Field and laboratory studies of net-spinning activity by Hydropsyche larvae (Trichoptera: Hydropsychidae)

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Abstract
Net-spinning activity of 3rd, 4th and 5th instars of Hydropsyche betteni Ross, H. slossonae, Banks and H. sparna Ross was observed at 3 different experimental temperatures and current speeds. The upstream species, H. slossonae, was intolerant of high temperatures (20.degree. C) while the other species were more tolerant. Colder temperatures (5 and 14.degree. C) decreased net spinning, of all species and instars. The slowest current velocity (10 cm/s) had little effect on 3rd-instar net spinning but reduced that of 5th instars. Faster currents (20 and 40 cm/s) raised the percentages of 4th and 5th instars producing nets but did not change 3rd-instar net spinning. Experiments investigating the absence of nets in the field during winter showed that net spinning was reduced when substrates developed thick accumulations of diatoms and detritus (ADD) and that larvae emigrated from low ADD areas, apparently in favor of high ADD areas. Winter photoperiods decreased net spinning. Resumption of net spinning in spring may be triggered by scouring of ADD from substrates during snowmelt and by longer day lengths.