Ch 10: Crop Pollination

Pollination takes place when pollen is transferred from the male part of a flower (the anthers) where pollen is produced, to the female part (the stigma) of another flower of the same species, where the pollen germinates. Pollinators (like bees, wasps, moths, bats) transport pollen between flowers, ensuring that flowers produce seeds and fruits. Many crops cultivated in East Africa require pollination.

Common crops that benefit from pollination include avocado, coffee, cowpeas, eggplant, mangoes, pigeon peas, pumpkins, okra, and tomatoes.

Indeed, studies have shown that 75% of all crop species, and over 80% of all flowering plant species are dependent on pollinators, primarily wild insects. Crops like passionfruit, cocoa, strawberries, eggplant, watermelon, cucumber, and pumpkin are wholly dependent on pollinators. For other crops, like coffee, avocado, mango and runner beans, pollinators contribute to increased yields and quality.

For trees, and plants in natural habitats, the contribution of pollination to ensuring habitats regenerate with healthy seeds and fruits is yet to be measured or fully understood.

Studies have estimated that pollination services provided by wild insects globally are worth over €150 billion (or over US $200 billion). In the Baringo region of Kenya alone, watermelons worth KES 900 million (US $9 million) are produced. All of these watermelons are the result of pollination by wild insects, primarily bees.

The following section describes pollination systems, and pollinators of some of the most common pollinator-dependent crops in our region.

Legume Crops

Runner Beans

Runner beans exports are one of Kenya’s most important horticultural earners. When pollinators visit runner bean flowers, and pollinate them, the resulting pods are larger, better shaped, and more nutritious.

Cowpeas and pigeon peas are widely grown by small-scale farmers in East Africa, and are especially important in rural areas for domestic consumption. Runner beans, cowpeas, and pigeon peas are almost entirely dependent on pollinators.

Legume crops have a specialised pollination system involving wild bees, allowing pollination to take place only when the flower is ‘tripped’ by the bee. The flowers are bilaterally symmetrical, and when the bee lands on the flower, it has to use its weight, or legs to ‘trip’ the flower. Tripping the flower involves spreading a part of the flower called the keel. This leads to the anthers being exposed, and the visiting bee being brushed with pollen. Wild bee species are the main pollinators of these crops.

Passionfruit

Passionfruit is nutritious, and delicious. It grows as a creeper, and spreads by way of numerous strong tendrils. Demand for passionfruit has largely been driven by the fruit juice market, with increasing numbers of farmers growing passionfruit in Kenya.

Passionfruit flowers are unique, and incredibly beautiful. They are also complex with anthers arranged above a ‘ring’, and nectaries having lids on them. It takes a hefty pollinator to lift the lid to the nectary, and the most efficient pollinators able to do this are large carpenter bees (mainly Xylocopa spp.).

To set fruit passionfruits are 100% dependent on pollinators like carpenter bees, and without them there would be no passionfruits for us to eat, or to make juice. It is unfortunate that farmers often mistake the large carpenter bees for beetles, or pests, and kill them.

Carpenter bees like to nest in wood, and are attracted to hedgerows with a variety of wildflowers.

Eggplant

Local demand for eggplants (also known as aubergines and brinjals) has increased significantly in East Africa. Eggplants are a high nutritious vegetable, and can grow in hot, dry areas where vegetables are harder to grow. Eggplants belong to the nightshade family (Solanaceae), which include tomatoes and potatoes.

Flowers of plants in this family have a specialised pollination system called ‘buzz pollination’ involving wild bees. Buzz pollination involves the bee holding the flower with its mouthparts, and causing it vibrate, by using the motion from its wing muscles to transfer the vibration energy into the flower. Only when this is done at the correct frequency is pollen forcibly released from the anthers.

Eggplants are 100% dependent on pollinators to produce fruit, with solitary (non-social) species of wild bees, being the exclusive pollinators of eggplant flowers.

Maintain natural habitat patches around fields of eggplant to support solitary wild bee pollinators.

Mango

Sometimes called the ‘king of fruits’, the area under mango cultivation has grown considerably in East Africa. It is eaten as fresh, or dried fruit, and used to make juice, jam, and chutney.

Kenya’s average mango yield per hectare is about 16 tons of fruit. A single large healthy mango tree can annually produce over one thousand fruits for sale. The farmer may be able to sell each mango for up to KES 50, giving him an annual income of KES 50,000 (around US $600) from each tree!

Mango trees flower in synchrony bearing many flowers at the same time. Mango trees are dependent on optimal weather, and sufficient numbers of pollinators to set fruit with success.

Mango tree flowers are simple, open, and small in size. They are visited, and pollinated by different kinds of wasps, and even ants, but flies, and stingless bees are thought to be the main pollinators of mangoes. As mangoes flower only seasonally, a grower of mangoes will need to maintain patches of natural habitat, and wildflowers, for wild insect pollinators to visit when the mango trees are not in flower.

Coffee

Kenya’s arabica coffee is well know, and sought after around the world. Coffee sales are a major contributor to the country’s Gross Domestic Product, and the industry employs millions of Kenyans.

