DBM development : are we measuring the right temperatures ?

We modelled the population phenology of the diamondback moth (DBM), Plutella xylostella (L.) (Yponomeutidae) with a simple degree day model implemented using DYMEX, a population dynamics modelling program. Predicted and observed developmental periods of DBM reared under field conditions during the months of October 1999 to February 2000 at Forthside Research Station, Tasmania (which is in the temperate zone) were compared. Threshold temperature values of the second and third instar larvae were modified using values from previously published data to obtain a marginally closer fit to the observed developmental periods. The model overestimated the developmental periods when standard daily Stevenson’s screen temperatures were used. Better predictions were obtained using daily temperatures recorded at 20 cm above ground level. We test this further by comparing observed and predicted development for DBM cohorts reared on potted Chinese cabbage under open, semi-shade, fully shaded and glasshouse conditions during August to November 2000 in Brisbane (which is in the sub-tropics). Temperatures recorded on the plant surface gave good predictions of development period from egg to adult emergence for all cohorts. The development model needs to be tested under a wider range of conditions, but our results suggest DBM development can be readily modelled. This work can form the basis of attempts to forecast DBM population dynamics and interpret population phenology.