How natural enemies and cabbage aphid (Brevicoryne brassicae L.) population dynamics affect organic broccoli harvest

The cabbage aphid (Brevicoryne brassicae L.) is the primary broccoli (Brassica oleracea L.) pest in Monterey County. These aphids' ability to contaminate a broccoli head has sometimes led to zero-tolerance spray thresholds. To improve on these management practices, aphid arrival time, within-plant colony location, and abundance of natural enemies were tested against infestation of organic broccoli heads at harvest. Cabbage aphids predominately colonized the outer leaves of a broccoli plant. These colonies, however, did not significantly influence infestation at harvest. Center-located aphids were correlated with head infestation for both field seasons, as were aphids on leaf 2 in 2002. Aphid arrival time into a field was strongly correlated with infestation at harvest, with early arriving aphids being less likely to infest a head. This was in part caused by natural enemies, particularly syrphid larvae (Syrphidae), which were in greatest abundance in response to early aphid colonizers. Natural enemies showed a capability to positively affect infestation at harvest, although their success seemed dependent on sufficient early season cabbage aphid arrivals. Therefore, future research efforts should focus on management practices that encourage the early establishment of natural enemies.