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
]. Hence, 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. The crops grown are dominated by the CMS line with CMS:MF ratios of 2:1 to 4:1 common [2
]. The MF cultivar is later removed after seed set and the seeds produced by the CMS cultivar are harvested for the commercial production of carrots [3
]. Hybrid carrot crops are generally considered to require supplementary pollination by managed pollinators such as honey bees (Apis mellifera
]. Earlier studies investigating open-pollinated carrot seed crops indicate honey bees to be abundant and carry high pollen loads in carrot crops [5
]. However, the reliability of honey bees as hybrid carrot crop pollinators appears to have diminished because of an increased variability between lines [1
This variability in honey bee mediated pollination has led to an increased focus on the use of alternative pollinators including other Hymenoptera and some Diptera [8
]. Despite enhanced pollination observed when alternative pollinators are used to supplement honey bee pollination services [10
], the presence of other pollinator species can be unreliable between seasons or locations, with many species also carrying few pollen grains on their bodies [11
]. As seed growers are able to hire honey bee hives to place within the carrot fields over the flowering period (December–January in Australia), managed A. mellifera
remain the primary pollinators in hybrid carrots.
The placement of managed honey bee hives used for crop pollination is largely based on current understanding of honey bee foraging behaviours, including foraging range.
Indeed, whilst bees may forage much farther away, they tend to forage within 300 m of their hive in the presence of suitable floral resources, and it is therefore recommended that bee hives should be placed around crops so that this foraging distance is not exceeded [13
]. Competition between hives that are grouped together encourages bees to forage farther within a crop and thus usual pollination practice is to cluster groups of honey bee hives together [14
]. When foraging, whether for pollen or nectar, bees use olfactory and visual cues (such as colour) and typically stay within one particular flower type [15
]. This constancy or ‘floral fidelity’ means that the pollen carried on the corbiculae of honey bees may only be of a single pollen type [17
]. However, this may not always be the case [18
], as a single floral resource may be in insufficient quantity to ensure adequate pollen is collected to maintain hive health [20
In general, the availability of food is of paramount importance to insect colonies such as honey bee hives. A food source must first be located and then its position communicated to other foragers so that it can be maximally exploited [21
]. Nectar and pollen collected from flowers form the basis of all food requirements of both adult and brood bees. Pollen provides protein, fat, vitamins and minerals, whilst nectar is a source of carbohydrates [22
]. Sources of nectar and pollen are ephemeral, so the proportion of workers collecting nectar or pollen at any one time varies to suit the composition and number of adults in the hive (males and workers), brood levels, the amount of food resources stored within the hive [23
] and the availability of forage and prevailing conditions [24
]. It follows that the food source quality is an important factor for bee forage preferences.
When honey bee numbers are low and crops are nectar-poor, bees may be lured to other plants that produce greater quantities of nectar [21
]. As the quality and quantity of nectar in hybrid carrots is often low [25
], many weeds and other plant species near crops may have a greater relative attraction to honey bees. To improve pollination, it has been suggested that carrot seed crops should not be located near other competing crops or floral resources, but preferably near habitats that support alternative pollinators with potential to pollinate the carrot umbels [26
In this study, we investigate possible causes to the observed low pollination rates of hybrid carrot seed crops despite the use of managed honey bee hives. We first examine honey bee foraging patterns among both nectar-collecting and pollen-collecting bees within hybrid carrot seed crops. Specifically, we examine visitation frequency and duration of honey bee visits on MF and CMS umbels. We then look at daily pollen collection activity of bees from hives placed adjacent to hybrid carrot seed crops in relation to climatic variables. Finally, we collect pollen balls from foraging bees returning to the hive and quantify carrot pollen content, relative to pollen collected from other flowering resources over the flowering lifetime of the hybrid carrot seed crop. By examining the pollen collected by honey bees in a hive adjacent to the crop, foraging patterns and any specific preferences for competing flowering plant species may be discerned.
