Insects are classified into groupings called 'orders'. The main orders that include species that use flowers as a food source are Butterflies and Moths (Lepidoptera), Bees and Wasps (Hymenoptera), Flies (Diptera) and Beetles (Coleoptera). These species may use pollen, nectar, or both of these as a food, either for adults or to feed their young.
Flowers exploit insects to achieve pollination; at the same time insects exploit flowers for food. It is a partnership. Each insect group has evolved different sets of mouthparts to exploit the food that flowers provide. From the insects' point of view collecting nectar or pollen is rather like fitting a key into a lock; the mouthparts of each species can only exploit flowers of a certain size and shape. This is why, to support insect diversity in our gardens, we need to plant a diversity of suitable flowers. It is definitely not a case of 'one size fits all'. While some insects (such as honeybees) are generalists and can exploit a wide range of flowers, others are specialists and are quite particular in their needs.
In the UK and the rest of Europe beetles do not play a major role in pollination, however certain flowers (especially yellow daisies such as Inula) attract distinctive small pollen beetles early in the summer. The main beetle visitors to flowers are Cantharid beetles such as the in one illustrated in close-up on the left, eating pollen displayed by the small florets in the centre of a perennial sunflower (Helianthus). Ladybirds, which are beetles, are sometimes found on flowers. Although they mainly eat other insects it is known that some species will eat pollen or nectar on occasions. Flowers in the Carrot or Umbellifer family (Apiaceae) seem to be particularly attractive to beetles.
Scientists believe that the very first flowering plants co-evolved with beetles as their pollination partners, about 200 million years ago.
The vast majority of Lepidoptera species in the UK are moths. Butterflies are a numerically smaller group but better-known because they fly by day. Their larvae (caterpillars) feed on the leaves of a range of food plants The adults ingest nectar, as a high-energy fuel for flight. They do not eat pollen. They have a long, very thin feeding tube (proboscis) that is used to suck nectar by capillary action. You can clearly see this in the picture. When not in use it is usually folded up like a coiled spring. They take nectar from small tubular flowers, or flowers such as Hemp Agrimony (Eupatorium) which consist of of small tubular florets grouped in larger inflorescences.
The Diptera is a large order. There are many different kinds of flies, including the Hoverflies (Syrphidae). The adults of a number of types of flies will drink nectar as well as taking other foods.
Typical Diptera mouthparts consist of a short fleshy tube which they use to suck up liquid or semi-liquid food. some also ingest pollen. The Drone Fly (illustrated left) is a common visitor to garden flowers. In appearance it mimics a honeybee. It is shown here sucking nectar from a floret in the central disc of a species of Aster.
Diptera tend to visit small, flat flowers which present their nectar openly; many flowers of the Carrot or Umbellifer family (Apiaceae), are of this kind. Other flowers of this type include some members of the Spurge family (Euphorbiaceae), and those daisies (Asteraceae) that have very small florets in the central disc of each flower. The fly illustrated right is taking nectar from Irish spurge, Euphorbia hyberna.
Although adult Hoverflies are mainly flower specialists, consuming nectar or pollen, hoverfly larvae are mostly carnivorous. The larvae are free-living and (unlike the larvae of bees and wasps) do not receive any form of parental food provision. The larvae of some Hoverfly species eat aphids. The larvae of some other species, such the Drone Fly, are semi-aquatic and live in stagnant pools of water, where they consume bacteria. Many hoverfly larvae live in rot holes or pools of water in old trees. A number of common species such as Episyrphus balteatus (above) can migrate from the continent. Small insects such as these are an essential part of the food chain that support insectivorous birds.
The Bee Fly (Bombylius major, pictured right) is one of a group of specialised flies which mimic bumblebees. They have a long thin proboscis which is inserted into small flowers such as Aubrietia and Primroses to drink nectar. They are common in gardens during the early months of the year. Unlike bumblebees they tend to hover in front of flowers, and their proboscis is rigid and does not fold up when they are in flight.
