Effect of laurel wilt invasion on redbay populations in a maritime forest community

Comparative Meta-analysis Effects of Nonnative Ants (Hymenoptera: Formicidae), Ground Beetles (Coleoptera: Carabidae), and Bark and Ambrosia Beetles (Coleoptera: Curculionidae) on Native Confamilials

Nonnative species often transform local communities to the detriment of native species. Much of the existing invasion ecology research focuses on the effects of a few extremely impactful species, and it is less clear how nonnative species which are not causing economic or ecological impacts alter closely related natives at risk of being displaced. Filling these knowledge gaps is critical because consequences of nonnative species are likely to vary depending on taxonomic scale, functional trait, and spatial or temporal niche. We conducted a meta-analysis to evaluate how biodiversity of native Formicidae (ants), Carabidae (ground beetles), and Scolytinae (bark and ambrosia beetles) species changes across a gradient of pressure from nonnative confamilials. We calculated Hill numbers for each group from data presented in literature and correlated native diversity metrics to proportion of nonnative species. Species richness of native ants was significantly negatively correlated with proportions of nonnative ants, whereas bark and ambrosia beetle metrics showed a nonsignificant negative correlation. Nonnative ground beetles had neutral effects on diversity of native ground beetles. Resulting contrasting patterns of invasive species effects on natives suggest complex biotic and abiotic factors driving effects of nonnative species in these groups. Our results suggest that a few extreme examples (e.g., red imported fire ants) drive most of the changes seen in native arthropod communities. To accurately assess impacts of invaders on native arthropod diversity, baseline data are needed, and community analyses must consider diverse functional traits of native taxa and improve the depth and breadth of community sampling.

Laurel Wilt: Current and Potential Impacts and Possibilities for Prevention and Management

In recent years, outbreaks of nonnative invasive insects and pathogens have caused significant levels of tree mortality and disturbance in various forest ecosystems throughout the United States. Laurel wilt, caused by the pathogen Raffaelea lauricola (T.C. Harr., Fraedrich and Aghayeva) and the primary vector, the redbay ambrosia beetle (Xyleborus glabratus Eichhoff), is a nonnative pest-disease complex first reported in the southeastern United States in 2002. Since then, it has spread across eleven southeastern states to date, killing hundreds of millions of trees in the plant family Lauraceae. Here, we examine the impacts of laurel wilt on selected vulnerable Lauraceae in the United States and discuss management methods for limiting geographic expansion and reducing impact. Although about 13 species belonging to the Lauraceae are indigenous to the United States, the highly susceptible members of the family to laurel wilt are the large tree species including redbay (Persea borbonia (L.) Spreng) and sassafras (Sassafras albidum (Nutt.) Nees), with a significant economic impact on the commercial production of avocado (Persea americana Mill.), an important species native to Central America grown in the United States. Preventing new introductions and mitigating the impact of previously introduced nonnative species are critically important to decelerate losses of forest habitat, genetic diversity, and overall ecosystem value.

Use of Semiochemicals for the Management of the Redbay Ambrosia Beetle

Simple Summary

Laurel wilt is a devastating exotic fungal disease that threatens avocado and related members of the laurel family in North America. This disease has killed over 300 million redbay trees and has caused cascading ecological impacts across the landscape. Management strategies, especially in natural forests, are limited. The ambrosia beetles that vector this disease respond strongly to odors produced by the trees, and our research indicates that it is possible to repel the beetles away from trees in a forest setting with the use of verbenone. Other compounds have been identified that can be used to trap the beetles. If used together, these strategies can be used to develop a single “push-pull” system to manage this disease in natural systems where other management strategies are not feasible.

Abstract

This review highlights current advances in the management of the redbay ambrosia beetle, Xyleborus glabratus, a primary vector of the pathogenic fungus, Raffaelea lauricola, that causes laurel wilt. Laurel wilt has a detrimental effect on forest ecosystems of southeastern USA, with hundreds of millions of Lauraceae deaths. Currently, preventive measures mostly focus on infected-tree removal to potentially reduce local beetle populations and/or use of preventative fungicide applications in urban trees. Use of semiochemicals may offer an opportunity for the management of X. glabratus. Research on attractants has led to the development of α-copaene lures that are now the accepted standards for X. glabratus sampling. Research conducted on repellents first included methyl salicylate and verbenone and attained significant reduction in the number of X. glabratus captured on redbay and swamp bay trees treated with verbenone. However, the death rate of trees protected with verbenone, while lower compared to untreated trees, is still high. This work underscores the necessity of developing new control methods, including the integration of repellents and attractants into a single push-pull system.

