Biology Of Culex Nigripalpus Theobald (Diptera: Culicidae)1: Part 1: Effects of Rearing Conditions on Growth and the Diurnal Rhythm of Pupation and Emergence

Abstract

The growth of larvae, diurnal rhythm of pupation and emergence and synchronization of pupation in Culex Nigripalpus Theobald were found to be affected by 5 major environmental factors—nutrition, population density, salinity of the rearing medium, temperature and light regimes. Embryogenesis in this species was controlled by temperature and eggs hatched on completion of embryogenesis. The onset of pupal ecdysis was temperature dependent, with a Q₁₀ value of slightly more than 2.0 for a 10°C range of temperature from 22–32°C. Higher salinity also affected the onset of pupal ecdysis to some extent, whereas different quantities of food above optimum and different densities of larvae used did not affect the onset. The duration of pupal ecdysis was affected mainly by the quantity of food available. Duration on basic and 2× basic ration, respectively, was 90 hr and 47 hr for the first 95% of the larval population. Greater larval densities and higher salinities under otherwise standard conditions did not lengthen the duration of pupal ecdysis, though a combination of higher densities, lower food and higher temperature tended to lengthen it by a maximum of 48 hr, resulting in a few more peaks of pupation. Under LD 12:12 on basic ration in 5% sea water pupal ecdysis followed a diurnal rhythm, the peaks of pupation having average periods of 23.61, 23.29 and 23.97 hr at 22°, 27° and 32°C, respectively. The Q₁₀ values for the periodicity for the range of 10°C is 1.01, thus showing a high degree of temperature compensation. Quantities of food greater than basic ration in 5% sea water produced a telescopic effect on pupal ecdysis, resulting in 1 or 2 main peaks of pupation with periods ranging from 15–20 hr, depending on the quantity of food available and temperature. Diurnal peaks of pupation appeared between 05^h00′ 19^h00′ of the LD 12:12 cycle, at a time set by temperature and nutrition. LD 12:12 regime affected the endogenous rhythm by producing an “advance peak” separated from other peaks by less than 20.0 hr. Occurrence of an “advance peak” is primarily due to the age threshold at the particular temperature. At higher temperatures pupation peaks showed smaller standard deviations than at lower temperatures, where a breakdown in the rhythm at the colder temperatures became evident. Entrainment of the pupation peaks took place under LD 12:12 only when larvae were reared on basic ration under crowded conditions (200 larvae/pan) at a high temperature. These conditions represented a stress factor on the developing larvae. The duration of the pupal stage was affected by temperature only. The emergence rhythm was dependent on the pupation rhythm, separated by an interval affected by temperature and not by light regimes. The sex ratio in an “advance peak” as well as in the main peaks of pupation was invariably 50:50, showing that the age threshold for pupation, set by temperature, was nearly the same for both sexes. Entrainment of the pupation rhythm to LD 12:12 and a high degree of temperature compensation in the periodicity of pupation peaks showed that the pupation rhythm is truly endogenous diurnal rhythm.