Economic benefits of biological control of Sitona obsoletus (clover root weevil) in Southland pasture

Sitona obsoletus is a serious pasture pest in New Zealand where its root-feeding larvae reduce white clover cover and nitrogen fixation. To maintain production, farmers may compensate by increasing inputs. The parasitic wasp Microctonus aethiopoides Loan was introduced for biological control of S. obsoletus and achieved parasitism rates exceeding 70%. In Southland, where S. obsoletus was first detected in 2010, unusually severe and prolonged infestations during 2013 and 2014 prompted intensive biological control releases in 2014 and 2015. This study evaluated if they were cost effective in 2015. On dairy farms biological control returned $14.78/ha/year or $2.3 million over the 158,017 ha. On sheep and beef farms, the estimated return was $6.86/ha/year or $4.7 million over 719,854 ha. Monte Carlo simulations were used to estimate returns ($/ha/year) using plausible ranges of model parameter values, and returns were positive in at least 97.5% of simulations. Biological control of S. obsoletus has returned a net benefit in Southland.