Bees and Wasps are closely related, and together with Ants and Saw flies they form the order Hymenoptera. The larvae of most species of Hymenoptera live on stored food left for them by an adult of the species. The provision of food for larvae is found through all species of Bees and Wasps and serves to distinguish the Hymenoptera from most other orders of insects. The presence of sociality (communities of individuals co-operating together) is another feature of many species of Hymenoptera.
The major difference between Bees and Wasps is that Wasps provide animal food for their larvae (in the form of other insects or spiders that they have captured) while Bees provide a store of pollen, or pollen mixed with nectar, for their larvae. For this reason there is a major co-dependency between bees and flowers. They have evolved together for at least 100 million years.
There are thousands of species of bees in the world. In the UK the honeybee now normally lives in a domesticated situation in managed beehives, rather than as a wild creature, although feral colonies can be found. Apart from honeybees there there are about 250 wild bee species in the UK including bumblebees and solitary bees. Many are uncommon because they require specialised habitats such as sandy banks or cliffs in which to build their nests, but there are are plenty of species commonly seen in gardens. These tend to be generalist species, and gardens are an important resource for them both in terms of food and nesting habitat. All bees provide pollen or a mixture of pollen and nectar for their larvae. Adults normally drink nectar as fuel for flight and in few cases will also eat pollen themselves. As well as honeybees, bumblebees and a few species of solitary bees in the UK live socially in colonies where individuals-co-operate.
The mouthparts of most bees species consist of a fairly rigid hollow tube (also called a proboscis, but often referred to informally as a 'tongue') through which they can suck nectar. This is usually folded away in flight. Many kinds of bees have an internal chamber, the so-called 'honey stomach', in which they store nectar as they collect it, for it to be regurgitated when they get back to the nesting site. The illustration shows a Common Carder bumblebee taking nectar from a yellow Buddleja.
The tongue length of bees closely dictates the shape and size of flowers that each species of bee can utilise. Tongue length varies between 1mm and 19mm according species, so a variety of flowers should be planted to meet their needs. Honeybees have a short to medium tongue length of about 6mm. Lists of 'bee plants' provided for beekeepers will concentrate on those flowers suitable for honeybees. A greater range of flowers is required to accommodate the needs of the whole spectrum of wild bees.
Some smaller species of solitary bees ingest pollen and store it internally until they get back to the nest site, when they regurgitate it in order to provision nest cells. Other species of bees have hair on their bodies which accumulates pollen while they are foraging. Many species then comb this pollen into small masses which they carry back on their rear legs (the so-called 'pollen baskets'). You can see pollen gathered onto hairs on the back legs of this small Mining Bee (Andrena) pictured right. It is only the female bees that gather pollen and provision the nest. Males have no role in collecting food for the young.
Some species of bees such as the Leafcutter Bees (Megachile spp., illustrated left) and Mason Bees (Osmia spp.) collect pollen on a hairy area under their abdomen called a scopa (the so-called 'pollen brush'). Mason Bees are common in gardens early in the year and are pollinators of fruit trees and bushes.
Although the role of other groups of insects in pollination is thought be significant, it is certain that bees, especially wild species, play the predominant role. They are important both economically, by pollinating many fruit and vegetables crops, and ecologically, by ensuring cross-pollination in wild flowers thereby maintaining the diversity of flora in wild habitats. Recent research (2011) suggests that most pollination in the UK is currently carried out by wild bee species, as problems with disease and parasites in recent decades have reduced the size of the domesticated honeybee population.
For more information about Mason Bees see my page about how to make a Bee Hotel; there is also a fact sheet about wild bees in the garden you can download from my Fact Sheets page.
Like bees, most species of wasps in the UK are solitary and many live in specialised habitats. Adult wasps of many species drink nectar on occasions. The common social species that most people recognise as wasps are attracted to sweet liquids such as the juice of rotting fruit, jam, and nectar. Wasps have short 'tongues'. A few plant species such as the Figwort (Scrophularia) and Gooseberry have evolved small brown flowers that seem particularly attractive to social wasps, which are possibly their main pollinator. Ivy (Hedera helix) flowers in late summer and autumn and produces exposed nectar that is particularly attractive to social wasps.
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