The induction of resistance in the urban pest mosquito, Culex quinquefasciatus, to chlorpyrifos and chlorfenvinphos and the cross-resistance spectra of these selected lines to propoxur and a number of organophosphate insecticides was studied. Distinct genetic factors governing intermediate (R1) and high (R2) levels of larval resistance to organophosphates were found to be allelic according to the single locus, multiple aliele hypothesis. Adults were less susceptible to malathion and fenitrothion after larval selection with chlorpyrifos, but the degree of resistance was found to be much less than that in the larvae. The implications of these results for the control of C. quinquefasciatus in Dar-es-Salaam, Tanzania, is discussed.

Implantation of additional corpora allata (CA) into the last instar larvae of Corcyra cephalonica resulted in the production of extra larval instars, larval–pupal intermediates and adultoid larvae. Moths metamorphosed from the extra larval instars were externally normal.

The juvenoid, hydroprene, when given to last instar larvae, prolonged the larval life span, induced extra larval moult and the moths metamorphosed from such larvae were larvoid adults. Extra pupal instars were produced when this juvenoid was applied to early pupae. Moths obtained after pupal treatment were adultoids. Another juvenoid, methoprene, prolonged larval life and produced giant larvae without extra moult. Incompletely moulted extra larval instars, larval–pupal intermediates and adultoid larvae were formed. Moths obtained after both larval and pupal treatments were externally normal. The percentage of moths obtained after the application of both the juvenoids was small.

The antiallatotropin, precocene II, had no creditable effect on the duration of larval or pupal life. Application to both larvae and pupae, pretreated with hydroprene, largely minimized the hydroprene-induced effects. Pupal treatment, however, produced pupal–imaginal intermediates. The moths obtained from precocene-treated larvae or pupae, with or without juvenoid pretreatment, were externally normal.

Wild-caught Xenopsylla brasiliensis from Kibwawa village, Chunya District, in south-west Tanzania were colonised in the laboratory and their offsprings used for determining the baseline susceptibility levels of the strain to DDT. A total of 120 fleas were tested with each concentration of DDT, using the techniques described by the WHO Expert Committee on Insecticides (1970).

Several filial generations were selectively raised, using DDT as a selection pressure. They were tested with various concentrations of the insecticide and their dosage–mortality regression lines compared with that of their parents.

About 5% of the parental generation (normal population) survived 1 hr exposure to 4% DDT. The LC50 of this population was 0.79% DDT. The mortality rates of the successive filial generations decreased gradually; their dosage–mortality regression lines moved to the right; and the slopes of these lines decreased. LC50s of F1, F2 and F3 were 1.6, 1.9 and 3.1% DDT, respectively. The highest DDT concentration (4%) killed 21.1 and 18.3%, of F4 and F5, respectively.

It was generally concluded that DDT resistance was present in the population, and that the selection pressure (DDT) was responsible for increasing the level of this resistance in the laboratory.