Understanding the foraging ecology of the larger grain borer, a forthcoming biosecurity threat to the US

Prostephanus truncatus (Horn), the larger grain borer, is endemic to southern North America and Central America, but invaded Africa with severe economic consequences for maize and cassava production. Its main competitor is the cosmopolitan Sitophilus zeamais (Motschulsky), the maize weevil. While competition between these two species has been studied extensively, there is little information on spatial partitioning in bulk storage of grain. Thus, our aims were to evaluate the short-term niche partitioning behavior of P. truncatus and S. zeamais when placed in a monolayer column of grain. The monolayers were divided into 24 equally-sized vertical and horizontal zones to track location of the insects on maize in each zone. After one and then seven days, monolayers were placed on their side, and the grain and insects from each zone were removed. We found that both species generally aggregated together and preferred the bottom of the grain mass. To determine whether semiochemicals played a role, volatiles were collected from maize with P. truncatus, S. zeamais, or both species using headspace collection and gas chromatography coupled with mass spectrometry. Then, preference was evaluated in a 4-way olfactometer among the stimuli. We found maize volatiles were important for orientation of P. truncatus, but not S. zeamais, suggesting a confluence of factors other than volatile semiochemicals may lead both species to choose the bottom of the grain column. These data may help improve targeting of pest control tactics by identifying the position of these insects in the grain mass.