EXPERIMENTAL ASSESSMENT OF A DEGREE-DAY MODEL FOR PREDICTING THE DEVELOPMENT OF PARASITES IN THE FIELD

The adequacy of a degree-day model for predicting the development of parasites, and hence their potential generation turnover rate, in the field was experimentally tested. The parasite-host system used was Aphelinus asychis Walker and Diuraphis noxia (Mordwilko). Input data for the model tested included: 1. developmental parameters (''t'' and ''K'') of A. asychis and 2. observed, and long-term average, daily maximum and minimum air temperatures. The parameters ''t'' and ''K'' were obtained under constant temperature conditions in the laboratory using least squares linear regression analysis of temperature on percent rate of development. The developmental parameters for the egg-to-mummy development period of A. asychis were estimated as t = 5.4-degrees-C, and K = 142.5 degree-days. The upper developmental threshold was observed to be above 29.4-degrees-C. A paired sample t-test indicated that predicted developmental times, based both on observed and long-term average temperature data, were not significantly different from observed developmental times in the field. On average, differences between observed and predicted developmental times did not exceed one day. The underlying errors to which the differences between predicted and observed developmental time's can be attributed to are discussed. Results obtained point to the potential of using simple laboratory and modelling techniques as aids in assessing the biological control potential of newly imported parasites, or explaining the field performance of established ones, during evaluation processes.