Southern pine beetle-specific RNA interference exhibits no effect on model nontarget insects

The efficacy and high specificity of the RNA interference pathway has prompted its exploration as a potential molecular management tool for many insect pests, including the destructive southern pine beetle, Dendroctonus frontalis Zimmermann, in which gene knockdown and mortality via double-stranded RNAs (dsRNAs) have already been demonstrated in the laboratory. The nucleotide sequence of dsRNAs requires an exact match of at least 16 nucleotides with the targeted messenger RNA to trigger knockdown of that gene. This allows vital genes in a target pest to be silenced and mortality induced while reducing the probability of adverse effects in nontarget organisms. However, prior to utilization in forest ecosystems, demonstration of the specificity of dsRNAs through laboratory bioassays evaluating potential nontarget effects on model insects is required for proper risk assessment analyses. Consequently, we evaluated three SPB-specific dsRNAs for lethal effects, sublethal effects (larval growth rate, adult emergence or adult fecundity), and relative gene expression in three model nontarget insects representing key functional guilds, including a predator, herbivore, and pollinator. The SPB-specific dsRNAs had no effect on survival of our nontarget insects. Additionally, no sublethal effects were found and the gene expression analyses corroborated bioinformatic analyses in finding no gene knockdown. Our findings support the high specificity of RNAi technology and provide support for its development and deployment for protection of conifer forests against SPB with minimal nontarget concerns.

Weak spatial-genetic structure in a native invasive, the southern pine beetle (Dendroctonus frontalis), across the eastern United States

The southern pine beetle, Dendroctonus frontalis, is a native pest of pine trees that has recently expanded its range into the northeastern United States. Understanding its colonization, dispersal, and connectivity will be critical for mitigating negative economic and ecological impacts in the newly invaded areas. Characterization of spatial-genetic structure can contribute to this; however, previous studies have reached different conclusions about regional population genetic structure, with one study reporting a weak east-west pattern, and the most recent reporting an absence of structure. Here we systematically assessed several explanations for the absence of spatial-genetic structure. To do this, we developed nine new microsatellite markers and combined them with an existing 24-locus data matrix for the same individuals. We then reanalyzed this full dataset alongside datasets in which certain loci were omitted with the goal of creating more favorable signal to noise ratios. We also partitioned the data based on the sex of D. frontalis individuals, and then employed a broad suite of genotypic clustering and isolation-by-distance (IBD) analyses. We found that neither inadequate information content in the molecular marker set, nor unfavorable signal-to-noise ratio, nor insensitivity of the analytical approaches could explain the absence of structure. Regardless of dataset composition, there was little evidence for clusters (i.e., distinct geo-genetic groups) or clines (i.e., gradients of increasing allele frequency differences over larger geographic distances), with one exception: significant IBD was repeatedly detected using an individual-based measure of relatedness whenever datasets included males (but not for female-only datasets). This is strongly indicative of broad-scale female-biased dispersal, which has not previously been reported for D. frontalis, in part owing to logistical limitations of direct approaches (e.g., capture-mark-recapture). Weak spatial-genetic structure suggests long-distance connectivity and that gene flow is high, but additional research is needed to understand range expansion and outbreak dynamics in this species using alternate approaches.

Revised and Potential New Tactics for the Suppression of Southern Pine Beetle Infestations

Cut-and-remove has been the recommended suppression technique for the suppression of infestations of the southern pine beetle (SPB), Dendroctonus frontalis Zimmermann (Coleoptera: Curculionidae), with cut-and-leave applied when conditions prevent removal of the felled pines. Changes in forest management practices and the timber industry, plus the expansion of SPB northward have led to an increased reliance on cut-and-leave in recent years. This change has resulted in an exploration of methods to improve the efficacy of cut-and-leave and the development of new tactics for suppressing SPB infestations. We describe these revised and new techniques and discuss their potential applications given current forest management conditions.

Semiochemicals for management of the southern pine beetle (Coleoptera: Curculionidae: Scolytinae): successes, failures, and obstacles to progress

The southern pine beetle, Dendroctonus frontalis Zimmermann (Coleoptera: Curculionidae: Scolytinae), is among the most destructive bark beetle pests of pines (Pinaceae) of the southeast and mid-Atlantic United States of America, Mexico, and Central America. Numerous volatile compounds can stimulate or reduce attraction of the beetle, but efforts to incorporate these into effective, practical technologies for pest management have yielded mixed results. Attractants have been incorporated into lures used in monitoring traps that are employed operationally to forecast outbreaks and detect emerging populations. The attraction inhibitor, verbenone, shows efficacy for suppressing southern pine beetle infestations but has not yet been adopted operationally. No effective semiochemical tree protectant has been developed for the beetle. We discuss complexities in the chemical ecology of the beetle that likely have impeded research and development of semiochemical management tools, and we describe basic science gaps that may hinder further progress if not addressed. We also report some supporting, original experimental data indicating (1) that a verbenone device can inhibit the beetle’s response to sources of attractant in a radius of at least several metres, (2) similar olfactory responses by the beetle to both enantiomers of verbenone, and (3) that pheromone background can cause conflicting results in semiochemical field tests.

Contrasting Competitor and Predator Responses to a Short-Lived Southern Pine Beetle Outbreak: A Case Study

The southern pine beetle (SPB) Dendroctonus frontalis Zimmermann, is the most important insect pest of pines in the southeastern United States, with outbreaks often resulting in thousands of hectares of pine mortality. Natural enemies and competitors have been cited as significant regulators of SPB populations and, therefore, outbreaks. A recent outbreak on the Homochitto National Forest (NF) in Mississippi provided an opportunity to undertake a case study comparing population fluctuations of SPB, its major predator Thanasimus dubius, and its competitors, Ips bark beetles. Trap catches of all three were tracked through the course of the outbreak on the Homochitto NF as well as in two other forests with low or no SPB activity. The number of predators collected initially increased on the Homochitto NF in response to the SPB outbreak, but their impact on reducing infestation numbers was unclear. Numbers of Ips trapped were similar across all three forests, indicating that other factors were regulating SPB populations. The outbreak only lasted a single year, and its brevity likely limited the availability of host resources for natural enemy and competitor populations. Additional studies are warranted to explore the mechanisms affecting the extent and duration of SPB outbreaks, such as active forest management. Study Implications: The widespread application of cut-and-leave treatments during a short duration southern pine beetle (SPB) outbreak on the Homochitto National Forest did not result in immediate increased collections of Ips bark beetles, competitors of SPB. Similar population trends of Ips bark beetles in areas with and without SPB infestations suggest that other factors, such as climate, regulate population numbers. Low numbers of the main SPB predator, Thanasimus dubius, immediately preceded an outbreak, and trap collections increased after outbreak onset, reaffirming the importance of this natural enemy in SPB population fluctuations.