Native ecotypic variation and the role of host identity in the spread of an invasive herbivore, Cactoblastis cactorum

Increased acclimation ability accompanies a thermal niche shift of a recent invasion

Globalization is removing dispersal barriers for the establishment of invasive species and enabling their spread to novel climates. New thermal environments in the invaded range will be particularly challenging for ectotherms, as their metabolism directly depends on environmental temperature. However, we know little about the role climatic niche shifts play in the invasion process, and the underlining physiological mechanisms. We tested if a thermal niche shift accompanies an invasion, and if native and introduced populations differ in their ability to acclimate thermal limits. We used an alien ant species-Tapinoma magnum-which recently started to spread across Europe. Using occurrence data and accompanying climatic variables, we measured the amount of overlap between thermal niches in the native and invaded range. We then experimentally tested the acclimation ability in native and introduced populations by incubating T. magnum at 18, 25 and 30°C. We measured upper and lower critical thermal limits after 7 and 21 days. We found that T. magnum occupies a distinct thermal niche in its introduced range, which is on average 3.5°C colder than its native range. Critical thermal minimum did not differ between populations from the two ranges when colonies were maintained at 25 or 30°C, but did differ after colony acclimation at a lower temperature. We found twofold greater acclimation ability of introduced populations to lower temperatures, after prolonged incubation at 18°C. Increased acclimation ability of lower thermal limits could explain the expansion of the realized thermal niche in the invaded range, and likely contributed to the spread of this species to cooler climates. Such thermal plasticity could be an important, yet so far understudied, factor underlying the expansion of invasive insects into novel climates.

Evolutionary history predicts high-impact invasions by herbivorous insects

A long-standing goal of invasion biology is to identify factors driving highly variable impacts of non-native species. Although hypotheses exist that emphasize the role of evolutionary history (e.g., enemy release hypothesis & defense-free space hypothesis), predicting the impact of non-native herbivorous insects has eluded scientists for over a century.Using a census of all 58 non-native conifer-specialist insects in North America, we quantified the contribution of over 25 factors that could affect the impact they have on their novel hosts, including insect traits (fecundity, voltinism, native range, etc.), host traits (shade tolerance, growth rate, wood density, etc.), and evolutionary relationships (between native and novel hosts and insects).We discovered that divergence times between native and novel hosts, the shade and drought tolerance of the novel host, and the presence of a coevolved congener on a shared host, were more predictive of impact than the traits of the invading insect. These factors built upon each other to strengthen our ability to predict the risk of a non-native insect becoming invasive. This research is the first to empirically support historically assumed hypotheses about the importance of evolutionary history as a major driver of impact of non-native herbivorous insects.Our novel, integrated model predicts whether a non-native insect not yet present in North America will have a one in 6.5 to a one in 2,858 chance of causing widespread mortality of a conifer species if established (R 2 = 0.91) Synthesis and applications. With this advancement, the risk to other conifer host species and regions can be assessed, and regulatory and pest management efforts can be more efficiently prioritized.

Assessing the Risk of Invasion by Tephritid Fruit Flies: Intraspecific Divergence Matters

Widely distributed species often show strong phylogeographic structure, with lineages potentially adapted to different biotic and abiotic conditions. The success of an invasion process may thus depend on the intraspecific identity of the introduced propagules. However, pest risk analyses are usually performed without accounting for intraspecific diversity. In this study, we developed bioclimatic models using MaxEnt and boosted regression trees approaches, to predict the potential distribution in Europe of six economically important Tephritid pests (Ceratitis fasciventris (Bezzi), Bactrocera oleae (Rossi), Anastrepha obliqua (Macquart), Anastrepha fraterculus (Wiedemann), Rhagoletis pomonella (Walsh) and Bactrocera cucurbitae (Coquillet)). We considered intraspecific diversity in our risk analyses by independently modeling the distributions of conspecific lineages. The six species displayed different potential distributions in Europe. A strong signal of intraspecific climate envelope divergence was observed in most species. In some cases, conspecific lineages differed strongly in potential distributions suggesting that taxonomic resolution should be accounted for in pest risk analyses. No models (lineage- and species-based approaches) predicted high climatic suitability in the entire invaded range of B. oleae—the only species whose intraspecific identity of invading populations has been elucidated—in California. Host availability appears to play the most important role in shaping the geographic range of this specialist pest. However, climatic suitability values predicted by species-based models are correlated with population densities of B. oleae globally reported in California. Our study highlights how classical taxonomic boundaries may lead to under- or overestimation of the potential pest distributions and encourages accounting for intraspecific diversity when assessing the risk of biological invasion.

Behavioral assays for studies of host plant choice and adaptation in herbivorous insects

The association of insect herbivores with their host plants is influenced by behaviors governing acceptance of those plants for feeding and oviposition. Behavioral changes accompany and may even precede host range expansion. Characterization and quantification of specific behaviors often form the basis of studies on host plant adaptation and chemical ecology. Behavioral assays of insects are usually designed to measure attraction for feeding or oviposition in relation to their host plants or specific chemistry. We review behavioral assays of insect herbivores with host plants or the volatiles they emit, with special consideration given to design, analysis, and interpretation to maximize ecological relevance. A toolkit of robust assays can help address fundamental issues at the intersection of ecology and evolution, such as the underpinnings of plant-insect interactions and the identification of genes involved in host race formation.