Genomic and transcriptomic insights into Raffaelea lauricola pathogenesis

BACKGROUND

Laurel wilt caused by Raffaelea lauricola is a lethal vascular disease of North American members of the Lauraceae plant family. This fungus and its primary ambrosia beetle vector Xyleborus glabratus originated from Asia; however, there is no report of laurel wilt causing widespread mortality on native Lauraceae trees in Asia. To gain insight into why R. lauricola is a tree-killing plant pathogen in North America, we generated and compared high quality draft genome assemblies of R. lauricola and its closely related non-pathogenic species R. aguacate.

RESULTS

Relative to R. aguacate, the R. lauricola genome uniquely encodes several small-secreted proteins that are associated with virulence in other pathogens and is enriched in secondary metabolite biosynthetic clusters, particularly polyketide synthase (PKS), non-ribosomal peptide synthetase (NRPS) and PKS-NRPS anchored gene clusters. The two species also exhibit significant differences in secreted proteins including CAZymes that are associated with polysaccharide binding including the chitin binding CBM50 (LysM) domain. Transcriptomic comparisons of inoculated redbay trees and in vitro-grown fungal cultures further revealed a number of secreted protein genes, secondary metabolite clusters and alternative sulfur uptake and assimilation pathways that are coordinately up-regulated during infection.

CONCLUSIONS

Through these comparative analyses we have identified potential adaptations of R. lauricola that may enable it to colonize and cause disease on susceptible hosts. How these adaptations have interacted with co-evolved hosts in Asia, where little to no disease occurs, and non-co-evolved hosts in North America, where lethal wilt occurs, requires additional functional analysis of genes and pathways.

A degradation debt? Large-scale shifts in community composition and loss of biomass in a tropical forest fragment after 40 years of isolation

Habitat loss and fragmentation are among the biggest threats to tropical biodiversity and associated ecosystem services. We examined forest dynamics in a mid-elevation 365-ha fragment in southern Costa Rica. The fragment was isolated in the mid-1970s and belongs to the Las Cruces Biological Station. A 2.25-ha permanent plot was established in the center of the old-growth forest (>400 m to nearest edge boundary) and all plants >5 cm DBH were censused, mapped, and identified to species in two surveys taken ~5–6 years apart (>3,000 stems/survey). Although the reserve maintains high species richness (>200 spp.), with many rare species represented by only one individual, we document a strong shift in composition with a two-fold increase in the number of soft-wooded pioneer individuals. The dominant late-successional understory tree species, Chrysochlamys glauca (Clusiaceae), and most species in the Lauraceae, declined dramatically. Turnover was high: 22.9% of stems in the first survey were lost, and 27.8% of stems in the second survey represented new recruits. Mean tree diameter decreased significantly and there was a 10% decrease in overall biomass. Such alteration has been documented previously but only in smaller fragments or within ~100 m of an edge boundary. Further penetration into this fragment was perhaps driven by a progressive invasion of disturbance-adapted species into the fragment’s core over time; the loss of once-dominant late successional species could be a contributing factor. The pattern found is of particular concern given that such fragments represent a substantial portion of today’s remaining tropical habitat; further studies in similar-sized fragments that have been isolated for similar prolonged periods are called for.

Twelve years of repeated wild hog activity promotes population maintenance of an invasive clonal plant in a coastal dune ecosystem

Invasive animals can facilitate the success of invasive plant populations through disturbance. We examined the relationship between the repeated foraging disturbance of an invasive animal and the population maintenance of an invasive plant in a coastal dune ecosystem. We hypothesized that feral wild hog (Sus scrofa) populations repeatedly utilized tubers of the clonal perennial, yellow nutsedge (Cyperus esculentus) as a food source and evaluated whether hog activity promoted the long‐term maintenance of yellow nutsedge populations on St. Catherine's Island, Georgia, United States. Using generalized linear mixed models, we tested the effect of wild hog disturbance on permanent sites for yellow nutsedge culm density, tuber density, and percent cover of native plant species over a 12‐year period. We found that disturbance plots had a higher number of culms and tubers and a lower percentage of native live plant cover than undisturbed control plots. Wild hogs redisturbed the disturbed plots approximately every 5 years. Our research provides demographic evidence that repeated foraging disturbances by an invasive animal promote the long‐term population maintenance of an invasive clonal plant. Opportunistic facultative interactions such as we demonstrate in this study are likely to become more commonplace as greater numbers of introduced species are integrated into ecological communities around the world.

