The effects of intraspecific hybridization on the host specificity of a weed biocontrol agent

Abstract

Hybridization can alter the host-specificity and fitness of herbivorous insects when the hybridizing populations are adapted to different hosts. It is less clear what the effects of admixture may be when genetically distinct populations are crossed that have similar and narrow host ranges. We tested the effects of hybridization between an Italian and Swiss population of the ragwort flea beetle, Longitarsus jacobaeae, a biocontrol agent for tansy ragwort, Jacobaea vulgaris, in the USA. Development success, development time, and fecundity of parental and first- and second-generation hybrids were assessed on the primary host and ten closely related plant species in no-choice larval transfer experiments. Four of the non-target species supported limited development but none represented novel host use of hybrids compared to the parents. Development time showed maternal effects on one of the non-target species where offspring of crosses with Italian mother developed slower. Percent larval development was significantly greater for one replication of a non-target species, indicating a plant genotype effect. Overall, hybridization did not result in changes of the fundamental host range or improved performance of hybrids on non-target species even when some hybrid lineages exhibited hybrid vigor on the primary host. These findings underscore the reliability of currently-employed host-specificity testing procedures that identify the fundamental host range of potential agents. Our results also support the newly advocated approach to promote intraspecific hybridization in biocontrol agents when the parental populations have similar and narrow host ranges to increase genetic variation and create novel genotypes to facilitate adaptation and persistence in the new range.