CHANGES IN WHEAT BULB FLY (DELIA-COARCTATA) POPULATIONS IN EAST-ANGLIA IN RELATION TO CROP ROTATIONS, CLIMATIC DATA AND DAMAGE FORECASTING

Records of wheat bulb fly egg (Delia coarctata) population densities i n fields sampled throughout East Anglia in the autumns of 1953-1990 ar e presented. In descending order of risk, fallow, potato, pea (mainly vining), sugar beet and oilseed rape are the main crops preceding whea t or barley which attract oviposition. A declining trend of egg popula tions observed over the study period may be associated with climatic c hanges as well as with the elimination of fallow in the rotation and t he dramatic increase in the use of insecticides against the pest. Nega tive correlations (P < 0.05) in mean annual egg numbers were found wit h departures from average of July temperature and January air or soil temperature; positive correlations (P < 0.05) with departure from aver age of August raindays. Stepwise regression analysis was used to ident ify the most important relationships of meteorological variables with mean annual egg numbers, or the proportion of fields sampled with egg numbers in excess of the action threshold of 2.5 million eggs/ha. Up t o 59% of the variation in the annual proportion of fields above thresh old was accounted for in a regression equation using departures from a verage of July temperature, August raindays and the percentage of aver age of October (preceding year) rainfall. Estimated mean annual egg po pulations and the proportion of fields above threshold showed a good f it with the observed values. The findings are discussed and compared w ith previous work. The forecasting model may be readily incorporated a s a regional risk-prediction component of a knowledge-based system for the management of wheat bulb fly control. Regional forecasts of wheat bulb fly oviposition from this work should be tested and modified as necessary according to experience or changing climatic or agricultural factors.