Contribution of insect flower visitors on macadamia nut set, retention and yield in central Kenya

The aim of this study was to evaluate the contribution of insects that visit macadamia flowers to nut set, retention, nut-in-shell, and kernel yields. The study was conducted at the Macadamia Research Centre in Kandara, Murang'a County, in three flowering cycles (cropping), from August 2020 to May 2022. Nut set, retention, and yield were assessed by randomly selecting twelve trees that were in full bloom at the study orchard in each of the flowering cycles. On each of the twelve trees, four branches of the same size were selected and tagged for the study. The branches were about 1 to 1.5 metres high from the ground. On each branch, one raceme at the bud stage was randomly selected and one of the four treatments administered. The treatments were (i) bagged during the day with mesh nets, (ii) bagged at night, (iii) bagged throughout the flowering season, excluding all insects, and (iv) racemes were left unbagged, thus having unlimited flower visitation by insects. There were significant differences on nut-in-shell yield (mass) (P≤0.05) in flowers that were unbagged (68.23 ± 4.03), bagged at night (61.50 ± 3.51), bagged during the day (6.53 ± 1.05) and those that were bagged throughout (4.45 ± 0.95). Racemes that had flower visitors fully excluded resulted in low nut-in-shell production. The initial nut set, retention, and ultimately the nut-in-shell yields were significantly increased in flowers that were left unbagged, thus insects had unlimited access. Macadamia nut set, retention, and ultimate yield (nuts-in-shell and kernel) is largely dependent on the presence of insect flower visitors.