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Evaluating Common Landscape Trees for Susceptibility to Asian Longhorned Beetle

James C. Sellmer, William D. Morewood, Patricia Neiner and Kelli Hoover
Pennsylvania State University, Depts. of Horticulture and Entomology

A paper from the Proceedings of the 12th Metropolitan Tree Improvement Alliance Conference (METRIA 12), Landscape Plant Symposium: Plant Development And Utilization, held in Asheville, NC, May 23-25, 2002, co-sponsored by the North Carolina State University, North Carolina Division of Forest Resources, USDA Forest Service Southern Region, North Carolina Landscape Association, North Carolina Association of Nurserymen, The Landscape Plant Development Center, The North American Branch of The Maple Society, and The International Ornamental Crabapple Society.


Abstract

Asian longhorned beetle (ALB) has been a known threat to the urban forests of the United States since its discovery in New York in 1996. Yet, since that time information about the suitability of common landscape trees as hosts for Asian longhorned beetle remains limited beyond observations from the infested areas of Chicago, New York, Austria, and China. The research results presented here represent screening methods being conducted in the U.S. Presently, screening trials are being conducted using three different methods, including oviposition preference trials on cut wood sections, larval performance trials on potted trees under greenhouse conditions, and oviposition preference trials on live trees under greenhouse conditions. Each of these distinctly different approaches has provided important preliminary information on host tree suitability for ALB and will continue to aid in identifying potential hosts and less preferred hosts for replanting in infested areas. Oviposition preference trials on wood sections show that northern red oak, white oak, and honeylocust are potentially suitable hosts. Larval performance trials further show that ALB larvae that were artificially inserted in northern red oak can develop up to at least 90 days. In addition, green ash appears to be a less suitable host for larval development. Oviposition preference trials on live trees of northern red oak, sugar maple, green ash, and red maple are presently underway. Preliminary choice trials conducted in a quarantine greenhouse suggest that ALB females initially preferred to feed on and oviposit in maples compared to the oaks and green ash. However, later in the 30-day trial period, the adults began to explore and chew oviposition niches on the oaks and the green ash as well. The oviposition choice trials are ongoing and, thus, the data on oviposition sites and preference are not available at this time. Oviposition and larval performance experiments on live trees will continue during the next couple of years in order to determine the suitability of common landscape trees as hosts for Asian longhorned beetle.

Introduction

Research on understanding and eradicating Asian longhorned beetle (ALB), Anoplophora glabripennis Motsch., from the United States has accelerated over the past couple of years. Across the nation groups of scientists, regulatory officials, and citizens are working to address this financially and environmentally devastating issue. Presently, research is being conducted on a variety of topics involving this insect including its biology; survey and detection within the U.S.; biological control; eradication including insecticide efficacy; and exclusion and prevention of new introductions (Fosbroke and Gottschalk 2001; Bohne 2002 http://www.uvm.edu/albeetle/index.html).

To date Asian longhorned beetle detection numbers are declining at both the Chicago and the New York Metropolitan area infestation sites; however, the geographical area where beetle activity has been found has grown over the past two years (Haugen 2002 http://www.na.fs.fed.us/fhp/alb/index.shtm). In Chicago, the Ravenswood, Addison, Summit, and Park Ridge communities, and O'Hare airport are among the infestation sites with a total of 1533 infested trees found since 1998. The Bayside-Queens, Greenpoint-Brooklyn, Islip, Amityville, and Manhattan communities of New York are among the infestation sites with a total of 5761 infested trees found since 1996. Most recently, two maples were found to be infested and were removed from Central Park in February 2002. Overall, 8077 trees have been destroyed since 1996 due to the threat of ALB in the U.S. While species within the genus Acer are the most commonly infested trees in New York and Chicago, trees within the genera Aesculus, Betula, Fraxinus, Salix, and Ulmus have also been attacked at these two locations (A. Sawyer, USDA-APHIS, personal communication). Recent projections suggest that if A. glabripennis spreads to urban trees across North America, there would be a loss of 35% of total canopy cover (1.2 billion trees) and a compensatory value loss of $669 billion (Nowak et al. 2001).

Given the potential damage that ALB may cause in urban forests, there is an urgent need to evaluate the suitability of commonly planted landscape trees for beetle oviposition and larval development. By proactively screening landscape trees, we can identify species and varieties of trees that are not suitable hosts for ALB. Replacing infested trees with species or cultivars that are not susceptible to ALB will reduce the overall vulnerability of the infested areas in New York and Chicago. This information will also help to prioritize species for surveys and could help to limit the damage to urban forests if populations spread or new populations are discovered in the future.

