Entomology, Ornithology & Herpetology: Current Research
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Diversity, Relative Abundance, and Diurnal Variation of Insect Visitors of Litchi (Litchi chinensis Sonn.) at Rampur, Chitwan, Nepal

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Introduction

Litchi (Litchi chinensis Sonn.) is considered as one of the most important sub-tropical evergreen fruits which is believed to be originated from southern China. Litchi is nut type fruit with fleshy aril edible part belonging to Sapindaceae family. Litchi is the third most important fruit after mango and banana in terms of area and production in Nepal and Chitwan is major litchi growing area of the country covering an area of 302 ha with productive area 148 ha, production 1,228 mt of fruit annually and productivity 8.3 mt/ha [1].

Litchi is a cross pollinated plant and its flower requires sufficient pollinating agents for pollination and fruit set. Nectar and pollen presence in huge amount in the self-sterile flowers attracts various insects, such as honeybees, flies, ants and wasp leading to entomophilic cross pollination. It is estimated that one-third of the total human food supply relies on insect pollination [2,3]. Flowering in litchi occurs during the month of March-April, which attracts and invite a number of insects for pollination [4]. Pollination of entomophilous crops by honeybees is regarded as one of the effective and cheapest methods for improving the qualitative and quantitative yield of crops [5]. Litchi flowers are visited by varieties of insects including, Hymenopteran, Dipteran, Coleopteran, Hemipteran, Homopteran and Lepidopteran insect orders [6]. Most common insect pollinators are the honeybees, Apis cerana F., Apis dorsata F., Apis mellifera L. and Apis florea F., and the syrphid flies Melanostoma univittatum De Geer. A total of 12 insect species, among which 6 species belonging to the order Hymenoptera, 5 species to Diptera and 1 species to Coleoptera were recorded visiting litchi flowers in India [4]. Minimum ecological conditions required for Apis species for commencement of flight activity is 15.5°C-18.5°C temperature, 600-1700 lx light intensity, and 9-20 mW/cm² solar radiation, respectively [7]. The foraging activity of A. mellifera at different hours of the day showed that maximum activity of bees was at 08:00 am followed by 10:00 am and 06:00 am with 128.83, 63.83 and 55.66 honeybees, respectively [8]. All farm inputs and varietal capacity goes in vain until an efficient and assured pollination guaranteed, which is major factor for quality and quantity production. Such study is scarce in Nepal. Therefore, a study was carried out to record the diversity, abundance and diurnal variation of pollinators of litchi.

Materials and Methods

The study was conducted in horticulture farm (latitude 27°37' N and longitude 84° 25' E) at the Agriculture and Forestry University (AFU) Rampur, Chitwan during the flowering stage (March to April, 2019). The monitoring of pollinators was done using a pan trap, sweep net and visual observations during the flowering stage of litchi. Three different colored pan traps, namely: blue, yellow, and white traps were placed hanging on branches of litchi tree three meters apart to each other in alternate ways concerning to color of pan trap. After twenty-four hours of installation of traps, these pan traps were removed, and trapped insects were collected and kept in insect collection bottle with 10% formalin. Similarly, visual observations were carried out to study the diurnal variation of pollinators at different times of the day at 6:00 am, 9:00 am, 12:00 (noon), 3:00 pm and 6:00 pm during peak flowering stage of litchi using a hand tally counter and chronometer (stopwatch). Sweep net was used to collect flower-visiting insects, and representatives of the insect species sampled were sorted out, pinned in wooden insect box and preserved as dry specimen, observed under a microscope, and identified with the help of dichotomous key and matching museum specimens. The pollinator diversity of insect visitors to litchi during the flowering period was measured using the Shannon-Wiener Diversity Index [9] : H=-Σ Pi(lnPi), where H represents the index of species diversity in a given locality, and Pi is the proportion of the total sample belonging to the ith species.

Results

Diversity of insect pollinators

The opening of flowers occurred during the first week of March, and 50% flowering was noticed in the mid-March, and flowering continued till the last week of March. The pollinator fauna of litchi included twenty-seven species from six different orders and ninteen families of class Insecta during the blooming period of litchi trees (Table 1). Among them, Hymenopteran was predominant comprising of twelve species from five different families, namely: Apidae, Megachilidae, Andrenidae, Vespidae, and Braconidae. Hymenopteran species were the most abundant insects (61.26%) visiting litchi flowers followed by Diptera (26.13%), Coleoptera (4.50%), Hemiptera (4.50%), Lepidoptera (2.70%) and the lowest Orthoptera (0.90%), respectively (Figure 1).

