Pollination is the process of transferring pollen grains from a blossom’s male anther to its female stigma. The process of pollination incorporates all events from maturation to zygote division. Camerarius, in 1964 first discovered that Pollination is necessary for seed production (Sharma R.,2006). 

A pollinator is the biotic agent, animals or vector that moves pollen from anthers to stigma of a flower. Insects and other animal pollinators are vital for the production of healthy crops for food, fibers, edible oils, medicines, and other products. Pollinators play an immense role in the production of many fruits, vegetables and field crops (Kleinetal., 2007).It is estimated

that more than 1,300 types of plants are cultivated around the world for food, beverages, medicines, condiments, spices and even fabric. Out of these, almost 75% are pollinated by animals. In fact, pollinators such as bees, birds and bats affect 35 percent of the world’s crop production which increased outputs of 87 of the leading food crops worldwide and obviously within these, fruits and vegetables are the most benefited items. Numerous studies have valued insect pollination as an eco-system service for agricultural food production at both global and national level (Gallai et al., 2009, Win free et al., 2011). With highest efficacy of honeybees, approximately 100,000 different species of animals around the world act as pollinators to 250,000 plant species on the planet. Honey bees are critically important for crop pollination worldwide (Klein et al., 2007) as it is assumed to contribute 80% of insect pollination (Robinson et al., 1989). Beside Bees, other insects like beetles, wasps, butterfly and moths also pollinate at a significant rate. Among mammals, bats are the principal pollinators, responsible for pollinating a large number of economically and ecologically important plants such as agave and cacti. Key pollinating birds are hummingbirds, honeyeaters, sunbirds and perching birds. Flies, ants, midges, monkeys,

lemur, possum, rodents, lizards, mosquitoes, flying fox, fruit bats, snails, slugs and even a gecko as well as human also function as effective pollinators elsewhere in the world. Investigating the financial advantages of a significant agricultural ecosystem service, like insect pollination of crops, is essential for managing farms and sustainably producing food. The possible effects of a persistent drop in insect pollinators on food output and food security are considered during valuation. However, it can also show how, by addressing pollination shortages within cultivated regions, appropriate management of insect pollination services can lower production risks and improve benefits (Abson DJ, Termansen M., 2011).

But now the population of wild, native and managed pollinators is declining at an alarming rate owing to alterations in their food and nesting habitats, shrinkage in natural ecosystems, pesticide poisoning, alien species, diseases and pests, over-collecting, human activity, climate change, smuggling and trading of certain rare and endangered species. Pollination services provided by honey bees have not been recognized and overestimated by farmers and policymakers, resulting in a decrease in agricultural yield (Joshi et al., 2003). Therefore, conservation of

pollinators' habitats and implementation of agro-environmental practices to enhance wild plants resources and nesting sites for bees in agricultural landscapes are vitally important.

Importance of honey bees in pollination:

Research confirmed that the desired outcomes may not be acquired in pollination without using honey bees (Chadha, 2001). Bees visit flowering plants regularly for nutrition, nectar, and pollens. Bees’ floral constancy is owing to their predilection for sugar-containing nectars and pollen with better nutritional contents. When one considers that a single honeybee colony may make up to 4 million journeys each year and that each trip visits an average of 100 flowers, the pollination potential of the colony becomes clear (Free 1993). This is due to their abundant body hair and distinct behavioural habits (Du Toit, 1988).Honeybee pollinators are responsible for providing 30 per cent of the world’s food supply, either directly or indirectly(Greenleaf and Kremen, 2006).This remarkable work of the honeybee plays a significant role in its position as an effective insect pollinator. In India, generally, four species of honey bees of commercial importance are found, i.e., Apis florae (little bee), Apis dorsata (rock bee), Apis cerana indica (Indian bee),Apis mellifera (Italian

bee). Among these first three are indigenous to India, but Apis mellifera was introduced to India in the 19th century, which is highly suitable for domestication in India (Chadha, 2001). Bees can work for a long time and their reproduction rate is very high. Honey bees improve the productivity level of different crops by cross-pollination. This is evident from the fact that when honey bees are used in cross-pollination, the income from farming is several times higher than its as worth as production of products like beeswax and honey. Furthermore, cross pollination is not required for self-sterile types; self-fertile plants will produce higher-quality seeds if pollinated by honey bees and other insects (Verma and Pratap 1993). Research conducted in the Kathmandu valley found that A. mellifera worker bees transported much more pollen from peach and plum blossoms than A. cerana indica worker bees. Honeybees in field work effectively in all directions and were uniformly distributed throughout flowers (Ingram et al., 1996).

