Flying in the cold

As you might expect, the majority of our moths sit out the winter as eggs, larvae or pupae, stages that are more resistant than the adult form. Among the macro-moths, just two-dozen species are active as adults during the wider winter period, underlining the difficult conditions that these fragile creatures face at this time of the year. The biggest challenge is the temperature, with subzero temperatures likely to kill any moth exposed to them. A few species remain active, however, and some have evolved behavioural or physiological adaptations that prevent them from freezing solid. While behavioural adaptations may see individuals seek out sheltered overwintering sites, physiological adaptations may involve the creation and addition of glycerol or other antifreeze chemicals into the vulnerable body cells.

Perhaps surprisingly, a number of British moths emerge from their pupal stage to take adult form during the winter, before seeking out a mate and depositing eggs. One of these is the winter moth, Operophtera brumata, which emerges during the final months of the year. While the males of this species are fully winged, the females are wingless and unable to fly. Newly emerged females face an arduous climb from the ground up onto the trunks of deciduous trees. Here they rest and release a pheromone to attract a mate. Once mated, the female resumes her climb towards the highest parts of the canopy, where she will lay her eggs before her brief life cycle comes to an end. When the eggs hatch in early April the emerging caterpillars should find that the leaf buds have just burst, providing an abundance of food.

The success of this strategy can be seen from the vast numbers of winter moth caterpillars that may be encountered during spring, sometimes to the extent that large areas of canopy are defoliated by their activities. It is for this reason that the winter moth may be familiar to gardeners and other horticulturalists growing apples and other fruits. The winter moth caterpillars also form an extremely important food source for nesting tits.

The names of some of our other moths also indicate adult activity during the winter months. Some readers will, for example, be familiar with the november moth, pale november moth and december moth. All three species may be encountered in Norfolk during the first half of the winter. Another important winter-active species is the mottled umber; like the winter moth it is one of the ‘looper moths’ that may cause widespread defoliation in years of peak abundance. Also like the winter moth, the female is wingless and uses pheromones to lure in prospective partners. Occasionally one of these moths may be attracted to the light of a kitchen window so do keep an eye out for them.

Winter moths

The freezing temperatures and snow of December should, you might have thought, have put a halt to the activities of most insects. However, one species of moth was seen at our window on several occasions over the Christmas period. This was the appropriately named ‘winter moth’ Operophtera brumata, a species whose name may be familiar to gardeners and fruit growers as a pest of apple trees. The grease bands seen around the trunks of apple trees are an attempt by fruit growers to reduce the impact of this species on the apple crop.

The winter moth is one of a small number of geometrid moths that are on the wing throughout the winter months. It is only the males that can be truly said to be on the wing, as the females of these species are flightless. Some authors describe the females as wingless but this is not factually correct since the females have wings but these are mere stubs. Interestingly, the pupal stage of these moths sports full-sized wing cases but these remain unfilled in female pupae. This might suggest that the loss of functioning wings in these females is a relatively recent evolutionary phenomenon. Male winter moths may sometimes carry the much smaller females in flight during copulation but females are more usually encountered climbing up walls or tree trunks.

The male winter moth is a rather drab creature, with soft brown tones and little in the way of patterning. In fact, it probably looks how most people would imagine a moth to look, even though many moths are brightly coloured. The single generation of adult winter moths is on the wing from October through into January, the resulting eggs not hatching until April. The timing of hatching is critical, since the caterpillars that emerge depend upon the flush of new leaf growth for food. By mid-June the caterpillars will have dropped to the ground and entered the pupal stage in which they will remain until the following winter.

Other moths may also spend the winter in the adult form but, unlike the winter moth and its geometrid relatives, these moths hibernate, favouring sheltered locations like tree cavities where the temperature is more even. Some of the winter-active geometrids have been shown to have amazing capabilities when it comes to freezing temperatures. The author and moth enthusiast Roy Leverton once described how he found a pale brindled beauty that was trapped in a pool of melt-water that had refrozen. Roy carefully chipped away a small chunk of ice, still containing the entombed moth, and took it home. Once the block and moth had thawed out, the moth flew off, seemingly unharmed by the experience. Now that’s a useful skill to have!

Winter insects hard to find

The winter months offer reduced opportunities for those of us interested in insects and other small creatures. Many species will sit out the cold conditions by burrowing under bark, burying themselves within the leaf litter or by squeezing into a nice dry crack in the brickwork. However, there are still insects and arachnids to be found at this time of the year.

Clearing away dead plant material from one of my borders the other day revealed several caterpillars, overwintering in this form before changing and emerging next year as adult moths. One of the ones I recognised was the caterpillar of the angle shades, a delightful moth that has featured at my moth trap during the summer months. Just below the surface of the soil, as I forked it through, was the large brown pupal case of a hawkmoth. Over two inches in length, it was almost identical to one that I had found last autumn. I managed to overwinter that one in the shed and on 5th June this year an adult privet hawkmoth emerged to be photographed and released. Other moths are still on the wing, including small brown ones that appear at lighted windows late in the evening. These are male winter moths, a species that is unwelcome in those gardens with fruit trees. Broad bands of sticky material can sometimes be seen wrapped around the trunks of fruit trees. These have been fixed there by gardeners, in an effort to trap the almost flightless female winter moth as she attempts to crawl up the trunk to lay her eggs. There are even moths on the wing in the house. Aptly named the white-shouldered house moth, these tiny moths appear on the curtains or at my reading light on most evenings.

There are other small creatures in the house, including two-spotted ladybirds that have hunkered down for the winter, pressed up against the corner of a window frame. A daddy-long-legs spider has taken up residence on the ceiling of the dining room. This spider has long legs and a cylindrical body and, if disturbed, will vibrate its web rapidly – a useful defence against potential predators. Another species of spider remains very much in evidence in the garden. This large spider has taken up residence on my larch-lap fencing. Known by its Latin name of Nuctenea umbratica, the females of this species are rather impressive. Reminiscent of a garden spider, but darker and with a flattened body, Nuctenea spends the day concealed in a crack. Once darkness falls, the spider emerges to wait for prey. Each panel of the fence seems to support seven or eight of these spiders, seemingly forming a healthy population and just one of the species to add interest during winter.