Figure 1. Abundance of different insect visitors/ pollinators during litchi flowering at Rampur, Chitwan, 2019.

Table 1: Diversity and abundance of litchi flower insect visitors at Rampur, Chitwan, 2019.

Relative abundance of insect pollinators

Honeybees were the abundant group of pollinators, viz. A.dorsata (30.63%), A. mellifera (9.46%), A. florea (6.76%) and A. cerana (3.60%) on litchi flowers. The average population of A. dorsata was recorded 10.15 insects/panicle/5 min, which represented 30.63% of population of total insect pollinators visiting litchi flowers on visual observation. Syrphids in totality were the second dominant insect visitors with an average of 9.57 insects/ panicle/5 min constituting 9.91% of total insect fauna. Among the Dipterans, Eristalis sp. frequently visited litchi flowers followed by green bottle fly (5.41%). Of all the insect flower visitors, A. dorsata and Eristalis sp. were the most frequent visitors in litchi orchard. Higher number of A.dorsata was recorded on litchi flowers with an average (10.15 bees/m²/5 min.) followed by Eristalis sp. (9.57 bees/m²/5 min.), A. florea (5.59 bees/m²/5 min.), A. mellifera (3.72 bees/m²/5 min.) and A.cerana (2.33 bees/m²/5 min), respectively.

Shannon-Wiener diversity index

The diversity index of insect visitors of litchi during the flowering was 1.07, less than 1.5 (Table 2), which revealed that there is low diversity of insect visitors. It may be due to the presence of some specific species in large numbers during the flowering stage of litchi at Rampur, Chitwan.

Table 2: Shannon-Wiener diversity index of insect visitors of litchi flowers at Rampur, Chitwan, 2019.

Diurnal variation of insect pollinators

Honey bees were the effective pollinators, whereas syrphid flies were frequent visitors on litchi flowers. The foraging activity of A. dorsata was maximum in the morning at 09:00 am, followed by 12:00 noon, 06:00 am and 3:00 pm with 22.37, 14.07, 7.06, and 4.95 bees/panicle/5 min, respectively (Figure 2). Its foraging activity was the lowest (2.34 bees/ panicle/5 min) at 06:00 pm. Similar foraging activity was performed by A. mellifera, A. cerana, whereas the foraging activity of A. florea was maximum at 12:00 (noon) followed by 03:00 pm, 09:00 am and 06:00 pm with 12.04, 8.66, 3.24, and 3.17 bees/panicle/5 min, respectively. Its lowest foraging activity was at 06:00 am (0.88 bees/panicle/5 min).

Figure 2. Diurnal variation of major pollinators on litchi flower (number/panicle/5 min) at Rampur, Chitwan, 2019.

Discussion

Diversity of insect pollinators

In general, the diversity of pollinating insects varies from region to region and locality to locality. Insect fauna belonging to Hymenoptera, Diptera, Coleoptera, and Lepidoptera have been reported to visit and pollinate the litchi flowers, which are attractive to insects for both pollen and nectar. Twenty-one species of insects were recorded visiting litchi flowers at Rampur, Chitwan, which is slightly lower in species than present finding [10]. Similarly, twenty pollinator species under 23 genera from 8 families belonging to orders Diptera, Hymenoptera and Coleoptera were reported in Muzaffarpur, Bihar [11]. In another study, 20 species under 16 genera from 8 families belonging to orders Diptera and Hymenoptera were also reported from district Haripur, Pakistan [6]. Again, only total of thirteen insect pollinators belonging to order Hymenoptera, Diptera and Coleoptera were recorded in Nadia district of West Bengal [12]. Similarly, as low as twelve insect species was observed belonging to the order Hymenoptera, and Diptera visiting the flowers during the entire blossom period of litchi [13].

