Laboratory and field studies and mathematical modelling of the three-dimensional distribution of adult Cryptolestes ferrugineus in wheat bulk

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S K, Vimala Bharathi
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Cryptolestes ferrugineus (Stephens), commonly referred to as rusty grain beetle, is a cosmopolitan pest and can survive in wide environmental conditions, including temperate and tropical areas with humid and dry environments. Understanding its movement and distribution in bulk grain would help to develop an effective stored grain management protocol. The one- and two-dimensional movement of the insects have been well established under laboratory conditions. The current work aimed to address the knowledge gap in the three-dimensional movement of rusty grain beetle under laboratory and field conditions. To understand the three-dimensional movement and distribution of C. ferrugineus, an experimental setup, consisting of 343 cubes arranged inside a wooden box, was designed. Using the designed setup, the experiments were performed in the laboratory at various uniform temperatures (20, 30, and 35ºC), moisture contents (12.5, 14.5, and 16.5%), insect densities (0.35, 1.77, and 3.53 adults (A) /kg), and movement periods (6, 24, and 72 h). The results of the study revealed that (i) at uniform environmental conditions, C. ferrugineus adults moved downward initially, then moved up and horizontally; (ii) the movement of adults was faster at higher temperature (35ºC) than at low temperature (20ºC); (iii) insects tend to disperse more at higher densities (3.53 A/kg); (iv) the effects of temperatures, moisture contents, insect densities, and movement periods observed in three-dimensional grain bulk were similar to those reported in one-dimensional grain columns and two-dimensional grain chambers. To perform a field experiment, about 300 t of wheat were filled inside an un-aerated, 10 m diameter corrugated steel bin in Winnipeg, Canada. About 75000 C. ferrugineus adults (insect density of 0.25 A/kg) were introduced at the centre surface of the bin in September 2019 and about the same number of Tribolium castaneum (Herbst) adults were introduced at the same location during September 2020. The bin was equipped with temperature sensors and insect sensors (Insectors®) and the hourly temperature and insect counts were monitored from September 2019 to October 2021. In addition, about 20-g grain samples were collected at each of the 36 locations (at various depths and radial locations) from September 2019 to August 2021 for every month. The movement and distribution of C. ferrugineus observed during the field study were similar to the laboratory study under similar environmental conditions. In addition, the analytical solutions to calculate the three-dimensional diffusivity of insect movement were proposed, and the diffusivity was calculated at various homogeneous environmental conditions.
Cryptolestes ferrugineus, Stored Grain, Three-dimension, Movement and distribution, Temperature, Moisture content