Day-degree requirement (>9.4°C) for initial oviposition (IO0.5) in Tetranychus urticae as a function of accumulated cold rest (hours with temperatures < 4.0°C) for all (y = 63.9 − 0.0198 x, P < 0.01, r2 = 0.65); coastal (triangles, y = 56.9 − 0.0160 x, P > 0.05, r2 = 0.40); and inland (circles, y = 78.3 − 0.0279 x, P = 0.01, r2 = 0.83) sites
Individual estimates of the thermal requirement for each
calibration population were negatively correlated with cold rest
duration ( P < 0.01, Fig. 6 ).
However, when cold rest duration was tracked in the model and the
thermal requirement adjusted, unbiased model predictions for the
calibration populations (Fig. 5 b) were obtained only when
separate cold rest regressions for coastal and inland sites
(Fig. 6 ) were used to adjust the thermal requirement.
Predictions were based on a start day of 27 November and a
a mean thermal requirement of 34.9 dd (>9.4°C); or
b a thermal requirement that varied with accumulated cold
rest (Fig. 6 ).
However, the observation is consistent with the need to use
separate regressions for the two sites with respect to the
relationship between the thermal requirement and cold rest
(Fig. 6 ), and may reflect biological differences related to
the milder climate at coastal sites..
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