John R. Linley

Laboratory tests of the effects of p-cresol and 4-methylcyclohexanol on oviposition by three species of Toxorhynchites mosquitoes.

ABSTRACT. Laboratory experiments tested the effects of p‐cresol or 4‐methylcyclohexanol at concentrations of 1,10 and 50 ppm, on oviposition by the mosquitoes Toxorhynchites brevipalpis Theobald, Tx. amboinensis (Doleschall) and Tx. splendens (Wiedemann). A 5+5 ppm mixture of the two chemicals was also tested. All three species laid significantly more eggs in cups containing p‐cresol, whereas only Tx. brevipalpis and Tx. amboinensis responded similarly to 4‐methylcycohexonol and to the mixture of both chemicals. Tx. brevipalpis was, to a relatively limited degree, the most responsive of the three species. Ancillary experiments indicated that the chemicals were acting as attractants, causing more females to fly to treated cups. No stimulant effects were detected either in terms of the proportion of females that initiated oviposition flight (after flying to the cups) or in terms of the number of looping flights executed prior to ejection of an egg.

Laboratory experiments on factors affecting oviposition site selection in Toxorhynchites amboinensis (Diptera: Culicidae), with a report on the occurrence of egg cannibalism*

ABSTRACT. Laboratory experiments tested the effects of water quality and the presence of conspecific and heterospecific immatures on oviposition by Toxorhynchites amboinensis (Doleschall). The females showed a highly significant preference for oak leaf infusion water over distilled water. When twenty starved third and fourth instar Tx. amboinensis larvae were present in the water, substantially fewer eggs were counted from pots containing these conspecifics, than from controls in which no larvae were present. Numbers of eggs from pots containing starved second instar larvae did not differ significantly from controls. Observations of larval behaviour while oviposition was occurring suggested that egg numbers were reduced in containers because of egg cannibalism with third and fourth instar larvae, and not because the larvae caused a deterrent effect. Subsequent experiments confirming the occurrence of substantial egg cannibalism by third and especially fourth instar larvae are described. As with larvae, the presence of Tx. amboinensis pupae in the water had little effect on oviposition. If placed in the water 24 h prior to test, pupae very slightly enhanced its attractiveness, but if introduced immediately before test there appeared to be a slight deterrent effect. With heterospecific larvae, twenty fourth instar Ae. aegypti larvae introduced into infusion water 24 h before test rendered the water slightly attractive, while water in which Ae. aegypti larvae had been reared for 48 h proved highly repellent.