Life history of a dipteran predator (Scathophagidae: Acanthocnema) of insect egg masses in a northern California stream

1. Little is known about the predators of insect eggs in fresh waters. This study describes aspects of the life history of a scathophagid fly (Acanthocnema sp.), whose larvae are predators of aquatic insect eggs. 2. Because the Acanthocnema predator oviposits its eggs on the surface of aquatic insect egg masses, all insect egg masses were collected regularly within a 200‐m reach of Redwood Creek (California, U.S.A.) between September 2003 and June 2007. Acanthocnema predators were found predominantly within egg masses of the caddifly Neophylax rickeri (Trichoptera: Uenoidae). 3. There was a mean of 0.25 Acanthocnema individuals per N. rickeri egg mass (n = 2367 egg masses). In general, N. rickeri egg masses were more commonly found clustered in aggregations (93.7%) than singly (6.3%), and Acanthonema were found more often within the aggregations of N. rickeri (98.7%) compared to singly laid egg masses (1.3%). 4. The duration of the Acanthocnema predator life stages was: egg 2.9 ± 0.8 (mean ± SD) days, larva 15.6 ± 10.2 days, pupa 80.3 ± 24.9 days and adult 7.2 ± 4.8 days. The short duration of the Acanthocnema egg stage (1–7 days) compared to that of its prey N. rickeri (2–4 weeks) raises the probability that the undeveloped eggs of N. rickeri would be available to the young predators upon hatching. Egg consumption of N. rickeri eggs by Acanthocnema averaged 262.6 eggs per larval period. 5. Acanthocnema had a bivoltine life cycle in which the first generation fed exclusively on N. rickeri egg masses in the winter and the second generation fed on the egg masses of several species, including other Trichoptera (Brachycentridae) and Diptera (Ceratopogonidae, Chironomidae) in the spring. These findings suggest differing feeding strategies by the first and second generations of Acanthocnema in response to the seasonal availability of prey species. This type of autecological information is important for understanding mechanisms of community interactions.