Introduction

Anthropogenic activities are resulting in shifts in species distributions (Parmesan and Yohe 2003; Parmesan 2006), with especially strong effects on the geographical distribution of plants and insects (Bergstrom and Chown 1999; Frenot et al. 2005). Temperature is an important determinant of successful establishment and spread of invasive aliens (Vitousek et al. 1997; Thuiller et al. 2007). The importance of thermal conditions on insects’ life cycle is particularly manifest in polar regions, where air and microclimate temperatures are consistently cool but are increasing at the fastest rates on the planet (Smith 2002; Bergstrom et al. 2006; Convey 2006). Overall, an increase in winter temperature is expected to have a more significant effect on the population dynamics of alien insects in cool sub-Antarctic regions than in moderately warm temperate ones, as these insects likely live close to their limit of low temperature tolerance (Convey 2001; Convey et al. 2002). Studying the low temperature biology of alien insects is thus particularly relevant as it determines their overwintering success and predicts whether the species could survive through seasonal climatic fluctuations and winter cold stress particularly in a changing climate context (Bale and Hayward 2010).

Thermal biology has been studied in a wide variety of insect species exposed to constant (Bale 2002; Sinclair et al. 2003) and fluctuating temperatures (Renault et al. 2004; Terblanche et al. 2010; Fischer et al. 2011). Several studies have demonstrated that fluctuating thermal regimes (FTRs), i.e. cold exposure interrupted by periodic short warming pulses, increase the duration of survival in several insect species (Coulson and Bale 1996; Nedvěd et al. 1998; Hanc and Nedved 1999; Renault et al. 2004; Colinet et al. 2006; Koštál et al. 2007; Colinet and Hance 2010; Renault 2011), potentially through physiological repair of accumulated injuries during warm bouts that are involved in the re-establishment of ion-gradient homaeostasis (Koštál et al. 2007), the metabolism of compatible solutes (Wang et al. 2006; Colinet et al. 2007a, Lalouette et al. 2007), the expression of heat shock proteins (Hsps) (Wang et al. 2006; Tollarová-Borovanská et al. 2009; Colinet et al. 2007b, 2010), the activation of antioxidant system (Lalouette et al. 2011), and the upregulation of various proteins (Colinet et al. 2007b). Another potential explanation is that under FTRs, the daily cold dose accumulated by the insects is reduced compared to those exposed at constant low temperatures (Hanc and Nedved 1999; Renault et al. 2004; Colinet et al. 2006, 2011). However, if the total amount of cold exposure is controlled for there are still survival advantages to FTRs, though fitness tradeoffs may be induced (Marshall and Sinclair 2010). However, although this repair of accumulated injuries and the subsequent increased duration of survival have been demonstrated in several insect taxa from temperate regions (Coulson and Bale 1996; Hanč and Nedvĕd 1999; Renault et al. 2004; Colinet et al. 2006), it has rarely been examined in those inhabiting Polar or alpine regions.

Merizodus soledadinus Guérin-Méneville (Coleoptera: Carabidae) [previously misnamed Oopterus soledadinus] is an opportunistic predator whose native range is restricted to the cold temperate zone of south America including islands off the southern coast of Cape Horn (Johns 1974). This species was introduced from the Falkland Islands to Port-Couvreux (Kerguelen Islands) in 1913 (Jeannel 1940), and more recently to South Georgia in 1963 (Convey et al. 2011; Darlington 1970). For the past 60 years (1941–2010) in the Falkland Islands (in Port Stanley and the Mount Pleasant airports), mean monthly temperatures range from 9.6 ± 1.1°C in February to 2.3 ± 1.3°C in July, with an annual mean of 6.6 ± 0.5°C (Steve Colwell, British Antarctic Survey, personal communication). Like thermal tolerance and performance are generally related to the temperatures, an organism experiences in its native habitat (Stevens 1989; Addo-Bediako et al. 2000; Ghalambor et al. 2006; Pörtner et al. 2006) M. soledadinus would a priori be expected to be stenothermal and cold tolerant.

