About two-thirds of all known species of living things are insects, and there are over 21,000 insect species in the UK. The plants that we grow for our food are a source of food for many insects, too. Some of the insects that visit these food plants are pests that damage the crop, but many of them are beneficial, either as pollinators of the crop or as predators and parasites of pests. The importance of insects to the growing of our food can be illustrated by observing them on broad bean plants in the school garden or in pots.

Growing the Broad Bean Plants

The broad beans should be sown in three batches at the beginning, middle and end of March to ensure that flowering plants are available in mid to late June. Use any spring-sown variety of seed (avoid varieties intended for autumn sowing) - see the instructions on the packet. Packets usually contain 25-60 seeds depending on packet size and supplier.

Broad beans are most easily grown in open ground in the school garden, if you have one, but can also be successfully grown in pots. In open ground, sow the seeds singly 5cm deep in rows or blocks, with about 22cm spacing between plants in a row and, if you plant more than one row, leave about 45cm between rows. In pots, sow the seeds singly in 12.5cm - 15cm diameter pots, using garden soil, or John Innes No.1, or a multipurpose compost.

Remember to water the growing plants during dry spells and to control weeds by hoeing or hand-weeding. If you have planted a tall-growing variety, you may need to stake the plants with bamboo canes, especially in windy areas.

By mid-June, all or some of the broad bean plants will be in flower, and ready for observation of their insect visitors as described below. In southern parts of England, the plants will also be bearing well-developed bean pods by this time. In northerly areas, however, the bean crop develops more slowly and the beans may not be ready to eat until nearer the end of the summer term. If pupils can also grow their own broad bean plant at home in a pot, then they can pick the mature pods later and open them to see the new beans inside.

Observing the Insects

In late June - preferably during National Insect Week (NIW, 23-29 June 2008) - pupils should observe the broad bean plants for a fixed time period (say 15 minutes) and make a record of the types of insect on the plants, where they were seen and what they were doing. Insect activity and behaviour is affected by weather, so the weather conditions should be noted: in general, insect activity is greater on warm calm sunny days, but some insects - especially blackfly - will be found on the plants in all weathers. The insects that are likely to be found are illustrated in the drawings below. These will help the children to recognize the insects they see, and colour pictures of some of these insects can be viewed here

Recording Form

An example of a recording form

click here to download a Recording Form

For easier recognition of the smaller insects, and to see more clearly what these are doing, a small magnifying glass is a very useful accessory and can be shared between pupils.

During this outdoor activity and afterwards, in the classroom, the following key points should be discussed with the class:

the variety of insects seen and what they were doing;
the importance of insect pollinators for plant reproduction;
the impact of pests on the growth of food plants;
the benefits of predators and parasites that kill pests; and
the wider significance of insects in human food production.

How to Recognize Insects Found on Broad Beans

You can download a copy of this drawing in pdf format click here. A laminated A5 recognition card with colour illustrations of all these insects has been produced for National Insect Week by Peregrine Productions. You can purchase these wipe-clean cards for £1.75 each by sending a cheque payable to Peregrine Productions with your order for NIW "Insects on Beans" laminated plates to Peregrine Productions, Thistledown, The Common, Little Blakenham, Ipswich, Suffolk IP8 4JX. To view a sample A5 plate of bumblebees, or to see their catalogue of other laminated recognition cards, visit their website.

Insect Activity and its Effect on the Broad Bean Plant

The Importance of Insects on Food Plants

The insects observed by your pupils on their broad bean plants demonstrate the importance of insects in food production world-wide. Everywhere that we grow food plants, there are insects that damage the crop, and other insects that are beneficial. Ask the class to think about the numbers of insects that they saw on their broad bean plants, and then to imagine how many insects there might be on all the bean plants grown in England every year to produce our annual harvest of about 450 thousand tonnes of field beans. The different roles that insects play in the growing of our food plants can be remembered as the 'Four Ps': Pollinators, Pests, Predators, and Parasites. In order to grow food successfully, farmers and gardeners need to know all about these Four Ps.

