Palaearctic leaf beetle Chrysolinafastuosa (Coleoptera, Chrysomelidae, Chrysomelinae) new to North America

Abstract Background The univoltine leaf beetle Chrysolinafastuosa (Scopoli, 1763) is native to in the Palearctic Region from eastern Siberia to western Europe. New information First North American records are presented for C.fastuosa (Scopoli, 1763) (Coleoptera, Chrysomelidae, Chrysomelinae), as confirmed by vouchered specimens from Canada: Nova Scotia. Additional citizen science records from USA: Vermont are also discussed. Diagnostic information is presented to distinguish C.fastuosa from other North American Chrysomelidae and a species distribution model to assess its potential spread in North America is presented. This insect is expected to cause some feeding damage to above-ground parts of ornamental and invasive Lamiaceae, especially species of Galeopsis L. The species distribution model and the range of its host plant Galeopsistetrahit, suggest the north-eastern US and south-eastern Canada, from the Atlantic coast to the west end of Lake Superior provide the most suitable conditions for this species. The United States of America and Canada are now known to be home to 70 or more species of adventive Chrysomelidae.


Introduction
The univoltine leaf beetle Chrysolina fastuosa (Scopoli, 1763) is native to the Palearctic Region from eastern Siberia to western Europe (Kippenberg 2010). It is a widespread and common species in some countries (du Chatenet 2002, Rheinheimer andHassler 2018), feeding on several weedy Palaearctic genera of Lamiaceae, including Galeopsis L., Lamium L., Leonurus L., Prunella L. (Garin et al. 1999) and also Urtica L. (Rheinheimer and Hassler 2018). Chrysolina fastuosa is known to complete its life cycle on Galeopsis spp. and occasionally Lamium spp. in open and semi-shaded habitats (Rheinheimer and Hassler 2018). We first became aware of citizen-science images of Chrysolina fastuosa in Nova Scotia (NS) Canada and Vermont (VT) USA, North America on the internet platform iNaturalist (iNaturalist contributors and iNaturalist 2023) thanks to coleopterist and iNaturalist user Boris Buche (Berlin), who alerted HD about citizen science records of this species in North America. We aimed to investigate these reports of Chrysolina fastuosa in North America and to complete a preliminary assessment of its potential spread using a Maxent species distribution model.

Materials and methods
Agriculture and Agri-Food Canada researchers PB, B Brunet and J Gleason conducted field investigations to investigate citizen science records of C. fastuosa. They searched for C. fastuosa adults on Lamiaceae for about three hours while visiting central Nova Scotia during 2022. We also contacted Nova Scotia entomologists and Dr. Donald Chandler, Emeritus Curator of the University of New Hampshire Insect Collection to search collections for additional specimens (none found). PB collected two adult specimens in Dartmouth NS within 20 m of an individual first reported by iNaturalist contributors earlier in 2022 (https://www.inaturalist.org/observations/122742243).
HD reviewed iNaturalist records of C. fastuosa in North America for verification. He also reviewed images of the similarly metallic Chrysochus auratus (Fabricius, 1775) (Chrysomelinae, Eumolpinae) from north-eastern North America to search for additional records. Except for the shorter lobes of tarsomere 4 in Chrysolina spp., most subfamilylevel diagnostic characters (Riley et al. 2002) were not visible in dorsal habitus photos (diagnostic information below). Specimen occurrences were mapped using SimpleMappr (Shorthouse 2010).
As a preliminary assessment of the potential distribution of C. fastuosa in North America, TWS prepared a species distribution model using the program Maxent version 3.4.4 (Phillips et al. 2017). All "research grade" iNaturalist records from Europe and Asia were downloaded from GBIF.org (2023a). Records were thinned to one observation per 10 minute grid cell, matching the resolution of the environmental variables (see below). Following thinning, 1021 observations were retained for model training. In addition, we added a point (the country centroid) for every country in which C. fastuosa is listed in Kippenberg (2010) for which there were no records in iNaturalist (n = 8: Afghanistan, Albania, Georgia, Greece, Kazakhstan, Liechtenstein, Macedonia, Turkey).
WorldClim bioclimatic variables (version 2.1, 10 minute resolution; Fick and Hijmans (2017) ) served as the environmental rasters for the Maxent analysis. These variables provide biologically meaningful summaries of temperature and precipitation data. Maxent is robust to collinearity amongst variables and differences in correlation structure in training and projection regions can reduce model transferability (Feng et al. 2019); accordingly, we used all 19 bioclim variables. We defined the background extent for model training as the set of Koppen-Geiger Climatic Zones (Kriticos et al. 2011) in which there was at least one record of C. fastuosa. We used the R package ENMeval (Muscarella et al. 2014) to tune model parameters, comparing regularisation values 1-3 and considering all possible feature classes (linear, quadratic, hinge, polynomial and threshold). Models were evaluated with 4-fold cross-validation using the hierarchical checkerboard option of ENMEval and the optimal parameters were selected using the information criteria approach of Warren and Seifert (2011). The default Maxent CLOGLOG output, which ranges from 0-1, was used to interpret projections, with values above the median (i.e. the 50 percentile) considered to be highly suitable, values between the 5 and 50 percentile as moderately suitable and values between the 1 and 5 percentile as low suitability.
We also downloaded records for the host plant Galeopsis tetrahit L. from GBIF.org (2023b) in order to compare the current distribution of the host plant to the potential distribution of C. fastuosa. Analyses and mapping were conducted in R version 4.2.3 (R Core Team 2023), with the packages terra ( Hijmans 2023), dismo (Hijmans et al. 2022) and sf ( Pebesma 2018). Code and data necessary to reproduce this analysis are included in Suppl. material 1.