The eastern part of the Kerguelen Islands, where M. soledadinus is widely distributed, is characterised by cool temperatures ranging from 7.7 ± 0.9°C in February to 2.0 ± 0.7°C in July with an annual mean of 4.6 ± 0.4°C (Meteo France data from 1951 to 2009). During the coldest month (July), mean minimum temperature is −5.0 ± 1.6°C (Meteo France data from 1951 to 2009). The population size and geographical distribution of M. soledadinus were relatively small at the Kerguelen Islands until the 1980s, with a rapid expansion in the 1990s at the expense of native invertebrate species (Chevrier 1996; Lebouvier et al. 2011). Temperatures on Kerguelen have warmed consistently since the 1980s: e.g. mean annual temperatures were 4.3 ± 0.4°C over the period 1951–1975 and 4.8 ± 0.3°C over the period 1976–2008 with a more significant increase during winter periods (Lebouvier et al. 2011). Thus, a changing climate may have improved winter survival of M. soledadinus over this time period. Despite this warming, temperatures at the Kerguelen Islands so far remain slightly cooler than the Falklands Islands.

In the present work, we aimed to describe the thermal sensitivity and responses to thermal fluctuations in M. soledadinus, as a first step towards predicting responses to climate change in the field. We measured survival at constant low temperatures and FTRs. We also examined the critical thermal limits (lower and upper) and the CO2 release in adults acclimated at several temperatures to examine the capacity of this species to compensate their metabolic rate to the range of temperatures experienced across the life cycle under warming conditions.

Materials and methods

Rearing and acclimation conditions

Wild imagoes of M. soledadinus (Coleoptera: Carabidae) were hand-collected in coastal areas at Port-aux-Français at the Kerguelen Islands (49°21′S, 70°13′E) in January 2008 and 2009. Imagoes were sampled under stones in a habitat where Acaena magellanica was dominant. The insects were then maintained under controlled conditions at +8°C (R.H. of 70 ± 5%), with a 15 h:9 h light/dark cycle, for 1 week before use in critical thermal limits and duration of survival at constant and FTRs, or up to 8 months before respirometry. They were supplied ad libitum with water and food consisting of larvae of Calliphora sp., Anatalanta aptera and Drosophila melanogaster.

Chronic low temperature survival

To avoid cold shock, adults of M. soledadinus were pre-acclimated at +4°C for 24 h prior to being used for the survival experiments. Groups of ten beetles were then directly transferred to Petri dishes which were randomly assigned to each one of the following low-temperature treatments: (1) Constant temperature (CT): −8, −6, −4, −2 and +4°C; (2) FTRs: −6 or −4°C for 22 h cycling with 0, 4, 8 or 12°C for 2 h, i.e. −6/0, −6/4, −6/8, −6/12, −4/0, −4/4, −4/8 and −4/12°C. During the chronic low-temperature exposure, beetles were food and water deprived and beetles held at constant +4°C were used as the controls. One Petri dish per thermal treatment was transferred to optimum conditions (+8°C with water ad libitum) at daily intervals to observe recovery. Survival was scored as the proportion of active insects (that exhibited coordinated walking activities) after 2 days of recovery at +8°C (preliminary tests demonstrated that survival then remained similar from 48 h onwards).

Critical thermal limits

Critical thermal limits (CTmin and CTmax) of individual adult M. soledadinus were assessed after 7 days acclimation at +4, +8 (control beetles) and +16°C with water and food supplied ad libitum. CTmin (N = 30 per treatment except for control, for which N = 26) and CTmax (N = 26 per treatment) of beetles was measured by knockdown using a method adapted from Powell and Bale (2006). Beetles were placed in a 35 × 5-cm jacketed glass column cooled with ethylene glycol circulated from a programmable alcohol bath (Haake F8-C50 Thermo Electron, Karlsruhe, Germany), through the outer chamber. The opening of the inner chamber was then closed with insulation foam. The air temperature in the inner chamber was monitored by thermocouples (±0.1°C) at the top and bottom of the column. As the beetles could not cling or climb up on the glass of the column, a 20 × 2 cm piece of white paper (90 g m−2) was positioned within the inner chamber. Preliminary experiments demonstrated that the beetles very often fell when the column was vertical; a fall that was stressful and that increased the locomotor activity of the insects as they quickly tried to stand on their legs again. The column was thus inclined to 21°, an angle that solved the problem. Each insect was placed individually in the column and allowed 10 min to acclimatise to the new environment. Temperature in the column was ramped from +8°C to the CTmin or to the CTmax at a rate of 0.5°C·min−1. CTmin and CTmax were determined as the temperatures at which the beetles lost the ability to cling to or climb the paper, and thus became motionless and fell out at the bottom of the column. The temperature was recorded, the insulation foam removed, and the insect was collected in a Petri dish. Timing of the duration of recovery from coma (both heat- and cold-induced) was then recorded at 10°C and began the moment the insect fell out of the column; the end point being the time when the insect exhibited coordinated walking abilities (i.e. the ability to stand on their legs and take a first step). This end point (rather than the commonly used measure of the first coordinated movement of the legs) was chosen as having more ecological relevance to the insect.

