Daylength and Temperature Control of Predation, Body Size, and Rate of Increase in Toxorhynchites rutilus (Diptera: Culicidae)

Abstract

Subtropical Toxorhynchites rutilus (Coquillett) frequently overwinter as 4th instars in treeholes of peninsular Florida. Exposure from hatch to short daylengths induced 4th instar diapause in over 50% of wild T. rutilus from southern Florida. Short daylengths and lower temperatures significantly reduced prey consumption rates. At a low frequency, diapausing larvae killed prey without consuming, which behavior increased with time in the 4th instar and was distinct from killing by prepupae. Termination of diapause by transfer of 4th instars to long daylengths was accompanied by an increase in prey consumption rate prior to pupation. Mean pupal masses were significantly affected by photoperiod and temperature treatments, confirming a correlation between heavier pupae and colder temperatures in the field. At 25°C and 21°C, female pupae were significantly larger than males, in contrast to no sexual size difference observed for continuous development at 27°C. The composite index of performance (r’), an analog of the per capita rate of increase, for diapausing cohorts was one-half to two-thirds of that estimated for non-diapausing cohorts, but temperature alone did not affect r’ between 25 and 21°C. Low rates of prey consumption during winter diapause may contribute to the relatively modest, albeit significant, effect of T. rutilus predation on mosquito abundance as estimated from long-term censuses of treeholes.