Discussion
We found that the effects of intraspecific hybridization were neutral in terms of fecundity and host acceptance. The reciprocal hybrids and the parental populations laid eggs on all knotweeds showing little preference towards any of the three species except for the Hokkaido parent that preferred to lay a greater number of eggs on Bohemian knotweed than Japanese knotweed. However, this host choice was maladaptive because of low survival of eggs to adulthood. On the other hand, the survival of the hybrid crosses was either intermediate between the parental populations or higher than the survival of at least one of the parents on given host plants. Thus, the sum effects of hybridization appear to be either neutral or positive in this system.
We did not find evidence for hybrid vigor with regards to fecundity since the reciprocal hybrid populations and the parental populations laid similar number of eggs when summed up across knotweed species. Heterosis is usually strongest in the first generation, and we used second generation hybrids in our experiments where the effects may be less pronounced. Yet, given that both parental populations have undergone long-term laboratory rearing some increase in fitness was expected upon hybridization, and second- and later-generation intraspecific hybrids showed higher performance than their parents in other insects as well (Hufbauer et al., 2013; Szűcs et al., 2012; Szűcs et al., 2017). It is possible that because of the small body size, high fecundity, and relative ease of rearing, A. itadori have been kept at large enough population sizes that buffered them from genetic problems, such as inbreeding or drift that hybridization could have alleviated.
When presented with a choice between three knotweed species, both the parental and hybrid A. itadori populations accepted all three species for oviposition (Fig. 2). This indiscriminate egg laying behavior was also observed in a recently collected distinct population (Murakami) of A. itadori in paired-choice tests (Camargo et al., 2022). Similarly, all three knotweed species were accepted for oviposition in no-choice tests by both the Kyushu and Hokkaido populations during host range testing (Grevstad et al., 2013). All the above experiments were conducted in different geographical regions, the one using the Murakami population in the Netherlands (Camargo et al., 2022), the host range testing in Oregon and the United Kingdom (Grevstad et al., 2013), and this study in Michigan, and they all used local knotweed populations for the experiments. Thus, it appears that a variety of genotypes of the three knotweed species are all recognized as potential hosts by A. itadori . This can be beneficial for field releases when mixed stands of different knotweed species co-occur, however, some of these choices do not reflect survival probability of psyllids.
The host choices of the Hokkaido population appear maladaptive since females from this population lay most of their eggs on Bohemian knotweed but can only develop on giant knotweed, which may be an artifact of over a decade of laboratory rearing and lack of exposure to different knotweed species. The developmental success of Hokkaido psyllids on Bohemian and Japanese knotweeds were under 1% (Fig. 2). Such suboptimal host choice behavior has been observed in many other herbivorous insects (Alred, 2021; Badenes‐Perez et al., 2006; Berenbaum, 1981; Davis and Cipollini, 2014; Faldyn et al., 2018; Ries and Fagan, 2003; Schlaepfer et al., 2005). Notably, monarch butterflies (Danaus plexippus ) may lay up to a quarter of their eggs on invasive swallow-wort vines (Vincetoxicum spp.) that are related to their milkweed hosts but do not support larval development (Alred, 2021; Casagrande and Dacey, 2014). The Kyushu population does not discriminate among knotweed species for oviposition, but it has relatively high development success (40-62%) on all three species. Our results for survival of both the Kyushu and Hokkaido populations on the different knotweed species are in line with findings during host-specificity testing (Grevstad et al., 2013).
Our development success results align with one other study by Fung et al. (2020) that compared the survival of the F4 FemHOK cross and the Kyushu parent on Japanese knotweed and found lower performance of the hybrids. The conclusion made by Fung et al. was that hybridization would not be beneficial for biological control in this system. However, we can place the effects of hybridization of A. itadori in a better context since we tested both reciprocal hybrids and compared their performance to both parental populations on all three knotweed species. We found that the survival of the reciprocal hybrids is somewhat intermediate between those of the parental populations on all three knotweed species (Fig. 2), which is a common outcome of intraspecific hybridization (Dingle et al., 1982; He et al., 2021; Szűcs et al., 2012; Tauber et al., 1986). This means that overall, hybridization improved performance compared to one parent and decreased performance compared to another parent. However, an intermediate developmental success is better than no development at all (see Hokkaido psyllids on Japanese and Bohemian knotweeds).