After determination of critical thermal limits and recovery times, beetles were kept individually in Petri dishes at +8°C with water and food ad libitum for 10 days and survival was scored every day as the number of beetles exhibiting coordinated walking activities, after which the beetles were sexed, and weighed (Sartorius M4 microbalance, d = 1 μg) after 6 days at 60°C (dry mass).

Metabolic rate

To examine the metabolic rate under various constant thermal conditions, fed insects were first cold-acclimated for 7 days at 0, 4, 8 (control beetles), 12 and 16°C (N = 8 insects for each thermal condition) and the metabolic rate measured at the temperatures at which the insects were acclimated. Body mass of the adults were measured prior to and immediately after each trial (Mettler-Toledo MX-5 microbalance (Mettler-Toledo, Columbus, OH, USA; d = 0.1 μg).

CO2 production was measured using a flow-through method modified from Williams et al. (2010) and described in Lalouette et al. (2011). Briefly, beetles were placed individually in a glass cylindrical respiration chamber (4 cm3) through which dry, CO2-free air was passed through the chamber at a rate of 25 mL min−1 controlled by a mass flow controller (Sable Systems International [SSI], Las Vegas, NV, USA) connected to mass flow valves (Sierra Instruments, CA, USA). Dry CO2-free air was supplied by a Purge Gas Generator (Parker-Balston, OH, USA) and further scrubbed of remaining CO2 and water vapour by passage through a Drierite–ascarite–drierite column. Temperature was controlled (±0.1°C) by SSI PELT-4 cooling cabinets and monitored by thermistors inside the unit. CO2 concentration of excurrent air was determined with a Li7000 infra-red gas analyser (LiCor, Lincoln, NE, USA). After 10 min equilibration, CO2 release was recorded every second over a 2.5 h period for each beetle and the analyses were done during the second half hour of the recording (in the time range 0.5–1 h). Data were baselined to an empty chamber recorded at the beginning and end of each run to correct for any instrument drift. Activity of beetles was monitored using infra-red activity detectors (AD-2, SSI), and the absolute difference sum over the relevant time period was calculated. The resulting slope was compared amongst temperatures as an index of activity (Lighton 2008). Data were captured and processed using a UI2 interface and Expedata software (SSI), averaged over 5 s and converted into mL CO2 min−1 using Expedata. We converted CO2 release (mL min−1) in O2 consumption (mL min−1), and then in Watts (J s−1) in assuming a Respiratory Quotient of 0.8 through the following formula: \( \frac{{{\text{CO}}_{2} \,{\text{release }} \times 1.25 \times 20.1312}}{60} \) (Lighton 2008).

Statistical analysis

Data are expressed as means ± SEM. Survival data were expressed as lethal times for 50 and 90% of the samples, Lt50 and Lt90, respectively calculated by probit analysis using MINITAB Statistical Software Release 13 (MINITAB Inc., State College, PA, USA), where time of exposure was an expression of dosage level at a particular temperature. Pearson correlations were used to test relationship amongst critical thermal limits, dry mass, and duration of recovery from chill coma, or amongst the duration of recovery from chill coma, dry mass, and the time spent in the column. Kruskal–Wallis tests (H test) and Mann–Whitney (W test) were performed to compare the temperature at the onset of chill coma and the time necessary to recover. Survival curves after the onset of chill coma were compared using log-rank (Mantel–Cox) test. χ 2 contingency tests were used to compare mortality using GRAPHPAD PRISM Software 5.02 (San Diego California USA, http://www.graphpad.com) with α = 0.05. The CO2 release (\( \dot{V}{\text{CO}}_{2} \)) data were log10-transformed to improve normality and homoscedasticity. We calculated the Q 10 over the range of temperatures measured using the equation Q 10 = 10(10a); where ‘a’ is the slope of the regression of log10 \( \dot{V}{\text{CO}}_{2} \) plotted against temperature. A general linear model with mass as covariate was performed on insects exposed at constant temperatures to determine effects of acclimation temperature on \( \dot{V}{\text{CO}}_{2} \). Pairwise post hoc comparisons were conducted to identify significant differences (Tukey’s tests). A linear regression was performed to determine whether the slopes of the absolute difference sums for activity (an index of the magnitude of signal in the activity channel) were correlated with temperature using R software 2.13.0.

