Aggregations during diapause are common among terrestrial arthropod species. They are usually considered as a defense strategy or a way to resist unfavorable abiotic conditions. However, systematic studies on the abiotic conditions as close as possible to the arthropods in their aggregation sites (= microsites) and individual survival in the aggregations are lacking. We studied whether or not aggregation sites of the lady beetle Hippodamia undecimnotata (Schneider) provide suitable abiotic conditions for its survival. We recorded hourly temperature and relative humidity throughout the overwintering aggregation period in three overwintering microsites and investigated if they 1) favor lady beetle survival and 2) prevent the proliferation of ectoparasitic fungi, a lethal threat for lady beetles. We found that overwintering microsites did not offer direct suitable conditions in terms of temperature for lady beetle survival and/or presented relative humidity and temperature conditions favorable to ectoparasitic fungi and therefore indirectly affected lady beetle survival.