Quantifying Corticolous Arthropods Using Sticky Traps

Summary

We describe a semi-quantitative approach of measuring characteristics of corticolous (bark-dwelling) arthropod communities. We placed commercially manufactured sticky traps on tree boles to estimate abundance, total length (a surrogate to biomass), richness, and Shannon diversity for comparison among tree species.

Abstract

Terrestrial arthropods play an important role in our environment. Quantifying arthropods in a way that allows for a precise index or estimate of density requires a method with high detection probability and a consistent sampling area. We used manufactured sticky traps to compare abundance, total length (a surrogate for biomass), richness, and Shannon diversity of corticolous arthropods among the boles of 5 tree species. Efficacy of this method was adequate to detect variation in corticolous arthropods among tree species and provide a standard error of the mean that was <20% of the mean for all estimates with sample sizes from 7 to 15 individual trees of each species. Our results indicate, even with these moderate sample sizes, the level of precision of arthropod community metrics produced with this approach is adequate to address most ecological questions regarding temporal and spatial variation in corticolous arthropods. Results from this method differ from other quantitative approaches such as chemical knockdown, visual inspection, and funnel traps in that they provide an indication of corticolous arthropod activity over a relatively long-term, better including temporary bole residents, flying arthropods that temporarily land on the tree bole and crawling arthropods that use the tree bole as a travel route from the ground to higher forest foliage. Furthermore, we believe that commercially manufactured sticky traps provide more precise estimates and are logistically simpler than the previously described method of directly applying a sticky material to tree bark or applying a sticky material to tape or other type of backing and applying that to the tree bark.

Introduction

Terrestrial arthropods play an important role in our environment. In addition to being of scientific interest in their own right, arthropods can be both detrimental and beneficial to other trophic levels (i.e., crops, horticultural plants, native vegetation, and food for insectivorous organisms^1,2,3,4). Thus, understanding the factors that influence arthropod community development and abundance is critical to farmers⁵, pest control managers⁶, foresters⁴, plant biologists⁷, entomologists⁸, and wildlife and conservation ecologists that study community dynamics and manage insectivorous organisms⁹. Arthropod communities vary in species composition and abundance both temporally and spatially across a variety of ecological landscapes including plant communities, plant species, and across various regions of individual plants. For example, studies have demonstrated significant differences in arthropod community metrics between the roots, bole and stems, and foliage, within the same individual tree^10,11. These findings are not surprising considering that different parts of the same plant, e.g., leaves versus barks of a tree, provide different resources for which arthropods have adapted to exploit. Thus, each part of the plant can support a different arthropod community. Because foliage dwelling arthropods can have such a large socioeconomic and environmental impact, substantial effort has been expended to measure community metrics using both qualitative and quantitative approaches¹². Alternatively, much less effort has been expended to develop approaches of quantifying corticolous (bark-dwelling) arthropod communities.

Like foliage-dwelling arthropod communities, corticolous arthropod communities can be important from both a socioeconomic and environmental viewpoint. Some forest diseases that are caused or facilitated by corticolous arthropods can be detrimental to economically viable timber harvest⁴. Additionally, corticolous arthropods can be an important component of the food chain in forest communities^13,14. For example, forest dwelling arthropods are the primary food source for many insectivorous bark gleaning song birds^15,16. Thus, understanding the factors that influence communities of corticolous arthropods is of interest to foresters and both basic and applied ecologists.

Understanding factors that influence arthropod community composition and abundance often requires the capture of individuals. Capture techniques can generally be categorized into qualitative techniques that only detect presence of a species for estimates of species range, richness, and diversity¹⁷, or semi-quantitative and quantitative techniques that allow for an index or estimate of abundance and density of individuals within a taxonomic group^18,19. Semi-quantitative and quantitative techniques allow researchers to estimate or at least consistently sample a specified sample area and estimate probability of detection or assume detection probability is non-directional and adequate as to not obscure the researcher's ability to detect spatial or temporal variation in abundance. Semi-quantitative and quantitative techniques for quantifying corticolous arthropods include suction or vacuum sampling of a specific area^20,21,22, systematic counting of visible arthropods^18,23, sticky traps²⁴, various funnel or pot-type traps^8,25, and entrance or emergent holes^26,27.

A number of spatial and temporal factors are thought to lead to variation in corticolous arthropod communities^11,14,28,29. For example, texture of tree bark is thought to influence the community structure of tree-dwelling arthropods¹⁴. Because of the more diverse surface area of the trunks of trees with more furrowed bark, trees with more furrowed bark are thought to support a greater diversity and abundance of arthropods¹⁴.

With this article we report a new semi-quantitative approach of enumerating corticolous arthropods that could be used to describe and test hypotheses regarding variation in corticolous arthropod communities across time and space with adequate precision to detect differences among tree species. Using sticky traps attached to the trunks of trees, we compared the abundance, total length (a surrogate for body mass), richness, and diversity of the arthropod community on the bole of white oak (Quercus alba), pignut hickory (Carya glabra), sugar maple (Acer saccharum), American beech (Fagus grandifolia), and tulip poplar (Liriodendron tulipifera) trees, trees that vary in bark texture.

