The onion (
More than 267 insect species belonging to seven insect orders are known to pollinate the onion during its entire flowering period (Bohart et al., 1970; Walker et al., 2011). Among all the pollinators recorded, honey bees are reported to assist in 60–95% of pollination (Kumar et al., 1985; Chandel et al., 2004) and the bee pollinated umbels are reported to have a yield up to ten to eleven times higher than the closed control (Munawar & Muzaffar, 1999). However, the use of bees or other insects in controlled pollination for producing pure line hybrids is almost impossible due to the difficulty in having control over the pollination activity of insects. Mayer & Lunden (2001) and Yucel & Duman (2005) tested several other cross pollination methods for onion hybrid seed production with little success and concluded that, due to sticky pollen, the pollination in onion has to be facilitated by insects (either solitary bees or honey bees) for optimum seed set.
Therefore, in the present study, such cross pollination techniques as hand pollination, brush pollination and controlled insect pollination were practised and their efficiency was analysed both qualitatively and quantitatively. Additionally, the pollinator diversity of onions in Uttarakhand, Indian Himalayan conditions was analysed, and the pollinator density per unit area in onion crop was calculated. The Indian bee (
The pollination studies on onions were conducted at the Experimental Farm, Hawalbagh, ICAR-Vivekananda Parvatiya Krishi Anusandhan Sansthan (VPKAS), Almora, Uttarakhand, India (29°38′01″ N and 79°37′49″ E, altitude 1250 amsl) in India's Alpine and Humid subtropical climatic zone. Onion bulbs of the VL Piaz-3 composite variety were directly sown in a 200 m2 field (14.0x14.3 m) on 12th December, 2020 (50th standard week) at a spacing of 50×50 cm. Fertilization, irrigation, earthing up, weeding and pest management were followed as per the agronomic practices recommended for VL Piaz-3.
To study the floral biology of onion variety VL Piaz-3, flower colour, number of flowers per umbel, required time for umbel formation, first flowering and last flowering from the date of sowing were recorded. The total number of umbels per plant and total number of umbels per square metre were counted in order to obtain the quantitative yield data. The data was recorded from a total of twenty-five plants selected randomly from the field and the values were averaged and presented as mean±SD values.
To assess the diversity of insects visiting onion flowers during the flowering period, the fields were monitored regularly for sixty-seven days (from initial flowering to seed set) through traversing zigzag manner three times a day (10.00, 13.00 and 16.00 h IST UTC+05:30). The traversing distance was up to 15 m in one direction and a total distance of 75 m (5 transect lines) was covered for the entire field once in a span of seven minutes. All the flower-visiting insects were counted and recorded in a field register. One representative specimen of each insect visitor was caught with a hand net and a repository of onion-flower visiting insects was prepared. For the characterization of coleopterans, the specimens collected from the fields were compared with the type specimen of the white grub beetles preserved in a Himalayan white grub species compendium at ICAR-VPKAS, Almora, Uttarakhand, India. However the technical bulletin on butterflies of Almora prepared by Stanley et al. (2018) was referred to for the identification of Lepidopterans. Furthermore, adult dipterans were identified through the pictorial, linear and dichotomous taxonomic keys designed by Buck et al. (2009) and Marshall et al. (2017). The hymenopterans were morphologically characterized through the literatures of Goulet & Huber (1993) and Seltmann (2004) with pictorial, interactive and dichotomous keys.
In order to assess the density of flower-visiting insects per unit area,
After initial assessment of pollinator density and diversity, three major insect pollinators’ viz.,
Entomophily and artificial pollination methods followed for pollinating onion flowers
T1- Indian bee ( |
One individual bee, actively foraging in onion fields was collected through a test tube and released on to a pre-marked umbel and bagged with plastic mesh of 500 micron diameters to prevent escape. One bee per umbel was released every day from initiation of flowering up to seed set. Duration of release 1 hour |
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T2- European bee ( |
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T3- Himalayan bumble bee ( |
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T4- Hand gloves (Cloth) | The pollens were collected from un-bagged umbel through the cloth hand gloves and gently rubbed over the bagged umbel and covered with plastic mesh after pollination. Umbels were gently rubbed for 3 minutes |
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T5- Hand gloves (Rubber) | The pollens were collected from un-bagged umbel through the rubber hand gloves and gently rubbed over the bagged umbel and covered with plastic mesh after pollination. Umbels were gently rubbed for 3 minutes |
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T6- Sponge puff | The pollens were collected from un-bagged umbel through the puff and gently rubbed over the bagged umbel and covered with plastic mesh after pollination. Umbels were gently rubbed for 3 minutes |
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T7- Camel Brush | The pollens were collected from un-bagged umbel through the camel brush and gently sprinkled over the bagged umbel and covered with plastic mesh after pollination. Pollens were sprinkled for 3 minutes |
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T8- Control (Open) | The umbels were not covered and allowed for free cross pollination by all the pollinating agents |
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T9- Control (Close) | The umbels were covered with plastic mesh and only self-pollination was allowed |
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Based on the results of pollinator density; the Indian bee (
To estimate the yield enhancement of onions through the entomophily and artificial pollination methods described above, the following yield data parameters were recorded: percentage of fruit set with respect to average number of flowers per umbel (842±132.51 number of flowers per umbel, average of 25 umbels), average number of seeds formed per umbel, weight of seeds obtained from each umbel, test weight of 1000 seeds in different treatments and yield per hectare (extrapolated values obtained from seed yield per 1 m2 area) in different treatments. A total of thirty umbels were bagged per treatment (each umbel was considered as a separate replication) and the data were recorded separately for each umbel under each treatment. For calculation of the test weight, seeds of all thirty umbels under each treatment were pooled and 1000 seeds randomly were counted and weighed. Moreover, to estimate the seed yield per unit area, the number of umbels per 1 m2 (36.66±11.43 umbels per 1 m2, average of 12 plots) was counted and the seed yield per umbel under different treatments was multiplied and the values were extrapolated as seed yield (in kg) per 1 hectare area.
