Importance of larval foodplant lifespan to British butterflies - with particular reference to the Brown Argus (Aricia agestis)

Roger Kemp

Most British butterflies have as their main larval foodplants perennials rather than annuals. This has the obvious advantage that young larval food material will be available year after year and at the correct period for egg laying and larval development. Sometimes, however, annuals are used and this can lead to rapid colonisation of new areas but equally rapid extinction once the annuals disappear. These colonies tend to be temporary rather than permanent.

A good example of this is the recent spread of the Brown Argus in my own area in the Vale of Aylesbury. For many years odd vagrants have been seen several miles from the nearest sites in the Chilterns of its normal larval foodplant the Common rock-rose (Heliantbemum chamaecistus). In August 1996 the few occurrences of second generation adults coincided with a large germination of Cut-leaved cranesbill (Geranium dissectum). Several young plants were noticed with single Brown Argus eggs laid on the upper surface of the cotyledon leaves.

These eggs eventually produced first generation adult butterflies in May 1997. Abundance of young Geranium foodplants at this time in turn enabled a large second generation to emerge in July and August 1997. However, because of the lack of rain until the end of August 1997 there were very few young Geranium plants available for female butterflies to oviposit during the adult flying period and hence few eggs were laid.

This highlights the temporary nature of butterfly colonies using annual foodplants. It may also partially explain why butterfly numbers fluctuate even when perennial foodplants are used. Weather conditions, particularly rainfall, during the larval phase of butterfly development clearly has an effect on availability of suitable young food material. Probably new plant growth is essential for healthy development as older growth may develop toxins against larval damage. It is probably the availability of such young growth which is one of the main determinants of numbers of adult butterflies emerging in any given year. It is a case of adult butterflies and young larval foodplant material both being in the right place at the right time.

Originally published in the Volume 57 of the Bulletin of the Amateur Entomologists' Society.

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