Chrysolina fastuosa (Scopoli, 1763) Diagnosis
Chrysolina fastuosa is 5.0-6.0 mm long and can be recognised in Canada and USA as belonging to genus Chrysolina by its elongate apical maxillary palpomere, non-connate th th th st th tarsal claws and metallic elytral colouration (Riley et al. 2002). It can be distinguished from other North American Chrysolina by its acute (Fig. 1, E) ventro-apical projection of protarsomere 5 (Rheinheimer and Hassler 2018 -misinterpreted as bidentate tarsal claws, Wilcox 1972). The apex of protarsomere 5 is uniformly narrowed in all other species with no projection. It is also distinguished by the non-concave anterior edges of the compound eyes. It has a brighter metallic green colouration than perhaps any native North American Chrysomelinae, blue metallic at the elytral suture and, in many, also a median longitudinal orange metallic area on the basal 2/3 of each elytron. The truncate aedeagal apex (Fig. 3) is somewhat like C. inornata (Rogers, 1856), but distinguished by its median projection.
Individuals of C. fastuosa can be distinguished from the similar-looking eumolpine, C. auratus by their non-flared metatibial apices without projections. Chrysochus auratus is also larger (6.5-10.0 mm) and also has a raised bead at the posterior edge of the prothorax that is greater than 2/3 the width of the base of antennomere 2. In C. fastuosa, any visible bead is less than half as wide as the base of antennomere 2. In C. fastuosa, antennomere 3 is longer than antennomere 4, while antennomere 4 is longer in C. auratus.   and iNaturalist 2023, Simpson et al. 2023). These photos were each identified by iNaturalist contributors as being of C. fastuosa and were confirmed by HD to match C. fastuosa and no other North American chrysomelid species. We present specimen and photographic evidence of 64 individuals from multiple sites in Nova Scotia Canada and Vermont USA, separated by over 900 km over 11 years. These led us to conclude that at least two populations of C. fastuosa are established in North America. We expect that these are the only two large populations in North America because this is a conspicuous, day-active species that is apparently readily documented by iNaturalist users.
Chrysolina fastuosa is known from the following localities in North America (Fig. 4)

Notes
The external morphology and male genitalia of the Canadian specimens closely matched taxon concepts of C. fastuosa. Specimens were confirmed as C. fastuosa using Mohr (1966) and Rheinheimer and Hassler (2018) and in comparison with

Analysis Potential Distribution
The optimal Maxent model parameters were regularisation = 1, with all feature classes retained; the mean AUC for the four calibration replicates of this model was 0.900, with a mean difference from the corresponding validation sets of 0.023. The suitability maps closely mirrored the distribution of C. fastuosa in its native range, with the highest suitability reported from central Europe, declining eastwards into Russia and northwards into Scandinavia (Fig. 5). The variables with the highest contribution to the model were annual temperature range (bio7, 32.8% relative contribution) and annual mean temperature (bio1, 16.9% relative contribution). The response curves for these variables suggest the optimal annual temperature range for C. fastuosa is between 20°C and 30°C, with suitability declining sharply with an annual range below 20°C or above 50°C. Suitability declines when annual mean temperature is below 3°C. Projecting this model to North America, the new records in Vermont and Nova Scotia fall in regions of moderate climate suitability (Fig. 6). Most of the north-eastern US and the Great Lakes region is classified as moderately suitable, with suitability declining towards the south and north west. The primary host plant for C. fastuosa, Galeopsis tetrahit L. (see below) is most abundant in the northern USA and Canada.