All the field experiments were set up in a completely randomized block design (CRBD) with nine treatments and thirty replications each. The quantitative yield data was analysed through the calculation of the average values with Microsoft Office Excel 2019 (Microsoft corp., USA) and the ANOVA was assessed at p<0.05 level of significance. The F-values, SE(m) and CV (coefficient of variation) values were calculated with SPSS software to compare the means of density of insect flower visitors, and post-hoc Tukey's-B test was conducted to analyse the statistically significant data of the pollination behaviour and foraging activity of
Onion composite variety VL Piaz-3 is highly cross pollinated with strongly protandrous flowers, wherein anther dehisces occurs well before the stigma becomes receptive. The colour of the VL Piaz-3 flowers varied from whitish to light bluish in colour with six perianth segments arranged in two whorls. The numbers of stamens were six and arranged in two whorls. Anthers were bilocular and the ovary was superior. The colour of pollens varied from dark greenish to light purplish colour. The additional details of floral biology are furnished in Tab. 2.
Floral biology of onion composite variety “VL Piaz-3”
Time of flower opening | From 6.20 h to 17.45 h (Throughout the day) |
Anthesis | Early morning hours from 6.25 to 7.25 h |
Anther dehisces | Observed from 7.45 h to 16.45 h |
Stigma receptivity | High between 9.15 h to 15.15 h (Examined through Hydrogen peroxide method proposed by Zeisler (1933)) |
Number of flowers per umbel | Ranged between 682 to 1211 (842±132.51 average of 25 umbels) |
Number of umbels per plant | Ranged between 4 to 13 (7.32±2.29 average of 25 plants) |
Number of umbels per m2 | Ranged between 19–65 (36.66±11.43 average of 12 plots) |
Days for umbel formation | Ranged between 131–144 days (137.44±3.79 days* average of 100 plants) (19th February to 4th March, 2021) |
Days for first flowering | Ranged between 143–155 days (149.96±6.54 days* average of 100 plants) (3rd march to 15th March, 2021) |
Days for last flowering | Ranged between 203–216 days (209.92±3.96 days* average of 100 plants) (2nd May to 15th May, 2021) |
Total flowering period of crop | Ranged between 60–73 days (67.76±3.83 days* average of 100 plants) (3rd March to 15th May, 2021) |
Total flowering period of each umbel | Ranged between 13–19 days (15.92±1.98 days average of 25 umbels) |
Values calculated from date of sowing of bulbs (12th December, 2020)
The pollinator diversity of onion was recorded for sixty-seven flowering days, and through visual observations, more than 120 insect species were recorded to visit onion flowers during the entire flowering period. Fifty-five of these insect species belonging to four insect orders (Hymenoptera, Diptera, Lepidoptera, and Coleoptera) were considered as pollinators, because they spent ample time foraging on the umbels (>45 seconds) and traces of pollens were observed on their bodies. These fifty-five flower-visiting insects were collected through the
List of insects visiting onion flowers under Himalayan conditions of Uttarakhand, India
Hymenoptera (19 species) | |
Lepidoptera (17 species) | |
Diptera (14 species) | |
Coleoptera (5 species) |
Diversity of insect pollinators of onion in Himalayas of Uttarakhand, India
Insect pollinator density obtained through in-situ method of sampling in onion crop at three time frames of the day during peak flowering period (*F-values calculated at 5% level of significance)
Based on the pollinator density results,
Foraging rate of pollen and nectar foragers of
Bee foragers | 10.00 h | 13.00 h | 16.00 h | F-value | p-value | 10.00 h | 13.00 h | 16.00 h | F-value | p-value | 10.00 h | 13.00 h | 16.00 h | F-value | p-value |
No. of flowers visited by pollen foragers | 24.52±1.91 a | 23.52±1.44 b | 19.71±1.63 c | 1.79* | 0.018 | 31.86±2.72 a | 29.86±1.79 a | 30.57±2.92 a | 0.41 | 0.863 | 27.48±6.49 b | 28.95±5.53a | 25.71±5.82 c | 0.14 | 0.0098 |
