Results
There was no difference in the fecundity of the parental and hybridA. itadori populations (F3, 20 = 1.03, P = 0.3988). The 25 females released within each cage (replicate) of the Kyushu parent laid 292 ± 57 (mean ± SE), the Hokkaido parent 447 ± 70, the FemKYU hybrid 396 ± 66, and the FemHOK hybrid 378 ± 65 eggs on average across the three knotweed species offered. The different psyllid populations tended to lay similar number of eggs (F3,20= 1.02, p = 0.4052) on the three knotweed species (F2,40= 2.56, p = 0.0902) except for the Hokkaido population (psyllid population*plant species: F6,40 = 2.79, p = 0.0233) (Fig. 1). Hokkaido females laid significantly more eggs on Bohemian knotweeds (206.4 ± 38.7) than on Japanese knotweeds (74.9 ± 23.3) while the Kyushu parent and the hybrids did not discriminate among knotweed species.
There were significant differences in development success of nymphs from the four psyllid populations on the three knotweed species (psyllid population*plant species interaction: χ = 833.87, df = 6, p < 0.001) (Fig. 2). The FemHOK hybrid was the only psyllid population that had similar developmental success on all three knotweed species. The Hokkaido and Kyushu parent had the highest survival on their optimal hosts of giant and Japanese knotweeds respectively, while the FemKYU hybrid performed best on Bohemian knotweed.
In the cross where the female is from the Hokkaido population and the male is from Kyushu (FemHOK) hybridization significantly increased survival on Bohemian (36%) and Japanese knotweeds (45%) compared to the Hokkaido parent (< 7%). However, hybridization also led to lower development success on giant knotweeds (37%) compared to the maternal parent (63%). For the other type of cross where the female was from Kyushu (FemKYU) there was some decline in survival on Bohemian knotweeds (52%) compared to the maternal parent (63%) and a larger reduction on Japanese knotweed (21%) compared to the Kyushu parent (61%). On giant knotweed both reciprocal crosses had similar performance as the Kyushu parent (Fig. 2).