Exploring aspects of pollination ecology on the watermelon plant Citrullus lanatus (Thunb.) and the effect of pesticide use.

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Pollination is an indispensable ecosystem service that results in an increase in food security and the improvement of livelihoods (Kwapong, Aidoo, Combey, & Karikari, 2010). It is the transfer of pollen grains (male sex cells) from the anther to the stigma (female reproductive organ) of a flower. This may occur by wind, water or biotic means and varies among plant species. Once the pollen gets into contact with the stigma, pollen tube germination takes place leading to the fertilization of ovules and then the plant continues the path of producing seeds and fruit after being fertilized. Pollination is therefore a requirement for seed and fruit production in most plants (Mayes, 2011) and many factors such as the flower physiology and morphology, pollinator characteristics as well as effects of weather influences the success of pollination.

Pollination plays an important role in flowering plant reproduction and fruit setting as pollination occurs during the flowering period. Estimates show that up to 90% of all flowering plant species rely on pollination by various kinds of animals (Richards, 1986; Buchmann & Nabhan, 1996).

Animal pollination requires an organism to transfer the plant’s male sex cells to a receptive stigma and this is carried out by many different species ranging from vertebrates to invertebrates such as insects. Insects provide more than 80% of the animal pollination in crops, of which bees are the most agriculturally important pollinators worldwide (Adamson, 2014).

The most recent approximation of the global economic benefit of pollination amounts to €265bn (Lautenbach, Seppelt, Liebscher & Dormann, 2012) estimated as the value of crops dependent on natural pollination. This cannot be stated as a “real” value, because should natural pollination end, it will be impossible to replace it with this amount of money thereby making its true value infinitely high.

The estimated production value of one tonne of a pollinator dependent crop is approximately five times higher than its equivalent for one of those crop categories that do not depend on living organisms for fruit and seed production (Kluser, 2010) therefore much effort must be put into conserving at all cost the “producers” of our food.

Humans have relied on bees and other insects for long to provide pollination services to our crops (Kevan & Phillip, 2001) and this is because they possess many traits that make them good pollinators, e.g., their numerous body hairs, their foraging behaviour and the fact that they collect food for themselves and their young. This last trait is very important; as other insect pollinators just feed on nectar and or pollen, but do not collect them therefore they may not be reliable enough to cause pollination although they supplement bees in pollination (Free, 1993). A sufficient transfer of the pollen grains is likely to occur during the collection process because a larger number of flowers will be visited during floral resource collection.

Generally many staple crops such as wheat, maize and sorghum are wind pollinated but insects do visit these crops for pollen while most fruits and vegetables are pollinated solely by animals. Plants with anemophilous flowers can produce seeds without animal pollination because in most cases wind will provide sufficient transfer of pollen. This notwithstanding, the presence of insect visitors has been shown to significantly increase seed set in combination with wind effects (Soderstrom & Calderon, 1971; Adams, Perkins & Estes, 1981) mainly in areas where wind velocity is not strong enough to cause a sufficient transfer of pollen.

Cross pollination is necessary for most plants and occurs when pollen from a different plant is deposited on a flower of the same species and even plants that undergo self-pollination often benefit from cross pollination.

There is a growing interest with regards to conservation of pollinators (Buchmann & Nabhan, 1996), as it is recognized that the productivity of many crops depends on the services of key pollinators. Insufficient pollinator service is a great concern in fruit production because when the flower receives too few visits from the pollinators, pollen limitation may result in reduced yields.

Worldwide declines in native and managed pollinators has led to an increased global discuss and focus on the potential factors that may be the underlying cause of these declines. Although a number of factors have been hypothesized as potential contributors to pollinator declines, no single factor has been isolated as the sole cause of the decrease in numbers. The available knowledge base suggests that pollinator declines are a result of multiple factors such as increased/improper pesticide use, as well as habitat loss for pollinators (Kluser, 2010) which may be acting in various combinations.

Research is being directed at identifying the individual and combined factors that are most strongly associated with pollinator declines but Walker (2013), identifies indiscriminate use of systemic pesticides, most notably a class of insecticides known as neonicotinoids as one of the major factors influencing pollinator decline.

According to MOFA (2011), the importation and use of pesticides in Ghana has increased rapidly as shown in table 1 in spite of the high cost of the products relative to the financial capacity of majority of farmers all in the bid to reduce production losses and this has been associated with the high incidence of plant diseases and pest attacks.

This research is aimed at exploring some aspects of the pollination ecology of the watermelon plant Citrullus lanatus (Thunb.) and the effect of pesticide use.

Watermelon belongs to the family Cucurbitaceae which is predominantly tropical, having 90% of the species in three main areas namely: Africa and Madagascar, Central and South America and Southeast Asia and Malaysia (Jeffrey, 1990). Economically, the family has many cultivated species and a number of wild species which are important for food, medicine and fodder (Njoroge, 1992; Njoroge & Newton, 1994). The watermelon plant has a short life span of between three to four months.

C. lanatus is one of the cultivated cucurbitaceous species thought to have their origin in Africa (Cobley, 1965; Masefield, Wallis, Harrison & Nicholson, 1969; Kirkbried, 1993). Its cultivation began in ancient Egypt and India and spread from there to other countries via the Mediterranean, to Asia.