Types of pollination:

Pollination is a method where pollen grains are picked from an anther, which is the male part of a flower and transferred to the flower’s female part called the stigma. Insects (butterflies, moths, bees, wasps, ants, beetles, etc.) numbering about

500 species are an important supplementary source of calories and proteins and fats in many regions of the world. Pollination is critical for food production and human livelihoods, and directly links wild ecosystems that many wild animals rely on for food and shelter with agricultural production systems. Without this service, many interconnected species inhabiting, and processes functioning within, an ecosystem would collapse. Reduced pollination is a key cause of declining horticultural production, which can be attributed to several reasons (Joshi et al., 2021). Self-pollination occurs when pollen grains fall directly from anther into the stigma of the flower. Cross- pollination refers to a complex type of pollination that allows the transfer of pollen grains from the anther of the flower into the stigma of another flower. Autogamy is pollination of a flower by its own pollen. Geitonogamy occurs between two different flowers present on the same plant. Homogamy refers to anthers and stigma of a flower mature at same time. In dichogamy, stigma becomes receptive before or after the pollens of the same flower are mature (Singh et al., 2014).

Pollination in various fruit crops:

Insects and other animal pollinators are vital to the production of healthy crops for food, fibers, edible oils, medicines, and other products. Pollination by bees therefore increases fruit production by 50% over that achieved by wind (Krishnan et al., 2012). In case of cashew major pollinator is A. mellifera and Trigona spinipes (Freitas et al., 2014). In sapota, it is Thrips (Thrips hawaiiensis and Haplothrips tenuipennis) (Reddy, 1989). In strawberry, it is Bombus lucorum and A. mellifera in greenhouses (Li et al., 2006). Major pollination mechanism of fruit crops is discussed below.

Mango:

Major pollinators found in mango plantation are honeybees (Apis cerana and Apis mellifera), allodapine bee (Braunsapis hewitti), sweat bees (Halictus sp. and Lassioglossum   spp.),     Chrysomya megacephala, Chrysomya pinguis, andMusca domestica (Sung et al., 2006, Fajardo etal.,2008).Large Diptera and the native bee (Trigona sp.), frequently moved from onetree to another and helps in cross pollination most effectively (Anderson et al., 1982). Sturrock (1944) also considered the flowers as self-fertile. It was supported by the earlier work of Popenoe (1917), who stated that though mango is self-fertile, but cross- pollination helps to increase fruit set. Fraser (1927) stated that fruit bud formation and pollination were the two big problems in

growing mangoes and Wolfe (1962) concluded that getting fruit to set was more difficult than getting trees too Du Toit (1994) found that fruit set was poor in both open- pollinated bagged inflorescences when honey bees were introduced into a South mango orchard. Singh (1989) had contrasting findings, showing that several for aging insects including the European honey bee significantly increased fruit set.

Citrus:

The pollination requirements for citrus are quite diverse (Sanford 2003; McGregor 1976), ranging from self fertile (Valencia oranges) to almost complete self- sterile (mandarin and mandarin-hybrid complex). Pollen must be transferred to these self-sterile or partially self-sterile flowers from those of different compatible type for maximum fruit production (Sanford2003). In others (Washington navel oranges), the plant is benefited if pollen is moved from flower to flower within the cultivar or within the species (Sanford 2003), and finally others such as lemons, have no known to be benefited from transfer of foreign pollen to the stigma (Sanford 2003; McGregor 1976). Some growers of seedless cultivars readily discourage honey bee pollination as seedless fruits are often more sought-after, demanding higher retail prices in comparison to seeded

varieties (Butcher, 1960). Krezdom, 1970 suggested that this by no means indicates pollination is not necessary in citrus. Although cross pollination is required, use of honey bees remains the most consistent, effective and economical means of ensuring adequate yields.

Pomegranate:

The presence of both male (unfertile) and bisexual (fertile) flowers on the pomegranate allow it to be self-pollinated as well as cross-pollinated. Several studies have shown that cross-pollination results in around about 20% increase in fruit set as well as an increase in overall fruit quality (Derin and Eti2001). Because of its heaviness, there is very little wind dispersal of the pollen and thus insects are mostly responsible for the transport of pollen between flowers. McGregor (1976) states that growers in California placed honey bee colonies in or near their fields, believing that their presence benefits pomegranate fruit production but there is little information available regarding the efficiency of honey bees in the pollination of pomegranate. Anonymous, (2006) stated that with subsequent crosspollination in pomegranate fruit set can increase up to 68% and additionally there is an increase in fruit quality (i.e. number of seeds per fruit, fruit size).