No. of flowers visited by nectar foragers | 23.42±2.13 b | 26.41±1.57 a | 23.54±1.95 b | 0.64* | 0.0069 | 16.33±1.98 b | 17.69±2.01 b | 19.11±1.38 a | 1.65* | 0.021 | 26.14±1.63 ab | 27.18±1.25a | 24.69±1.87 c | 0.36* | 0.0089 |
No. of umbels visited by pollen foragers | 4.95±1.28a | 4.19±0.94c | 4.57±1.10b | 3.68* | 0.0261 | 3.24±0.74a | 3.60±1.01a | 3.45±1.03a | 1.31* | 0.32 | 4.14±0.79b | 4.71±1.31a | 4.19±1.03b | 5.06* | 0.0083 |
No. of visited by nectar foragers | 3.98±1.01b | 3.87±0.69c | 4.23±0.98a | 0.42* | 0.0085 | 2.13±0.47c | 3.01±0.57a | 2.94±0.73b | 0.67* | 0.0099 | 4.23±1.03b | 3.99±1.07c | 4.57±0.87a | 0.89 | 0.0053 |
post-hoc Tukey's-B test conducted at 5% level of significance
Moreover, the studies conducted to assess the time spent by individual pollen and nectar foragers (Tab. 5) showed that the pollen foragers spent statistically on par time per each flower during the pre- and post-peak flowering periods, while they swiftly collected the pollen from flowers and spent significantly less amount of time per flower during the peak-flowering period. However in contrast, nectar foragers were recorded spending significantly more time per flower during the peak flowering period to collect nectar from deep seated nectaries in the flowers and significantly less time spent per flower during the pre- and post-peak flowering periods because of fewer flowers per umbel and more time spent searching for nectar bearing flowers. The number of pollen and nectar foragers visiting the onion flowers at three time intervals and three flowering periods were recorded (Tab. 6). The number of pollen foragers was noted to be significantly lower in the morning hours (10.00 h), increased drastically throughout the day and was statistically significant in the evening hours (16.00 h).
Time spent in seconds by individual pollen and nectar forager bee of
Bee foragers | 10.00 h | 13.00 h | 16.00 h | F-value | p-value | 10.00 h | 13.00 h | 16.00 h | F-value | p-value | 10.00 h | 13.00 h | 16.00 h | F-value | p-value |
Pollen foragers | 1.76±0.26 a | 1.82±0.51 a | 1.80±0.36a | 1.91* | 0.16 | 1.47±0.28a | 1.34±0.31b | 1.44±0.50a | 0.80* | 0.0058 | 1.79±0.4a 3 | 1.78±0.42a | 1.77±0.45a | 1.30* | 0.32 |
Nectar foragers | 3.04±0.61c | 3.12±0.70b | 3.30±0.62a | 11.75* | 0.0002 | 3.46±0.60b | 3.28±0.62c | 3.92±0.54a | 2.57* | 0.0077 | 2.96±0.63b | 2.87±0.62c | 3.08±0.62a | 1.53* | 0.0025 |
post-hoc Tukey's-B test conducted at 5% level of significance
Pollen and nectar foragers of
Bee foragers | 10.00 h | 13.00 h | 16.00 h | F-value | p-value | 10.00 h | 13.00 h | 16.00 h | F-value | p-value | 10.00 h | 13.00 h | 16.00 h | F-value | p-value |
Number of pollen foragers | 21.29±3.99 c | 27.00±5.29b | 30.71±5.99a | 0.27* | 0.0094 | 19.29±2.81c | 39.00±7.90b | 42.57±6.55a | 1.72* | 0.002 | 26.00±3.65c | 43.43±4.79b | 46.43±4.12a | 1.96* | 0.0014 |
Number of nectar foragers | 78.71±3.99a | 73.00±5.29b | 67.86±3.34 c | 0.69* | 0.0006 | 80.71±2.81a | 61.43±6.97b | 57.43±6.55c | 1.77* | 0.0018 | 74.00±3.65a | 56.57±4.79b | 53.57±4.12c | 0.87* | 0.0048 |
post-hoc Tukey's-B test conducted at 5% level of significance
Similar results were recorded during all the flowering periods for the pollen foragers. Highly contrasting results were recorded for nectar foragers, while statistically significant numbers of nectar foragers of
The quantitative seed yield data obtained through the entomophily and artificial pollination methods in onion was analyzed and presented in Tab. 7. Open pollinated onion umbels were recorded to attract the maximum number of insect flower visitors which resulted in a higher percentage of fruit set, followed by
Quantitative seed yield data of onion obtained through entomophily and artificial pollination methods
46.79 | 394±6.35b | 1.38±0.18b | 4.2±0.01b | 503.70 | |
36.46 | 307±9.02c | 1.05±0.22c | 3.9±0.02c | 383.25 | |
21.26 | 179±4.36d | 0.82±0.11d | 2.8±0.02d | 299.30 | |
