Polygenic adaptation contributes to the invasive success of the Colorado potato beetle

Effects of elicitors on plant host selection by adult Leptinotarsa decemlineata (Coleoptera: Chrysomelidae)

The Colorado Potato Beetle, Leptinotarsa decemlineata Say, is the principal defoliator of potato crops globally. It is well known for its propensity to rapidly develop resistance. Thus, new control options which are resilient to the pest's resistance capabilities are a critical need. The use of chemical ecology in integrated pest management (IPM) programs has been proposed as a means to delay resistance. Elicitors are chemical growth regulators that activate plant defenses. These plant defenses provide numerous opportunities to integrate chemical ecology into IPM programs, including changes to a plants volatile profile. In this laboratory study, we provide evidence that elicitors which mimic jasmonic acid (JA) and salicylic acid (SA) can be used to attract or repel L. decemlineata respectively. Adult beetles are highly attracted to potato plants sprayed with the JA mimicking elicitor Blush 2X, while plants sprayed with SA mimicking elicitor, Actigard 50WG, appear to be repellent. Additionally, residency time on plants sprayed with Actigard 50WG was significantly shorter than with control plants. The potential use of elicitors within IPM program is discussed.

Testing for adaptive changes linked to range expansion following a single introduction of the fall webworm

Adaptive evolution following colonization can affect the impact of invasive species. The fall webworm (FWW) invaded China 40 years ago through a single introduction event involving a severe bottleneck and subsequently diverged into two genetic groups. The well-recorded invasion history of FWW, coupled with a clear pattern of genetic divergence, provides an opportunity to investigate whether there is any sign of adaptive evolution following the invasion. Based on genome-wide SNPs, we identified genetically separated western and eastern groups of FWW and correlated spatial variation in SNPs with geographical and climatic factors. Geographical factors explained a similar proportion of the genetic variation across all populations compared with climatic factors. However, when the two population groups were analysed separately, environmental factors explained more variation than geographical factors. SNP outliers in populations of the western group had relatively stronger response to precipitation than temperature-related variables. Functional annotation of SNP outliers identified genes associated with insect cuticle protein potentially related to desiccation adaptation in the western group and genes associated with lipase biosynthesis potentially related to temperature adaptation in the eastern group. Our study suggests that invasive species may maintain the evolutionary potential to adapt to heterogeneous environments despite a single invasion event. The molecular data suggest that quantitative trait comparisons across environments would be worthwhile.

Rapid and Repeated Climate Adaptation Involving Chromosome Inversions following Invasion of an Insect

Following invasion, insects can become adapted to conditions experienced in their invasive range, but there are few studies on the speed of adaptation and its genomic basis. Here, we examine a small insect pest, Thrips palmi, following its contemporary range expansion across a sharp climate gradient from the subtropics to temperate areas. We first found a geographically associated population genetic structure and inferred a stepping-stone dispersal pattern in this pest from the open fields of southern China to greenhouse environments of northern regions, with limited gene flow after colonization. In common garden experiments, both the field and greenhouse groups exhibited clinal patterns in thermal tolerance as measured by critical thermal maximum (CTmax) closely linked with latitude and temperature variables. A selection experiment reinforced the evolutionary potential of CTmax with an estimated h2 of 6.8% for the trait. We identified 3 inversions in the genome that were closely associated with CTmax, accounting for 49.9%, 19.6%, and 8.6% of the variance in CTmax among populations. Other genomic variations in CTmax outside the inversion region were specific to certain populations but functionally conserved. These findings highlight rapid adaptation to CTmax in both open field and greenhouse populations and reiterate the importance of inversions behaving as large-effect alleles in climate adaptation.

Whole genome analyses reveal weak signatures of population structure and environmentally associated local adaptation in an important North American pollinator, the bumble bee Bombus vosnesenskii

Studies of species that experience environmental heterogeneity across their distributions have become an important tool for understanding mechanisms of adaptation and predicting responses to climate change. We examine population structure, demographic history and environmentally associated genomic variation in Bombus vosnesenskii, a common bumble bee in the western USA, using whole genome resequencing of populations distributed across a broad range of latitudes and elevations. We find that B. vosnesenskii exhibits minimal population structure and weak isolation by distance, confirming results from previous studies using other molecular marker types. Similarly, demographic analyses with Sequentially Markovian Coalescent models suggest that minimal population structure may have persisted since the last interglacial period, with genomes from different parts of the species range showing similar historical effective population size trajectories and relatively small fluctuations through time. Redundancy analysis revealed a small amount of genomic variation explained by bioclimatic variables. Environmental association analysis with latent factor mixed modelling (LFMM2) identified few outlier loci that were sparsely distributed throughout the genome and although a few putative signatures of selective sweeps were identified, none encompassed particularly large numbers of loci. Some outlier loci were in genes with known regulatory relationships, suggesting the possibility of weak selection, although compared with other species examined with similar approaches, evidence for extensive local adaptation signatures in the genome was relatively weak. Overall, results indicate B. vosnesenskii is an example of a generalist with a high degree of flexibility in its environmental requirements that may ultimately benefit the species under periods of climate change.

Rapid evolution of insecticide resistance in the Colorado potato beetle, Leptinotarsa decemlineata

Despite considerable research, efforts to manage insecticide resistance continue to fail. The Colorado potato beetle (CPB), Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae), epitomizes this problem, as it has repeatedly and rapidly evolved resistance to>50 insecticides. The patterns of resistance evolution are intriguing, as they defy models where resistance evolves from rare mutations. Here, we synthesize recent research on insecticide resistance in CPB showing that polygenic resistance drawn from standing genetic diversity explains genomic patterns of insecticide resistance evolution. However, rapid gene regulatory evolution suggests that other mechanisms might also facilitate adaptive change. We explore the hypothesis that sublethal stress from insecticide exposure could alter heritable epigenetic modifications, and discuss the range of experimental approaches needed to fully understand insecticide resistance evolution in this super pest.