Wednesday, 6 January 2010

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!

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