Coffee bushes flower synchronously at different times of the year, usually at the beginning of the rains. Coffee varieties can be self-pollinated, but the presence of pollinators improves the quality, and size of the coffee beans. Some varieties of coffee in East Africa, such as robusta coffee, appear to be completely dependent on pollinators.

The main pollinators of coffee flowers are different kinds ofbees. Honeybee hives can be placed amongst coffee bushes to improve coffee yields. Other pollinators of coffee are many different kinds of wasps, large

flies, butterflies, moths, and even sunbirds.

To support wild bee pollinators of coffee it is important to provide other sources of nectar. These can include areas of wildflowers, or indigenous shade trees along the edges of coffee plantations.

Protecting pollinators of coffee from pesticides is important, and should be managed carefully.

Spraying when coffee is in flower could negatively impact the pollinators, and therefore coffee yields.

Watermelon, Squashes and Cucumber

Watermelon, and other melons flourish in the dryland areas of Kenya. Watermelon is an important source of nutrition for many people, and a high value crop.

Watermelons grow as creepers, have separate male and female flowers on the same plant, and are pollinated by many different kinds of bees and flies. It takes many thousands of pollen grains transferred onto the stigma of a receptive watermelon flower to produce a large, tasty fruit. The amount of pollen deposited correlates to the quality, and flavour of the fruit. Large amounts of pollen produce the best fruits. Wild solitary bees, stingless bees, and hoverflies have been recorded as good pollen transporters, and pollinators of watermelon flowers.

Watermelons, sweet melons, butternut squashes, and pumpkins are all in the same family, Cucurbitaceae. All of these crops are highly dependent on pollinators.  Leave some areas of natural habitat to encourage pollinators, and manage pesticides carefully during flowering.

Papaya

Papaya, also known as pawpaw, is an important fruit crop for many smallholder farmers in East Africa, who are fully dependent on pollinators to pollinate their papaya.

Papaya has separate male and female plants. Male flowers are produced in large numbers on male plants, while female flowers are produced is smaller numbers at the base of the leaves on female papaya plants. Papaya pollinators have to visit male flowers, collect pollen, then visit female flowers to ensure pollination, and fruit set.

The pollinators of papaya in East Africa are primarily hawkmoths, and a few skipper butterflies.

Hawkmoths are efficient pollinators as they can move rapidly between different papaya plants on a farm.

 Hawkmoths, and skipper butterflies need other plants for nectar. They also require wild plant species, called host-plants, to provide a place to lay their eggs, and from where caterpillars can feed, and grow.

Spices and Seeds

Spices are tasty, high value, and often health supporting components of our diet. Pollinators contribute by pollinating the flowers that develop into the seeds and fruits that we harvest, and use as spices. Examples of spices that are pollinated by insects in East Africa include chillies (pollinated by bees), bell peppers (pollinated by bees), cardamom (pollinated by bees), and coriander—also called cilantro (pollinated by bees, wasps, flies).

Traditional Vegetables

Different kinds of indigenous vegetables are grown as traditional food plants across East Africa, and contribute to food, and nutritional  security in rural areas. Each region often has special plants that make the cuisine, and diet of that area distinctive. Common traditional vegetables in the region include the blackbean (njahe), wild spinach (terere), leafy amaranth varieties, and a number of legumes.

Bees, and other insects pollinate traditional vegetables. For example, in western Kenya, ‘Mitoo’, Crotolaria brevidens, is a legume, which has a specialised pollination system that involves leafcutter bees, and carpenter bees serving as its pollinators. ‘Mchicha’, Gynandropsis gynandra, is another popular plant whose long brush-like flowers are pollinated by both hawkmoths, and bees.

Pollination systems of most of our traditional vegetables have not been studied.

Forage and fodder plants

Forage, and fodder plants are crops that comprise livestock feed. Different animals will feed on different kinds of plants, depending the region, and habitat.

Important forage crops for cattle in East Africa are legumes like lucerne (also known as alfalfa), which help increase meat, and milk production. Wild bees are the main pollinators of Lucerne.

Other legume species used by livestock include: acacias, Indigofera, and Crotolaria.

All of these are dependent on pollinators.

The highly nutritious pods of the Umbrella Thorn Acacia, Acacia tortilis, are widely consumed by livestock in arid and semi-arid regions. Without these pods, it would be harder for livestock to survive in arid areas. The pods are so valuable for supporting livestock that Acacia tortilis trees are carefully guarded, and passed as an inheritance from parents to children in parts of northern Kenya. Acacia pods are the result of pollination by a wide range of insects, primarily wild bees.

Indigofera are small sturdy shrubs that grow in the desert, and are an important forage plant for camels. In many parts of the Horn of Africa, free- ranging camels browse on the leaves, and pods of Indigofera. Without them camels would not be able to survive in these areas. Indigofera is dependent on wild bee species for pollination.

These examples show how pollinators are connected to us, not just through cultivated crops, but through our livestock too.