Net-spinning activity of third, fourth, and fifth instars of Hydropsyche betteni, H. slossonae, and H. sparna was observed at three different experimental temperatures and current speeds. The upstream species, H. slossonae, was intolerant of high temperatures (20 °C) while the other species were more tolerant. Colder temperatures (5 and 14 °C) decreased net spinning of all species and instars. The slowest current velocity (10 cm/s) had little effect on third-instar net spinning but reduced that of fifth instars. Faster currents (20 and 40 cm/s) raised the percentages of fourth and fifth instars producing nets but did not change third-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 also 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.
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Fuller et al. (1980) studied this question.