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

Summary

Effective traps to attract and capture the emerald ash borer (EAB) are a key element of detecting and managing this invasive pest. Double-decker traps, placed in full sun near ash trees, incorporate visual and olfactory cues and were more likely to capture EAB than other trap designs in field trials.

Abstract

Emerald ash borer (EAB) (Agrilus planipennis Fairmaire), the most destructive forest insect to have invaded North America, has killed hundreds of millions of forest and landscape ash (Fraxinus spp.) trees. Several artificial trap designs to attract and capture EAB beetles have been developed to detect, delineate, and monitor infestations. Double-decker (DD) traps consist of two corrugated plastic prisms, one green and one purple, attached to a 3 m tall polyvinyl chloride (PVC) pipe supported by a t-post. The green prism at the top of the PVC pipe is baited with cis-3-hexenol, a compound produced by ash foliage. Surfaces of both prisms are coated with sticky insect glue to capture adult EAB beetles. Double-decker traps should be placed near ash trees but in open areas, exposed to sun. Double-decker trap construction and placement are presented here, along with a summary of field experiments demonstrating the efficacy of DD traps in capturing EAB beetles. In a recent study in sites with relatively low EAB densities, double-decker traps captured significantly more EAB than green or purple prism traps or green funnel traps, all of which are designed to be suspended from a branch in the canopy of ash trees. A greater percentage of double decker traps were positive, i.e., captured at least one EAB, than the prism traps or funnel traps that were hung in ash tree canopies.

Introduction

The emerald ash borer (EAB) (Agrilus planipennis Fairmaire) (Coleoptera: Buprestidae) has killed hundreds of millions of ash (Fraxinus spp.) trees since it was first identified in 2002 in the greater Detroit, Michigan metropolitan area and in nearby Windsor, Ontario, Canada. Inadvertent spread of infested ash trees, logs and firewood, along with natural dispersal of beetles, have resulted in EAB establishment in at least 27 states, and two Canadian provinces to date¹. Recent reports indicate the EAB has also invaded Moscow, Russia, where it is killing landscape ash trees^2,3, generating additional concern about its potential spread into Europe. Interspecific variation in EAB host preference and resistance among North American ash species has been documented^4,5,6,7,8,9, but virtually all ash species in North America are likely to be suitable hosts. Catastrophic levels of ash mortality have been recorded in areas of Michigan and Ohio^10,11,12, with associated ecological and economic impacts^13,14,15,16.

Effective methods to detect new EAB infestations and monitor low density populations are critical aspects of managing this invasive pest in urban, residential and forested settings. Early detection provides time to develop a strategy, secure funding and implement activities to reduce impacts of the EAB. For example, municipal officials and homeowners can begin treating valuable ash in landscapes with systemic insecticides before damage caused by increasing larval densities limits efficacy of these products^17,18. Similarly, identification of a new infestation and reliable information about local EAB distribution gives foresters and property owners the opportunity to implement timber sales, species conversion or other activities to reduce economic costs or ecological effects of ash mortality.

Early detection, delimitation, and effective monitoring of low-density EAB populations, however, remain difficult. Visual surveys to identify newly infested ash trees are not reliable because ash rarely exhibit external signs or symptoms of EAB infestation until larval densities build to moderate or even high levels^4,19. The most effective means of detecting very low densities of EAB involve using girdled ash detection trees^19,20,21,22. Ash trees are girdled in spring or early summer by removing a band of outer bark and phloem around the circumference of the trunk, which stresses trees, increasing their attraction to adult EAB beetles. Girdled trees can be debarked in autumn or winter to identify EAB larval presence and density. Although girdled ash trees have been used operationally for EAB detection^19,23,24,25, they are problematic. Debarking girdled trees can be labor intensive and locating suitable trees for girdling can be difficult, especially in urban or residential areas or when surveys must be conducted for multiple years¹⁹.

