Pollination rates in hybrid carrot crops remain limited after introduction of honey bee hives. In this study, honey bee foraging behaviour was observed in commercial hybrid carrot seed crops. Significantly more visits were made to male-fertile (MF) rather than cytoplasmically male-sterile (CMS) flowers. Pollen was collected from bees returning to a hive, to determine daily variation in pollen loads collected and to what level the bees were foraging for carrot pollen. Honey bees visited a wide range of alternative pollen sources and made relatively few visits to carrot plants throughout the period of flowering. Visitation rates to other individual floral sources fluctuated but visitation to carrot was consistently low. The underlying rate of carrot pollen visits among collecting trips was modelled and estimated to be as low as 1.4%, a likely cause of the limited success implementing honey bee hives in carrot crops.

Although there is some literature published on insect visitation to open pollinated carrot seed crops in several regions around the world, there is none in Australasia and none on insect visitation to hybrid carrot seed crops worldwide. With a growing hybrid carrot seed crop industry in southern Australia and New Zealand, a survey of insect visitation to carrot seed crops is timely as the first step in elucidating which insects may be pollinators. Surveys of insect visitation were thus undertaken over two consecutive seasons in the south of Tasmania in order to identify potential pollinators in carrot seed crops. Sticky traps, water traps and observations were conducted within five carrot crops at three geographically independent sites to determine the variety and quantity of insects present in carrot crops. Nearly 42,000 insects were counted from observations, sticky traps and water traps. More than 100 morphological groupings were discerned. Small insects (B5 mm), most of which were thrips, made up 78% of the catch on sticky traps. Among the insects observed visiting umbels, nectar scarabs (Phyllotocus spp.) were the most abundant taxa but honey bees (Apis mellifera), muscoid flies and ladybirds were more often observed but at lower numbers. Nectar scarabs, soldier beetles and muscoid flies warrant further investigation of their role as pollinators of carrot flowers.

Insect crop visitations do not necessarily translate to carriage or transfer of pollen. To evaluate the potential of the various insects visiting hybrid carrot flowers to facilitate pollen transfer, this study examines insect visitation rates to hybrid carrot seed crops in relation to weather, time of day and season, pollen carrying capacity, inter-row movement, and visitation frequency to male-fertile and male-sterile umbels. The highest pollen loads were carried by nectar scarabs, honey bees, and the hover fly Eristalis tenax (Linnaeus). Honey bees and muscoid flies were observed to forage mostly within the male fertile carrot row while nectar scarabs and E. tenax foraged across rows, carrying equal pollen loads regardless of their distance from the pollen source. All observed insect taxa were more frequently seen visiting male-fertile than male-sterile umbels. In contrast to other visiting insects, honey bees were abundant and frequent visitors and were observed carrying high pollen loads. Consequently, we suggest both optimizing honey bee management and improving the attraction of carrot lines to honey bees to improve pollination rates for hybrid carrot seed crops.

Cytoplasmically male-sterile (CMS) carrot cultivars suffer from low pollination rates. In this study, insect visitation varied more than eightfold between 17 CMS carrot cultivars in a field-based cultivar evaluation trial. The visitation rates of honey bees, nectar scarabs, muscoid flies, and wasps each significantly differed among these cultivars. No significant difference in visitation rates was observed among cultivars of different CMS type (brown-anther or petaloid) or flower colour, but cultivars of Berlicumer root type had significantly higher insect visitation rates than Nantes. Six cultivars were further compared in regard to selected umbel traits: as umbel diameter increased, so did the visitation of soldier beetles, while that of honey bees decreased. Finally, nectar of these six cultivars was analysed for sugar content, which revealed monosaccharides to be the most common sugars in all. There was high variation in the levels of sugars from individual umbellets but no significant difference in nectar sugar composition among cultivars, suggesting that nectar sugar composition is of minor importance regarding pollinator attraction to hybrid CMS carrot umbels.