Guava:

In guava, studies showed that the distribution of cross-pollination by insects is about35%and it is said that cross-pollination was the most common form of pollination in guava (Vinod et al., 2018). Guava bears a hermaphrodite flower and can self-pollinate. However, self-pollination is unusual without the help of pollinators (Kadam et al., 2012). Besides, cross-pollination are considered as the most frequent form of pollination in guava [6]. Several studies on guava pollination mentioned that bees, especially honey bees are their major pollinators (Amin et al., 2019, Singh et al., 2017, Alves et al., 2006) contribute about 25.70 to 41.30 %cross pollination. The authors added thatcross pollination in guava could lead to a better fruit yield, higher production of fruit mass and number of seeds. Generally, if flowers are not fertilized early, they may fall early or insufficient pollination may lead to distorted fruits. Even auto-pollinated varieties produce more and better fruits when they are cross pollinated than when auto-pollinated (Pashe et al., 2015).

Papaya:

Papaya plants produce separate male and female flowers on the same plant, a characteristic known as “monoecious.” This means that both male and female flowers

exist on a single papaya tree. Papaya flowers are primarily pollinated by wind and insects, although some self-pollination can occur. Wind plays a crucial role in transferring pollen from male to female flowers. The breeze carries the pollen from the male flowers to the female flowers. Pollination by insects (hawkmoths in Australia) and by wind have been studied. Evidence in favour of wind pollination is the fact that the pollen are light and high papaya pollen count (10-18% of total aero pollen) has been recorded on the outskirts of Kolkatta (Chakraborty et al, 2007). Pollen remains viable for 2 days before and after anthesis, with maximum viability on the day of anthesis. At room temperature, and 50% relative humidity, pollen remains viable for 48 hrs.

Cause of pollinator’s decline:

The reasons of the drop and restoration of pollinators; increase study on non-honeybee pollinators; conservation of pollinators and increasing awareness of pollinator services. Globally habitat loss due to land use transformation and land management changes following agricultural intensification are the main reasons (Dicks et al., 2020). Degradation and fragmentation of near and semi-natural habitats could alter pollinating populations and harm insect pollinator’s communities (Cardoso et al.,

2020). Declining honey bee numbers and biodiversity is posing a significant danger to agricultural productivity. Natural habitat loss, parasite and pesticide exposure, loss of floral richness, and low genetic variety owing to increasing land usage, long distance transportation of colonies, exotic plants and bees, and climate change might all be factors contributing to a significant reduction in the bee population. In addition, the rate of genetic erosion due to habitat fragmentation and degradation by reducing the genetic flux between demes would increase the probability of extinction in populations and species. The intensification of agriculture appears to be an important factor in the decline of the insect populations. Increase on the use of agrochemicals, leading to possible habitat deprivation within agricultural regions (Raven et al., 2021) those results in major declines in insect biomass and diversity. The intensification of agriculture leads to misuse of pesticides particularly herbicides and insecticides that may pose as imminent threat to pollinators (Potts et al., 2010). The use of insecticides may also have killed useful pollinators by direct poisoning (Alston et al., 2007) and may lead to local changes in the diversity and abundance (Brittain et al., 2010). Moreover, fertilizers and herbicides may indirectly influence pollinators by

reducing the inflorescence availability (Gabriel et al., 2007). All handled or wild pollinators must be protected against excessive use of pesticides and other chemicals that may interrupt reproduction when these chemicals reduce the sources of nectar for pollinators, reduce nesting materials for bees, and destroy host plants of butterflies and moths larvae (Kluser et al., 2007).

Pollination leads to seed production and is important for breeding in many crops around the world. There is less value in improving resistance to crops and diseases if it is not accompanied by high quality, so it is necessary to give priority to the development of good fruit quality. The pollination of insects preserves the genetic diversity of plant populations and offers benefits such as increasing the quality and quantity of fruits, seed production and fertility, leading to a better vigour of the next generation. Honey bee accomplishes this activity throughout their life. A decrease in their number will have a pessimistic effect on our life as well as on nature also. Both beekeepers and farmers will benefit from encouraging the use of beekeeping for the pollination of horticultural crops. More studies are required to highlight the importance of these poorly studied pollinators.

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