This paper reports 2 studies. (i) After a year of baseline data collection, lambdacyhalothrin-treated bed nets were introduced into 3 of 5 villages in north-west Thailand, the remaining 2 being treated with placebo. Human bait collections were carried out in each village on 2 nights per month, for 8 months of each year, and the biting densities were compared between the first year and the second year. The treated bed nets did not have any significant impact on the density or parous rates of Anopheles sawadwongporni and A. maculatus s.s. populations. The results for A. dirus s.l. were not conclusive because of the low number caught. Significant reductions in biting and parous rates of A. minimus species A were observed in only one of the 3 treated villages, and there was no overall difference between treated and control groups. However, the trial suffered from the washing of nets by villagers and the low rate of reimpregnation. (ii) A short-term study involved 4 villages in a cross-over design, and lasted 48 d. For the first 24 d, residents of 2 villages were given new treated nets while the other 2 villages retained their own untreated nets. For the second 24 d, this situation was reversed. Daily light-trapping revealed no significant difference in the indoor densities or parous rates of A. minimus species A between the periods with treated or untreated nets.(ABSTRACT TRUNCATED AT 250 WORDS)

Anopheles minimus Theobald was studied in four villages that had experienced differing levels of DDT application during malaria control operations in the foothills of northern Thailand. During times of high population level, the behaviour of the mosquito was assessed with respect to man-biting indoors and outdoors and feeding from domestic bovids. Its responses in villages recently treated with DDT differed from those in villages that had not been treated for several years, so that the insecticide had little direct lethal effect. Furthermore, the influence of the insecticide on the behaviour of the insect was considerably longer than is the lethal effect of the chemical. There was a suggestion that the taxon A. minimus may include two or more morphologically-cryptic species.

Females of Anopheles minimus Theobald at a site in northern Thailand that were caught on man or domestic bovids and were released showed a significant tendency to return to the type of host upon which they were first caught (F = 0-03). From a total of 3526 individuals marked and released, 51 were recaptured. A simple explanation is that the taxon is in fact a mixture of two or more morphologically cryptic species, a suggestion already indicated by previous studies.