Environmental determinants of the abundance of the fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae)

Abstract

Expanded global trade and improved modes of transportation have shortened the duration of movement between countries but have also increased and aided the spread of alien species. The latter has been aggravated by the changing climate and is expected to worsen in the coming years. Invasive insect species are a particularly serious problem when they infest agricultural crops. One such alien invasive species is the fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), a notorious pest of many grass crops, including maize (Zea mays L.). FAW is native to tropical and sub-tropical regions of North, Central and South America. It invaded South Africa in January 2017. This pest does not exhibit diapause and therefore temperature has a crucial influence on its fitness and geographic distribution. During harsh winter temperatures, FAW migrates to areas with milder temperatures and re-establishes when temperatures are conducive. This study aims to determine (i) critical thermal minima (CTmin) and maxima (CTmax) of FAW and how life stage (first instar, sixth instar and adults), thermal history and diet affect the critical thermal limits of this insect. I also determine FAW infestation prevalence and relate this to the critical thermal limits of the pest to the recorded hourly temperatures in two maize fields in South Africa over 14 months. FAW populations were fed on three different diets (maize, wheat, and chickpea-based artificial diet) and acclimated for 24 hours at one of three temperatures (20, 25 and 30°C). Critical thermal limits were determined by placing individual insects into a test tube immersed into a water bath and decreasing (for CTmin) or increasing (for CTmax) the temperatures at a constant rate (0.1°C.min-1) until the insect lost muscle function. Both CTmin and CTmax differed significantly among life stages. CTmin of first instar larvae exhibited plasticity only in response to acclimation temperatures, whereas sixth instar larvae were sensitive to both acclimation and diet. Adult CTmin was not affected by either acclimation or diet. Acclimation and diet influenced the CTmax of first instar larvae, but only diet affected the CTmax of sixth instars. Only acclimation had an influence on adult CTmax. There were also complex interaction effects in both cold and heat tolerance. To determine FAW infestation prevalence, maize fields at three farms, Loskop Prison farm at Brits, North West province, Baviaanspoort (Pretoria) and Zonderwater (Cullinan) Prison farms, Gauteng province, South Africa, were inspected. Hourly weather data near these three locations were obtained for a duration of 14 months and compared to the CTmin and CTmax of the life stages of FAW. There was a high infestation of FAW larvae on Loskop Prison farm (67 and 88 % infestation), but no FAW presence was observed at Zonderwater Prison or Bavaiaansport Prison farms. This was mainly attributed to the cultivars planted but may have also been due to the presence of natural enemies and state of the maize plants. The Brits farm cultivated non-Bt maize whereas the Zonderwater Prison farm grew Bt maize. The maize field at Baviaanspoort Prison farm was too dry for FAW infestation. Temperatures measured near these locations did not go above the CTmax of any FAW life stage, regardless of the season. However, winter temperatures did go below the CTmin of all life stages, although at differing frequencies and durations. These winter temperatures below CTmin may not be occurring at a frequency high enough and duration long enough to significantly reduce the FAW population in these regions. The difference in frequency and duration of environmental temperatures lower than CTmin between life stages indicates how temperature stress can vary for individual life stages of the same species, which may have implications for the implementation of pest control strategies. This study demonstrates the complexity of thermal tolerance and emphasizes the importance of including different life stages in thermal biology studies. The biochemical changes responsible for the observed differences in thermal biology between life stages need further investigation.