Pollination Ecology of Watermelon and Other Global Food Crops

Abstract

<p>Managed and wild pollinator taxa are responsible for greater than one third of global food production and their services benefit more than 85% of wild plants. As the production of pollinator-dependent crops expands globally, an understanding of the mechanisms underlying pollinator behaviour and pollination service delivery is becoming increasingly important. This thesis investigates the efficiency and behaviour of insect pollinators that visit global food crops with a focus on watermelon (<i>Citrullus lanatus</i> (Thunb.) Matsum. & Nakai" family: Cucurbitaceae) as a model crop species.</p> <p>First, I reviewed the literature concerning watermelon pollination biology to ascertain existing knowledge gaps. Second, I collated and synthesized the literature on watermelon floral visitors and their efficiency across different growing regions globally through a systematic literature and meta-analysis. Third, I used field surveys and pollen deposition trials, to explore floral visitor composition and effectiveness of watermelon insect visitors across 15 commercial farms in five geographically distinct growing regions" Riverina (NSW), Katherine (NT), Gumlu (QLD), Lakeland (QLD) and Chinchilla (QLD) in Australia. Fourth, I investigated the pollen morphological traits across diploid and triploid watermelon genotypes and compared the fate and outcome of pollen grains in inter-planted genotypes (hand pollinated crosses" triploid x triploid and triploid x diploid) in a hybrid cropping system. Finally, I examined how insect flower visit duration relates to foraging behaviour, pollination success and the level of crop pollinator dependency using field data collected from 11 insect-pollinated crop species, including watermelon, across 56 farms in Australia, New Zealand, and USA.</p> <p>I found that knowledge gaps exist concerning the identity of pollinator assemblages across cultivars, genotypes, and growing regions. In particular, few published studies have focused on seedless cultivars due to their relatively recent cultivation. The results of this review indicated that different watermelon genotypes require specific management to ensure optimal production. In the global meta-analysis of watermelon floral visitors, I recorded over 265 watermelon floral visitors (including 5 orders, 18 families and 75 genera) from 54 records across 17 countries. Honey bees (<i>Apis mellifera L.</i>) and other bees were equally effective at depositing pollen on stigmas, but varied in their total contribution to fruit and seed set. In Australian field surveys of watermelon flower visitors, I found that the insect species community composition differed significantly among regions, but honey bees were the dominant watermelon flower visitor at all sites, with relative abundance varying from 73 - 94%. However, native bees (<i>Tetragonula</i> spp., and bees from the Families Megachilidae, and Halictidae including the genera <i>Lasioglossum, Homalictus, Lipotriches</i>), and flies (Syrphidae spp.) also visited and transferred pollen onto watermelon stigmas. In particular, native stingless bees deposited similar amounts of pollen as honey bees in some regions. In my investigation of pollen morphological traits across diploid and triploid watermelon genotypes, I found that pollen from triploids and diploids was visually distinguishable, as pollen in triploids had more deformities (44%), tetrads (43.5%), and abnormal growth of callose plugs in their pollen tubes. Contrary to previous reports, I found that triploid watermelons produced low numbers of viable pollen grains that geminated (8±3%) and produced pollen tubes (6.5±2%). Whilst honey bees can collect and deposit pollen from triploids onto stigmas, the impact of this behaviour on hybrid watermelon reproduction is likely minimal due to the low germination rate of pollen from triploids. Finally, I found that flower visit duration varies across insect taxa and crop species and was generally not associated with either the number of pollen grains deposited on the stigma or fruit set. Flower visit duration was only marginally positively related to pollen tube growth. However, there was an interaction between taxa and visit duration, indicating that the outcome upon plant reproductive success as a result of visit duration, varied according to different floral visitor groups. I also found that floral visitors spent more time on flowers when foraging for nectar compared to pollen and their visit duration was greater with increasing pollinator dependency of the crop species.</p> <p>This thesis demonstrates the importance of understanding local, regional and global variation in pollinator assemblages across crop genotypes. Identifying wild pollinator taxa, deploying managed populations of native taxa (e.g. native stingless bee colonies in Australia), employing pollinator-safe land management practices, and exploring methods for increasing the efficiency of managed honey bee colonies, are all measures likely to contribute to improving watermelon production in Australia and globally. Greater understanding of pollinator behaviour, efficiency and plant-pollinator interactions is necessary to ensure ongoing effective and resilient pollination services to watermelon cropping systems, and to develop best practice pollination management for pollinator dependent crops to improve global food security.</p>