Harnessing Nature’s Pest Control: How Predators and Parasitoids Fight Brassica Pests
by Jessa Thurman, March 2025
Jessa Thurman in the field establishing field cages, which are a great tool for assessing the impact of beneficials.
In the battle against crop pests, nature provides a powerful line of defense—beneficials (also known as natural enemies). My recent study explores how beneficials such as parasitoid wasps and predators help suppress populations of the diamondback moth (Plutella xylostella), a major pest of Brassica crops like cabbage, broccoli, and canola. During my PhD at the University of Queensland, I set up sentinel larvae of the diamondback moth on plants in cages that were exposed or excluded from beneficials. This allowed me to quantify how many larvae died from parasitism and predation on organic farms. I then conducted several trials throughout the crop season and across three years to determine how environmental factors influence this ecosystem service.
Graphical abstract for the new paper, “Biocontrol of diamondback moth (Plutella xylostella) in organic crops: Spatial and seasonal dynamics” published in Agriculture, Ecosystems & Environment by Dr. Jessa Thurman and her supervisor, Prof. Mike Furlong.
What did I find?
- Parasitism rate fluctuates with the seasons, with the highest levels occurring in cooler months. However, predation remained relatively stable across the crop seasons. Surveys in the field recovered lots of generalist predators like ants, ladybird beetles, spiders, and lacewings, which likely contributed to this pest suppression.
- Only one species of parasitoid wasp, Diadegma semiclausum, was recovered from larvae, but it contributed the most to mortality of the diamondback moth. This is likely because this wasp is supported in the field with early season releases by farmers.
- Proximity to non-crop habitat may influence predation and parasitism, as this habitat can be a refuge for beneficials. But we found no effect with the distance from non-crop (from 1m into the crop to the centre of the field at 90m).
The proportional survival of diamondback moth (DBM) and parasitoid wasps (Diadegma semiclausum) across the accumulated day-degrees experienced in the field. B) The marginal death rate of the diamondback moth due to parasitism or predation was also compared across day-degrees experienced in the field. Figures are from Thurman and Furlong, 2025.
What does this mean for growers?
These results highlight the importance of timing and environmental conditions when relying on beneficials for pest control. If parasitoid activity is highest in cooler months, then adjusting planting schedules or not spraying pesticides that impact parasitoids during this period could enhance their impact. For warmer seasons, alternative species like Cotesia vestalis should be considered for inoculative release. Meanwhile, maintaining a diverse predator community throughout the season can provide a more stable layer of protection, however predicting individual species impact can be hard as they can work together as a team to contribute to predation.
The average survival sentinel diamondback moth cohorts were followed from their first larval instar (L1) through to adulthood across cage (open or closed) and distance from non-crop habitat (1 m, 30m, and 90 m) treatments. Open cages had cohorts exposed to beneficials, while closed were excluded. Figure from Thurman and Furlong, 2025.
The full paper is open-access and now available online:
Thurman, J.H. and Furlong, M.J. (2025) ‘Biocontrol of diamondback moth (Plutella xylostella) in organic crops: Spatial and seasonal dynamics’, Agriculture, Ecosystems & Environment, 385. Available at: https://doi.org/10.1016/j.agee.2025.109567.