Discussion:
Understanding what factors promote and what factors inhibit the
establishment of candidate biological control agents is critical if
biological control is to transition from a reactionary science to a
predictive one (Kimberling, 2004). There has, justifiably, been a major
focus on predicting the potential for non-target impacts (Barratt, 2011;
Barratt et al., 2010), leading to extensive modeling of the potential
extent of a candidate’s introduced range based upon its native host’s
range (Barton, 2004; Kaser & Heimpel, 2015; Raghu et al., 2007).
Increasingly, studies are conducting pre-release ecological niche
modeling to identify geographic regions from which candidate biological
control agents might be selected (for some examples see Banerjee et al.,
2019; Manrique et al., 2014; Mukherjee et al., 2011; Zalucki & van
Klinken, 2006). These types of pre-release comparisons can even help
prioritize the suitability of different strains of biological control
agents (Manrique et al., 2014), thus reducing both the environmental and
political risks of introducing ineffective agents (McClay & Balciunas,
2005). Of course, these types of models are only estimates and make
numerous assumptions to quantify and reduce complex biological and
ecological processes. For example, models that integrate biological
knowledge in addition to climatic variables produce more accurate models
than those based on climate data alone (Low et al., 2020). In addition,
the choice of climate variables can have important implications on
results (see Booth, 2021). Furthermore, climate variables have been
found to be more closely associated with some species compared to
others, even when the species have overlapping distributions (Shabani et
al., 2016). Most importantly, these approaches fail to account for the
evolutionary potential of species (e.g., Bean et al., 2012, Diamond,
2018). Yet, despite these limitations, ecological niche models can prove
useful as part of a larger discussion of factors that might influence
species distributions (Warren, 2012). Here we find that of the three
strains of the biological control agent Aphalara itadoricurrently being considered or currently being released for the
biological control of invasive knotweed species (Reynoutriaspp.), only the Hokkaido strain is predicted to be suitable in both
Europe and North America based on climate comparisons between the
current distributions of knotweed species in these two regions and the
source localities. The suitability of the other two strains differs by
location and target species and are discussed in more detail below.
In our analyses and the analyses published in Andersen and Elkinton
(2022), the Kyushu strain is found to have no-to-low climate suitability
for any of the target knotweed species in either Europe or North America
(Figure 4 and 5; Andersen & Elkinton 2022). The lack of climate
suitability of the Kysuhu strain mirrors field observations in North
America, where to date, efforts to establish the Kyushu strain have been
hindered by both biotic and abiotic factors (Andersen & Elkinton, 2022;
Grevstad et al., 2022; Jones et al., 2021; Jones et al., 2020). In
contrast, our analyses of the Murakami strain shows that this agent has
greater potential, particularly in North America. Analyses, based on the
distributions of all three species of knotweed in North America, suggest
that the Murakami strain has medium-to-high climate suitability in this
region (Figures 4 and 5). In addition, laboratory testing has shown that
this strain is capable of laying eggs on all three target species in a
choice experiment, and that feeding results in significant reductions
for all three species in plant height (8% total reduction), and rhizome
biomass for R. × bohemica , and R. sachalinensis(35% and 50%, respectively) (Camargo et al., 2022). Unfortunately, our
climate analyses suggest that this strain has no-to-low suitability
based on the distributions of all three target species in Europe
(Figures 1-3). Analyses based on records of the Hokkaido strain suggest
that this agent has at least some climate suitability based on knotweed
records from both Europe and North America. We should note that this
strain has been shown in the laboratory to have reduced fitness on
species of knotweed other than R. sachalinensis (Grevstad et al.,
2013); however, it is possible that additional populations feeding on
the other target knotweed species might be present on Hokkaido as well,
and we encourage further investigations in this region.
In an effort to create datasets with large enough geographic
distributions to be statistically meaningful, these continental-wide
analyses might mask more localized locations where climate suitability
of the different strains might be achieved. For example, despite our
previous findings that the Kyushu strain has no-to-low climate
suitability for most of North America against R. japonica(Andersen & Elkinton, 2022), in the spring of 2022 we did note the
presence of 15 overwintering adults at one field release site in western
Massachusetts (Andersen & Elkinton, unpublished data). Similar reports
of small numbers of overwintering adults of the Kyushu strain were also
reported in coastal Rhode Island (Dr. Lisa Tewksbury, personal
communication) and in North Carolina (Dr. Fritzi Grevstad, personal
communication), and individuals of the Murakami strain have been
observed persisting and dispersing in the field in the Netherlands (Dr.
Suzanne Lommen, personal communication), despite our results suggesting
they have no or low climate suitability. While records from several more
years will be necessary before we can consider these localized
populations “established”, they do suggest that even in areas where
suitability might be predicted to be low based on our analyses, that
persistence and eventually establishment might be possible.
Lastly, we would like to point out an interesting result from our
analyses. Our climate models, based on the invasive distributions of
each knotweed species, tend to predict low-climate suitability to areas
across much of the Japanese archipelago. On one hand, readers should
interpret this result as an indication that our ecological niche models
are capturing only a portion of the factors that shape the potential and
realized niches of an organism (as reviewed above). On the other hand,
we believe this result highlights the fact that local adaptation has
occurred in this system. Invasive knotweeds have been present in North
America and Europe for nearly 200 years (Conolly 1977), and that during
that time they have successfully adapted to the local environments in
their introduced ranges – this evolutionary potential is likely one of
the reasons that they are listed amongst worlds 100 most invasive
species (IUCN 2021). Given that coevolutionary forces form the basis of
sustainable biological control services (Holt & Hochberg 1997), this
potential mismatch between the newly evolved realized niche of an
invasive species and the existing potential niche of the natural enemy
in its native range, such as A. itadori , could have profound
implications on the “success” of a biological control program if the
target and the natural enemy no longer share the same climatic
constraints.