Wednesday, 19 April 2017

Overwintering moths and butterflies: three years of tunnel surveys at Newhaven Fort

We completed our third winter of surveying moths and butterflies in the tunnels at Newhaven Fort in February. The previous two years set a trend in which our first visit in mid-December produced better results than the second in early or mid-February. That trend was bucked in February as we saw a better return than any of the previous visits.

Why? There are environmental variables at work which affect the numbers of insects that we see. These include day length, seasonal weather, population abundance prior to diapause (and the many factors affecting seasonal abundance), the activity of predators and pathogens during the winter, and unintentional human disturbance (e.g. use of artificial lighting). The tunnels are a suitable hibernaculum during the winter because they are dark yet they still indicate changes in day length, they are ventilated and the temperature is cool and fairly stable. The tunnels offer protection from environmental shocks such as sudden cold snaps, flooding, strong winds and the potential disturbance or destruction of habitats. The tunnels offer a fairly stable environment and catastrophic change is less likely; therefore the instincts which drive insects into places like tunnels are more likely to result in their survival. Any combination of these and other variables would have contributed to the abundance of moths and butterflies we saw in February. The autumn saw an extended period of warm and dry weather. The 2016-17 winter was cooler than the two previous years and there have been more frosts. The cool winter weather extended to the day or two prior to our February visit. These are likely to have influenced the numbers we saw and might help to explain why we saw significantly more this February than in February '15 and '16.

Here's a summary of the six visits we have made since December 2014, which allows us to compare numbers of each of the seventeen species we've recorded during the three year period. The most important species we've seen is the Bloxworth Snout (Hypena obsitalis) and this is the main reason for the survey. It has been present in consistent numbers during the first three years of the survey and marks Newhaven Fort out as one of the best-known sites for this moth. Larvae have yet to be found, but its foodplant Pellitory-of-the-wall (Parietaria judaica) is present at the Fort and Castle Hill LNR and it is tempting to think that it is breeding locally.

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