Quantifying Corticolous Arthropods Using Sticky Traps

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^{19,23,25,35,36}. Each of these approaches have benefits and shortcomings.

With chemical knockdown, a pesticide is sprayed over a predefined area and arthropods are allowed to drop onto a drop cloth as they die, where they are then collected and quantified¹⁹. Alternatively, with visual location, live arthropods are located in the predefined area and collected by hand for later quantification²³. Both of these methods are instantaneous relative to our method, thus provide a more quantifiable estimate of area sampled for use in estimating density. A further attribute to chemical knockdown as well as visual inspection is, because it is somewhat instantaneous, the estimate is limited to the time at which the survey was conducted. Because it only samples arthropods present at the time of sampling, this method provides an accurate estimate of the size of area sampled, facilitating an estimate of density. These approaches, however, often disregard variation in the non-resident arthropod population, arthropods that temporarily inhabit the tree boles such as flying arthropods or arthropods that use the surface of tree boles as travel routes from the ground to higher forest foliage. Because many of the arthropods that influence other trophic levels use bark for short periods as part-time residence, nearly instantaneous samples from the visual observation and chemical knockdown method likely will not adequately depict the entire suit of arthropods that use tree bark as a substrate^8,35,36.

To better depict the corticolous arthropod community that occurs over longer periods, longer-term methods such as funnel and sticky traps have been developed^{25,26,27,28,29,30,31,35,36}. Funnel traps are attached to tree boles and are designed to funnel arthropods into bottles of preservative, thus are beneficial in that they can be used for long periods of time (weeks to potentially months) while still preserving the arthropods. The limitation of these traps is their limited ability to trap flying arthropods that land on the tree boles. Alternatively, sticky traps are effective at capturing both crawling and flying arthropods.

With the original sticky traps, a sticky material was placed directly onto the tree to trap both crawling and flying arthropods over a predetermined time³⁷. While this approach was effective at trapping both crawling and flying arthropods, it is difficult to spread the exact same amount of material for each trap, thus maintain a consistent sampling area and trapped arthropods have to be identified and quantified in the field under often less than ideal weather conditions, potentially leading to additional variation in the estimates due to misidentification or miscounting. An improvement was offered by Collins et al.³⁶ when they spread the sticky material on tape, then, after trapping for a predetermined amount of time, covered the tape with cellophane and removed the tape so arthropod identification and quantification could be conducted later in the laboratory, where conditions were much more appropriate for the activity. While this method is an improvement over the previously described methods, it is still messy, and still difficult to consistently spread the same amount of sticky material at each trap. As an improvement to this method, we propose using commercially manufactured sticky traps to address both of these deficiencies.

Commercially produced sticky traps have been used to trap flying arthropods over water³⁸, at various elevations of vascular vegetation³⁹, and in the foliage of trees⁴⁰, but to our knowledge have not been used to sample arthropods on tree bark. Commercially produced sticky traps provide an improvement over previously used approaches in that the sticky material is adhered to the cardboard backing in the factory and because they are commercially manufactured, the surface area of the material is very consistent. Additionally, the traps can be placed on the trees with the trap intact, preventing flying arthropods from landing directly on the trap, as was done in our study, or the cardboard cover could be removed so the trap is catching both crawling arthropods and flying arthropods landing directly on the trap. Additionally, the traps are easily removed from the tree, covered with cellophane and transported to the laboratory where they can be stored in a freezer and quantified at a later date. The trap's stiff cardboard construction also facilitates viewing the traps in the laboratory under a dissecting microscope allowing for more precise identification, quantification and measurements of arthropods, reducing some of the detection error that would likely occur when conducting this activity in the field. Finally, sticky material on trees can become saturated, reducing the ability of the trap to capture arthropods⁴¹. The method we describe allows researchers to easily replace the sticky traps to maintain effectiveness, allowing long-term monitoring of individual trees.

As demonstrated by our results, this approach appears to provide adequate precision to address most ecological or environmental questions regarding variation in corticolous arthropod communities. Detection of arthropods from sticky traps used to quantify corticolous arthropods with this method was adequately precise to provide an SEM that was <20% of the mean for all community metrics used in this study. This level of precision was achieved with a reasonable sample sizes of only 7 to 15 individual trees. With this level of precision and moderate sample sizes, we detected differences in total length (a surrogate for biomass), total abundance, total richness, and Shannon diversity among species of trees. We did not partition the variance between measurement error (variance associated with variation in proportion of area trapped among traps or variation in detection probability) and variance among individual trees within a tree species, however, these results clearly indicate that this method has adequate detection probability to prevent measurement error from obscuring results to important ecological or environmental questions.

We describe this method as being semi-quantitative because although we believe our detection probability to be high and provide adequate precision to address most ecological questions, we have no way of estimating detection probability. Thus, we have no way of estimating potential negative bias associated with our point estimates. Additionally, a fully quantitative method that could be used to estimate overall abundance or density, requires an accurate estimate of the sampling area⁴². Unlike the visual inspection or chemical knockdown methods, the sampling area with funnel traps and with this method is uncertain because it is not instantaneous, the traps are placed on the tree for a predetermined amount of time and arthropods going about their normal activities are trapped when they cross the surface of the sticky traps. Thus, the size of area being trapped is dependent on the activity level of the arthropods. Arthropod activity level varies with time of day, by season, by species, or by individual⁸. Because arthropod activity level varies, the sampling area will vary based on activity level. It will be important for researchers to consider how activity level influences inference from results when using both this and the funnel trap method. We argue, however, neither methods that provide a more accurate estimate of the sampling area because they are more instantaneous nor methods that provides a less accurate estimate of sampling area but a better depiction of the arthropod community over time is better. Instead, the two types of methods address different questions. The chemical knockdown and visual inspection methods describe the community during a very specific point in time, while the funnel and sticky trap methods describe the community over a period of hours or days, depending on how long the traps are left in place. We believe, however, when researchers are interested in identifying and describing spatial and temporal variation of corticolous arthropod communities utilizing the bark surface over a substantial time (days to weeks), the method described here is the most convenient and accurate approach.

Finally, the primary objective of our original study was to better understand how mesophication of southeastern deciduous forests is likely to impact forest dwelling insectivorous birds and mammals, thus, we combined arthropods into guilds⁴³. We see no reason, however, why these capture techniques could not be used to quantify arthropods at the species or any other taxonomic level.