In 1857, David Livingstone reported the existence of a large wild species of watermelon in Botswana. As a result of prolonged cultivation and selection, new forms of watermelon have evolved that have no resemblance to the ancient African forms (Fehe’r, 1993).

The plant is grown in Ghana for its flesh that is extremely refreshing mostly during the dry season because it contains abundant water and minerals and the fruit is also rich in α and β-carotene. In some other parts of Africa, especially West Africa, C. lanatus is grown for its seeds, which contain high levels of unsaturated linoleic acid and various amino acids (TCN, 1996).

The main fungal diseases of this crop are anthracnose, powdery mildew Sphaerotheca fuliginea (Schlecht.) and downy mildew which is caused by the fungus Pseudoperonospora cubensis (Berkeley & Curtis). Other production problems are as a result of insect attacks, e.g. melon ladybird Henosepilachna elateri (Rossi.), aphids Aphis gossypii (Glover.), melon fly Dacus cucurbitae (Coquillet), red spider mite Tetranychnus sp. (Koch) and thrips Cerathothripoides cameroni (Boyhan, Darbie, Granberry & Kelley, 2000).

Table 1. Pesticides Import Statistics for 2007 – 2010






Formulated Pesticide Product

Solids (Mt)

Liquids (Lt)

Solids (Mt)

Liquids (Lt)

Solids (Mt)

Liquids (Lt)

Solids (Mt)







































Others e.g. Plant growth regulators e.t.c


















Grand Total





Source: MOFA, GCAP 2011


Pollination of one third of the foods we consume require a living organism to unite the reproductive parts of the flowers (Mayes, 2011) but these organisms especially honey bees are gradually declining in population (Potts et al., 2010). This might be due to loss, modification, destruction or fragmentation of their habitat, the misuse of pesticides among others (Bhattacharya, 2010).

This decline in the population of pollinators has a negative effect on our food production as a nation. It leads to a reduction in the quantity and quality of food that can be produced because flowers that are not fertilized might end up producing no fruit at all.

Some plants require a specific number of visits by pollinators to ensure adequate pollination. Eight or more bee visits to the watermelon flower is superior to four or fewer visits, as the former results in a high fruit yield and quality (Stanghellini, Ambrose, & Schultheis, 1997).

In a watermelon field, adequate pollination would be indicated by a high percentage of melons in the number 1 category that is symmetrical, completely developed throughout, and of satisfactory weight and sweetness (McGregor, 1976). Appearance such as the colour and shape of the fruit is often affected by pollination and this is important during marketing.

Although the cultivation of this crop is gaining popularity in countries such as Kenya (Njoroge, Gemmill, Bussmann, Newton & Ngumi, 2004) and Ghana, data is not available on the effective pollinators of watermelon as well as the effect of pesticide use on the pollinators and fruit yield in the region. According to Corbet, Williams & Osborne, (1997) studies in other regions have shown that adequate pollinator visitation contributes positively to productivity of the plant hence the need to ensure consistent and sustainable pollinator populations.


Utilization of pollinators especially honeybees is considered as one of the cheapest and most eco-friendly approach in maximizing the yield of cross-pollinated crops (Free, 1970). Many investigations have consistently confirmed that yield levels can be increased to an extent of 50 to 60 per cent in fruits and plantation crops, 45 to 50 per cent in sunflower and sesame and 100 to 150 per cent in cucurbitaceous crops through good management of pollinators (Melnichenko & Khalifman, 1960).

Insect pollination of crops is an essential crop management practice and should be utilized skilfully by harnessing the activity of honey bees, wild bees and other pollinators including solitary bees. Achievement of desired pollination to increase the qualitative and quantitative parameters of crop yield lies in the planned and efficient utilization of the pollinators, as well as ensuring the safety and health of the various pollinators hence the need to identify through this research the impact of indiscriminate pesticide use on the pollinator population.

According to Samnegård, (2011) it was previously believed that the most limiting factors influencing the formation of seeds and fruit in plants were nutrients and water supply but in no way related to pollination because pollen is produced and spread in very large quantities. Nevertheless later research has revealed that pollen limitation occurs in many plant species and could be a major reason for reduced fruit and seed production (Burd, 1994; Larson & Barrett, 2000), especially for plants in fragmented landscapes (Aguilar, Ashworth, Galetto, & Aizen, 2006). This indicates the essential role pollinators play in many plants reproductive success.

Pollination failure can occur at different stages; before, during or after pollen dispersal (Wilcock & Neiland, 2002). Reasons for pollination failure can be pollen feeders depleting the pollen, not viable pollen, lack of pollinators or pollinator activity leading to a diminution of the reproductive success of the plants involved. The reproductive success of plants is related to whether progeny will survive into the future or not. A large number of unfertilized ovules results into a reduction in the reproductive success of plants and this could be a consequence of too few pollen grains arriving at the stigma or an excess of non-matching pollen (Wilcock & Neiland, 2002), which will be as a result of too few pollinators or non-efficient pollinators working the flowers.

To curb this, an identification of the most efficient pollinator is required and efforts made towards their conservation as is sought to be done through this work.