Exploiting asynchronous host-parasitoid emergence distributions to optimise insecticide applications and protect beneficial insects– a case study using the citrus gall wasp Bruchophagus fellis (Hymenoptera: Eurytomidae)

Abstract

The citrus gall wasp (CGW), Bruchophagus fellis (Girault) (Hymenoptera: Eurytomidae), is a serious pest of citrus in southern Australia. Severe infestations can result in yield loss and a reduction in fruit size. Several parasitoid species attack CGW, with Megastigmus brevivalvus (Girault) (Hymenoptera: Torymidae) (MBV) being the dominant species. Both CGW and MBV are univoltine and adult wasps emerge from galls in the spring. Median emergence of MBV lags behind that of CGW by 2–3 weeks. We modelled the effect of spray timing for foliar-applied, non-systemic insecticides on the relative exposures of CGW and MBV based on their emergence distributions and estimated the optimal spray timing that maximises the control of CGW while minimising the impact on MBV. Three temporal emergence distribution scenarios for the two species and 28 periods of residual insecticide activity were considered in the investigation. The results showed that the optimal spray timing was always before the peak emergence of CGW. Depending on the residual activity of the insecticide, the optimal spray date pre-dated the peak emergence of CGW by 1–24 days, with longer lead times for insecticides with longer residual activity. By contrast, the optimal spray date varied little across the different emergence scenarios, suggesting that the results apply over a wide range of locations and seasons. A single insecticide application at the optimal timing may not guarantee satisfactory control of CGW. When the residual activity of the insecticide is short, multiple applications with a combined residual activity of 16 days are needed to ensure insecticide contact with ≥90% of all CGW that emerge in a season.