Results

Survival at chronic low temperatures

Survival at constant low temperatures was significantly increased with increasing temperatures. The lowest survival duration was obtained at −10°C (data not shown on the Fig. 1, mortality occurred in less than 5 min and 100% mortality after 2.4 h). The longest duration of survival was obtained at −2°C (Lt50 = 12.6 ± 1.0 days) and was similar to that of the control (+4°C) (Fig. 1).

Fig. 1
figure 1

Lt50 (black bars) and Lt90 (grey bars) (mean ± SE) in adults of M. soledadinus kept at constant −8, −6, −4, −2 and 4°C. Bars with distinct letters are significantly different (P < 0.05)—lower case for Lt50 and upper case for Lt90

Full size image

Survival at fluctuating temperatures

Overall, survival of the ground beetles was significantly higher in the FTRs using −6°C as the low temperature than in the constantly cold exposed to −6°C (Fig. 2a). Within the different FTRs using −6°C as the low temperature, increasing the warmer temperatures (from 0 to 12°C) during the daily pulses had no more beneficial effects on the duration of survival (Fig. 2a). For exposure to the less severe cold temperature (−4°C): duration of survival at the Lt50 was increased by exposure to pulses of either 0 or +12°C, whilst survival duration at the Lt90 was sensitive only to pulses above +4°C (Fig. 2b).

Fig. 2
figure 2

Lt50 (black bars) and Lt90 (grey bars) (mean ± SE) in adults of M. soledadinus kept at constant (CT) −6 (a) and −4°C (b); and cycling thermal regimes (FTR, −4 or −6°C for 22 h with a daily return at 0, 4, 8 or 12°C for 2 h. Bars with distinct letters are significantly different (P < 0.05)—lower case for Lt50 and upper case for Lt90

Full size image

Critical thermal limits

There was no significant difference between male and female critical thermal limits (Males: CTmin = −4.20 ± 0.96°C, N = 69 and Females: CTmin = −4.49 ± 1.07°C, N = 51; T 102 = 1.42; P > 0.05; Males: CTmax = 37.63 ± 1.98°C, N = 33 and Females: CTmax = 37.75 ± 0.82°C, N = 43; T 40 = 0.35; P > 0.05), so sexes were pooled for subsequent analyses. Neither CTmax nor CTmin were correlated with dry mass (CTmin: r 90 = 0.004, P > 0.05; CTmax: r 68 = 0.167, P > 0.05), nor to the duration of recovery from chill coma (CTmin: r 90 = 0.000; P > 0.05; CTmax: r 68 = 0.218; P > 0.05). The recovery of coordinated walking was neither correlated to dry mass (CTmin: r 90 = −0.104; P > 0.05; CTmax: r 68 = 0.236; P > 0.05) nor to the duration the insects stayed in the column (CTmin: r 90 = −0.000; P > 0.05; CTmax: r 68 = 0.238; P > 0.05).

Low temperature acclimation significantly decreased CTmin in adults of M. soledadinus (H2 = 64.46; P < 0.01) (Fig. 3a) but had no effect on the duration from chill coma recovery (CCR, H2 = 0.09; P > 0.05; Fig. 3b). There was no mortality in the 10 days after the experiment in any of the groups.

Fig. 3
figure 3

Critical thermal minima (CTmin) (a) in M. soledadinus acclimated for one week at 4, 8 or 16°C, and the duration of recovery from chill coma (b). Circles represent mean values, lines within box are the medians. Different letters indicate significant differences (P < 0.05)

Full size image

Conversely, acclimation had no effect on CTmax (H2 = 3.00; P > 0.05) (Fig. 4a) but the time needed to fully recover walking activities decreased significantly (heat coma recovery HCR H2 = 7.88; P < 0.05) with increased acclimation temperature (Fig. 4b), indeed 16°C-acclimated beetles had the shortest duration of recovery (W = 492.5; P < 0.05) and the highest longevity after 10 days of the experiments (χ 2 = 12.06; P < 0.01) (Fig. 5).