Relative abundance of insect pollinators

The Hymenopterans insects were the most dominant flower visitors constituting 74.69% of the insect visitors and 25.21% others [11]. In another study, they were observed as the most dominating pollinators, which represented 73.51%, among which A. dorsata (21.82%), A.mellifera (26.84%) and A. cerana indica (7.60%) were observed on litchi flowers [4]. Honeybees were the dominant flower visitors of litchi, and their abundance was in the order: A. mellifera (47.52%)>A. cerana(20.59%)>A. dorsata(5.38%) >A. florea (0.37%) and 25.21% others [11]. Diptera were observed as the second most abundant insect visitors constituting 26.47% of the total pollinators of litchi flowers, among which Syrphid flies were observed as the most frequent visitors on litchi flowers [4]. In a very recent study, A. dorsata, A. mellifera, A. cerana indica, A. florea, LassioglossumM sp., Vespa tropicalis L. and Camponotus compressus F. belonging to Hymenoptera, Episyrphus balteatus De Geer , Eristalis sp., Syrphus sp., Sacrcophaga sp. and Lucilia sericata Meigen belonging to Diptera and Coccinella septumpunctata L. belonging Coleoptera were reported in litchi orchard [12].

A wild honeybee, A. dorsata immigrate towards southern areas of Chitwan valley from northern part during November-December and maximum colonies aggregate in March with maximum staying of eight months and return back to northern part during March-July [14] which support the dominance of A. dorsata at Rampur, Chitwan during flowering stage of litchi. The present finding reveals that A. dorsata, A. mellifera, A. cerana and A. florea were the most dominant and efficient pollinators of litchi flowers representing more than 65% of the total pollinating insects which are in line with the finding of other studies [7,15]. The honeybees, A. dorsata, A. mellifera, A. cerana and A. florea were the dominant flower visitors of litchi, of which A. dorsata (50.11%) was predominant visitor followed by A. cerana indica (11.80%), A. florea (8.68%) and A. mellifera (7.12%) [12,13]. Among Apis spp., the highest activities of A. dorsata (3.30 and 3.22 foragers/panicle/5 min), A. mellifera (2.92 and 2.98 foragers/panicle/5 min), A. cerana indica (1.24 and 1.38 foragers/panicle/5 min) has been reported [4]. The abundance of Hymenopterans (77.71% insect fauna/inflorescence/5 min.), among which A. dorsata was the predominant species ranking first, which recorded 2.53 per inflorescence and represented 50.11% followed by Syrphids with an average of 1.00 insect per inflorescence constituting 22.27% of total insect fauna as the second dominant insect visitors to litchi flower [12], which support present finding at the Horticulture Farm of Agriculture and Forestry University in Rampur, Chitwan, Nepal.

Diurnal variation of insect pollinators

The foraging behavior of all honeybee species followed the same general pattern as temperature, solar radiation, light intensity, nectar sugar concentration and inversely with relative humidity between commencement and cessation [7]. The peak foraging behavior of honeybee species A. cerana, A. mellifera and A. dorsata were recorded around 12:00 noon to 2:00 pm, where the highest of all three bee species were recorded at 2:00 pm and the lowest at 10:00 am in rapeseed flower [16]. The foraging speed (time spent in second per flower per forager) of all the honeybees were observed the highest during 9-11 am, which gradually declined towards the succeeding time of the day [12]. Again, the foraging frequency of A. dorsata reached peak with speed of 5.38 second in the morning 9-11 am, which was minimum in late afternoon 3-5 pm for all the honeybees, which are similar to the present finding. The mean foraging speed was maximum in A. dorsata (4.64 second) and minimum in A. florea (3.32 second) [12]. On the contrary, the foraging rate (number of flowers visited per minute per forager) was the highest in afternoon 3-5 pm for all the honeybees and the maximum and minimum mean foraging rate was recorded in A. florea (12.29) and A. dorsata (10.54), respectively [12].

Conclusion

The monitoring of litchi pollinator recorded a total of twentyseven species of insects from six different orders and ninteen families. Honeybees were the abundant group of pollinators, viz. A.dorsata (30.63%), A. mellifera (9.46%), A. florea (6.76%) and A. cerana (3.60%) on litchi flowers. It is concluded that litchi being an entomophilous fruit tree requires pollinating agents, so safe pesticides should be sprayed to control insect pests before and after the flowering stage, especially during the evening to protect pollinators ’ abundance in litchi orchard. Hence, pollinators ’ friendly proper bee management approach, like the timely establishment of colonies in sufficient numbers in litchi orchards and collaboration with beekeepers and growers, would lead to the enhancement of litchi fruit yields and conservation of insect pollinators.

Acknowledgements

The authors thank Directorate of Research and Extension, Department of Entomology , Department of Horticulture, Agriculture and Forestry University, Rampur for their administrative support to accomplish the research.

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