Restricted gene flow according to host plant in an herbivore feeding on native and exotic watermilfoils (Myriophyllum : Haloragaceae)

A native weevil is a candidate for biological control of the introduced aqu atic weed Eurasian watermilfoil (Myriophyllum spicatum) in North America. E uhrychiopsis lecontei feeds on both the exotic Eurasian watermilfoil (EXO) and the native Northern watermilfoil (Myriophyllum sibiricum; NAT) in the U nited States. In theory, the ability of weevils that have shifted hosts to adapt to their new host species depends in part on restriction of gene flow between host races. We investigated components of gene flow between weevil s on the two host plants. Weevils raised on the exotic EXO preferentially o viposited on EXO. Larvae tended to stay on the same host species when given the option to switch. Females had greater fecundity when raised on EXO tha n on NAT. Weevils raised on the native NAT oviposited on both plants. Thus, there is the potential for gene flow from NAT weevils into EXO weevil popu lations but less so in the reverse direction because of host fidelity. Gene flow is further reduced by the relatively low fertility (number of eggs an d the percentage hatched) of crosses between host races compared to crosses between different populations but within hosts. More offspring resulted fr om crosses between EXO weevils from different lakes in Vermont and from one Wisconsin lake than between EXO and NAT weevils from lakes 5 km apart in V ermont. Host race divergence among the Vermont weevils has begun, <11 yr (3 3 generations of weevils) after the invasion of the novel host into lakes. Thus, this change is taking place very quickly.