Abstract.1Pissonotus quadripustulatusis a brachypterous planthopper that feeds on the clonal salt marsh plant, sea oxeye daisy,Borrichia frutescens. Asphondylia borrichiaeis a macropterous gall fly that creates galls on the apical meristems. MostBorrichiaoccurs in isolated patches consisting of more than one genotype.2Densities ofPissonotusandAsphondyliado not significantly differ within patches but they do differ between patches. I tried to find out why some patches ofBorrichiasupport consistently higher densities ofPissonotusandAsphondyliathan others.3Reciprocal transplants ofBorrichiabetween patches showed that for bothPissonotusandAsphondyliarecipient site (local environment) had a strong effect on population densities. However, the best sites forAsphondyliawere generally the worst sites forPissonotusand vice versa.4Donor site (local plant genotypes) was not significant forPissonotusbut was marginally significant forAsphondylia.However, forAsphondyliarecipient site had a greater effect. Local plant genotype effects may be greater for gall‐forming insects, which tend to be specialists, than for more generalist sapsucking insect