Honey bee, Apis mellifera, visitation affects onion seed set in the Sacramento Valley

The onion thrips, Thrips tabaci, previously considered of minor importance to hybrid onion seed production in California, vectors the newly introduced iris yellow spot virus, a serious disease of onions. Insecticide applications to control onion thrips have increased, coincident with a steep decline in onion yields, especially in Colusa County, a significant onion seed production area in California. We examined a number of possible contributing factors to the hybrid onion seed yield decline and found a strong positive correlation between honey bee activity and onion seed set, indicating that a lack of pollination may be contributing to yield declines in commercial production fields. Field size influenced honey bee, Apis mellifera, activity whereby larger fields had less activity than smaller ones. In addition, bee visits to onion flowers were negatively correlated with the number of insecticide applications per field. Thus, reduced onion seed yields in recent years could be associated with an increase in insecticide use for onion thrips control that may be repelling or killing honey bees, important pollinators of this crop.