Critical pollination chemistry: Specific sesquiterpene floral volatiles in carrot inhibit honey bee feeding

Abstract

Although many plant species are reliant on insect pollination, agricultural plant breeding programs have primarily focused on traits that appeal to growers and consumers, rather than on floral traits that enhance pollinator attraction. In some vegetable seed production systems, this has led to declining pollinator attraction and poor seed yields. We predicted that low-yielding crop varieties would be less attractive to pollinators due to deficiencies in nectar rewards or volatile floral attractants. To test our prediction, we used a chemical phenotyping approach to examine how floral chemical traits of five carrot lines affect honey bee visitation. In bioassays, honey bees avoided feeders containing nectar from all carrot lines indicating a general non-attractant effect. Certain compounds in carrot flowers and nectar not only failed to elicit attraction but functioned as repellents, including the sesquiterpenes alpha-selinene and beta-selinene. Others enhanced attraction, e.g. beta-ocimene. The repellent sesquiterpenes have previously been implicated in plant defense suggesting a fine balance between pollination and plant protection, which when disrupted in artificial selection in plant breeding programs can impact the crop yield. These new insights highlight the importance of bioactive compounds in attracting pollinators toward floral resources in both ecological and agricultural settings.