Fig. 4
figure 4

Critical thermal maxima (CTmax) (a) in M. soledadinus acclimated for 1 week at 4, 8 or 16°C, and the duration of recovery from heat coma (b). Circles represent mean values, lines within box are the medians. Different letters indicate significant differences (P < 0.05)

Full size image
Fig. 5
figure 5

Survival after recovery from heat coma in adult M. soledadinus that were acclimated 1 week at 4, 8 and 16°C. N = 26 for each experimental condition

Full size image

Respirometry

The fresh masses did not differ between temperature acclimation (F 4 = 0.33; P > 0.05) and were included in the analysis as a covariate of \( \dot{V}{\text{CO}}_{2} \) (Table 1). \( \dot{V}{\text{CO}}_{2} \) of adult M. soledadinus increased with increasing acclimation temperature (F 4 = 39.52; P < 0.001) (Table 1). The Q 10 after acclimation to temperatures ranging from 0 to 16°C was 2.49 (Fig. 6). There was no relationship between activity and temperature (F 1,40 = 2.776, P = 0.601.

Table 1 CO2 release, metabolic activity and fresh mass (mean ± SD) in adults of M. soledadinus after 1 week acclimation at 0, 4, 8, 12 or 16°C
Full size table
Fig. 6
figure 6

The relationship between temperature (in °C) and log10 standard (resting) metabolic rate (in mL CO2 min−1) in adults of M. soledadinus (N = 8 for each temperature), after a 1 week acclimation to the measurement temperature. All values were obtained using flow-through respirometry

Full size image

Discussion

We found that adult M. soledadinus have the ability to survive cold exposures to temperatures as low as −6°C for a few days, and that acute cold injuries start to accumulate below −8°C. These results are consistent with populations from South Georgia, where adult M. soledadinus have supercooling points of −5 to −6°C (Block and Somme 1983; Ottesen 1990), and which suggests that the threshold in mortality that we observed between −6 and −8°C may be accompanied by freezing. The chronic cold tolerance of adults of this ground beetle was moderate (ca. 2 weeks at −2°C) compared to some other arctic or sub-Antarctic species, for example, the carabid Pelophila borealis from South Georgia, which survived 7 weeks at −5°C and 7 months at −3°C (Sømme 1974). However, the similar survival duration obtained at −2 and +4°C suggests that adult M. soledadinus mortality at −2°C was related to water deprivation rather than to cold exposure. Adult M. soledadinus supplied with water ad libitum can survive about 60 days at −4°C (D. Renault, unpublished data), it thus is unlikely that mortality occured as a result of the exhaustion of energy reserves in any of the survival experiments in the present study.

Work on temperate and tropical insect species has demonstrated a strong relationship between duration of chilling exposure and the amount of accumulated injuries (Rojas and Leopold 1996; Koštál et al. 2004, 2007) and the beneficial impact of FTRs on adults’ survival (Renault et al. 2004; Colinet et al. 2006; Renault 2011). By contrast, we found only a small benefit of FTR on chronic cold survival of adult M. soledadinus. Under the temperature cycles, adult M. soledadinus survived for longer than constantly cold-exposed individuals. Although there was a slight effect of the high temperature on survival at an FTR to −4°C, modulating the highest temperature applied during the pulses for the −6°C FTR had no effect on survival, suggesting that at this latter temperature there is a limited ability to repair cold injuries. This weak ability to exploit intermittent periods of favourable thermal conditions in our experimented cycles matches with the geographical distribution of M. soledadinus in cold temperate but not extreme and sub-Antarctic habitats where thermal variation is of low amplitude (Bergstrom and Chown 1999). Indeed organisms’ flexibility in thermal tolerance and performance are thought to be proportional to the range of the thermal fluctuations they undergo in their living environment (Stevens 1989; Addo-Bediako et al. 2000; Ghalambor et al. 2006; Pörtner et al. 2006). FTRs with cold exposures of −4°C increased survival to levels comparable to that seen at + 4°C, likely resulted from water deprivation. Todd and Block (1997) have previously described low desiccation tolerance in adult M. soledadinus, and this species is largely nocturnal, possibly to prevent desiccation (Ottesen 1990).

