Cucurbit Pollination: Mechanisms and Management to Improve Field Quality and Quantity

FACTSHEET 02/19 CP 118: Cucurbit pollination: Mechanisms and management to improve field quality and quantity Action points that may have been boosted by the temporal ● Preserve wild flowers within, and on the edge availability of courgette nectar from an earlier of fields as a way of attracting pollinators into crop. However, bees will also need alternative courgette fields. Key courgette pollinators were floral resources, such as those provided most abundant in fields with more wild flowers by wildflowers, to fulfil their long-term nutritional requirements ● Wild flowers facilitate pollination services to courgettes by providing additional floral resources ● Use the BEE-STEWARD model to help to pollinators. There was no evidence that wild develop pollination management practices. flowers provide competition by ‘distracting’ The model simulates the effects of different bees from courgette flowers. Conservation of management practices on pollinator population Honeybees (A. mellifera) and buff-tailed bumblebees dynamics, health, and its overall impact on (B. terrestris) will improve courgette pollination pollination services levels. These species were the most abundant ● Consider growing parthenocarpic (fruit set pollinators of courgettes. B. terrestris had a more in the absence of pollination) varieties if equal preference for visiting male and female pollinators are scarce, in combination with courgette flowers and carried more loose pollen other environmentally considerate practices, grains than A. mellifera, therefore it is likely to be to increase stable pollinator levels. Parthenocarpic the most effective pollinator of courgette varieties could be advantageous when there ● There was no evidence that artificially introduced is a pollination deficit, as it will help improve managed colonies of B. terrestris increase fruit uniformity caused by lack of pollination. courgette yield. Management practices should In return, these parthenocarpic crops can focus on maintaining natural population numbers continue to aid pollinator survival by providing in areas of high pollinator abundance nectar and pollen resources for wild and managed bees, and other flower-visiting insects in an aim ● Plant courgettes adjacent to recently grown to improve the long-term pollinator availability courgettes to use local bumblebee populations Introduction The findings showed the value of pollination for courgettes grown in the UK and how effective native AHDB Horticulture Project CP 118 investigated the value species are at pollinating courgettes. of pollination to courgette yield and examined which native UK species are the most effective pollinators for Interestingly, 56% of the fruit was able to reach marketable field-grown courgettes.It was also explored whether size and shape without any pollination. This is due to a wildflower presence in courgette field margins had any process known as parthenocarpy – fruit set in the impact on overall pollination levels of courgettes and absence of pollination. In the project, experimental fruits whether their presence affected pollinator richness and were classed as ‘aborted’ if they did not meet minimum abundance. Additionally, it was explored whether the commercial standards, i.e. they were less than 14 cm artificial introduction of commercial bumblebees long, 30 mm wide (at the midpoint), and over 5° in improves pollination levels and consequent yield. Using curvature, or showed any obvious signs of bacterial field data combined with computer simulations, the effect damage, such as blossom end rot. that courgette nectar and pollen availability had on Evidence of parthenocarpy in courgette inspired a bumblebee population dynamics and health was literature review, which showed that methods to induce modelled, for the first time, to investigate whether its parthenocarpy (selective breeding, hormone application, availability aided health, conservation and long-term and genetic modification) were able to increase fruit quantity survival of bumblebee populations. without adversely affecting quality in 18 crop species that This project was one of the first to explore the benefits traditionally require insect pollination for fruit set. of pollination for courgette yields in the UK. Prior to this study, there was no information available on the Despite the natural parthenocarpic tendency of effectiveness of natural UK pollinator species for courgettes, no parthenocarpic varieties are currently courgette pollination services and no work had been grown on a commercial scale in the United Kingdom. done on the pollinator dependence of ‘Tosca’, a popular What are most effective pollinators variety of courgette grown in the UK; it was unknown of courgette? whether this crop was experiencing