Overcrowding of Culex sitiens (Diptera: Culicidae) larvae: population regulation by chemical factors or mechanical interference.

Crowding of Culex sitiens Wiedemann larvae was investigated to determine whether pupation was delayed by the presence of chemical factors in the water or by mechanical interference. As the larval density increased from 0.05 to 1.0 larvae per milliliter of water, the duration of larval development increased from 7.5 to 23.0 d and larvae mortality increased from 1.7 to 99.6%. At high larval densities (2 larvae per milliliter), the larvae did not pupate, but remained alive as 4th instars for at least 50 d, even after their abundance subsequently declined due to mortality. When not crowded, 80% of the larvae fed at the surface; the remainder fed mainly on the bottom (even at a depth of 35 cm). Removal of chemical retardants by changing the water daily increased pupation to 65% (compared with 4% in controls, in which the water was not changed), but this was less than the effect of reducing mechanical interference by increasing the surface area, which increased pupation to 98%. Therefore, the main density-dependent factor was mechanical interference during feeding, but prolonged development to 19 d showed that chemical retardants also were important. The chemical retardant was not stable, so that 2nd generation larvae reared in the same water were unaffected, but its effect was irreversible, because most crowded larvae moved to clean water and a lower density did not pupate.