas of the UK. The fly lays its eggs on bare or partially covered soil in July and August. Wheat and barley are attacked, normally when sown after crops such as potatoes, sugar beet and vining peas or on land which was fallowed (e.g. set-aside) during the previous summer. In most years, egg hatch and larval invasion of the crop starts in January and continues until late March. Each larva feeds within an individual stem or tiller and migrates between shoots as it grows, eventually completing three larval instars (stages) prior to pupation in April. Attacked shoots turn yellow and die, giving rise to the well known 'deadheart' symptoms.

The chemical control of wheat bulb fly has traditionally relied heavily on the use of organophosphorus insecticides. In recent years, many of these have been withdrawn, leaving a much diminished product range with which to combat the pest. Seed treatments and egg hatch sprays are available as preventive treatments designed to kill larvae before they penetrate the plants. Egg hatch sprays are applied to coincide with the start of egg hatch in early January. In contrast, deadheart sprays, in the form of dimethoate, are the final line of defence against the pest. This systemic insecticide cannot prevent the initial phase of larval invasion but kills maggots already feeding within plants and prevents the secondary phase of damage as larvae move between shoots.

The three-year HGCA research project titled 'Improving chemical control options for wheat bulb fly' started in autumn 1996. The project was done at field sites in Cambridgeshire and North Yorkshire and comprised of two distinct parts, the overall aim of which was to refine recommendations and guidelines to obtain maximum efficacy from two wheat bulb fly insecticides. In Part I, the efficacy of dimethoate against wheat bulb fly was investigated in relation to range of factors capable of influencing the performance of this product. In Part II, the efficacy of the recently released synthetic pyrethroid seed treatment, tefluthrin (Evict) was evaluated in comparison with chlorpyrifos egg hatch sprays, alone, or in combination with follow-up sprays of dimethoate.

HGCA Project Number: 1996
Price: £7.50

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