Ecological studies of Culex (Lutzia) vorax, with special reference to the dispersion pattern and the predatory behaviour

Abstract

The distribution pattern of the larvae of the predatory mosquito, Culex (Lutzia) vorax in a ditch was analyzed by the index of aggregation (Iδ, Morisita, 1959). The aggregation of the larvae of C. vorax was comprised of two groups with different size. Smaller aggregation was about 10cm in width and the other was dispersing over a range of about 2m including some of the former in it. The larger group was considered to be consisted of the larvae dispersing from one egg raft. The C. vorax population was, therefore, considered to be intermingled with the population of the prey, C. pipiens fatigans, not individually but as a group. Consequently, the heaviest reduction of the prey population was found in the areas where C. vorax is occurring. The larvae dispersed quickly after hatching in the laboratory but ceased to spread after 24 hours when almost all the larvae had attained to the second instar. The distribution pattern, however, was still aggregative after the stabilization. The relationship between the prey population and the quantity of the predation was recognized as Ivlev's (1961) predation curve, fitting the formula r=R(1-e^<ξN>). As for first instar larvae, however, the expected values obtained by this formula hardly fitted to the observed ones. Those values calculated by the following formula, r=R{1-e^<ξ(N-a)>} fitted quite closely to the observation. The asymptote obtained here was lower in the more advanced instar. On the other hand, the first instar larvae had higher predatory efficiency, and fed on more larvae at higher density of prey population and this higher predatory efficiency differed discontinuously from those of the second and later instars. Such a higher predatory efficiency in the first instar seemed to cause the faster growth of the larvae within the instar. This phenomenon was recognized by the growth curve of this species in the formula, Y=5/(1+e^<-0.579(x-1.702)>). The cannibalism of C. vorax was examined experimentally. Four fourth instar larvae of both prey and predator were released into a vial and the reduction of individuals of each species was counted in time sequence. The reduction of the number of the prey larvae always occurred first, but the cannibalism among individuals of the predator larvae did not occur when a certain number of prey larvae was present. This phenomenon seemed to be attributed to the specific behaviour of the predator such as settling and waiting to capture prey even if no prey was present. The settling behaviour of the predators was apparently reducing the chance of the contact among themselves, therefore, the cannibalism did not occur when the prey, which moves more frequently, was present.