Adventive Species Biology
Adult Chrysolina fastuosa are known to feed on leaves of Galeopsis spp., Lamium (spp.) and Urtica spp in their native range. Larvae are thought to specialise on leaves and floral parts of Galeopsis spp. (especially G. tetrahit) and perhaps also on Lamium album L. and L. maculatum L. All species of Galeopsis and Lamium are of Eurasian origin and not native to North America (Kartesz and BONAP -The Biota of North America Program 2015); Galeopsis tetrahit is weedy in ornamental and agricultural settings and can cause serious yield reductions in canola, wheat, oats and alfalfa (O'Donovan and Sharma 1987). However, North America is home to three native species of Urtica L. (Lamiaceae), including the common and widespread Urtica dioica ssp. gracilis (Aiton) Selander (Boufford 2023).
Chrysolina fastuosa uses plants in this genus for adult feeding (Rheinheimer and Hassler 2018). Chrysolina fastuosa could possibly harm ornamental ground cover plantings of Lamium. However, such feeding may be partly beneficial because Lamium species also invade crop fields and native habitats (Swearingen and Bargeron 2016). Lamium amplexicaule L. and L. purpureum L. are further agriculturally harmful as winter annual weeds of crop fields on which the soybean cyst nematode, Heterodera glycines Ichinohe, 1952 (Tylenchida, Heteroderidae), can complete development (Creech et al. 2007). Thus, C. fastuosa presents both potential beneficial impacts on introduced weeds and an ecological threat to native plant species. More detailed study will be needed before the magnitude of these potential impacts is understood. The distribution of C. fastuosa matches prior observations (Douglas et al. 2021) that most of the accidentally established non-indigenous Chrysomelidae in North America are European weed-associated species that have established in eastern Canada and northeastern USA. It is possible that C. fastuosa was also accidentally first introduced into North America with woody ornamental plants with soil during 1960 to 1965 as hypothesised by Douglas et al. (2021) for several other weed-associated European Chrysomelidae. If this is true, then North American populations have been present for more than 50 years and their expansion has been gradual. Under this scenario, we can expect continued gradual population expansion outwards from central Vermont and central Nova Scotia unless further human-mediated transportation occurs. Such slow spread is consistent with the biology of C. fastuosa because most individuals disperse by walking rather than flying (Rheinheimer and Hassler 2018). If North American populations are more recent, then they may be spreading more quickly.
The distribution model for C. fastuosa indicates that the distribution of suitable climatic conditions in North America largely corresponds to the distribution of its host plant Galeopsis tetrahit: primarily, the area from New Jersey to Nova Scotia, west to Lake Superior. While G. tetrahit occurs across the prairies and into British Columbia, climatic conditions in the continental interior may present a barrier to the natural dispersal of C. fastuosa through this region. However, while C. fastuosa is relatively rarely observed in regions with low climate suitability in its native range (Fig. 5), it is not entirely absent, so we must be cautious in interpreting the distribution limits suggested by our model. The analysis presented here is preliminary and more detailed study, including physiological data, is necessary to produce a more rigorous assessment. Furthermore, our analysis is subject to the limitations of all correlative distribution models: un-or under-reported occurrences and misidentifications in the source data (iNaturalist) may introduce bias. However, we note the records we use correspond closely to the distribution reported in the Palaearctic Catalogue of Coleoptera (Kippenberg 2010), which suggests they are adequate for this analysis.
This new North American record, added to the species counts by Douglas et al. (2021) and Douglas et al. (2022) indicate that Canada and the USA are together known to host between 70 and 80 species of adventive Chrysomelidae. Of these, 53 to 61 adventive species of Chrysomelidae are known from Canada and 57 to 67 are known from the USA. This is the eighth or ninth adventive member of subfamily Chrysomelinae established in Canada and USA, of which four to six were introduced accidentally (Douglas et al. 2021).

Conclusions
Chrysolina fastuosa has been established in North America in Canada: Nova Scotia and USA: Vermont on introduced Galeopsis L. (Lamiaceae) plants. Numbers of recorded adventive Chrysomelidae for Canada and America north of Mexico are updated to reflect this finding.