Crop pollination by insects in small-scale agroforestry farming in Tanzania

Context. The decline of pollinating insects worldwide threatens pollination services for wild angiosperms and important food crops. The importance of insect pollination services for food production has been documented for crops that are available on the global markets, and which stem from large-scale farming systems. Little effort has been directed towards understanding the role of insect pollinators in small-scale farming systems in developing countries, even though these systems feed a substantial part of the World’s population. Objective. I studied crop pollination by insects in a small-scale agroforestry farming system in the Kilimanjaro and Arusha regions in northern Tanzania. I assessed to which degree crop yield was limited by insect pollination; how environmental context and agricultural practices influenced pollinators, and consequently pollination and crop yield; to which degree local farmers were aware of pollinators; and local farmers’ potential vulnerability to changes in pollination services, in terms of declines in household income and food availability. Methods. I carried out a combination of observational and experimental ecological studies in a total of 24 local small-scale agroforestry type farms, and interviewed 147 local farmers, to assess the importance of insect pollination for production of watermelons (Citrullus lanatus Thunb., Cucurbitaceae). Insect pollination is essential for fruit development in watermelons, and watermelon is an important cash crop to local farmers in my study area. Main results. Results from hand-pollination experiments and observational studies of relationships between flower visits by insects and fruit quantity and quality, showed that watermelon crop yield was limited by pollination services. My findings indicate that local farmers can double the number of marketable fruits and increase sugar content of the watermelons by approximately 10%, if the watermelon flowers are sufficiently pollinated throughout the blossom period. The main groups of visitors to watermelon flowers were wild honeybees (Apis mellifera; 87.8%), followed by hoverflies (Syrphidae; 8.5%) and other Hymenoptera (3.7%). Environmental context influenced pollinators, and consequently pollination and crop yield; visitation rates by insects to watermelon flowers increased with abundance of co-occurring flowers of other plants, especially at high tree cover in the field surroundings. Visitation rates by non-honeybee visitors were higher at the edge compared to centre of crop fields. Pesticide spraying reduced visitation rates by 50% from the lowest to the highest observed frequencies of pesticide application. Increasing inputs of fertilizer and watering had little effect on crop yield, compared to enhanced pollination. Only 7% of the local farmers were aware of pollinators and their importance for crop pollination, although 67% of crops grown by local farmers for household food and income depended on insect pollination to a moderate to essential degree. Watermelon crops contributed nearly 25% of household income and were grown by 63% of the interviewed farmers. Management Implications. It is critically important that small-scale farmers understand the role of pollinators and their importance for agricultural production. Agricultural policies to improve yields in developing countries should include measures to improve pollination services, such as education and advisory services to local farmers on how to develop pollinator friendly habitats in agricultural landscapes. The seemingly alarming negative impact of pesticide use on flower visits by insects need to be addressed by the responsible management authorities, who should develop a sustainable strategy for managing pests and ensuring increased agriculture yield.