Diversity of Insect Pollinators and Their Contribution to Macadamia (Macadamia Integrifolia) Nut Yield in Murang’a County, Kenya

Abstract

Macadamia is the most lucrative nut that is cultivated for its nutrient-rich kernel and oil that is of great health and economic importance. However, optimal production of macadamia in Kenya is still hindered by both abiotic and biotic constraints, such as inadequate pollination. Pollination, which is mainly animal mediated in macadamia, is required for the successful nut set, retention, and development of nuts to maturity. This study aimed to contribute to the sustainable management of macadamia pollination in Kenya by identifying the diversity and abundance of insects that visit macadamia flowers, generating annual population changes of the key insect flower visitor, assessing pollen removal efficiency from the stigma by insects visiting macadamia flowers, and, further, evaluating how supplementary pollination influences nut yield. The research was done at the KALRO Macadamia Research Centre in Kandara within Murang’a County, between August 2020 and May 2023. Weekly surveys of insects that foraged macadamia flowers were conducted for 12 successive months in 2021, starting from January to December. Assessment of the effects of insect flowers on nut set, retention, and final yields was done from August 2020 to May 2022 at the Macadamia Research Centre by use of mesh nets to bag inflorescences to limit insect access during the day; other inflorescences were bagged during the night, and some had their inflorescences during the blooming season fully bagged, thus excluding all insects, while some inflorescences remained unbagged, allowing insects unlimited access. The investigation of pollen removal efficiency from the stigma by different insects visiting macadamia flowers was done during peak flowering months, namely, August, September, and October in 2021 and 2022 at the Macadamia Research Centre. This was done by evaluating the macadamia pollen removed from the stigma by an insect after visiting macadamia xvii flowers as well as investigating its foraging behaviour. Pollen grain counts were done on the body parts of the insects that were captured using a sweep net after they completely foraged macadamia flowers and just before they left the flowers. Assessment of the effects of supplementing pollination was done by having racemes bagged to exclude pollinators, those that were left to open pollinator visitation, and some hand pollination administered for two cropping cycles from August 2021 to May 2023. Five honey bee (Apis mellifera) colonies were also introduced at the Macadamia Research Centre to enhance pollination and mitigate deficits. Data was subjected to analysis of variance and t - tests, using the R statistical package, 2018. Means were separated using Fisher's least significant difference (LSD) test with an alpha value of p ≤ 0.05. A total of 61 species of diverse insects were observed foraging flowers of macadamia in the research centre, and Apis mellifera (Hymenoptera: Apidae), was recorded as the most frequent and abundant visitor. Weather seasons influenced the mean number of the insect species that were recorded foraging flowers of macadamia during the study, with long rains followed by cold season having the higher mean, whereas short rains and the dry season were associated with the lowest mean. The Simpson diversity index showed significant differences across the weather seasons, namely, cold, short rains, long rains and dry (p < 0.0005), as well as in the sparse and dense flowering patterns (p < 0.0005). Macadamia pollen transfer depended on insects that visited and foraged flowers during the day. There were significant differences in the racemes that were bagged, open and bagged throughout, with those that had insect visitors limited access to flowers completely declining in nut-in-shell. Nut set and subsequent retention was significantly greater in inflorescences that were not bagged. Yields of nuts-in-shell and kernels also increased xviii significantly in racemes that were not bagged allowing insects access to flowers. In this study, the honey bee (Apis mellifera) was found to have significantly higher visitation time, more frequent visitor and collected more pollen from the stigma on its body. Other flower visitors, such as hoverflies and wild bees, carried significant numbers of pollen grains. The results demonstrate that macadamia is pollen limited and pollination is highly reliant on insect pollinators. There were significant differences among the bagged, open and hand pollinated racemes. The initial nut set and retention was increased significantly in hand pollination depicting pollination deficit, with yield difference of 21.54%. Supplementary pollination resulted to higher nut set, retention, nut-in- shell and kernels yields. It is therefore concluded that a plethora of insect species visits and pollinate macadamia flowers but honey bee is the most dominant and efficient pollinator. Macadamia growers are encouraged to introduce honey bee colonies in their farms to supplement during peak flowering seasons for optimal pollination that would result in better nut set to improve yields and also address pollination deficits. This study recommends ecological intensification that supports the conservation of the macadamia anthophiles, through diversification of their habitats and floral resources, in order to enhance ecosystem services that would increase macadamia productivity in Kenya. Further research should be conducted on the strategies to develop and conserve non-bee macadamia flower visitors.