The brown argus is another blue butterfly that is not blue; it is a member of the family hesperiidae. The problem with the brown argus is that it is very like the females of some other blue butterflies, especially the female common blue with which it shares habitat preferences and so it may be either under recorded or even over recorded through misidentification. It is, however, a much more consistent brown colour (if that makes sense) and the dots along the wing are very distinct orange rather than also part silver and black. That is very well if the butterfly is stationary bit what if it is in flight? In flight the underwing of the brown argus is silvery grey whereas the common blue has a darker, more blue underwing. Easy ...?
The brown argus has a preference for common rock-rose as its larval food plant and rock-rose is a plant of chalk and limestone grassland so if there is no rock-rose then think 'common blue'. The books suggest there are two broods of brown argus which means they can be seen from early May until late September but with a lull at the end of June/beginning of July. In the Nature of Dorset tweets database there are twenty five records showing their first emergence in week 17 which is last week April/first week May and then reports weekly until week 22 in Mid-June. Reports there after are much more intermittent until week 35 in September but there are notably records for weeks 41 to 43, well into October. That could be an indication of a third brood here in the south or maybe it just means the second brood survive longer if the weather remains favourable?
There are records from twenty four sites and they are almost exclusively sites on chalk or limestone with Ballard Down near Swanage seemingly good for them. Surprisingly, perhaps, there are no reports from Portland (yet).
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