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Journal article

Moth populations and bad weather – four speculative observations

2013

Abstract

There is no doubt in my mind that fifty years ago substantial defoliation of more than just spindle and bird cherry trees was not really unusual; that the regular cleaning of car headlamps and even radiator grills was necessary in summer; that garden buddleia and valerian were always plastered with butterflies and other insects practically the whole summer long; and that an open lavatory window with the light left on made a reliably high-yielding moth trap. These things seemed fairly constant, though I suppose it is possible that my memory has dumped the bad years. If I never again experience the (quite noisy!) steady rain of caterpillar faeces falling from oak woodland canopy in spring, or myriad moths fluttering in the car headlights down every country lane, it is because various long term trends over the past half century (unrelated to global warming per se, albeit with similar roots) have progressively made Britain unsuitable for high populations of most species of Lepidoptera. However, there are also shorter-term effects (that is, ones that may not persist) that result simply from runs of unsuitable weather. Whether or not it is right to categorise recent events as aspects of real climate change (and even then, whether or not as a consequence of global warming), such runs are not new and are to be expected simply on a stochastic basis. So optimists can say they may not last – though there is no denying that in the past the British environment has undoubtedly presented much more favourable conditions for moth and other insect populations to recover than at present. My main interest in Lepidoptera is as hosts for parasitic wasps, so I habitually try to collect caterpillars, leaf mines, etc. in bulk in order to rear these fascinating and littlestudied insects. Often, in order to obtain livestock of particular parasitoids for experiments, I have collected the same species of common Lepidoptera caterpillars over several years, sometimes witnessing declines and even local extinctions of species apparently as a result of abnormal runs of unsuitable weather. This has led to a few observations and ideas that might be worth sharing, although unfortunately I had not usually collected the kind of quantitative data that would raise my recollections and comments above being merely anecdotal and speculative.

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