Thermal biology of the two-spotted stink bug, Bathycoelia distincta (Hemiptera: Pentatomidae) a major pest of macadamia

Abstract

The physiological functions of insects are mainly dictated by temperature because they rely on the external environment to regulate their body temperatures. This temperature dependence drives their performance, with profound implications on abundance and distribution. Daily environmental temperature fluctuations may outpace an insect’s thermal tolerance capacity, which requires physiological plastic mechanisms to survive. In the context of global warming, insects may be vulnerable to temperature variations, and may ultimately determine their population dynamics. The two-spotted stink bug, Bathycoelia distincta (Distant) (Hemiptera: Pentatomidae) is an important pest of macadamia in South Africa. It causes damage by direct feeding on the kernel and comprises more than 80 % of the shield bugs in the orchards. Increased crop losses due to stink bug damage threaten macadamia nut production. Modern pest management practices require an understanding of the biology of pests and ecology. An important question to pose is, how does an organism's thermal plastic traits affect its ecological population dynamics? First, this study aimed to quantify the effects of temperature on biological parameters of B. distincta life stages, such as the development rate, development duration, survival, adult longevity, pre-oviposition period, oviposition period, and life table parameters, to determine its thermal requirements and population growth at constant temperatures ranging from 19 to 29 °C. In addition, the effect of diet (macadamia nut and sweetcorn) on development, survival, and sex ratio was investigated at 25 °C (Chapter 2). Second, to quantify the phenotypic plasticity of B. distincta life stages. Two thermal tolerance indices were explored: rapid hardening (rapid heat hardening: RHH and cold hardening: RCH) and acclimation (critical thermal maximum: CTmax and minimum: CTmin). RHH and RCH were determined by exposing B. distincta life stages to extreme temperatures of 41 and -8 °C, respectively. Acclimation effects on CTmax and CTmin were quantified by exposing B. distincta life stages to 48 h at 20, 25, and 30 °C. Temperature was ramped up and down at a rate of 0.2 min-1 to score survival at high (CTmax) and low (CTmin) critical temperature points (Chapter 3). Thirdly, the development rate was monitored (on the host plant) at temperatures ranging from 18 to 40 °C to acquire the total heat required to complete development (degree-days) and thermal requirements of each life stage of B. distincta. iv Lastly, the physiological traits, degree-days, and thermal requirements were used to predict seasonal generation turn-over, heat and cold thermal stress, thermal safety margin (TSM), and relative fitness in macadamia orchards along an elevational gradient (705 - 1493 m a.s.l.) to determine the effects of climatic zones (Cwa climate zone: Monsoon-influenced humid subtropical climate, Cwb: Subtropical highland climate or Monsoon-influenced temperate oceanic climate, and Bsh: Hot semi-arid steppe climate) (Chapter 4). B. distincta developed at a wide range of temperatures on sweetcorn (18 to 29 °C) and macadamia nut (18 to 35 °C). The survival rate was high (51 to 100 %) between temperatures with a monotonic increase of population growth from 19 to 29 °C. The total number of heat units required to complete development was 783 DD. All life stages of B. distincta displayed thermal plasticity, but instar 2 was the most plastic stage except in response to cold acclimation. Response to extremes varied more at low extreme temperatures compared to high extremes. As expected, the number of generations decreased with increasing elevations from the Cwa (Arbor: 2.4 generations) to the Cwb climate zone (Highfield: 1.1 generations). None of the life stages experienced thermal stress. TSM and relative fitness were highest at the Cwb climate zone of the highest elevation. These findings suggest that B. distincta will potentially cause more damage in response to global warming because of its estimated population growth rate at elevated temperatures. Although relative fitness was highest in the Cwb climate zone, damage could be expected in the Cwa zone due to increased number of generations. This study can also help identify macadamia orchards in climate zones vulnerable to climate-related consequences such as outbreaks. Climatic data combined with the DD model can be used to predict the phenology of B. distincta and timing of chemical applications. The impacts of global warming on crop losses due to insect pests are evident worldwide, and this study has shown that macadamia orchards in certain climatic zones (e.g., Cwa climate zone) could be at risk of increased abundance of B. distincta. Thus, integrated pest management strategies should be of priority to macadamia farmers for effective management of B. distincta. Given that South Africa is the largest producer of macadamia globally and its major pest is thermal plastic, linking the physiological traits of B. distincta life stages to climatic conditions of all macadamia growing regions in South Africa will help understand its distribution limits.