Respiration-based monitoring of metabolic rate following cold-exposure in two invasive Anoplophora species depending on acclimation regime.

The Asian and Citrus longhorned beetles, Anoplophora glabripennis (ALB) and A. chinensis (CLB) respectively, are two closely related invasive species with overlapping native ranges. Although both species have rather similar biological characteristics, they differ in their invasion patterns. ALB shows numerous, but local, outbreaks in urban areas of North-East America, Western and Central Europe, whereas CLB has colonized a large part of Northern Italy. Temperature is pivotal in setting distribution limits of ectotherms. Low temperature may be limiting for larvae since they are the main overwintering stage for both species. To investigate whether differential cold tolerance may contribute to setting the respective limits of the range invaded by each species, we monitored larval metabolic rate before and after exposure to a one-week ecologically relevant moderate cold stress (-2/+2°C, 14/10h). We tested two distinctive fluctuating regimes before the cold exposure to check whether larval acclimation significantly altered their cold tolerance. Survival was high in all conditions for both species. Visual examination showed temporary locomotor inactivity during the stress but respiration rates were not altered after the stress suggesting that larvae could rapidly resume their initial metabolic activity. The respiration rate was globally higher in ALB than in CLB. Together, these results tend to indicate that both species have similar tolerance to the moderate cold stress tested, but also that ALB may be better at maintaining metabolic activity at cold than CLB. These observed differences could affect phenology in both species and in turn their establishment potential.

Metabolic fingerprinting of the responses to salinity in the invasive ground beetle Merizodus soledadinus at the Kerguelen Islands.

Salinity is an abiotic factor that may impact survival and fitness of terrestrial insects in coastal environments. Meanwhile, some terrestrial arthropods can survive in hypersaline environments, and counterbalance osmotic stress by intra- and extracellular buildups of organic osmolytes. The ground beetle Merizodus soledadinus originates from South America and it is distributed in forests and riparian zones, where salinity levels are considerably low. This species has been introduced at the Kerguelen Islands a century ago, where it colonized coastal areas (tide drift lines), and must thus withstand salinity variations due to tide, spray, and organic matter deposited therein. In the present study, we addressed the physiological plasticity of M. soledadinus to saline conditions, by monitoring body water content and survival in adults experimentally subjected to different salinities. We also investigated possible metabolic adjustments involved at three contrasted salinity levels (0‰, 35‰, 70‰) at 4 and 8°C. We hypothesized that this invasive ground beetle can withstand a broad range of salinity conditions thanks to the plastic accumulation of compatible solutes. The study revealed a progressive drop in body water content in individuals exposed to 35‰ and 70‰, as opposed to the controls. Metabolic fingerprints showed compatible solute (erythritol, alanine, glycine and proline) accumulation at medium and high salinity conditions (35‰ and 70‰). We concluded that the osmo-induced accumulation of amino acids and polyols was likely to modulate the ground beetles' body water balance on medium saline substrates, thus enhancing their survival ability.

Habitat phenotyping of two sub-Antarctic flies by metabolic fingerprinting: evidence for a species outside its home?

Metabolic fingerprinting can elucidate rearrangements of metabolic networks in organisms exposed to various environmental conditions. Maintenance of organismal performance occurs by alterations in metabolic fluxes and pathways, resulting in habitat-specific metabolic signatures. Several insects of sub-Antarctic Islands, including the wingless flies Anatalanta aptera and Calycopteryx moseleyi, are exposed to saline organic matter accumulated along littoral margins. However, C. moseleyi has long been considered restricted to a habitat of lower salinity, the Kerguelen cabbage. High C. moseleyi densities identified in saline decaying seaweeds are intriguing, and may involve osmoregulatory adjustments including accumulation of osmoprotectants. In the present work, we examined quantitative metabotypes (metabolic phenotypes) among wild C. moseleyi individuals from seaweeds versus non-saline Kerguelen cabbages. They were compared to metabotypes from wild A. aptera, a common fly on seaweed. Statistical procedures designed to magnify between-class differences failed to clearly separate C. moseleyi metabotypes from cabbage and seaweed, despite contrasted morphotypes, diets, and salinities. A. aptera exhibited higher glycerol, inositol, trehalose, and other osmoprotectants concentrations that may enhance its performance under saline environments. Seaweed may represent a secondary niche in C. moseleyi, promoted by the marked reduction in Kerguelen cabbage frequency subsequent to climate change, and herbivorous pressures caused by rabbit invasion.

Starvation resistance and effects of diet on energy reserves in a predatory ground beetle (Merizodus soledadinus; Carabidae) invading the Kerguelen Islands.

The relationship between nutritional requirements and the availability or quality of food is a prime parameter in determining the geographical expansion of invasive insects. At the sub-Antarctic Kerguelen Islands, the invasive ground beetle Merizodus soledadinus becomes the main invertebrate predator when it colonizes new habitats, leading to the local extinction of native fly species. Such changes in the structure of prey communities may alter the energy management (storage and expenditure) of this predator. In this species, we monitored survival and body mass during food deprivation, in addition to evaluating the effects of two distinct diets (maggots versus enchytraeids) on the consumption and restoration of body reserves (sugars and triglycerides). We found that adults can starve for more than 60 days, and feed every 3.76 days on average when food is available. We recorded higher predation rates on maggots, associated with steeper body mass variations, compared to enchytraeids. Sugars and triglycerides were significantly consumed during food deprivation and restored after refeeding, but varied similarly among individuals supplied on the distinct diets. Other parameters may determine the food preferences observed, such as salt content in prey tissues, because M. soledadinus mainly feeds in hypersaline foreshore habitats, and may limit the consumption of osmotic conformers.