Abstract
Lethal time50 (LTime50) and lethal temp (LTemp50) are commonly used laboratory indices of arthropod cold tolerance, with the former often being employed to predict winter survival in the field. In the present study, we compare the cold tolerance of different life‐history stages (nondiapausing and diapausing females, as well as males and juveniles) of a major agricultural pest: the two‐spot spider mite Tetranychus urticae Koch (Acarina: Tetranychidae). Diapausing females from European populations of this species are shown to be freeze avoiding, supercooling to −23.6 ± 0.37 °C and with an LTemp50 of −23.2 °C. However, nondiapausing females [supercooling point (SCP) –19.1 ± 0.49 °C, LTemp50 –14.32 °C], males (SCP –21.27 ± 0.52 °C, LTemp50 –16 °C) and juveniles (SCP –25.34 ± 0.29 °C, LTemp50 –18.3 °C) are subclassified as strongly chill tolerant juveniles. LTime50 is 148.3 days for non‐acclimated diapausing females, whereas nondiapausing females, males and juveniles reach 50% mortality by 21.7 days. When individuals are acclimated at 10 °C for a period of 7 days, no effect is found. Cold tolerance is suggested to be a major contributor to the successful spread of T. urticae across temperate countries, although it is dependent on a diapause trait, suggesting a potential target for control. Winter field trial data from diapausing females indicate that LTime50 is a reliable indicator of winter survival even within diapause, supporting the use of these indices as a valuable component within environmental niche models for the prediction of future pest invasions.
Highlights
Increasing evidence suggests that understanding the thermal of a species tolerance can enhance predictions of their potential to invade new geographical regions (Terblanche et al, 2006, 2007; Mitchell & Hoffmann, 2010; Alford et al, 2012)
The present study aimed to investigate the capacity of diapause and nondiapause stages of European T. urticae populations to survive different U.K. winter conditions, at the same time as comparing the efficacy of different laboratory-based cold tolerance indices (LTemp and Lethal time (LTime)) with respect to predicting winter survival
LTemp resulting in 50% (LTemp50) (Fig. 1a) and 90% (LTemp90) (Fig. 1b) mortality of the acclimated and non-acclimated life stages indicated that non-acclimated nondiapausing adult females were the least cold tolerant (LTemp50 = −14.2 ∘C), ing their cold survival with acclimation significantly enhanc(LTemp50 = −17.2 ∘C; non-overlapping fiducial limits) (Fig. 1)
Summary
Increasing evidence suggests that understanding the thermal of a species tolerance can enhance predictions of their potential to invade new geographical regions (Terblanche et al, 2006, 2007; Mitchell & Hoffmann, 2010; Alford et al, 2012). Ecological and environmental factors contribute to the environmental niche model (ENM), which takes the known distribution of a potentially invasive species and correlates it with these variables to predict population establishment in new regions (Elith & Leathwick, 2009; Jiménez-Valverde et al, 2011; Hill et al., 2013). Laboratory and field based stress physiology experiments often form the basis of establishment risk assessments with respect to providing a license for the use of novel biological control agents (BCAs) in non-native countries (van Lenteren et al, 2006). Studies on the dark sword grass moth (Agrotis ipsilon) suggest that a lack of cold hardiness may explain why this pest is unable to establish a permanent population in the U.K. (Bale, 2002), whereas increased high temperature tolerance in the mite Halotydeus destructor corresponds with predicted niche shifts and extended distributions of this species in Australia (Hill et al, 2013).
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