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Dynamics of life history traits in Tetranychus ludeni Zacher in response to fluctuating temperatures

Tetranychus ludeni Zacher (Acari: Tetranychidae) is an invasive spider mite that currently occurs in Europe, Asia, Africa, America, and Australasia (CABI 2011; Migeon & Dorkeld 2018). It feeds on more than 300 plant species in 60 families and causes significant damage to a number of economically important crops such as eggplant, pepper, tomato, bean, pumpkin and other cucurbitaceous plants (Zhang 2002, 2003; Gotoh et al. 2015). In their study on effects of constant temperatures on several spider mite species, Gotoh et al. (2015) suggest that T. ludeni may be better adapted to warmer climate and predict that it could replace T. urticae to become a major pest of crops in the world. However, temperature in nature is fluctuating, typically higher during the daytime and lower during the nighttime, which could have different effects on organisms (Fischer et al. 2011; Paaijmans et al. 2013; Bowden et al. 2014; Gotoh et al. 2014; Zeh et al. 2014; Nachman & Gotoh 2015; Bayu et al. 2017). So far, how fluctuating thermal conditions affect life history traits of T. ludeni is unknown. In the present study, we investigated the plasticity of life history traits in response to fluctuating temperatures in T. ludeni to provide information for predicting its potential to invade and to become a major pest in different regions or conditions. We maintained a breeding colony of T. ludeni on 20 potted kidney bean plants (Phaseolus vulgaris L.) in the Massey University Entomology and IPM Laboratory, Palmerston North, New Zealand, at 25 ± 1°C temperature and 50 ~ 70% RH with a photoperiod of 16L:8D hours. We performed four fluctuating temperature treatments (15~21, 22~28, 29~35 and 36~42°C) with 20 replicates per treatment in growth chambers (I-36VL, Percival Scientific Inc., Perry, Iowa, USA). The temperature setting protocol is shown in Figure 1. For each replicate, we randomly collected 20 female and 4 male adults from the colony, introduced them onto a bean leaf disc (3 cm × 3 cm) placed upside down on a water saturated cotton pad in a Petri dish (5.5 cm diameter × 1.0 cm height), and allowed them to stay on the leaf disc for 24 hours at 25°C. We kept 50 eggs on the leaf disc, removed the redundant eggs using a soft paintbrush, and then placed the Petri dish with 50 eggs into a growth chamber at a test temperature. We replaced the leaf disc with a fresh one once every five days. We took each Petri dish from the test temperature once a day for a few minutes and recorded egg hatch, immature stage mortality and adult emergence under a stereomicroscope (Leica MZ12, Germany) at 25°C. During the few minute’s observation each day, we also sexed and removed emerged adults and recorded development period (from egg to adulthood) of each sex. To determine the effect of treatments on adult body size, we randomly selected two females and two males of the resultant adults per replicate and measured their size under a digital camera (Olympus SC30, Japan) connected to the stereomicroscope and a computer with adequate imaging software (CellSens® GS-ST-V1.7, Olympus, Japan) installed. To obtain information on the effect of treatments on reproductive outputs and adult longevity, we randomly selected 20 newly emerged females and 20 newly emerged males (< 1 d old) from each of the above treatments and individually