Artificial traps baited with EAB attractants eliminate many concerns associated with using girdled ash detection trees. In contrast to other important forest pests such as gypsy moth (Lymantria dispar L) and some Scolytinae bark beetles that produce long distance sex or aggregation pheromones, to date, no effective long-distance pheromones have been found for EAB. A short range sex pheromone, cis-lactone, may facilitate mating^26,27, but in field trials, cis-lactone lures have not consistently increased EAB attraction to artificial traps²⁸. Adult beetles rely on volatile compounds emitted by ash leaves, bark and wood to identify their host trees^29,30,31 and encounter potential mates. Several volatile compounds have been evaluated for use in lures to attract adult EABs to artificial traps^27,32. Currently, traps used operationally for EAB detection surveys in the U.S. are baited with lures containing cis-3-hexenol, a common green leaf volatile produced by ash foliage^30,33. In previous years, EAB traps used for U.S. surveys have also been baited with Manuka oil, which is extracted from the New Zealand tea tree (Leptospermum scoparium Forst and Forst ) or Phoebe oil, an extract of the Brazilian walnut tree (Phoebe porosa Mez.); both contain several sesquiterpenes that are also present in ash bark²⁹. Problems with inconsistent supplies of these natural oils, however, have restricted their use.

In addition to host-produced volatiles, adult EAB beetles respond to visual stimuli, including color and light^20,32,34,35. Early studies showed EAB adults, which are relatively agile fliers, were rarely captured by black funnel traps baited with various ash volatiles (DGM and TMP, unpublished data). Other trap designs, such as cross-vane traps, were evaluated but the aversion of EAB beetles to dark spaces and shadows limited the effectiveness of these traps.

Development of a three-sided prism, which can be coated with clear insect trapping glue³⁵ to capture beetles, was a substantial improvement in trap design. Attraction of adult EABs to color has also been extensively evaluated in field trials and in laboratory studies with a retinograph³⁴. Results show EAB beetles are consistently attracted to specific shades of green and purple^32,36. Prism traps fabricated from colored corrugated plastic are now widely used in EAB survey activities in the U.S. and Canada.

Because EAB adults are strongly attracted to light, beetles are much more likely to colonize open-grown trees than shaded trees^20,21. Guidelines for EAB detection surveys in the U.S. required individual prism traps to be suspended from a mid-canopy branch in an ash tree growing along a road or the edge of a wooded area³⁷. In theory, this should ensure that at least one panel of the prism is exposed to sunlight. Operationally, however, prism traps may be partially shadowed by overhead branches or by adjacent or nearby trees. Sticky panel surfaces are frequently blown into foliage, resulting in leaves adhering to and obscuring at least a portion of one or more panels.

Double-decker (DD) traps were developed to integrate multiple visual and olfactory cues to enhance attraction of EAB beetles. Each DD trap is comprised of one green and one purple corrugated plastic prism attached to a 3 m tall schedule 40 polyvinyl chloride (PVC) pipe (10 cm diameter), which is supported by sliding the PVC pipe over a t-post. Using both green and purple prisms is designed to attract both sexes of EAB beetles^32,36,38,39. Additionally, rather than being suspended from a branch in the canopy of an ash tree, the DD traps are placed in full sun, 5-10 m away from ash trees along the edge of a wooded area or in the midst of scattered, open-grown ash trees.

Protocol

1. Prepare the Green and Purple Panels

1. Acquire green and purple corrugated plastic panels (120 cm × 60 cm) for EAB trapping from a commercial distributor of pest management supplies. Use a box-cutter or utility knife to score two fold lines in each panel by partially cutting through the plastic along the vertical corrugations, 40 cm from the two short edges of the panel. This allows the panel to be folded into a 3-sided prism (each face will be 40 x 60 cm). Panels are usually scored by the distributor to facilitate folding, so this step may not be necessary.
2. Drill two pairs of holes into each panel using a ½ inch (1.3 cm) bit as indicated in Figure 1. Drill holes 4 inches (10 cm) from the top of the panel and 4 inches on either side of the left fold. Drill the second pair of holes 19 inches (48 cm) from the top of the panel and directly below the top holes (i.e., 4 inches on either side of the left fold) .

