Habitat manipulation in lucerne Medicago sativa: arthropod population dynamics in harvested and ‘refuge’ crop strips

Abstract

Summary In order to realize the full potential of natural enemies in integrated pest management, it is necessary to understand their population dynamics, particularly in relation to the role of refuges. Harvested and unharvested plots were established in a commercial hay lucerne Medicago sativa crop and the potential for the unharvested vegetation to constitute a refuge for natural enemies of Helicoverpa spp., one of the key pests, was evaluated. The temporal and spatial dynamics of arthropod populations were evaluated using sequential samples from directional traps (pitfall and Malaise) and vacuum sampling in both crop treatments. The spatial dynamics of predation were measured using ‘sentinel’ cards baited with Helicoverpa spp. eggs. There was no significant directional movement of arthropods captured in pitfall traps, except for a 3‐day period immediately after harvest when there was a net movement of the natural enemies Dicranolaius bellulus, Carabidae adults, spiders and the phytophagous mite Halotydeus destructor from harvested to unharvested plots. There was net movement of adult Netelia producta, D. bellulus, Coccinella transversalis and Syrphidae as well as the pest Helicoverpa spp. from harvested to unharvested plots for 24 h following a harvest. Densities of D. bellulus and C. transversalis within unharvested plots decreased with increasing distance from the borders with harvested plots. Predation rates of Helicoverpa eggs placed in unharvested plots were broadly consistent with predator densities and decreased with distance from the harvested plots. Predation rates of Helicoverpa eggs placed in harvested plots declined with increasing distance from unharvested plots, suggesting that natural enemies from the latter dispersed into regrowing lucerne. Strips narrower than the plot width used (30 m) may be adequate to accommodate natural enemies displaced by harvesting. Unharvested refuge strips should ideally be spaced less than 30 m apart so that the natural enemies can contribute to biological control of pests over the entire width of harvested strips.