This study was conducted in the Ozark and Shawnee Hills ecological sections of the Shawnee National Forest (SNF) in southwestern Illinois. During July 2015, we identified 18 (9 dominated by oak/hickory and 9 dominated by beech/maple) sites with the USFS stand cover map for the SNF (allveg2008.shp) in ArcGIS 10.1.1. In the xeric sites, the dominant species were pignut hickory and white oak and in mesic sites, the dominant species were American beech, sugar maple, and tulip poplar. To compare bole arthropod community among tree species, at each data collection site, we identified the three of the five (white oak, pignut hickory, sugar maple, American beech and tulip poplar) focal species trees >17 cm diameter at breast height (d.b.h.) closest to the center of a 10 m radial circle. If fewer than three appropriate trees were present, the circle was expanded and the closest tree fitting the criteria was selected. For each tree chosen, we installed four sticky traps at breast height, one facing in each cardinal direction: north, south, east and west.

We collected arthropod data from the boles of 54 individual trees (12 pignut hickories, 15 white oaks, 8 American beeches, 12 sugar maples, and 7 tulip poplars) among the 18 sites. We grouped arthropods according to a simplified guild classification by diagnostic morphological characteristics indicative of closely related orders from current phylogenetic records, similar to that of "operational taxonomic units"^30,31 (Appendix A). Based on this classification, we captured representatives of 26 guilds in our traps that were each in place for 9 days (Appendix A). Because our study focused on trophic interactions between tree species, corticolous arthropods, and bark-gleaning birds, we removed all arthropods smaller than 3 mm from analysis because their importance as a food resource is minimal for bark-gleaning birds. We used a mixed model that included either arthropod length (surrogate to body mass), abundance, Shannon diversity and, richness as the dependent variable, tree species and effort (proportion of tree covered with traps) as fixed variables, and site as a random variable. Because all traps from a single tree were combined as one sample, individual trees were not included as a random variable.

Protocol

1. Placement of a trap on the tree

1. Measure the diameter of a tree at breast height. At breast height in each cardinal direction, for an area the size of the pre-manufactured sticky trap (glue board), use a bark shaver to remove bark until an area the size for the sticky trap is smooth enough to staple the sticky trap onto the tree so that there is no space for arthropods to crawl under the trap. Label the back of the trap using a dark colored permanent marker with the date, trap number, location and other pertinent information.
   1. To trap arthropods, either (a) capture both flying and crawling arthropods, by opening and removing the sides and cover of the sticky trap by cutting the cardboard along the edge of the sticky material, (b) or exclude flying arthropods from landing directly on the trap, by opening the trap as directed on the box.
2. Place one trap on each previously shaved location so that the openings are oriented vertically (one opening facing up, the other opening facing down) to maximize capture of arthropods crawling up and down the tree boles. For traps with the tops removed to capture both flying and crawling arthropods, orient traps so the end that was the opening prior to the removal of the cardboard cover is oriented vertically, to maintain trapping consistency.
3. Staple traps to the tree by placing one staple at each corner and one staple in the center bottom and center top of the trap. Start stapling in the bottom right corner, then the bottom center, the top right corner, the top right center, the bottom left corner, and finally the top left corner. Be careful to ensure the entire bottom and top of the traps are flush against the tree to minimize arthropods crawling under the trap.
4. Leave traps in place for desired amount of time. Be certain all traps are left in place the same amount of time.
   NOTE: In areas where arthropods are extremely abundant, for example during moth outbreaks, traps may become saturated within hour or days. Under these circumstances, traps will need to be regularly replaced prior to be saturated to maintain constant capture probability.

2. Removing the trap from the tree

1. After the desired amount of time trapping, cover the entire trap, except for the staples, with polymeric cellulose film (e.g., cellophane).
   NOTE: Placing the film on the traps prior to removal will reduce the likelihood of disturbing the trapped arthropods.
2. Remove each trap by taking a large flat screwdriver and prying each staple partially from the tree, adequate to facilitate the grasping of the staples using needle nose pliers. Take large needle nose pliers or a similar grasping tool and pull the staples from the tree.
3. Place the traps in a rigid box of some type for transportation to a laboratory for analysis. If traps are to be stored for more than 12 h, store traps in a freezer to preserve content.

3. Laboratory analysis

1. Using a dissecting scope, examine content of a trap recording the number of individuals to desired taxonomic level.
2. Use sorted arthropods to estimate richness (total number of taxonomic groups), diversity indices, or abundance (total arthropods). If estimated biomass is a desired result, measure length and width of arthropods to the nearest mm and use published length/width, biomass regressions to estimate biomass^32,33,34.
3. Subtract the total width of the 4 traps from the diameter at breast height for each tree to estimate trapping effort (proportion of tree covered by the traps) for each tree.
4. Because samples from multiple traps on the same tree are not independent, either sum samples from the same tree or include individual tree as a random variable in all analysis to avoid pseudo-replication.

Results

Based on the mixed model results, the model that included tree species best explained variation in total arthropod length, abundance, and diversity, neither of independent variables explained substantial variation in richness, although the models that included tree species trapping effort were competitive with the null model (Table 1). In addition, proportion of the tree trapped appears to have no influence on abundance, total length, and Shannon diversity, with only mini...

Discussion

Although alternative techniques such as suction or sweep nets have been used, most previously published attempts at quantifying arthropods on tree boles used some version of either quantifying arthropods by visually inspecting tree boles in the field, using chemical pesticides to kill arthropods in a specified area then quantifying the recovered arthropods, or placing funnel traps or a sticky substance directly onto the tree^19,23,25

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Straight Draw Bark Shaver, 8"	Timber Tuff	TMB-08DS
PRO SERIES Bulk Mouse & Insect Glue Boards	Catchmaster	#60m
Staple gun	Stanley	TR45D