The role of year-round vegetation in agricultural ditches as refugia for key insect pests and their natural enemies in Ventura County

Agricultural ditches can maintain vegetation year-round providing resources and refuge to arthropods, both insect natural enemies and pests. In Ventura County, California, three insect pests, Plutella xylostella, Lygus herperus, and Frankliniella occidentalis, cause millions of dollars in crop damage. We studied the role of ditch vegetation as refugia for these pests and their predator-parasitoid complex. Arthropods were sampled between November 2023 and September 2024 at 8 drainage ditches adjacent to vegetable and berry crops. Both insect pest and general predator abundance increased significantly in June and September, but general predators were present year-round. P. xylostella and L. herperus were also significantly more abundant in plant species of the family Brassicaceae, while F. occidentalis did not show a preference. General predators used a large variety of plants as resources changed through the seasons. Percent vegetation cover was negatively correlated to P. xylostella abundance and positively correlated for F. occidentalis. Increased proportion of bare ground in ditches decreased abundance of L. herperus and increased it for F. occidentalis. For general predators, an increase in vegetation cover significantly correlated with higher abundance while bare ground had a small negative effect. Interestingly, plant species richness had a negative effect on general predator abundance, but it did not impact the abundance of the insect pests. This research thus far identified more than 4,000 parasitoid Hymenoptera from 18 different families. The findings suggest that improving vegetation management in ditches could help reduce pest sources and enhance the effectiveness of beneficial arthropods.