Influence of Trap Distance From a Source Population and Multiple Traps on Captures and Attack Densities of the Redbay Ambrosia Beetle (Coleoptera: Curculionidae: Scolytinae)

The redbay ambrosia beetle, Xyleborus glabratus Eichhoff, is the principal vector of laurel wilt disease in North America. Lures incorporating essential oils of manuka plants ( Leptospermum scoparium J. R. Forster & G. Forster) or cubeb seeds ( Piper cubeba L.f.) are the most effective in-flight attractants to date. Using grids of traps baited with these essential oil lures, we evaluated 1) the effect of trap distance from a source beetle population on beetle captures, 2) the feasibility of trapping out low-density beetle populations, and 3) the effect of trap spacing on beetle captures. In the first experiment, increasing trap distance up to 300 m from a source X. glabratus population had little effect on beetle captures. In a second experiment conducted in a study area with very low beetle densities, trapping for 5 mo prior to deploying freshly cut, uninfested redbay bolts had no effect on subsequent attack densities. In a third experiment, numbers of X. glabratus captured in traps in the center of a grid of nine traps spaced 1 or 5 m apart were compared with lone baited or unbaited traps 30 m away. Relative to the more distant traps, the grid of baited traps neither increased captures in the unbaited center trap nor decreased captures in the baited center trap, regardless of spacing. The results suggest that the effective trapping distance of essential oil lures for X. glabratus is <1 m, and that newer, more attractive lures will be needed to be useful in managing X. glabratus populations.

Orchid-pollinator interactions and potential vulnerability to biological invasion

Mutualistic relationships between plants and their pollinators have played a major role in the evolution of biodiversity. While the vulnerability of these relationships to environmental change is a major concern, studies often lack a framework for predicting impacts from emerging threats (e.g. biological invasions). The objective of this study was to determine the reliance of Platanthera ciliaris (orange-fringed orchid) on Papilio palamedes (Palamedes swallowtail butterfly) for pollination and the relative availability of alternative pollinators. Recent declines of P. palamedes larval host plants due to laurel wilt disease (LWD) could endanger P. ciliaris populations that rely heavily on this butterfly for pollination. We monitored pollinator visitation and fruit set and measured nectar spur lengths of P. ciliaris flowers and proboscis lengths of its floral visitors in Jackson County, MS, USA. Papilio palamedes was the primary visitor with minimal visitation by Phoebis sennae (cloudless sulfur butterfly). Lengths of P. ciliaris nectar spurs were similar to proboscis lengths of both pollinator species. Fruit set was moderate with access to pollinators (55 ± 10.8 %), yet failed (0 %) when pollinators were excluded. Visitation increased with inflorescence size, but there was no such pattern in fruit set, indicating that fruit set was not limited by pollinator visitation within the range of visitation rates we observed. Our results are supported by historical data that suggest P. palamedes and P. sennae are important pollinators of P. ciliaris. Although P. sennae may provide supplemental pollination service, this is likely constrained by habitat preferences that do not always overlap with those of P. cilaris. Observed declines of P. palamedes due to LWD could severely limit the reproductive success and persistence of P. ciliaris and similar orchid species populations. This empirical-based prediction is among the first to document exotic forest pests and pathogens as an indirect threat to plant-pollinator interactions.

Attraction of Redbay Ambrosia Beetle, Xyleborus Glabratus, To Leaf Volatiles of its Host Plants in North America

The redbay ambrosia beetle, Xyleborus glabratus, is an important pest of redbay (Persea borbonia) and swamp bay (P. palustris) trees in forests of the southeastern USA. It is also a threat to commercially grown avocado. The beetle is attracted to host wood volatiles, particularly sesquiterpenes. Contrary to other ambrosia beetles that attack stressed, possibly pathogen-infected, and dying trees, X. glabratus readily attacks healthy trees. To date little is known about the role of leaf volatiles in the host selection behavior and ecology of X. glabratus. To address this question, an olfactometer bioassay was developed to test the behavioral response of X. glabratus to plant leaf volatiles. We found that X. glabratus was attracted to the leaf odors of their hosts, redbay and swamp bay, with no attraction to a non-host tree tested (live oak, Quercus virginiana), which served as a negative control. Gas chromatography-mass spectrometry (GS/MS) analysis of leaves revealed the absence of sesquiterpenes known to be attractive to X. glabratus and present in host wood, suggesting that additional leaf-derived semiochemicals may serve as attractants for this beetle. An artificial blend of chemicals was developed based on GC/MS analyses of leaf volatiles and behavioral assays. This blend was attractive to X. glabratus at a level that rivaled currently used lures for practical monitoring of this pest. This synthetic redbay leaf blend also was tested in the field. Baited traps captured more X. glabratus than unbaited controls and equivalently to manuka oil lures. We hypothesize that leaf volatiles may be used by X. glabratus as an additional cue for host location.