Behavioural and physical reactions of the Colorado potato beetle, Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae) walking on a slanted surface.

A natural reaction of the adult Colorado potato beetle [Leptinotarsa decemlineata (Say)] is to walk uphill on sloped surfaces. The geotaxis reaction of the beetle was observed on slopes of 20 degrees or steeper. It was demonstrated that the uphill orientation behaviour was not a consequence of physical limitation for across-slope locomotion. The walking speed of insects deviating from the fall line did not change within the range of slope angles tested. The speed of adult beetles decreased with an increase in the slope of the substrate as a reaction to the increased gravitational force vector opposing uphill movement. The larger size of the hind legs might make uphill locomotion more efficient than traversing a sloped surface. As the angle of the slope increased, the gait changed from a 3/3 to a 5/1, as did the posterior and anterior extreme position of the legs. This behaviour might be triggered by the need to maintain balance on slanted surfaces as the vertical projection of the centre of mass on the substrate moved outside the support base pattern at the steeper angles. In one experiment beetles were made to pull a load when walking over a horizontal surface. The loads pulled were equivalent to the gravitational loads opposing forward motion when walking up a slope. No differences in forward speed or gait were observed at the lower-angle equivalent compared to beetles walking on slopes. Differences in speed were noted at slope angles higher than 40 degrees indicating that adaptation of the walking strategy might be needed on steeply slanted surfaces.