Pollinators

Insects are the main agents of pollination of most flowering plants, including many of the crops that form the staple food of almost the entire human population of over 6 billion people. In rich countries with intensive agriculture, advanced farmers may grow modern crop varieties that do not require natural cross-pollination but, in most countries of the world, the flowers of food plants have to be pollinated before they will produce the grains, fruits and nuts that we harvest. Without insects to pollinate them, a large proportion of all flowering plant species would eventually become extinct and entire ecosystems would be destroyed.

Many different insects, including flies, moths, butterflies, wasps and beetles, pollinate flowers but the most important are the bees, especially bumble bees and honey bees. Bees are attracted to the sugary nectar produced by flowers and also collect pollen as protein-rich food for the larvae in their colonies. Broad bean pollen is unusual in being grey rather than golden and the children may be able to see this by the colour of the pollen baskets on the rear legs of bees that are visiting the broad bean flowers. Many modern broad bean varieties are also unusual in not being entirely dependent on insect pollination in order to set seed. Only those bumble bee species that have long mouthparts can reach the nectar by entering the open broad bean flower, but other bumble bees and honey bees sometimes 'rob' the nectar from the back of the flower.

Pests

It is estimated that up to 20% of potential global food production is lost to insect pests. Losses are particularly high in tropical regions but, even in our temperate climate, farmers and gardeners in the UK have to be constantly on their guard against pest damage. Broad beans are especially susceptible to attack by the aphids known as blackfly, which are capable of massive population growth in the summer months with generation times as short as one week and each adult giving birth to 70 - 120 young. The blackfly seen on the stems and leaves of the broad bean insert their needle-like mouthparts into the plant tissue to drink the sap. This activity steals nutrients from the plant and reduces the yield of beans. Very heavy infestations of blackfly can kill the plant. When blackfly move from one plant to another they can transmit plant diseases, which themselves may reduce bean yield or kill the plant.

The other main pest of broad bean plants is the pea and bean weevil. You may not see the weevils themselves but the damage they cause is very distinctive. The adults nibble at the edges of the leaf, causing neatly-rounded notches up to 5mm across. Serious attacks significantly diminish the leaf area available to absorb sunlight for the plant to make sugars from carbon dioxide and water by photosynthesis, and therefore reduce the crop yield.

Predators and Parasites

Several species of ladybirds, hoverflies and lacewings prey on blackfly and help the farmer and gardener by reducing the numbers of these pests. In most of these it is the immature stages - the larvae - that are predators. However, adult ladybirds also attack blackfly, and the two commonest species (2-spot and 7-spot ladybirds) are often seen searching for them on the broad bean plant.

Blackfly numbers are also controlled by parasites, especially by tiny parasitic wasps that lay their eggs inside young blackfly. The developing wasp gradually eats the body of the aphid, eventually killing it, and then bites its way out, leaving a circular hole in the blackfly's empty skin (the 'mummy'). Deadly parasites of this type are known as parasitoids.

Such predators and parasites provide natural control of pests. As an alternative to chemical pesticides, these biological control agents are increasingly used in pest management systems.

Food Facts

Pulses (grain legumes) such as beans, peas and lentils are a major and important source of protein in the human diet, especially for people who consume little or no meat or fish. Global production of these pulses is about 60 million tonnes per year, i.e. an average of 10 kg per person (but this includes pulses used in animal feeds). Worldwide, 3 million hectares of land are used to grow faba beans (broad beans and field beans) - an area about one quarter the size of England.

Additional Information

For further information, click here to see the list of books about insects elsewhere in this Education and Resources section of the NIW website.

You may like to try the following additional activity in your study of insects on broad bean plants. In the week before National Insect Week, ask the class to see if they can find any trails of black ants around the school playground or garden, and move one or two of the pots of broad bean plants next to a trail. The ants will be attracted onto the broad beans in order to feed at the black nectaries near the junction of the leaf stalks and the stem. This should be to the plant's advantage since the ants then act as 'bodyguards' by attacking herbivorous insects that would damage the plant and threaten their supply of nectar. However, this does not protect the broad beans from blackfly because the ants themselves harvest honeydew from these aphid pests and therefore defend them from predators. Ask your pupils to record which insects the black ants attack and which ones they leave alone. The observations can be used in discussion to illustrate the complexity of interactions between the 'Four Ps', even in a simple system on a single plant.