Anatomical impact and notes on viviparous development related to infestation by neotenic females of Stylops advarians (Strepsiptera: Stylopidae) on adult female Andrena milwaukeensis (Hymenoptera: Andrenidae)

Stylops advarians Pierce (Strepsiptera: Stylopidae) is a prevalent parasite of adult Andrena milwaukeensis Graenicher (Hymenoptera: Andrenidae) in Saskatoon, Saskatchewan, Canada. By dissecting adult bees and examining histological sections, we sought to determine how neotenic females of S. advarians impact female hosts of A. milwaukeensis anatomically. Adult bees with 1–3 females of S. advarians within their gasters were compared to nonstylopised bees (control). The presence of a single female parasite inhibited development of the host’s ovaries. The bee’s foregut shifted laterally when one parasite occupied the gaster and ventrally when two or three were present, thereby reducing the crop’s expandable capacity and the amount of nectar and pollen that stylopised bees can ingest. The midgut and hindgut were less significantly affected by stylopisation. Female parasites typically occupied the host’s gaster dorsolaterally, where each was supported by one of the host’s air sacs. If a third female parasite was present, she resided dorsally along the midline of the host’s gaster, mostly supported by the two female parasites to either side. Asynchronous development within neotenic female parasites was demonstrated, wherein mature first-instar larvae occupied the cephalothorax and abdomen at the same time that the adult female was still supporting multiple embryos.