Merizodus soledadinus enters chill coma between −3 and −5°C, which is similar to other sub-Antarctic insects (e.g. Klok and Chown 2003), however due to concerns about the outcome of differences in the methodological procedures for measuring critical thermal limits from one study to the next, inter-study comparisons must be taken with care (Terblanche et al. 2007). The CTmax of about 37°C is relatively low compared to temperate insects (Terblanche et al. 2006), but is considerably higher than the thermal conditions likely to be encountered by M. soledadinus in the field, where maximum temperatures rarely exceed 25°C (Lebouvier et al. 2011). In the present work, acclimation had a significant effect on CTmin and on the duration of recovery from CTmax in adult M. Soledadinus. Our results support the idea that physiological adjustments to cold and heat stress are at least partially decoupled in terrestrial arthropods (Klok and Chown 2003).

Recovery from thermal coma in M. soledadinus showed large individual variation (as has been reported in other species, e.g. David et al. 1998), and whilst warm acclimation significantly reduced recovery time from heat coma, cold acclimation had no effect on recovery from chill coma, which supports the hypothesis that recovery and onset of coma have different underlying mechanisms (Sinclair and Roberts 2005). Our observations are also consistent with observed decoupling of low and high temperature tolerances in insects (Addo-Bediako et al. 2000). Although we did not observe an effect of acclimation on CCR time, such an effect has been observed in many other species, including Acheta domesticus (Orthoptera: Gryllidae) and Drosophila melanogaster (Diptera: Drosophilidae) (Anderson et al. 2005; Lachenicht et al. (2010). We did not find an acclimation effect on heat knockdown time, but in the duration of recovery time from heat coma. This underlines the fact that recovery time and knockdown temperature are not necessarily interchangeable measures, but may give very different interpretations of thermal adaptation. A comprehensive exploration of the relationship between the two measures is called for.

Despite a 1 week acclimation period, respiration in M. soledadinus increased strongly with increasing temperature. The relationship between \( \dot{V}{\text{CO}}_{2} \) and temperature is roughly linear on a log scale, indicating that the measurement temperatures lie within the physiological temperature range (Knies and Kingsolver 2010), and that the highest measurement temperature is not physiologically stressful for these beetles. The Q 10 of 2.49 is comparable to typical acute temperature sensitivities of ectotherms, both at the whole organism and enzyme level (e.g. Glanville and Seebacher 2006) due to the exponential relationship between the rate of biochemical reactions and temperature (Arrhenius 1915). Because the metabolic rate of M. soledadinus conforms to Arrhenius kinetics in spite of acclimation, it appears that the response of this species does not show metabolic compensation. The metabolic rate of M. soledadinus is lower than that of similar-sized Coleoptera from sub-Antarctic climates (Sømme et al. 1989; Crafford and Chown 1993), which in turn is lower than that of temperate beetles (Keister and Buck 1974). Todd (1997) also found a higher respiratory metabolism in Trechisibus antarcticus than in M. soledadinus at any given temperature, both being introduced on South Georgia. These relatively low baseline metabolic costs may come at the expense of decreased maximal performance (Pörtner 2006), and are a hallmark of relatively stenothermic organisms.

Our study shows that the invasive beetle M. soledadinus on Kerguelen Island is only moderately cold tolerant, and exhibits relatively little thermal plasticity. The lower thermal limits of M. soledadinus are relatively close to the low temperatures experienced in winter on Kerguelen Island, but the upper thermal limits are well above summer conditions, suggesting that this species has abundant scope to deal with current climate change (Lebouvier et al. 2011). Furthermore, the thermal sensitivity we observe, alongside a relatively steep increase in metabolic rate with warm acclimation, suggest that recent warming on Kerguelen Island may be releasing this species from abiotic constraints. Indeed population size and geographical distribution of M. soledadinus on the island was relatively constrained until the 1980s, with a rapid expansion in the 1990s (Chevrier 1996). Recent field observations (2003–2010, D. Renault, personal observation) suggest that the densities of M. soledadinus are also considerably higher than that reported for 1992–1993 by Chevrier (1996). Given that M. soledadinus is an opportunistic predator, continued release from thermal constraints could allow considerable spread and increased impact on indigenous insect populations.