Optimal environmental search and scattered orientations during movement of adult rusty grain beetles, Cryptolestes ferrugineus (Stephens), in grain bulks – suggested movement and distribution patterns

An understanding of insect movement and distribution within grain bulks is essential to effectively control insect pests. In this paper, opinions published in the literature and related to random, downward, and bias movements (the tendency to move in non-random directions) of adults of Cryptolestes ferrugineus have been compared and the net displacement of C. ferrugineus adults in stored-grain bulks was calculated. The average net displacement of the adults inside a stored-wheat mass was 0.435 ± 0.229 m/d. The causes of the scattered orientations were analyzed and the following points were suggested: 1) both experimental observations and theoretical analyses did not support the view that adults of C. ferrugineus exhibit a positive geotactic behavior; 2) the downward movement might be caused by adults' drifting; 3) during their bias movements in responding to environmental factors such as temperature gradients, adults did not significantly decrease their random movements; and 4) their dispersal inside granaries might be mainly influenced by their random movement, drift, and scattered orientations. At the individual level, they were always moving toward optimal environmental areas. At the population level, the random movement plus scattered orientations resulted in a diffusion distribution pattern. Comparison between the suggested diffusion pattern and the insect distributions published in the literature in the following areas was conducted: theoretical analyses of insect diffusion, movements and distributions under laboratory conditions, and dispersal in full-size granaries. Results show that the published insect distributions were consistent with the suggested diffusion pattern.