Honey bees are the most abundant visitors to Australian watermelon but native stingless bees are equally effective as pollinators
- Despite the benefits of a diverse approach to crop pollination, global food production remains reliant on a low diversity of managed pollinators, especially the European honey bee (Apis mellifera). To facilitate more robust pollinator management and improve the resilience of the production system, it is necessary to understand regional variation in the pollination ecology of global food crops. Watermelon (Citrullus lanatus [Thunb.] Matsum & Nakai) is a highly insect pollinator-dependent crop and even though it is grown globally across many different climate zones, little is known about its pollination ecology across the diverse growing regions of Australia, spanning from the tropics to the arid zone.
- We compared the species composition, visitation rates and effectiveness of the dominant floral visitors on 15 farms across five major watermelon-growing regions of Australia.
- We found that insect species composition differed significantly among regions, but honey bees were the dominant watermelon flower visitor, with relative abundance varying from 73% to 94%. However, native bees (including stingless bees Tetragonula sp., and bees from families Megachilidae and Halictidae such as Lasioglossum, Homalictus and Lipotriches) and flies (particularly Syrphidae sp.) also visited and transferred pollen onto watermelon flowers.
- In particular, native stingless bees were common visitors in several growing regions and deposited similar amounts of pollen to honey bees.
- Our findings indicate that the Australian watermelon industry utilizes honey bees, but the diverse assemblage of available native pollinating taxa provides an additional opportunity for growers in specific growing regions. Pollination service delivery could be increased by deploying managed populations (e.g., native stingless bee colonies), employing pollinator-safe land management practices as well as exploring methods for increasing the efficiency of managed honey bee colonies.
Morphological characteristics of pollen from triploid watermelon and its fate on stigmas in a hybrid crop production system
Hybrid crop production is more reliant on pollinators compared to open-pollinated crops because they require cross-pollination between a male-fertile and a male-sterile line. Little is known about how stigma receipt of pollen from male-sterile genotypes affects reproduction in hybrids. Non-viable and non-compatible pollen cannot fertilise plant ovules, but may still interfere with pollination success. Here we used seedless watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai) as a model hybrid plant, to evaluate the morphology, physiology, and movement of pollen from inter-planted genotypes (diploids and triploids). We found that pollen from triploids ('Exclamation' and 'Royal Armada') and diploids ('SP-6', 'Summer Flavor 800', and 'Tiger') was visually distinguishable. Pollen in triploids had more deformities (42.4–46%), tetrads (43–44%), and abnormal growth of callose plugs in pollen tubes. The amount of pollen in triploids to germinate on stigmas was low (8 ± 3%), and few pollen grains produced pollen tubes (6.5 ± 2%). Still, contrary to previous reports our results suggest that some viable pollen grains are produced by triploid watermelons. However, whilst honey bees can collect and deposit pollen from triploids onto stigmas, its effect on hybrid watermelon reproduction is likely to be minimal due to its low germination rate.
A global assessment of the species composition and effectiveness of watermelon pollinators and the management strategies to inform effective pollination service delivery
For most food crops the identity and efficiency of pollinators across key growing regions remains a significant knowledge gap that needs to be addressed before we can develop crop-specific approaches for pollination service delivery. Here, we conducted a systematic literature review and meta-analysis on watermelon (Citrullus lanatus (Thunb. Matsum. & Nakai)), a globally important fruit crop, to identify the floral visitors and their efficiency across different growing regions. We found that 265 insect species visit watermelon flowers (including 5 orders, 18 families and 75 genera) across 17 countries and 6 continents. Bees and flies were the most abundant flower visitors overall, but show distinct regional differences. Honey bees were the majority visitor in 53% of growing regions (range: 0 - 94%), whilst wild bee species were more abundant in 42% of regions (range: 3.4 - 100%). Honey bees and other bees were equally effective at depositing pollen on stigmas, but varied in effectiveness for fruit set and seed set. Pollination data from global studies appear to be limited for the largest-scale watermelon producers, namely: China, Turkey, and India, with the majority (56%) of data available from North America. This synthesis identified four key themes for improving pollination in watermelon: increasing honey bee densities on crops where local polices and environmental conditions are suitable; introducing other managed pollinators; identifying key wild pollinator taxa to encourage within crops; and improving local and landscape management practices to support pollinators.