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.

Mountain Pine Beetle Impacts on Health through Lost Forest Air Pollutant Sinks

The mountain pine beetle (MPB) destroys millions of coniferous trees annually throughout Western US forests. Coniferous forests are important air pollutant sinks, removing pollutants from the air such as PM2.5 (particulate matter < 2.5 μm in diameter), O3 (ozone), SO2 (sulfur dioxide), NO2 (nitrogen dioxide), and CO (carbon monoxide). In this paper, US Forest Service data on MPB tree mortality in the Western US is combined with a forest air pollution model (i-Tree Eco) and standard health impact functions to assess the human mortality and morbidity impacts of MPB-induced tree mortality. Modeling results suggest considerable spatial and temporal heterogeneity of impacts across the Western US. On average, MPB is associated with 10.0–15.7 additional deaths, 6.5–40.4 additional emergency room (ER) visits, and 2.2–10.5 additional hospital admissions per year over 2005–2011 due to lost PM2.5 sinks. For every 100 trees killed by MPB, the average PM2.5 mortality health costs are $418 (2019$). Impacts on other criteria pollutants are also estimated. Several sensitivity checks are performed on model inputs. These results have important policy implications for MPB management and on our understanding of the complex couplings between forest pests, forest health, and human health.

Tree Mortality following Thinning and Prescribed Burning in Eastern Oregon, U.S.

We examined causes and levels of tree mortality one year after thinning and prescribed burning was completed in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) forests at Pringle Falls Experimental Forest, Oregon, U.S. Four blocks of five experimental units (N = 20) were established. One of each of five treatments was assigned to each experimental unit in each block. Treatments included thinning from below to the upper management zone (UMZ) for the dominant plant association based on stand density index values for ponderosa pine followed by mastication and prescribed burning: (1) 50% UMZ (low density stand), (2) 75% UMZ (medium density stand), (3) 75% UMZ Gap, which involved a regeneration cut, (4) 100% UMZ (high density stand), and (5) an untreated control (high density stand). Experimental units were thinned in 2011 (block 4), 2012 (block 2), and 2013 (blocks 1 and 3); masticated within one year; and prescribed burned two years after thinning (2013–2015). A total of 395,053 trees was inventoried, of which 1.1% (4436) died. Significantly higher levels of tree mortality occurred on 100 UMZ (3.1%) than the untreated control (0.05%). Mortality was attributed to prescribed fire (2706), several species of bark beetles (Coleoptera: Curculionidae) (1592), unknown factors (136), windfall (1 tree), and western gall rust (1 tree). Among bark beetles, tree mortality was attributed to western pine beetle (Dendroctonus brevicomis LeConte) (881 trees), pine engraver (Ips pini (Say)) (385 trees), fir engraver (Scolytus ventralis LeConte) (304 trees), mountain pine beetle (D. ponderosae Hopkins) (20 trees), Ips emarginatus (LeConte) (1 tree), and Pityogenes spp. (1 tree).

Identification of genes and gene expression associated with dispersal capacity in the mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae)

Dispersal flights by the mountain pine beetle have allowed range expansion and major damage to pine stands in western Canada. We asked what the genetic and transcriptional basis of mountain pine beetle dispersal capacity is. Using flight mills, RNA-seq and a targeted association study, we compared strong-flying, weak-flying, and non-flying female beetles from the recently colonized northern end of their range. Nearly 3,000 genes were differentially expressed between strong and weak flying beetles, while weak fliers and nonfliers did not significantly differ. The differentially expressed genes were mainly associated with lipid metabolism, muscle maintenance, oxidative stress response, detoxification, endocrine function, and flight behavior. Three variant loci, two in the coding region of genes, were significantly associated with flight capacity but these genes had no known functional link to flight. Several differentially expressed gene systems may be important for sustained flight, while other systems are downregulated during dispersal and likely to conserve energy before host colonization. The candidate genes and SNPs identified here will inform further studies and management of mountain pine beetle, as well as contribute to understanding the mechanisms of insect dispersal flights.