Dispersal of the invasive pasture pest Heteronychus arator into areas of low population density: effects of sex and season, and implications for pest management

The African black beetle, Heteronychus arator (Scarabaeidae), is an exotic pest of pastures in northern New Zealand. Both adults and larvae feed on pasture grasses. Adults disperse by walking (short range) or flying (long range). Dispersal flights are triggered by warm night temperatures in spring and autumn. Short range adult dispersal in search of mates, food or oviposition sites is poorly understood. This study investigated ground dispersal of black beetle adults over three seasons in New Zealand pastures. Black beetle adult activity was monitored using pitfall traps along fence lines on a dairy farm in the Waikato region of New Zealand in the spring of 2013. In spring 2014 and autumn 2015 black beetle ground activity was also monitored in two 40 x 40m pasture plots that were 10m apart using a 10 x 10m array of pitfall traps. A biopesticide bait was applied to reduce beetle populations within one plot; the second plot was not treated. A mark-release-recapture study was conducted simultaneously with marked beetles released on the same day the biopesticide was applied. Pitfall traps were emptied twice weekly for 24-26 days post-treatment. Trap catches along the fence lines were correlated with air temperatures in 2013. Trap catches were male biased in spring compared with autumn. Trap numbers in the untreated plots were nearly double that of treated plots. More beetles were caught in the pitfall traps at the edges of the treated plots than in the centre. Trap catches were consistent throughout the untreated plot in spring 2014 but in autumn 2015 more beetles were caught in the centre of the untreated plot than at the edges. Few marked beetles were recaptured with dispersal rates estimated as <0.5m per day. Warmer temperatures encouraged short range dispersal in black beetles. Males were more active than females during the spring mating season. Edge effects were strong and should be considered in the design of field experiments.