Effect of insect mediated pollination in culinary melon, Cucumis melo var. acidulus Naudin

Abstract

The present study entitled “Effect of insect mediated pollination in culinary melon Cucumis melo var. acidulus Naudin” was conducted at Department of Agricultural Entomology, College of Agriculture, Vellayani, Thiruvananthapuram, Kerala during 2022 to 2024. The objectives of the study were to assess the foraging behaviour of insect visitors, major pollinator-flower interaction and effect of pollination in culinary melon. The popular culinary melon variety, KAU Vishal was used in this study, and the crop was raised both under open field conditions and augmented (bee hive installed) condition. Observations on floral structures viz., flower spread, stamen characters, pistil characters etc., were taken after 10 percentage bloom. Insects visiting culinary melon were documented and the foraging behavior of major pollinator was assessed at weekly intervals for two months. The developmental changes in culinary melon (flower, fruit and seed development) due to bee visits were assessed by installing an Indian bee hive with six frame bee strength among the treatments; augmented, open- pollinated, hand pollinated and in pollinator excluded conditions. Studies on the floral biology of culinary melon revealed that flower bud initiation and anthesis was earlier in male flowers (31.30 days and 32.10 days) when compared to female flowers (38.70 and 49.80 days). Variation in the time of anthesis was also observed among the male (7:45 am) and female flowers (8:04 am). The count as well as the viability of pollen grains on anther and number of pollen grains deposited on the stigma was highest during 1100 -1300 h, which coincided almost with that of the stigma receptivity. A total of 19 insect visitors belonging to 18 genera, 8 families, and 4 orders were recorded, of which 15 species were identified as pollinators in terms of their foraging behaviour. Among them, six were nectar foragers and nine were both nectar and pollen foragers. Based on Shannon-Wiener diversity index, peak pollinator diversity was recorded during 0900-1100 h. Of these pollinators, Apis cerana indica Fab., Nomia 70 strigata Fab. and Ceratina binghami Cockerell recorded highest relative abundance (7.68, 3.70 and 3.04 bees mֿ ² 5 min ⁻¹) and number of loose pollen grains (60,000, 22,000 and 12,500). Thus, A. c. indica was ranked as the efficient pollinator of culinary melon in terms of the pollination efficiency index. Foraging behaviour of A. c. indica exhibited significant temporal variation throughout the day. Peak visitation rates were recorded during 1100-1300 h (11.83 ± 0.68 flowers min⁻¹) when compared to early morning (0700-0900 h: 7.79 ± 0.31 flowers min⁻¹) and late evening (1700-1900 h: 6.16 ± 0.68 flowers min ⁻¹). The number of bees visiting a single bloom was highest during 1100-1300 h (1.92 ± 0.05 bees 5 min⁻¹), followed by 0900 -1100 h (1.76 ± 0.13 bees5 min ⁻¹) and 1300-1500 h (1.62 ± 0.08 bees 5 min ⁻¹). The average time spent by bees on anthers as well as on the stigma was significantly high in the morning hours (0900-1100 h: 11.34 ± 0.84 sec and 0900-1100 h: 65.05 ± 29.65 sec), while, the number of returning foragers was highest during 1300- 1500 h (10.83 ± 0.83 bees 5 min ⁻¹). These findings highlight mid-morning to early afternoon as the optimal period for pollination activity in culinary melon. The impact of bee visits on flower, fruit and seed development under augmented (1-12 visits), open-pollinated, hand pollinated and in pollinator excluded conditions was assessed. Studies on the quantification of endogenous hormone levels such as Auxin (IAA) and Gibberellin (GA₃) revealed that open-pollinated flowers exhibited the highest auxin concentration (19.50 ± 0.20 ppb) which was followed by flowers with seven bee visits (6.32 ± 1.53 ppb). GA₃ was not detected in any treatment, indicating either its negligible role at anthesis in pollination response or due to the concentrations below detection limits. The quantitative and qualitative yield parameters of culinary melon as part of the fruit and seed development among the treatments were also observed. The fruit set percentage (0 to 100%) in culinary melon varied with the number of bee visits, with 100 per cent fruit set both in nine bee visits and open pollinated ones. Though percentage fruit set increased with bee visitation, the percentage of malformed fruits were also found increasing in flowers with 10 visits and beyond. Maximum fruit weight (2.43 ± 0.11 kg), fruit length (46.00 ± 1.00 cm), fruit diameter (11.46 ± 0.41 cm), flesh thickness (3.90 ± 0.10 cm), and TSS content (3.03 ± 0.06 °Brix) were recorded from 71 seven bee visit treatment. The seed quality parameters viz., seed set (1265.67 ± 140.10 seeds per fruit), total seed weight (11.02 ± 0.07 g), 100-seed weight (2.37 ± 0.06 g), and seed germination percentage (97.33 ± 0.57 per cent) were also maximum in the seven bee visits treatment. The present study revealed that A. c. indica was the dominant pollinator in terms of the Pollination efficiency index with peak foraging activity during 0900 to 1300 h. The count as well as viability of pollen grains on anther and number of pollen grains deposited on the stigma coincided almost with that of the stigma receptivity. The syngenesious type of anthers in culinary melon flowers with more than half of them inside the corolla cup and the similarity in length of Indian bee proboscis and stamen, signifies the relevance of bee pollination. Developmental changes in flower, fruit and seed among the pollination treatments indicated that both open-pollinated flowers and those receiving seven bee visits exhibited elevated auxin (IAA) levels, while the treatment, seven bee visits exhibited superior fruit and seed traits. These results highlight the critical role of A. c. indica in pollination, yield and quality of fruits in culinary melon.