Hand gloves (cloth) | 11.40 | 96±3.00f | 0.38±0.11g | 3.3±0.02e | 138.70 |
Hand gloves (rubber) | 8.79 | 74±3.61g | 0.34±0.09h | 2.9±0.03g | 124.10 |
Sponge puff | 17.34 | 146±4.73e | 0.66±0.19e | 3.1±0.02f | 240.90 |
Camel brush | 17.10 | 144±4.58e | 0.59±0.15f | 2.6±0.01h | 215.35 |
Control (open) | 85.99 | 724±7.00a | 3.90±0.47a | 4.7±0.01a | 1423.50 |
Control (close) | 7.84 | 66±4.04h | 0.31±0.04i | 2.2±0.01i | 113.15 |
SE(m) | 12.98 | 0.215 | 0.013 | ||
C.V. | 3.141 | 64.95 | 6.913 | ||
F-values | 2461.78* | 27.41* | 38.67* | ||
p-values | 0.00014 | 0.00007 | 0.000011 |
Note: F- values calculated at 5% level of significance,
DMRT- Duncan's multiple range test (DMRT) was carried out for comparing the significant differences in the mean values of various treatments
Assessment of per cent fruit set with respect to total number of flowers per umbel under different pollination treatments.
Onion (
Recently Walker et al. (2011), Devi et al. (2015) and Saleh et al. (2021) reviewed the floral biology of onion and importance of insect pollinators in improving seed yield of onion. Walker et al. (2011) reported a total of 18,407 individual insects belonging to 264 species and twenty-eight families of seven insect orders visiting the onion flowers during the entire flowering period, whereas Bohart et al. (1970) showed the presence of 267 species of insect pollinators visiting onion flowers. Our studies formed a close concurrence with the previous studies, in which through visual observations we recorded more than 120 species of insects from five orders visiting onion flowers. Out of which fifty-nine species were collected from fields and identified up to species level with the help of taxonomic keys.
Among all the insect pollinators visiting onion flowers, bees had an important role in efficient pollination and optimum seed set. Jadhav et al. (1981), Kumar et al. (1985) and Chandel et al. (2004) showed that 60–95% of onion pollination is assisted by honey bees of family Apidae. Moreover, Saeed et al. (2008) and Saeed & Masood (2008) reported four bee species and four wasp species and twelve true fly species to be the major insect pollinators of onion. However, Chandel et al. (2004), in the sub-temperate climates of India's Himachal Pradesh (altitude >1200 m) recorded two honey bee species (
Based on the previous studies carried out by Rao & Suryanarayana (1989), Chaudhary & Sihag (2003) and Chandel et al. (2004) and the results of our studies, we selected the Indian bee (
Although onion is a cross pollinated crop and insects play an important role in pollination, the exploitation of insects or domesticated bees in hybrid seed production system is still in infant stages in the temperate and sub-temperate areas of Indian Himalayas (Pratap & Pratap, 1997; Chandel et al., 2004). Moreover, the production of pure lines of breeder seeds is a very cumbersome procedure due to unavailability of suitable crossing techniques in onions (Van der Meer & Van Bennekom, 1968; Mayer & Lunden, 2001; Yucel & Duman, 2005). Therefore, several crossing methods were tested and it was noted that, only controlled pollination by insects helps in efficient pollination and optimum seed set. Similar crossing studies conducted by Matherson et al. (1996) and Sajjad et al. (2008) concluded that, honey bees were the most efficient onion pollinators and assisted in the production of high quality seeds. Quantitative yield analysis studies showed that the insect-pollinated umbels recorded superior yield data in comparison to other mechanical methods. Gjorgji & Rukie (1997) concluded that, the test weight of onion seeds pollinated by honey bees was 10.57% higher than those pollinated through hand pollination. Similar results were recorded in our study, in which
As a result of this study, a total of 120 species of visiting insects were observed to visit onion flowers for collecting pollen and nectar, out of which Indian bee (