Efficacy and host specificity compared between two populations of the psyllid Aphalara itadori, candidates for biological control of invasive knotweeds in North America

Abstract

Invasive knotweeds are large perennial herbs in the Polygonaceae in the genus Fallopia that are native to Asia and invasive in North America. They include Fallopia japonica (Japanese knotweed), F. sachalinensis (giant knotweed), and a hybrid species F. x bohemica (Bohemian knotweed). Widespread throughout the continent and difficult to control by mechanical or chemical methods, these plants are good targets for classical biological control. We examined the suitability of two populations of the psyllid Aphalara itadori from Japan as biological control agents by comparing their impact on the target weeds and assessing their fundamental host ranges. Both populations were capable of halting knotweed plant growth and reducing both above and below ground biomass by more than 50% in just 50days. Moreover, the psyllids caused mortality of several of the plants during this period. The two populations differed markedly in their reproductive potential on the different knotweed species. The Kyushu psyllid performed best on F. japonica and F. bohemica and the Hokkaido psyllid performed best on F. sachalinensis. Both were found to be specialized to knotweeds, with only very low occurrence of development on a small number of related non-target plant species. For the few non-target plant species that supported development, choice tests and multi-generational tests were used to further evaluate the likelihood of non-target host use. We conclude that A. itadori would be both effective and low risk as a biological control agent for invasive knotweeds and that both the Kyushu and Hokkaido populations may be needed to effectively control the entire knotweed species complex.