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BiologyThe flight period of this butterfly is variable, but usually occurs between early June and mid-August (3). This species has one brood a year; eggs are laid singly on the upper surface of leaves of the foodplant, and hatch after a week (2). The caterpillars hibernate at the bottom of the food plant or on the ground (3), and complete their development the following spring (2). Recent genetic studies have shown that single-brooded populations once thought to be northern brown argus occurring in the Peak District, Yorkshire Wolds and north Wales are actually brown argus (Aricia agestis). Populations in Co. Durham once thought to be Durham argus (A. a. salmacis) are actually mainly A. a. artaxerxes (3).