28. Insect abundance
TH Sparks and IP Woiwod

Abundance of insects in Britain shown in relation to temperature variables

The abundance of common footman moth in Britain in relation to mean July-August temperatures in Central England

The abundance of common blue butterfly in Britain in relation to mean June-August temperatures in Central England

The abundance of peach-potato aphids at Rothamsted, Herts, in relation to mean January-February temperatures at Rothamsted

Specific indicators
The same surveys as used for Indicator 27 also provide specific indicators of the abundance of aphids, common blue butterflies and common footman moths.
[Source: Rothamsted Research, Rothamsted, Harpenden, Hertfordshire AL5 2JQ; Centre for Ecology & Hydrology, Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire PE17 2LS]

Relevance
Changes in the abundance of insect pest species and of species which are potential carriers of disease are particularly important as they can pose a considerable economic threat. On the other hand, there are a number of beneficial insect species which help control pest infestations and a number of endangered species of conservation value whose populations are currently protected. The three indicators reported here are the abundance of the peach-potato aphid (Myzus persicae) at Rothamsted, the common blue butterfly (Polyommatus icarus) and the common footman moth (Eilema lurideola) averaged across UK recorded sites. The former species is a serious crop pest, the latter are widespread species of grassland and woodland respectively.

Sensitivity to climatic and other factors
The numbers of insects surviving to adulthood, and their activity once emerged, are both related to climate. Warm, dry weather generally encourages faster development of the early stages leading to lower mortality and lower predation. However, there are some potential problems if conditions become too dry, or if food supplies become desiccated, so it is hard to make generalisations.
All three of the species selected here show high sensitivity to temperature. The correlation between the abundance of the peach-potato aphid (transformed to a logarithmic scale) and mean January-February temperatures taken from the Rothamsted meteorological station is strongly positive. There is also a strong positive correlation between the log abundance of the common blue butterfly and mean temperatures in June-August in Central England. However, there was some suggestion of an optimum relationship here with abundance falling off in the warmest summers. Finally the log abundance of the common footman is closely correlated with mean temperatures in July-August in Central England.

Change over time
There is some suggestion of an upward trend with time in the abundance of all three species. However, the record is dominated by the year-to-year variation in abundance in response to temperature. Thus, peach-potato aphids were particularly abundant in the warm summers of 1976, 1989 and 1995.

The relationships with temperature suggest that 1ºC warming would increase aphid, common blue and common footman populations by factors of 2.5, 1.7 and 1.1, respectively. Clearly, we would expect insects to be more abundant in a warmer climate, especially aphids, except perhaps in years with severe droughts