Where do pollinators go in winter?

Winter is quite a difficult time for pollinators. It is cold and often rainy and the first frosts will wipe out most of the flowers, and thus drastically reduce the availability of food. Pollinators have evolved various strategies to overcome this problem. In the following, I describe some of these strategies.

Winter is a difficult time for insects

Bumblebees, at least from our perspective, have evolved quite a radical way to deal with the problem. The whole colony including the workers and the old queen die before winter and only the young queens reared in the colony will survive to start new colonies in the following spring. In autumn, they drink a huge amount of nectar to fill their honey stomach and to build up body fat and then they go searching for a suitable hiding place for hibernation. These hiding places are mostly underground, under tree roots and hedgerows or at the base of walls. If the young queens have not reached a certain weight before the onset of winter they are likely not to survive winter. Therefore it is really important they find enough nectar-rich flowers in autumn such as comfrey, buddleia and heathers.

In spring, the queens emerge from hibernation and feed on early flowers such as fruit tree blossom before starting to build new colonies. The earliest queens to emerge are those of the Buff-tailed Bumblebee (Bombus terrestris), and you can usually see them searching for flowers on the first warm days in March. They are quite big and often a bit clumsy after their long hibernation, and thus easy to spot. The aptly named Early Bumblebee (Bombus pratorum) is the earliest among the UK’s bumblebee species to establish colonies and the first worker bumblebees you see flying in spring are likely to be of this species.

A Buff-tailed Bumblebee queen searching for nectar

One exception to the rule is the Buff-tailed Bumblebee, Bombus terrestris, which can maintain winter colonies in urban and sub-urban habitats with a good supply of winter-flowering shrubs such as mahonia, strawberry tree (Arbutus unedo) and winter heathers. So if you see bumblebees flying around in your garden in December don`t be surprised: you have a Buff-tailed Bumblebee winter colony somewhere in the vicinity.

Winter-active Buff-tailed Bumblebee collecting pollen in January

Solitary bees do not maintain colonies and have neither queens nor workers, and therefore they have evolved different strategies to survive winter. Adult bees are normally active for only about 3 month in the year (mostly spring/summer depending on the species) and then die after the business of mating and nest building is finished. The offspring survives winter either as young adults (most of the early-emerging bee species), as pupae (many of the later-emerging bees) or as larvae (usually found among species emerging very late) inside the nest cells which, depending on the exact species, have been built in hollow reeds, in holes in wood or walls, in dead stems or in underground chambers. 

For example, the adults of the common Red Mason Bee (Osmia rufa) emerge early and are active between March and June. They build their nests in hollow stems, in walls or in holes in wood and readily use ‘bee-hotels’ (see here for more information: http://www.urbanpollinators.blogspot.co.uk/2012/08/the-bee-bee-in-your-garden-or-how-to.html). The larvae pupate in late summer and after emergence from the pupal stage the young adults spend autumn and winter in an unanimated state inside their nest cells, ready to emerge in spring.

Mason Bee nesting chambers (closed with mud) in cardboard tubes

Another example is the Ivy Bee (Colletes hederae). Adults of this bee species emerge, often in great numbers, very late in the year (late August/September), around the time when ivy (Hedera helix), their main food plant, is starting to flower. They build underground nesting chambers in south facing, loose, and often sandy soil. In this species, individuals overwinter as young larvae which will grow on and pupate in the next year. 

Honeybees differ from bumblebees in as far as they are overwintering as a whole colony inside a beehive. When the temperature drops below 10°C, they stop foraging and begin to cluster around the queen in the central area of the hive. To protect the queen from the cold winter temperatures outside, the workers, by means of shivering, keep, the temperature in the centre of the cluster at around 27 °C which is raised even further to 37 °C later on when the queen resumes egg-laying. The workers work in ‘shifts’ on the outside of the cluster and periodically have time inside the cluster where they do not exactly rest but are nonetheless able to enjoy a bit of warmth themselves. Honeybees consume their stored honey throughout winter in order to be able to produce the body heat required to maintain the temperature in the cluster.

