Background: Knowledge on insecticide resistance in target species is a basic requirement to guide insecticide use in malaria control programmes. Malaria transmission in the Mekong region is mainly concentrated in forested areas along the country borders, so that decisions on insecticide use should ideally be made at regional level. Consequently, cross-country monitoring of insecticide resistance is indispensable to acquire comparable baseline data on insecticide resistance.

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Analysis of mitotic karyotypes of wild specimens of larvae of the Bactmcera tau-like fruit flies (Diptera: Tephritidae) in Thailand has revealed seven distinct chromosome forms, based on the amount and distribution of heterochromatin in sex chromosomes and autosomes. Such cytological differences are perfectly correlated with morphological observations and molecular genetics data. These findings clearly suggest that B. tau is a cluster of at least seven closely related species temporarily designated as species A (= B. tau), B, C, D, E, F and G. On the basis of the gross quantity of heterochromatin accumulation in the genome, three groups of mitotic karyotypes can be recognized. Group 1 comprises species A and E. Species E specifically occurs only in fruits of Shychnos thorelii while species A attacks many kinds of host plants. Group 2 contains species B, F and G. Species B has been found only in Siphonodon celastrineur fruit, whereas species F and G attack the same host species, the medically important plant, Hydnocarpus anthelminthicus, albeit in different localities. Group 3 includes species C and D, each of which comprises larger amounts ofpericentric heterochromatin in all chromosomes than the other two groups. Hence, these two species are cytologically remote from those of groups 1 and 2. Species C and D occur allopatrically but they attack the same host plant species, Momordica c0chinchinenSi.s. Thus, genetic differentiation at the chromosomal level to specific host plant species and geographic isolation seem to play an important role in speciation of members of the B. tau complex. 0 2000 The Linnean Society of London ADDITIONAL KEY WORDS:-Bachocera tau complex -metaphase chromosome heterochromatin -karyotypic evolution.