Seasonal and Spatial Occurrence of Aphid Natural Enemies in Organic Brassica Fields: Diversity, Phenology, and Reproduction

Abstract

A pest monitoring dataset collected for organic Brassica fields in south coastal British Columbia, Canada, was examined to test three possible scenarios explaining poor aphid control by natural enemy insects: the lack of diversity within the enemy guild, slow colonization of fields by enemies, and poor natural enemy reproduction. Both parasitoids and predators were detected, but the predator guild was composed primarily of syrphid larvae and very few coccinellids. We detected a lag in the colonization of fields by enemies: aphids were detected in >20% of samples, but enemies in <5% within 3 weeks of transplanting. Within the first week of transplanting, aphids exceeded the spray action threshold of 10% colonized plants. Although we predicted that Brassica plants located next to field margins would have more parasitism and syrphid oviposition, because of their proximity to floral resources (pollen and nectar) in weeds and shrubs of field margins, the levels of parasitism were equal in both interior and edge plants, and the highest levels of syrphid oviposition activity were observed in plants within fields. These findings suggest that solutions for enhancing biological control of aphids in organic Brassica fields should focus on earlier establishment of enemies within fields, and that aphid control could be enhanced by increasing the diversity of the predator guild, focusing especially on highly mobile predators that are aphidophagous as adults. One possible strategy would be to introduce ladybird beetles on host plants to fields prior to transplanting the crop.