Saturday, 5 January 2013

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.

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