Modification of the triangle method of degree‐day accumulation to allow for behavioural thermoregulation in insects

Abstract

SUMMARY The ability to predict insect development in field situations is fundamental to pest management programmes and in the understanding of insect phenology. Basic modelling techniques, however, fail to take into account behavioural thermoregulation by larval or nymphal insects, which has been shown to result in substantial increases in body temperature relative to ambient, particularly in species which bask. 2. The triangle method of degree‐day accumulation formulated by Sevacherian, Stern & Mueller (1977), which incorporates daily maximum and minimum air temperatures, is modified here to include thermoregulation data in the form of a linear body/ambient temperature relationship and daily sunshine hours. Using developmental and larval thermoregulatory data for the nymphalid butterfly Aglais urticae (small tortoiseshell), it was calculated that there was a mean increase of 75% in the availability of degree‐days for larval development, for the period April to September inclusive (using 10 years of meteorological data for Birmingham, UK). 4. Implications for this species' phenology was demonstrated by constructing a simple model. By taking larval thermoregulation into account, the modified method performed better than the original triangle method in predicting patterns of adult emergence: it is suggested that without larval thermoregulation, A. urticae would typically be univoltine in central England, whereas it actually achieves two generations per year.