Impacts of weed management on ladybird (Coleoptera: Coccinellidae) abundance and diversity on resident vegetation in a citrus orchard

Citrus fruits are markedly affected by aphids on the island of Réunion (France). These pests are generally controlled with pesticides, which have adverse effects on human and environmental health. Furthermore, resident vegetation in orchards is often eliminated with herbicides due to suspected competition with citrus trees for resources. Yet this resident vegetation could serve as a habitat for natural enemies of crop pests, particularly ladybirds (Coleoptera: Coccinellidae), which are well-known aphid predators, thus reducing the need for pesticide treatments. Results of a 2-year study conducted in an experimental citrus orchard, on the impacts of four different weed management treatments on ladybird populations, viz: i) M: hedge-trimmer mowing; ii) T: disk harrow tilling; iii) HM: hammer mill crushing/mulching; iv) H: herbicide spraying are presented. Ladybirds were captured over a 5 week period each year using Malaise-derived flight interception traps. A total of 690 ladybird specimens, belonging to 12 species distributed across four subfamilies, were captured. These were mainly aphidophagous, with Scymnus constrictus Mulsant being largely dominant. There was a significant effect of distance to a field border with diversified vegetation on ladybird abundance and diversity. Ladybirds were also significantly more abundant on tall vegetation cover than on short cover. Although proportionately fewer ladybirds were trapped in the ‘H’ treatment than in the other three, there were no clearcut effects of weed management treatments on ladybird abundance. This study nevertheless confirmed the efficacy of interception traps for assessment of ladybird population abundance and diversity. It also demonstrated the potential of these natural enemies as bioindicators of habitat disturbance.