Maturation of field-collected female <i>Agriotes obscurus</i> and <i>Agriotes lineatus</i> (Coleoptera: Elateridae) over their swarming period

Larvae of two species of click beetle, Agriotes obscurus and Agriotes lineatus, are important pests of vegetable and field crops in both Europe and North America. Both species have been long and extensively studied, but to date little is known regarding the maturation and egg development of female beetles relative to their swarming periods. This knowledge is important for developing wireworm management tactics that target female beetles, as these would ideally eliminate the beetles before they oviposit. This paper is an attempt to address this knowledge gap. We dissected 2,450 female A. obscurus and 477 female A. lineatus collected in southwestern British Columbia in 2015-2020, and describe how their abdominal lipid content and number of mature eggs change during their swarming period. Based on the presence of mature and immature eggs, active ovarioles, and lipid content, we propose 6 consecutive beetle maturity stages. Beetles collected early in the season have high lipid content and no eggs. Over time, immature, then mature eggs appear and the lipid content decreases dramatically. Ovarioles are generally active throughout the swarming period, even when lipids are no longer present, suggesting that for these species egg laying may continue until the end of the season, and that fecundity depends both on a beetle's original lipid content at emergence, and subsequent diet.