a pollination deficit or not. With Defra putting an economic value of insect In 2016 and 2017, 228 pollinator transects showed that pollination at £3,398/ha to courgette growers, the study honeybees (Apis mellifera) and buff-tailed bumblebees aimed for further understanding of sustainable practices (Bombus terrestris) (Figure 3) were the most abundant that can enhance pollination levels in courgette fields, pollinators of courgettes. Swabbing 17 buff-tailed while synergistically aiding the long-term conservation bumblebees and 4 honeybees with glycerine jelly, and of valuable native pollinator species. counting pollen grains under a microscope showed that How important is pollination for determining buff-tailed bumblebees carried more loose pollen grains courgette fruit set? than honeybees, and therefore have a higher pollination potential. Buff-tailed bumblebees also had a more equal Pollination experiments were conducted over two years preference for visiting both male and female courgette to explore pollinator dependence and pollination deficit flowers, which is desirable for optimum pollen transfer. in field-grown courgettes in the UK. Pollination treatments included hand (maximal) pollination, open (natural) Traditionally, it is believed that introduced, managed pollination, and no pollination (pollinator exclusion). colonies of bumblebees and honeybees may increase Experiments were conducted on 160 flowers per treatment. yields at sites with low levels of wild pollinators, but no evidence for this was found in this study (Figure 2). Results showed that there was no significant difference This was investigated by stocking one field (2.58 ha) between full (hand) pollination and open (natural) in Cornwall with six commercially reared colonies of pollination, and that pollination increased courgette yield buff-tailed bumblebees (the recommended stocking by 39% when compared with pollination exclusion (Figure 1), density for courgette by Koppert Biological Systems) as well as the size, weight and growth rate of courgettes. 1 1 90 8 95 70 E 60 ± 50 90 40 ruit set 30 8 2 Percentage fruit 1 0 8 Hand pen o Pollinated Pollinated Pollination ith colonies ithout colonies Figure 1. Average fruit set of field-grown courgettes subjected to three Figure 2. Percentage of fruit set with and without commercial different pollination treatments (hand pollination, open pollination B. terrestris colonies open. Data were combined for the 5 days and no pollination) when colonies were open and the 5 days when colonies were closed Are courgette flowers a valuable floral resource? Courgette provides vast quantities of nectar and pollen to ensure high pollinator visitation rates and pollination potential. While buff-tailed bumblebees showed strong fidelity to courgette flowers for nectar, courgette pollen was not found in any pollen loads of returning foragers (Figure 5). This was based on 394 bees, sampled from 42 colonies across 14 sites. This demonstrates the importance of providing additional floral resources to aid population survival by fulfilling bees’ long-term nutritional Figure 3. Buff-tailed bumblebees foraging on a male requirements, particularly for pollen, and allowing them to courgette flower capitalise on the temporal resources provided by courgettes. and comparing the yield of courgettes when colonies were open with when colonies were closed. The results showed no significant difference in yield when colonies were open compared with when colonies were closed, which is likely due to the already high levels of natural pollinators at the studied sites. Management practices should prioritize conserving natural pollinators, instead of artificial supplementation. How can these pollinators be encouraged? Figure 5. Sampling pollen loads from foragers returning to commercial Pollinators rely on flowers for food – nectar is rich in bumblebee colonies that were situated within courgette fields sugars, a valuable source of energy, and pollen is rich in protein, which is essential for growth and development. Using a bumblebee population model called BEE- In 2016, flowers and pollinators were surveyed in STEWARD, courgette nectar was shown to be a valuable courgette fields in Cambridgeshire (2 fields), food resource for bumblebees. This is because crop Worcestershire (2 fields), and Cornwall (5 fields), three flowering coincides with when bumblebee foragers are times within a growing season. Results showed that wild most active (Figure 6). This means more bumblebees in flower species richness in courgette fields was the most the crop, and therefore pollination, as well as more nectar important factor for increasing buff-tailed bumblebee being returned to the colony, which will improve abundance. Therefore, encouraging wild flower growth on bumblebee

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