Building Double-decker Traps for Early Detection of Emerald Ash Borer

Both the design and placement of DD traps exploit the attraction of adult EAB beetles to specific shades of color and to light. The green prism on the top of the PVC pipe is most attractive to male beetles, who spend their life span feeding on ash leaves, as well as mating^32,36,38,39. The lower purple prism ensures traps are also attractive to female beetles³². Like males, female beetles feed on ash foliage throughout their life span, but mature females spend large amounts of time on the bark of branches or the trunk where they lay eggs³⁹. While detecting new infestations only requires capturing a beetle of either sex, the ability to attract and capture female EABs may be particularly important. Mature EAB females are physiologically capable of flying further than males⁴⁰ and population models suggest long-distance dispersal of mated females contributes substantially to the expansion of new EAB infestations^41,42,43.

Placing DD traps in the open serves multiple purposes. Once assembled, DD traps are highly apparent and the silhouette of the tall, vertical trap resembles that of a small ash tree. Using two prisms enhances the silhouette of the trap, but perhaps more importantly, doubles the surface area available for trapping beetles. The green and purple prisms on the stand-alone DD traps are unlikely to be obscured by foliage from trees or vegetation. In contrast to baited prism traps hung from the branch of an ash tree, lures on the DD traps do not have to compete with compounds emitted by live ash surrounding the trap. Instead, lures on the DD traps provide the beetles with a distinct point source of ash-related volatiles they can readily identify and locate. Moreover, setting the DD trap in the open exploits the strong and consistent preference for sunny conditions exhibited by EAB beetles.

Finally, the DD traps are simple to install, easy to check and withstand strong winds and summer thunderstorms. The traps are not only highly apparent to EAB beetles; they are also easy for survey personnel to locate for mid-season checks or recovery. Vandalism has never been problem in any of the dozens of sites where DD traps have been monitored over the past ten years. The unappealing sticky panels likely serve to quell the enthusiasm of would-be vandals.

The DD traps can also serve as outreach tools to increase public awareness of EAB. Displaying a sign can be helpful. Simple and inexpensive signs with a photo of a DD trap and an explanation of the purpose of the trap can be printed with a desktop color printer, laminated, glued to a wood or fiberboard backing, then attached to a t-post in areas where people are likely to encounter traps at the site. Explanatory signs and the obvious effort underway to detect EAB typically generate goodwill as well as functioning as an educational tool.

Obviously, DD traps with two panels, a t-post and PVC pipe cost at least twice as much as suspending a single prism trap in the canopy of ash trees. The PVC pipe and t-post, however, can be re-used for many years. Additionally, much of the cost of detection surveys for invasive forest pests such as EAB reflects the time needed for survey crews to locate a suitable site, install the trap, check the traps periodically, and to sort and identify trap catches. Costs of false negatives, e.g., failure to detect a low density infestation of EAB, must also be considered if a less effective trap is used. Results from studies encompassing more than 30 different sites have shown that in recently infested areas with low density EAB populations, DD traps were consistently more likely to capture EAB beetles than prism traps hanging in the canopies of ash trees^22,31,44.

Distinct advantages and disadvantages are associated with each EAB trap design evaluated in previous field trials. Funnel traps are initially more expensive than the other trap designs, but can be re-used for multiple years. Prism traps are the least expensive design, while DD traps are intermediate in cost. The most expensive components of DD traps (t-posts and PVC pipe) are readily available in hardware and home improvement stores and can be re-used for multiple years. Non-toxic, non-ethanol antifreeze or insecticidal strips are needed to capture insects attracted to funnel traps. Prisms, whether suspended in ash canopies or on DD traps, must be coated with sticky insect glue to capture insects and cannot be reused. All insects captured by funnel traps must be collected and returned to the laboratory for sorting and identification. Panels on prism and DD traps must be examined and suspect beetles removed with forceps for subsequent species confirmation in the laboratory. Some expertise is useful for distinguishing EABs (or at least buprestid beetles) in the field, but training surveyors is not difficult. Hanging funnel traps and prism traps on branches in ash tree canopies can require considerable time, especially if trees are large. Crews often need long poles or large slingshots to access branches and braided line or cord is needed to suspend the traps. Double-decker traps, which are placed in the open near ash trees, can be easily set up anywhere. High winds have occasionally blown prisms out of trees or caused lines bearing funnel or prism traps to wrap around upper branches, preventing access to the traps from the ground. In contrast, DD traps have remained intact during severe storms, including storms with straight line winds^22,44.

It is important to note that EAB detection and survey methods are not mutually exclusive. Different detection methods can be integrated into survey strategies appropriate for specific situations or local conditions. For example, low value or declining ash trees along fence lines, roads or in forested settings can be girdled and debarked, to function as detection tools. Less expensive prism traps or reusable funnel traps can be distributed more broadly in large scale systematic surveys. Free-standing DD traps may be especially appropriate for high risk areas such as campgrounds or recreation areas where potentially infested ash firewood presents an ongoing threat. Surveyors can elect to place DD traps in a variety of locations or settings, as long as the traps are near ash trees and in the open, so the prisms are exposed to sun. Double-decker traps have been successfully used in highway medians and along railroads, in powerlines running through forested areas, around the perimeter of sawmills and waste wood disposal yards, and along rivers, drainage ditches and other riparian areas. Although the DD traps are baited with an ash volatile (cis-3-hexenol) and were designed for EAB surveys, native buprestid species are also frequently captured on the sticky panels. Further studies to assess potential modifications to DD traps or lures that might increase attraction of other buprestid species could be useful.