2. Prepare the PVC Pipe

1. Obtain one section of schedule 40 polyvinyl chloride (PVC) pipe, 3 m long and 10 cm in diameter, from a commercial hardware store or other supplier for each trap. Drill four holes into the PVC pipe, as shown in Figure 2. Using a ½ inch (1.3 cm) drill bit, drill a pair of holes 4 inches (10 cm) from the top of the PVC pipe and 3½ inches (8.9 cm) apart. Align the holes horizontally.
2. Drill a second pair of holes, 3½ inches apart, and 52 inches (1.3 m) below the top of the pipe, directly below the first set of holes .

3. Install a T-post

1. Use a post pounder to install a t-post in the desired location. Drive the post deep enough so that the flange is just below the soil surface, but there is no need to go any deeper. The t-post will provide plenty of support for the DD trap, even in high winds. Guy wires or additional supports are not necessary. Although the t-post does not need to be set deeply into the soil, it is a good idea to check for underground utilities or buried pipes before driving the post into the ground.

4. Assemble the DD Trap

1. Fold a purple panel into a prism. If panels are pre-glued, be sure the sticky side is on the outside. Feed the two small flaps through the slits and use two 8 inch (20 cm) cable ties to fasten the panel ends together. Repeat with a green panel.
2. Support the PVC pipe horizontally above the ground. Slide the purple prism onto the PVC pipe and down to the lower set of holes in the PVC pipe.
3. Feed a large, 2 foot (60 cm) long cable tie horizontally through the top left hole in the prism, then through the pair of holes in the PVC pipe, and finally through the adjacent hole on the right side of the prism. Be sure the prism is snug against the solid portion of PVC pipe that lies between the two holes. Close and tighten the cable tie, making sure the cable tie is not twisted.
4. Feed a second large cable tie through the bottom hole in the prism, around the pipe and through the corresponding hole on the adjacent side of the prism. Close the cable tie and tighten, again taking care to ensure the cable tie does not twist before it is tightened.
5. Slide the green prism onto the PVC pipe and attach it to the upper set of holes in the PVC pipe with two 60-cm cable ties, following the same steps as before. There will be a 21 inch (54 cm) gap between the upper and lower prisms.

5. Apply Insect Glue and Set Up the Trap

1. Apply clear insect trapping glue to panels that are not pre-glued. Even pre-glued panels often require additional glue to capture EAB beetles. To apply insect trapping glue, set the base of the PVC pipe on the ground and prop the inner face of the upper prism on the t-post.
2. Using latex (or similar) gloves, scoop up two to three handfuls of glue and smear it across the surface of a panel, making sure the surface of the panel is thoroughly coated. Rotate the PVC pipe to access all three surfaces on both prisms. Glue does not need to be applied to the PVC pipe.
3. Pick up the PVC pipe, hold it vertically and slide the lower end over the t post. Avoid touching the sticky panels.

6. Bait the DD Trap

1. As pre-glued prisms usually arrive with a small hole (4 mm) near the bottom edge of one panel of the prism, use a 4 inch (10 cm) cable tie to attach one pouch of cis-3-hexenol to the lower edge of the top (green) prism. If a hole is not present, use a handheld hole punch or a pocketknife to cut a small hole or slit into the plastic, then use the small cable tie to attach the lure.
   NOTE: The cis-3-hexenol compound in the lures is slowly released over about a 10 week period. If traps need to remain effective for more than 10 weeks, replace the lures as needed.

7. Checking Traps for EAB Beetles

1. Check traps at 2-4 week intervals if possible, to avoid losing captured beetles, which occasionally drop off the traps, especially if heavy rain occurs. Examine all three sides of the lower prism for EAB beetles. Then, using disposable latex gloves, lift the PVC pipe straight up and off the t-post and prop the inner face of the upper prism on the t-post, exposing one panel of both prisms.
2. Use forceps to remove any suspect EAB beetles and place them in a re-sealable plastic specimen bag.
3. Rotate the PVC pipe to examine all three panels on both prisms for suspect EAB beetles, then slide the pipe back over the t-post.
   NOTE: Flies, click beetles and various other insects will inevitably be captured on the traps. It takes a keen eye to spot an EAB beetle. Carrying a photo reference card can be helpful when checking traps for the first time.
4. To confirm species identification, return suspect beetles to the lab and soak them in a non-toxic histological clearing agent for a day or two to remove the insect trapping glue. Beetles can then be examined under a dissecting microscope.

