Biotypes of Dasineura tetensi, differing in ability to gall and develop onblack currant genotypes

Variation in damage levels on certain black currant, Ribes nigrum L., genot ypes, caused by the black currant leaf midge, Dasineura tetensi (Rubs.) (Di ptera: Cecidomyiidae), has been observed in northern Sweden. I investigated whether this variation is due to variation in virulence among midges. From a field population of midges, I successfully selected for virulence and av irulence, respectively, on the resistant black currant genotype cultivar 'S torklas' (called resistant genotype). The performance of avirulent and viru lent midge larvae on two black currant genotypes were studied in experiment s where first or second instar larvae were artificially transferred. There were no differences in larval survival and developmental rate between the t wo midge types when transferred to the susceptible currant genotype '7801-3 1' (called susceptible genotype). Larvae of the virulent strain established galls and developed on 'Storklas' but development was initially slower the re than on the susceptible currant genotype. Larvae of the avirulent strain suffered high mortality or remained in first instar on that same currant g enotype when transferred alone, but developed readily if transferred togeth er with virulent larvae. Larvae transferred in second instar to host plants susceptible to the larvae resumed feeding and developed further to maturit y. Second instar larvae were also able to establish new galls even though t hese galls were not as well developed as those caused by first instar larva e. Black currant plantations in northern Sweden were surveyed and local mid ge populations were found to be composed of either avirulent, virulent or a mixture of both midge types. Virulent midges were not restricted to planta tions where resistant current genotypes were grown. I conclude that, at lea st, two biotypes of the midge exist, and that those two are distinguished b y the ability to gall and survive on 'Storklas'.