Biocontrol of diamondback moth (Plutella xylostella) in organic crops: Spatial and seasonal dynamics

Biocontrol presents a sustainable alternative to insecticides by using natural enemies to reduce pest populations. However, the occurrence of biocontrol agents and the services that they provide, varies across space and time. Factors that influence service provision for a key economic pest, the diamondback moth (Plutella xylostella) were investigated in field studies on commercial organic farms in southeast Queensland, Australia. Cohorts of the pest were placed on cabbage plants within cages that were either open or closed to natural enemies and then placed at varying distances from adjacent non-crop habitat to determine spatial variation in pest mortality. Seasonal variability was captured in repeated trials over three years. Plutella xylostella mortality was significantly higher in cages open to natural enemies. Marginal death rates due to parasitism from Diadegma semiclausum, which was the only parasitoid reared from larvae, were significantly higher than any other mortality factor. Beta regression models found parasitism and predation were predicted by seasonal, but not spatial treatments. Mortality due to parasitism was highest in cooler seasons, while predation was variable, but significantly lower in cooler seasons. This suggests that P. xylostella populations are primarily regulated by D. semiclausum on organic farms in southeast Queensland, but pest suppression is reduced when warmer temperatures occur, risking pest outbreaks. This study provides rare direct quantification of the contribution of arthropod natural enemies to pest population suppression on farms. The deployment of additional biocontrol agents may increase this ecosystem service and its stability across seasons.