Introduction:
Biological control is a safe and cost-effective approach for the landscape-wide management of weedy species (Van Driesche et al., 2010). On numerous occasions, insects and pathogens have been identified and promoted to reduce the environmental and economic damages of weeds in locations from across the globe (Van Driesche, 2012). Despite concerns about the non-target effects of biological control agents (Barratt et al., 2010; Howarth, 2000; Simberloff, 2012), modern biological control programs implement a system of safe-guards to reduce unwarranted damage to non-target species – particularly when the agent is being introduced from a novel region through a practice known as classical biological control (Heinz et al., 2016; Messing, 2001). As a result, the use of biological control as an alternative to other labor and chemically intensive methods is increasingly becoming a part of both conservation and organic management practices (Baker et al., 2020; Van Driesche et al., 2016)
The control of invasive knotweed species, Reynoutria spp. Houtt. (Caryophyllales: Polygonaceae), has received much attention, with cultural, mechanical, and chemical control options all being implemented (e.g., Delbart et al., 2012; Kadlecová et al., 2022; Martin et al., 2020). Interest has also been directed towards harvesting knotweeds, as the plants have unique chemical properties (Metličar & Albreht 2022; Metličar et al. 2021) and may themselves be an important source for biopesticides (Dara et al. 2020). However, for landscape-wide efforts biological control is likely the most effective strategy, and as such an international effort was established to identify and promote the natural enemy, Aphalara itadori (Shinji) (Hemiptera: Aphalaridae), which was observed feeding and causing damage upon wild populations ofR. japonica Houtt. on the Japanese island of Kyushu in 2004 (Shaw et al., 2009). Prior to field releases, a laboratory reared population of the Kyushu strain of A. itadori was then used for host-range testing and candidate biological control reviews were conducted (Grevstad et al., 2013; Shaw et al., 2009; Shaw et al., 2011), resulting in the first approved biological control agent in the European Union (Shaw et al., 2009). A second population of A. itadori , feeding on R. sachalinensis (F. Schmidt) Nakai, was subsequently collected in 2007 near Lake Toya on the Japanese island of Hokkaido, and similarly brought to the laboratory for host range testing and candidate biological control review (APHIS, 2020). Both strains were subsequently approved for release in Europe and North America, and recently a third strain, the Murakami strain, was identified from near the Japanese city of Murakami and has been released in the Netherlands against R. ×bohemica Chrtek & Chrtková (Camargo et al., 2022). Review of the Murakami strain for release in North America is currently underway.
As part of the review prior to introduction in North America, climate suitability models for the Kyushu strain and the Hokkaido strains were developed using the software program CLIMEX (Hearne Software, Melbourne, Australia). These models predicted a strong climate match for both the Kyushu and Hokkaido strains to potential release locations across North America (Grevstad et al., 2012). However, despite this predicted climate match, there have been no documented accounts of establishment of this species anywhere it has been released. Note: here we use establishment to indicate a self-sustaining population that is present in a location for at least three consecutive years without importation or release of additional individuals. We choose to use this more conservative definition, though “establishment” has historically been reported in the literature after only one year (see Van Driesche et al. 2008). Unfortunately to date in locations where the psyllids have been released and individuals have been observed in the field during post-release monitoring, neither reduction in plant densities nor biomass have been observed. We previously suspected that environmental constraints might be limiting the success of the Kyushu strain in North America (Andersen & Elkinton, 2022), and noted a poor climate match to the source locality of the Kyushu strain based on North American records ofR. japonica using a different modeling approach, MaxEnt (Phillips et al., 2006; Phillips & Dudik, 2008). The MaxEnt models based onR. japonica records did, however, predicted medium-to-high suitability for the source localities of the Hokkaido and Murakami strains (Andersen & Elkinton, 2022). Therefore, we were curious as to whether climate suitability estimates conducted in MaxEnt for each target knotweed species based on records from Europe and North America could help provide insights into factors influencing the success ofA. itadori releases in each region and for each target species of knotweed.
To address this, we collected public records of all three target knotweed species from the Global Biodiversity Information Facility (GBIF) from Europe and North America. Using the MaxEnt software platform, we estimate climate suitability envelopes based on records from the invasive regions of each species, and we compared the predicted suitability of the source locality of each A. itadori strain compared to other localities in Japan for each species of knotweed and geographic region combination.