Control of wheat bulb fly in winter wheat: I. Chemical methods II. Varietal susceptibility

Abstract

From 1987 to 1991, sixteen trials were done in eastern England to evaluate a range of chemical control strategies for the control of wheat bulb fly (Delia coarctata) in winter wheat. The risk of wheat bulb fly damage was estimated by sampling egg populations in the autumn. Four trials were done in each of the following categories of risk: Category A, low risk, fewer than 1.0 million eggs/ha; Category B, medium risk, 1.0 - 2.5 million eggs/ha; Category C, high risk, 2.5 - 5.0 million eggs/ha; Category D, very high risk, more than 5.0 million eggs/ha. Each trial included early (October) and late(November) sowing dates to create a varying order of crop susceptibility to wheat bulb fly damage. There were four core treatments at each site: 1) untreated; 2) omethoate, applied when larval damage was first seen (dead-heart spray); 3) a full treatment of fonofos seed treatment, fonofos granules applied at sowing, fonofos spray applied at the start of egg hatch and omethoate spray applied at the dead-heart stage; 4) fonofos seed treatment. The full treatment was not applied as a commercially viable recommendation but to give maximum elimination of wheat bulb fly for damage assessment purposes. In addition to the core treatments, combinations of preventive treatments (including fonofos seed treatment, fonofos granules at sowing, fonofos spray at sowing or at the start of egg hatch and a mixture of chlorpyrifos plus dimethoate applied at peak egg hatch) were used in conjunction with the varying levels of risk to wheat bulb fly damage.

Two successive mild winters, in 1988/89 and 1989/90, stimulated crop growth and offset the severity of wheat bulb fly damage. Assessment of risk, based on wheat bulb fly egg populations, was in some cases unreliable owing to a large mortality of eggs or larvae prior to plant invasion, particularly in the very high risk Category D sites. This observation is reflected in the reduced levels of pest attack and the lack of yield response to treatment in the Category D sites.

Wheat sown before November did not show statistically significant or economically profitable yield increases in response to treatment when subjected to wheat bulb fly egg populations less than the accepted threshold for autumn sowings of 2.5 million wheat bulb fly eggs/ha. November and December sowings were confirmed as being more vulnerable to wheat bulb fly damage than earlier sowings, which is reflected in the higher levels of preventive treatments proposed for late-autumn sowings. Of the 159 insecticidal treatments considered, a total of 23 statistically significant (P < 0.05) yield increases over control occurred in the late sowings, compared with 10 in the early sowings.

Fonofos seed treatment was more effective in the late sowings than the early sowings, confirming the value of wheat bulb fly seed treatment as a low-cost preventive measure. Seedbed treatments, egg hatch sprays and dead-heart sprays all demonstrated varying degrees of control of wheat bulb fly and their relative merits are discussed. Multiple treatments were shown to be cost-effective in some of the high-risk sites; however, they would not be economically viable in the absence of a severe attack of the pest. The differing levels of pest control and yield responses of each treatment have been used to support a range of proposed chemical control strategies for the various levels of risk based on egg numbers. The need for a more precise means of forecasting the severity of wheat bulb fly damage, allied with improved management of wheat bulb fly control strategies, is called for and discussed.