Honeybees stop foraging if temperature falls below 10 °C

Hoverflies have two strategies to survive winter. They either hide in the soil as fully-grown larvae or they hibernate as adults in sheltered places such as the nooks and crannies of old trees. Hoverflies overwintering as adults are the ones you see flying around on the first warm days in spring, searching for aphid-infested plants to lay their eggs on, as the larvae of most hoverflies are voracious consumers of aphids. Later on in the season, they are joined by the newly emerged hoverflies of those species that overwinter as larvae.

Hoverfly enjoying the last warm days in autumn

Butterflies and moths display a wide range of strategies to survive winter. Depending on the species, they can overwinter as egg, as caterpillar (= the larva of moths and butterflies), as chrysalis (= the pupa of moths and butterflies), as hibernating imago (= the adult moth or butterfly), as active imago, or by migration to warmer climates.

More moth species than butterfly species overwinter as egg, but to name just one example of a butterfly species to overwinter as egg stage, the Silver-washed Fritillary is one such species. Among butterflies, a much more common strategy for overwintering is to overwinter as caterpillar. Caterpillars often hibernate among the vegetation, in seed pods, in silken nests, and in rolled-up leaves, and the caterpillars of some species will resume feeding during mild spells. In most such species, caterpillars spend winter as individuals, but in some species they are aggregated into groups. Small Copper, Meadow Brown and Speckled Wood are all examples of butterfly species overwintering as caterpillar, with the latter species also being able to overwinter as pupae.

Meadow Brown butterflies overwinter as caterpillars

By contrast, hawk moths overwinter as pupae and spend winter in warm cocoons underground. Orange Tip, Holly Blue and Large and Small White butterflies (with the latter two species sometimes referred to as ‘Cabbage Whites’) are examples of butterflies overwintering as pupae.

Hawk moth caterpillars look for suitable overwintering sites in late summer

Other butterflies such as Small Tortoiseshell, Peacock, Brimstone and Comma survive winter as adults and hibernate in sheltered places such as holes in trees, behind loose bark and in man-made structures such as cellars. If you come across a hibernating butterfly in your house leave it be if it is in a cool and undisturbed place (for example your cellar or an unheated spare room). Try to relocate any disturbed butterflies to a suitable location that is frost-free and undisturbed, but please do not chuck the butterfly out of your house if it is freezing outside, as it then is surely going to die. 

Small Tortoiseshell`s hibernate in sheltered places to survive winter

In a minority of moth and butterfly species, the adults remain active during winter. These species have evolved the capability to produce an anti-freezing agent which stops ice-crystals forming in their ‘blood’ if temperatures are below 0 °C (without such an anti-freezing agent, exposure to subzero temperatures is a sure way to kill an insect). Examples of such species include the aptly named Winter Moth, which is active between November and February and which is a pest of fruit trees and the December Moth which you can encounter even on very cold days, usually in November and December.

Butterfly migration is not very common, but many readers may have heard about the spectacular migration of the monarch butterfly in North America. Not as spectacular, but nonetheless quite amazing, two of our native UK butterflies migrate as well. Painted Ladies come to our shores in the thousands in a good year, and they migrate back to Africa in autumn to escape the hostile conditions of our North European winters. The Red Admiral is also a migratory species, and after spending the summer in the UK, a part of the population will migrate back to continental Europe. However, thanks to global warming, an increasing number of Red Admiral individuals now stay in the southern counties of the UK, where they hibernate in sheltered places.

Painted Ladies migrate back to Africa before winter sets in

I hope you enjoyed reading about the ingenious ways of pollinators to survive our winters. Look out in spring for the lucky ones, the pollinators which have survived winter, and celebrate a new year of bees, butterflies, hoverflies and other pollinators buzzing and busily flying around our flowers and crops.

Long may it continue ... .