Understanding the thermal biology of macadamia nut borer, Thaumato(bia batrachopa (Meyrick) (Lepidoptera: Tortricidae): a pest of macadamia in South Africa

Abstract

Responses and adaptaSon of Thaumato<bia batrachopa to environmental changes and the impact of how it responds and perhaps adapts to these changes and fluctuaSons are complex subjects. Thaumato<bia batrachopa’s life stages, body temperatures, and various life cycle occurrences schedules are primarily influenced by temperature and other ecological factors. There is currently no evidence on the role of temperature on the development duraSon of T. batrachopa life stages, nor the outcomes regarding developmental rates and thermal responses. Understanding the mechanisms through which the temperature affects T. batrachopa’s life stages should contribute to developing a model of the potenSal responses of this pest to climate change. Thermal biology studies should reveal the thermal breadth of T. batrachopa, which can be applied to current and future climate scenarios to plan and execute integrated pest management (IPM) strategies. Therefore, we aimed to explore the thermal biology of T. batrachopa, to build up some knowledge on the impact of temperature on populaSon phenology and predict the temperature-dependent populaSon abundance at short and intermediate Sme scales in this study. This was achieved through the following objecSves; i) To study the effects of temperature on the development rate of T. batrachopa life stages by monitoring temperature-dependent development duraSon from egg to adult stage, sex raSo, longevity, and fecundity of adults under a range of constant temperatures, ii) To determine thermal tolerance responses of T. batrachopa life stages, establishing criScal thermal limits (CTLs) in response to acclimaSon, lethal limits, and heat and cold hardening, and iii) To evaluate degree day models for T. batrachopa, predicSng generaSons it can complete in the orchards, and invesSgate linkages between variability of microclimaSc data and populaSon dynamics of T. batrachopa in macadamia orchards at different climate zones. Effects of temperature on the development duraSon of T. batrachopa life stages were explored by rearing from eggs to adults at four constant temperatures. Fecundity and longevity were assessed through mated pairs of opposite sex and recording their mortality. The temperature-dependent models were derived by quanSfying the development rate at each temperature, followed by fi[ng the nonlinear models to select the model that best esSmates the thermal requirements for development. The thermal tolerance study focused on acclimaSng T. batrachopa life stages at three temperatures and evaluaSng effects through CTL assays on acclimated individuals. Rapid hardening was explored by exposing T. batrachopa life stages to cold and warm temperatures, before assessing their responses to sublethal temperatures. The impact of climate zones on generaSon number, thermal stress (heat and cold), thermal safety margin (TSM), and relaSve fitness across four macadamia orchards was examined using phenological model setup and simulaSon of thermal requirements and physiological traits from Chapter 2 and Chapter 3, respecSvely. The development duraSon of eggs, larvae, and pupae of T. batrachopa egg was significantly shorter in high temperatures. Mated adults lived longer at 16°C, with females deposiSng more eggs than at 25 and 31°C temperatures. The temperature-dependent development model that best fi'ed the development rate of T. batrachopa life stages was HarcourtYee_82 but could not esSmate Tmin. Therefore, Briere1_99 esSmated the lower development temperature (Tmin) for all life stages, and the upper development temperature (Tmax), and opSmum temperature (Topt) were esSmated by HarcourtYee_82. The Degree Day required for T. batrachopa from eggs to adults was 564.72DD. The life stages of T. batrachopa showed significant variaSon in their thermal tolerance. The acclimaSon ability of T. batrachopa varied with acclimaSon temperatures and life stages, and the results were more pronounced for larval stages. The larval stages had higher criScal thermal minimum (CTmin) and criScal thermal maximum (CTmax) than adults across all acclimaSon temperatures. Instar 4 larvae were the most heat and cold-tolerant aier acclimaSon across all temperatures. Pre-exposure and exposure for two hours didn’t significantly improve each life stage's survival. However, the adults had a higher %survival than larval stages aier rapid heat hardening (RHH) and rapid cold hardening (RCH). The predicted generaSons were high at low elevaSons (6.8 generaSons). The T. batrachopa populaSon was thermally heat-stressed with the highest TSM and poor relaSve fitness in high-elevaSon climate zones. Our findings show that T. batrachopa develops rapidly at warm temperatures (25 and 31°C), leading to more generaSons in the summer months. This might result in higher infestaSons and increased damage to macadamia nuts. The T. batrachopa’s ability to acclimate at low and high-temperature variaSons implies it can adapt, survive, and persist under climate change. PredicSons show that Orchards in low-elevaSon climate zones will experience higher infestaSons and greater damage to macadamia nuts due to the increased number of T. batrachopa generaSons.