Considering developmental success alone we concur with Grevstad et al. (2013) that the Kyushu population is best suited for release on Bohemian and Japanese knotweeds and the Hokkaido population should only be used for releases on giant knotweed. However, the Kyushu population could also be used on giant knotweeds if the Hokkaido population is not available since they have relatively high developmental success on this species. In cases where cold adaptation traits from the Hokkaido population may be desirable, such as in Michigan, the release of hybrids might increase chances of overwintering success and should be considered as a viable alternative to either parental population.
An additional climate factor is photoperiod, which combined with temperature is used by most insects, including A. itadori to decide when to enter diapause (Danilevskii, 1965; Grevstad et al., 2022). Native species are locally adapted to use the cues from shortening daylength to prepare for winter and they will switch from a reproductive phase to a non-reproductive phase at a critical photoperiod (Danilevskii, 1965; Masaki, 1999). The Hokkaido population that was derived from collections made at 42.6° latitude enters its non-reproductive stage at a longer critical photoperiod than the Kyushu psyllids originating from a latitude of 32.8° N. This means that Hokkaido psyllids will diapause earlier than their Kyushu counterparts in the field (Grevstad et al., 2022). This appears to be a desirable trait in southern Michigan that is located at the same latitude as Hokkaido. However, the Hokkaido population will have low survival on the prevailing Japanese and Bohemian knotweeds in southern Michigan. On the other hand, because of their shorter critical photoperiod, Kyushu psyllids may start a new generation later in the season that they cannot complete before cold temperatures set in.
In sum, based on mismatches in temperature, photoperiod, and host plant availability neither the Kyushu nor the Hokkaido population may be ideal for releases in southern Michigan. Given that the hybrids between these two populations are able to develop and lay eggs on any of the three knotweed species and that they will have a mix of genotypes from both parental populations it is likely that there will be individuals with traits that can confer better survival and performance than those of either parent. Hybrids could also adapt faster to altered climates and photoperiod regimes because of their likely higher genetic diversity. We know from other weed biocontrol systems that critical daylength can evolve rapidly and that hybridization can alter the timing of diapause (Bean et al., 2012; Szűcs et al., 2012). For example, rapid evolution of the critical photoperiod was found in Diorhabda carinulata that allowed the southward expansion of this agent used to controlTamarix spp. In the USA (Bean et al., 2012). In ragwort flea beetles (Longitarsus jacobaeae ) used against the invasiveJacobaea vulgaris intraspecific hybridization altered the summer diapause response (Szűcs et al., 2012) and increased the biocontrol potential of hybrids in the field (Szűcs et al., 2019).
Additionally, as a hybrid species, Bohemian knotweeds can possess a greater degree of genetic diversity, which has been proposed as a main characteristic in this species’ invasive potential (Clements et al., 2016; Parepa et al., 2014). In such a case where genotypes of a target species are diverse, hybrids may be well suited in that they maintain a greater degree of diversity of their own compared to the parental populations, potentially benefitting establishment probability through the increased likelihood of adaptive evolution (Szűcs et al., 2017).
The release program of A. itadori in North America is still in its early stages, so monitoring of initial Kyushu and Hokkaido field populations will be essential to assess the establishment success and control potential of these two parental populations. In addition, a new population of A. itadori was collected in Niigata prefecture in Japan on the island of Honshu in 2019, called the Murakami population (Camargo et al., 2022). This population performs best on Bohemian knotweed and based on collection location its climate adaptation may be somewhat intermediate between the Kyushu and Hokkaido populations (Camargo et al., 2022). The first releases of the Murakami population were conducted in the Netherlands in 2020 (Camargo et al. 2022). However, it may take years before this population is approved for field release in the United States, therefore, releases for the foreseeable future have to focus on the populations that are currently available. Hybridization between the two populations already approved for biocontrol release might prove to be a relatively simple and efficient method of increasing rates of establishment.
Data Archiving Statement: Data for this study is available as Supplementary Information.
Acknowledgements: We are grateful to F. Grevstad for providing us with the Hokkaido and Kyushu psyllid populations. We thank B. Blossey and Zs. Szendrei for comments on an earlier version of the manuscript. This work was supported by the Michigan Department of Natural Resources Michigan Invasive Species Grant Proposal (award # IS19-5003). M. S. was supported by the United States Department of Agriculture National Institute of Food and Agriculture (USDA NIFA) Hatch projects 1017601 and 1018568.