GENETIC-VARIABILITY WITHIN AND BETWEEN ENGLISH POPULATIONS OF THE DAMSON-HOP APHID, PHORODON HUMULI (HEMIPTERA, APHIDIDAE), WITH SPECIAL REFERENCE TO ESTERASES ASSOCIATED WITH INSECTICIDE RESISTANCE

The damson-hop aphid, Phorodon humuli (Schrank), is a serious pest of hops in England. It is holocyclic (with obligatory sexual phase) and h ost alternating. From suction trap data, P. humuli aerial densities ar e known to be greatest in the main hop growing regions of Herefordshir e and Kent (mid-west and south-east England, respectively), some 260 k m apart. The aphid is now resistant to several insecticides. This is i n part conferred by elevated carboxylesterase activity, ranging from l ow in susceptible to high in very resistant strains. Enzyme markers, i ncluding carboxylesterases (EST-4 to -7), separated electrophoreticall y from individual insects, have been used to examine the degree of gen etic heterogeneity among I? humuli sub-populations on both its hosts - Prunus spp. (primary overwintering host) and hops, Humulus lupulus (s econdary summer host). The esterase data revealed heterogeneity among subpopulations collected from wild, unsprayed hosts in regions less th an 30 km in area, with a higher mean frequency of elevated esterase va riants in the commercial hop growing regions of Herefordshire and Kent , compared with samples from a non-commercial region around Rothamsted . Esterase distributions remained similar over consecutive years. Simi larly, allele and genotype frequencies for another enzyme (6-phosphogl uconate dehydrogenase, 6-PGD) were also heterogeneous among subpopulat ions sampled at less than 30 km apart (especially from Prunus) in each of the three regions surveyed, whilst allele and genotype frequencies sometimes remained stable over a number of summers. In addition, 6-PG D genotype frequencies were mostly congruent with Hardy-Weinberg expec tations, even for parthenogenetically-reproducing aphids colonizing ho ps. These data suggest that the 6-PGD alleles tested are selectively n eutral; that gene flow (= migration) is restricted between aphid popul ations, even within a single region (less than or equal to 30 km) and, that the autumn migration from hops to Prunus is probably of shorter range (perhaps less than 20 km) compared with the spring migration fro m Prunus to hops.