The objective of this presentation is to highlight some of the results from recent screening experiments that have taken place both by Michigan State University and Penn State University. Colleagues at Michigan State (Dr. Deb McCullough and Laura Lazarus) have conducted oviposition suitability and early larval development studies utilizing cut wood sections of sugar maple (Acer saccharum), northern red oak (Quercus rubra), white oak (Quercus alba), honeylocust (Gleditsia triacanthos), eastern cottonwood (Populus deltoides), sycamore (Platanus occidentalis), and tulip poplar (Liriodendron tulipifera) (Ludwig et al. 2002). At Penn State, our research efforts have focused on host suitability for oviposition and larval development using living trees under greenhouse conditions (Ludwig et al. 2002).

Screening Experiments

1. Oviposition preference on wood sections

Two trials were conducted using 5 x 20 cm (2.0 x 8 in) cut wood sections of sugar maple, northern red oak, white oak, honeylocust, eastern cottonwood, sycamore, and tulip poplar. Screening experiments were conducted by placing one male-female pair of adult beetles in a 3.8 L (1 gal) glass jar containing a section of wood of one of the test tree species. Sugar maple twigs were included in each jar to ensure that beetles would survive during the test period. Jars with beetles were held in a growth chamber at 21 °C (70°F) and 16:8 light:dark photoperiod. The number of host species that could be tested was determined by availability of healthy beetles (Ludwig et al. 2002).

Four male-female pairs of beetles were used in Test 1 to evaluate oviposition on northern red oak, honeylocust, eastern cottonwood, and sugar maple. Four different pairs of beetles were used in Test 2 to evaluate white oak, sycamore, tulip poplar, and sugar maple. Beetles used in Test 1 were 25 to 48 days old, and those used in Test 2 were 54 to 55 days old when bioassays began.

In Test 1, three pairs of beetles were randomly assigned to northern red oak, honeylocust, or eastern cottonwood, while one beetle pair remained on sugar maple for the duration of the test. Beetles were allowed to mate, feed, excavate egg niches, and oviposit on the wood in each jar for four days. After four days, each mating pair was placed into a new jar containing fresh sugar maple twigs and a sugar maple wood section for two days to allow beetles to recover. Each mating pair was then assigned to a new species for four days, followed by a two-day recovery period on sugar maple. This process was repeated again, so that each pair of beetles was exposed to three different host species. Test 2 included white oak, sycamore, and tulip poplar along with sugar maple and followed the same methods described for Test 1. A 61-day-old female of one of the mating pairs died during the second resting period in Test 2, so another mating pair of beetles was used to complete the test. At the end of each 4-day period, wood sections were examined to determine the number of egg niches on the wood where the female beetle had clearly used her mandibles to scrape the bark. Wood sections were placed on end in a 21°C (70°F) growth chamber for rearing, then were carefully dissected 21 days after completion of each test. Number of eggs and first instar larvae were recorded for each section.

Table 1. The mean and standard error for niches created, eggs laid, and 1st instar larvae found
on wood sections of six tree species; n=3 logs per species. Sugar maple was used as a comparison1

Test 1
  Red oak Honeylocust Cottonwood Sugar maple
Niches 9.3±2.40 a 5.3±1.33 b 1.7±1.20 c 13.3±0.88 d
Eggs 7.7±1.45 a 5.3±1.2 a 0.0 a 7.3±3.28 a
Larvae 3.0±1.53 a 1.7±0.67 b 0.0 a 7.0±3.46 a
Test 2
  White oak Sycamore Tulip Poplar Sugar maple
Niches 40.7±1.20 a 2.7±1.45 c 0.0 c 18.0±3.46 b
Eggs 5.3±0.88 a 0.0 a 0.0 a 2.3±1.45 a
Larvae 4.7±1.45 a 0.0 c 0.0 c 1.3±0.88 b

1 Within row means followed by the same letter are not significantly different (Kruskal-Wallis test and
multiple comparison procedure, P<0.05).

Results

Based on these results, it appears that northern red oak and white oak may be acceptable hosts for ovipositing A. glabripennis beetles, especially in situations where more preferred hosts are not available. This may also be true for honeylocust. In contrast, females laid no eggs on eastern cottonwood, sycamore, or tulip poplar. These results should be considered preliminary given that relatively few beetles were exposed to each of the species we tested. Additional research is needed to further assess whether the response of female beetles to a variety of live trees during choice experiments is similar to their response to individual cut sections of wood that were used in this study.