Honey bees from hives located adjacent to carrot seed crops displayed a preference for foraging in alternative species to carrot, with only a small percentage, estimated at 1.4%, of the pollen load on bees found to be carrot pollen. Of the bees visiting carrots, a preference for MF flowers, containing both pollen and nectar, was recorded. Between 63% and 87% of honey bee visits were to MF rather than CMS flowers, and nectar collecting bees were more likely to carry low pollen loads on their body than pollen collecting bees. As CMS flowers contain nectar but no pollen, only nectar collectors would be expected to visit these flowers. The likelihood of low pollen loads on nectar collecting bees visiting the CMS flowers, combined with the low frequency of visits to CMS in comparison to MF flowers in the crop, and a preference to visit alternative species to carrots, can help to explain the poor seed set observed in hybrid carrot seed crops despite the placement of hives near the crops [31
It has previously been recommended that open-pollinated carrot seed crops should not be located near other crops which may provide competition for the attention of honey bees [26
]. The results of our study extend the recommendation to hybrid carrot seed crops utilising CMS parent lines. Our results support similar findings by Galuszka and Tegrek [7
], who also reported low pollination in carrot crops when alternative pollen and nectar sources were available. Similar crop management problems have been demonstrated in avocado crops which failed to rival competition from citrus flowers [32
], alfalfa crops which were out-competed by roadside gumweed up to 1 mile (1.6 km) away [33
] and of onions where between only 6% and 8% of pollen collected from returning bees was found to be onion pollen [34
Several studies have established that bee activity is restricted by lower minimum temperatures [35
]. As A. mellifera
are heavily utilised as pollinators of carrot seed crops, the consequence of flowering over a period when morning temperatures are cool would most likely lead to lower seed set during these periods. Although daily maximum temperature alone was not a significant factor in relation to the amount of pollen collected by the honey bee colonies on any one day, the rate of evaporation, which tends to increase with increasing temperature, was. Furthermore, pan evaporation also depends on the temperature difference between the air and the evaporating surface, the relative humidity, solar radiation and wind speed, all factors that are known to impact on honey bee foraging activity [37
The examination of pollen from the corbiculae of returning honey bees showed that A. mellifera
from a single hive were prepared to visit a highly diverse range of other flowering plants. If there are one or more specific alternative plants that bees are visiting, seed growers may be able to manage their crops by manipulating the flowering time of their carrot crops to a time when competition is least likely, thereby reducing the pollination deficit. Alternatively, if there is low crop visitation of A. mellifera
due to general competition from surrounding flora then crop site selection or removal of competitive flowering plants should be considered where possible [13
]. However, this study suggests the removal of competing floral resources is not likely to be feasible due to costs or scale of the removal required.
Honey bees showed a distinct preference for visiting MF carrot umbels rather than CMS carrot umbels as has been observed repeatedly elsewhere [7
]. This lower visitation rate led to lower carrot pollen loads both on honey bees’ bodies and within pollen collected on the corbiculae. This finding may be due to honey bees preferentially foraging more frequently along rows, rather than across rows, which has been observed elsewhere [12
]. Whilst A. mellifera
were reoccurring visitors, they did not appear to be frequent visitors to the carrot crops relative to other flowering resources outside the crop, and even less frequent visitors to the CMS umbels compared to the MF umbels located within the crop. Therefore, it appears that the attractive floral attributes of carrot umbels successfully used by honey bees in open-pollinated seed crops may have been lost during the plant breeding process or that other repulsive attributes may have been accidentally bred into many hybrid carrots lines. Indeed, further work is required to elucidate the differences in quality and quantity of the floral cues and subsequent rewards provided to pollinators including honey bees.
It is currently believed that bees are unable to identify and preferentially forage for pollen based on its protein content alone and therefore may not always collect pollen that infer greater nutritional benefit to the colony [39
]. However, it does appear that bees are able to discern pollen with higher amino acid [41
] and lipid [42
] content and to learn pollen scents [40
] and potentially use these cues when pollen foraging, there-by maintaining their floral fidelity [45
]. More recently, some aspects for nectar quality and floral volatiles from hybrid carrot flowers were reported [46
In conclusion, the results of our study, although performed over a limited number of fields, point to poor retention of bees foraging within the carrot fields studied. Similarly, the diversity of pollen collected from a single managed honey bee hive indicated an extremely low preference for the hybrid carrot pollen compared to the floral resources outside the crop. To better understand these findings we recommend further, replicated studies to determine the consistency across fields to better inform growers on how and where hives should be placed. Furthermore, replicated studies examining the variability in individual hive responses focused on hive nutritional or queen laying status may aid in the improvement of hybrid crop pollination. By extending future investigations to cover both nectar and pollen attributes in unattractive hybrid carrot, and attractive open-pollinated carrot cultivars, coupled with behavioural assays of these cues and rewards in isolation, a better understanding of the key factors behind the poor bee-attraction to hybrid carrot flowers should be ascertained. Once understood, these qualities could be included as traits desired by seed production companies during the development of new cultivars in the future.