Results

In a large scale study, three artificial trap designs as well as girdled ash trees were deployed systematically, 10-20 m apart, across a newly infested 16 ha forested area with a very low density of EAB²⁵. Artificial trap designs tested included purple prisms baited with Manuka oil and suspended >3 m high from a branch in the canopy of ash trees, 3 m tall DD traps with two green panels, and DD traps with two purple panels supported by t-posts. Double-decker trap...

Discussion

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 male...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Light green corrugated plastic panel: 120 cm x 60 cm	Great Lakes IPM; www.greatlakesipm.com	IPM-EAB GR	All three surfaces of each prism need to be covered with clear insect trapping glue, even if the panels are pre-glued. Pre-glued panels are often not sticky enough to consistently capture or retain EAB beetles.  Other clear insect trapping glue products are available but are considerably more difficult to apply.
Light purple corrugated plastic panel: 120 cm x 60 cm	Great Lakes IPM; www.greatlakesipm.com	IPM-EAB  LP
Large cable tie (4): 60 cm with a 79 kg capacity	Cabletiesandmore.com; http://www.cabletiesandmore.com/cableties.php	CT-24-NU-100PK
Medium cable ties (4): 20 cm with a 22.7 kg capacity	Cabletiesandmore.com; http://www.cabletiesandmore.com/cableties.php	CT261
Small cable tie: 10 cm with a 8.2 kg capacity	Cabletiesandmore.com; http://www.cabletiesandmore.com/cableties.php	CT204
cis-3-hexanol pouch	Synergy Semiochemicals; http://www.semiochemical.com/html/buprestids.html)	3136	Lures used to bait DD traps consist of pouches containing cis-3-hexenol, a non-toxic compound present in ash leaves.  One pouch is attached to the lower edge of the top prism using a small cable tie.  Each pouch of cis-3-hexenol has a release rate of approximately 50 mg/day. Note that cis-3-hexenol is sometimes written as Z-3-hexenol.
Aphinity Hexenol	Sylvar Technologies
Lure GLV4 emerald ash borer	Chemtica, Heredia, Costa Rica
cis-3-hexanol pouch	WestGreen Global Technologies; http://www.westgreenglobaltechnologies.com/
Clear insect trapping glue	Hummert International; http://www.hummert.com/product-details/8196/pestick	01-3522-1
Histoclear II histological clearing agent	National Diagnostics; www.nationaldiagnostics.com	HS-202	Histoclear II will be needed to remove the sticky insect glue from suspect beetles.  Other histological clearing agents are available but may not remove the glue and some products dissolve plastic, an important consideration if plastic containers are used for soaking the beetles.
Histoclear II histological clearing agent	Great Lakes IPM; www.greatlakesipm.com	10011	Histoclear II will be needed to remove the sticky insect glue from suspect beetles.  Other histological clearing agents are available but may not remove the glue and some products dissolve plastic, an important consideration if plastic containers are used for soaking the beetles.
t-post: 1.5 m	multiple sources		A t-post (5 feet tall) (1.5 m) is used to support the PVC pipe.
post pounder	multiple sources		Use a post pounder to set t-posts into the ground. No additional support is necessary.
HDPE (high density polyethylene) PVC pipe : 3 m x 10 cm diameter	multiple sources
Forceps (rigid)	multiple sources		Forceps (tweezers) will be needed to remove suspect beetles from the traps. Rigid forceps work better than flexible forceps.
Latex gloves	multiple sources		Latex gloves are needed for applying the insect trapping glue to the prisms and for checking the traps to collect EAB beetles.
Baby oil or baby wipes	multiple sources		Baby oil or baby wipes are helpful for removing the trapping glue from hands and equipment.
Re-sealable plastic specimen bags: 5 cm x 8 cm	multiple sources		Small re-sealable plastic specimen bags are useful for collecting beetles from traps.  Each bag should be labelled, either with pre-made, adhesive labels or with soft felt pens.
Guides to help with distinguishing EAB from beetles native to North America are available on the national EAB website at www.emeraldashborer.info.