Abstract
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.
Highlights
In commercial hybrid carrots, cross-pollination is required between a cytoplasmically male-sterile (CMS) line, which does not produce pollen, and an F1 male-fertile (MF) line that does produce pollen [1]
We look at daily pollen collection activity of bees from hives placed adjacent to hybrid carrot seed crops in relation to climatic variables
Compared to the CMS carrot lines (n = 252; t = 5.16, df = 91, P < 0.0001; Figure 1). This significant difference was driven by observed differences in the number of pollen foraging bees, with no difference observed in the abundance of nectar foraging bees between CMS and MF lines (z = −1.764, P = 0.078)
Summary
Cross-pollination is required between a cytoplasmically male-sterile (CMS) line, which does not produce pollen, and an F1 male-fertile (MF) line that does produce pollen [1]. Pollinator visits to the CMS line are for nectar only, with self-fertilisation of the seed-producing CMS line prevented by the absence of pollen on the CMS flowers. Hybrid carrot crops are generally considered to require supplementary pollination by managed pollinators such as honey bees (Apis mellifera Linnaeus) [4]. Earlier studies investigating open-pollinated carrot seed crops indicate honey bees to be abundant and carry high pollen loads in carrot crops [5]. The reliability of honey bees as hybrid carrot crop pollinators appears to have diminished because of an increased variability between lines [1,6,7]
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