Climate change and potential selection for non-diapausing two-spotted spider mites on strawberry in southwestern British Columbia

Abstract

A validated model of the timing of post-diapause oviposition in the two-spotted spider mite, Tetranychus urticae Koch, was used to predict when 50% of strawberry leaflets with T. urticae also have T. urticae eggs (1005) in each year from 1954 to 2006 at Langley, British Columbia. This timing was studied in relation to hours of frost occurring before and after oviposition. Historically, 1005 occurred before there were frost-free days, but there was a clear threshold at 390 h with temperatures <0°C after 1005, which was not exceeded. This suggests that there is selection pressure for early oviposition, but also a limit to the extent of selection. The subzero temperature profile ~1 month before oviposition was clearly different from that after 1005. The number of hours with subzero temperatures 1 month before oviposition, and the standard deviation of those estimates, were negatively correlated with year and indicated that there could be oviposition in January - rather than February - by 2015. Cumulative hours with temperatures <0°C between 27 November (the empirical estimate of the time when T. urticae begins accumulating degree-days for post-diapause oviposition) and 30 April was negatively correlated with year, and extrapolation of a linear regression suggested that there could be selection for continuous armual oviposition by 2050. There was considerable variation in the data, but considered in combination with published evidence for climate change, these results will be important in developing pest management strategies, and furthermore, will impact many aspects of agriculture in the Fraser Valley of British Columbia.