horticulturae Article Efficacy of an Eco-Friendly Bloom Thinning Formulation on Mango Trees and Its Olfactory Effect on an Insect Pollinator, Apis mellifera Tae-Kwon Son 1 , Md Munir Mostafiz 2 , Hwal-Su Hwang 2 , Nguyen Truong Thạnh 3 and Kyeong-Yeoll Lee 2,4, *   Citation: Son, T.-K.; Mostafiz, M.M.; Hwang, H.-S.; Tha . nh, N.T.; Lee, K.-Y. Efficacy of an Eco-Friendly Bloom Thinning Formulation on Mango Trees and Its Olfactory Effect on an Insect Pollinator, Apis mellifera. Horticulturae 2021, 7, 62. https:// doi.org/10.3390/horticulturae7040062 Academic Editor: Esmaeil Fallahi Received: 12 February 2021 Accepted: 22 March 2021 Published: 24 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Farm Management, College of Agriculture and Life Sciences, Kyungpook National University, 80, Daehak ro, Buk-gu, Daegu 41566, Korea; apple@applebio.info 2 School of Applied Biosciences, College of Agriculture and Life Sciences, Kyungpook National University, 80, Daehak ro, Buk-gu, Daegu 41566, Korea; munirmostafiz12@gmail.com (M.M.M.); bgtwo2@naver.com (H.-S.H.) 3 Plant Protection Research Institute, Vietnam Enasa JSC, No. 26, Chung cu 8X PLUS, P. Tan Thoi Nhat, Q 12, Hochiminh 729930, Vietnam; ntthanh.enasa@gmail.com 4 Institute of Agricultural Science and Technology, Kyungpook National University, 80, Daehak ro, Buk-gu, Daegu 41566, Korea * Correspondence: leeky@knu.ac.kr; Tel.: +82-53-950-5759 Abstract: In various orchard fruit trees, thinning of blossoms and fruits is important to increase fruit size and quality and to promote a new bloom in the following season. Several chemical thinning agents are currently commercially available, but they are inconsistent and produce side effects in crop plants and insect pollinators. Because of environmental concerns, developing alternative eco-friendly bloom thinning agents is necessary. We developed an eco-friendly bloom thinning formulation (BTF) using minerals and extracts of various medicinal plants. Our BTF spray (0.1%, w/v) decreased the number of fruits per tree (46.5%) and fruit yield per tree (81.5%) but increased the fruit weight (196.8%) compared with the control treatment; the spray induced a small number of larger mango fruits in the treated trees. We also investigated the effect of BTF on the olfactory behavior of Apis mellifera L. (Hymenoptera, Apidae), a major insect pollinator. We analyzed the behavioral changes of adult workers at two different concentrations (0.1% and 1%) of nine different BTF spray components using a Y-tube olfactometer. The behavioral responses of honey bees to nine BTF components showed significant differences. However, honey bees showed no clear attraction or repellent behavior towards the tested BTF components. Our results suggest that the newly developed eco-friendly BTF is practically applicable in mango orchards without interrupting honey bee behavior. Keywords: fruit trees; mango; orchard management; insect pollinator; thinning agents 1. Introduction When fruit trees produce more flowers and fruits, the size and quality of fruit are reduced. Hence, thinning is necessary to reduce crop load and increase the market value of tree crops [1–4]. Thinning promotes bloom again in the following season in some orchard trees, such as apples and plums [4,5]. Thinning methods have been developed using hand, mechanical, and chemical methods [1,5]. Hand thinning is the most time- consuming method and is unprofitable because of the labor intensity and prolonged time [1]. By contrast, mechanical thinning using rope, club, or high-pressure spray guns can be used; however, these methods easily bruise and damage fruits, making them commercially unacceptable [1]. Using a hand-held electrical device can be useful for the post-bloom mechanical thinning of peach trees [6]. The chemical thinning method is more economical compared with the hand or me- chanical thinning methods [7]. Several chemical thinning agents (i.e., ethephon, ammonium Horticulturae 2021, 7, 62. https://doi.org/10.3390/horticulturae7040062 https://www.mdpi.com/journal/horticulturae