Predicting spring emergence of blackcurrant leaf midge (Dasineura tetensi) from air temperatures

Abstract

AbstractThe mean duration of post‐diapause development of overwintered Dasineura tetensi larvae (in cocoons) was 72.8 (SD=11.4), 45.9 (SD=8.6), 28.7 (SD=6.0), 15.9 (SD=4.3), 10.4 (SD=1.9) and 10.2 (SD=1.8) days at constant temperatures of respectively 10, 12.5, 15, 17.5, 20 and 25 °C in the laboratory. No perceptible development occurred at 5 or 7.5 °C and complete mortality occurred when larvae were held at 30 °C for prolonged periods. The relationship between development rate (r days−1) and temperature (T °C) was sigmoidal between 10 and 25 °C, the logistic equation r=0.0158+0.085/(1+exp(−0.696(T−17.0))) accounting for 98% of the variation. Larvae entered the winter in diapause. Populations of cocoons were greatest in the surface soil in the centre of bushes adjacent to the crown, 69, 15, 9 and 6% of cocoons occurring in the top 0–1, 1–2, 2–3 and 3–4 cm of the soil, respectively. The time of termination of diapause in the field varied greatly between individuals and from season to season but a significant proportion (>40%) had broken diapause by the end of January in each of the three seasons studied. Diapause was not terminated in the laboratory by chilling over‐wintered larvae in cocoons at −2.5, 2.5 or 10 °C for up to 28 days nor when held in a L16:D8 photoperiod. A computer‐based phenological forecasting model was constructed using the development rate values (using the INSIM software developed at The Agricultural University, Wageningen, The Netherlands). The model accumulated daily development amounts calculated from daily maximum and minimum air temperatures from 1 February, the end of the coldest period of the year on average and before significant post‐diapause development occurred. The model uses boxcar trains to simulate dispersion. The model predicted the time of first emergence of D. tetensi adults in spring at HRI‐East Malling generally to within 6 days of the observed time of emergence, and to within 11 days at worst. There was poorer agreement between observed and predicted times of emergence when daily maximum and minimum soil temperatures (depth ca. 3 cm) were used. The use of the model to time insecticidal sprays in relation to the flowering time of blackcurrant is discussed.