2. Larval performance in potted trees under greenhouse conditions

Two larval insertion experiments were conducted within an 111m2 (1200 sq ft) quarantine greenhouse at The Pennsylvania State University. This facility was equipped with screened cages measuring 3 x 2.7 x 2.1 m (10 x 9 x 7 ft) in which the trees and larvae were confined. Sugar maple, green ash (Fraxinus pennsylvanica), and northern red oak trees averaging 2 cm in caliper were cultured in #20 containers in a pine bark medium (Ludwig et al. 2002).

First instar ALB larvae were implanted into the trees by making a downward tangential incision to a depth of 5 mm along the trunk of each tree at a predetermined height with a scalpel through the bark, near the bark-cambium-phloem interface, creating a bark flap. One larva was inserted under each bark flap. The flap was carefully closed and sealed with a 5 x 15 cm (2 x 6 in) piece of plastic wrap and anchored with tape at both ends to confine the larva to the insertion site. The plastic wrap was removed after 14 days and larval status was observed and recorded when possible without harming the larvae. In the second experiment, the insertion area was covered with gauze after the plastic wrap was removed. The gauze remained in place for 28 days. Both experiments were terminated by destructively harvesting the implanted sections of trunk, carefully splitting open the section with a chisel and hammer, and removing each implanted larva. Weight, length, gallery size measurements, and observations on gallery characteristics were recorded.

creating a bark flap larava in tree
insertion point tree in the greenhouse

Figure 1: A composite picture illustrating the implantation, cultural parameters and activity from an ALB larva on a tree under trial conditions.

The first insertion trial was initiated in September 2000 as a randomized complete block design with two cages each containing two sugar maple, northern red oak, and green ash trees. Larval insertions were made in each tree at heights of 1, 1.5, 1.75, and 2 m (3.3, 4.9, 5.7, and 6.6 ft) above the soil line following the implantation protocol described above. The mean caliper and standard error of the mean (± SEM) in mm for each tree species measured at 0.15 m (6 in) above the soil line were: green ash 33.7 ± 0.9, sugar maple 25.3 ± 0.7, and northern red oak 20.8 ± 0.6. Sixty days after implantation the trees were destructively harvested.

The second trial was initiated in April 2001. Larvae were inserted into each of the three tree species at heights of 1.0, 1.25, 1.5, 1.75, and 2.0 m (3.3, 4.1, 4.9, 5.7, and 6.6 ft) above the soil line. The insertions were replicated in three cages containing two trees of each species. The mean caliper (±SEM) in mm for each tree species measured at 0.15 m (6 in) above the soil line were: green ash 33.0 ± 1.0, sugar maple 23.2 ± 1.1, and northern red oak 22.6 ± 0.5. Ninety days after implantation the trees were destructively harvested and data collected as described above.

Table 2. Mean (±SEM) caliper (mm) and percent larval survival at four heights above the soil line for each tree species in trial 1; n=4 trees per species.
 
Height above soil line (m)
  1.00 1.50 1.75 2.00
Green ash
Caliper (mm) 36.9±1.6 35.4±0.9 33.4±0.3 29.3±3.8
Larvae Survival 0% 0% 75% 25%
Sugar maple
Caliper 26.9±2.2 26.5±2.0 24.3±2.0 23.6±3.3
Larvae Survival 50% 50% 75% 50%
Northern red oak
Caliper 22.4±0.5 21.2±1.2 19.8±2.3 19.6±3.1
Larvae Survival 25% 50% 75% 100%

Table 3. Mean (±SEM) caliper (mm) and percent larval survival at five heights above the soil line for each tree species in trial 2; n=6 trees per species.
 
Height above soil line (m)
  1.00 1.25 1.50 1.75 2.00
Green ash
Caliper (mm) 35.9±3.0 37.8±3.3 35.6±1.7 29.3±4.2 20.8±2.6
Larvae Survival 0% 50% 17% 0% 17%
Sugar maple
Caliper 23.2±5.9 26.4±2.8 27.6±2.1 24.8±2.2 23.0±5.9
Larvae Survival 33% 50% 67% 83% 33%
Northern red oak
Caliper 22.6±3.0 25.7±2.3 24.4±2.1 23.0±1.7 20.9±1.9
Larvae Survival 67% 67% 50% 100% 50%

graph 1
Figure 2. The overall percent larval survival in each tree species from each insertion trial.

graph 2
Figure 3. Mean larval weight (±SEM) in grams of A. glabripennis larvae reared in green ash,
northern red oak, and sugar maple; n=4 trees per species. Different letters indicate a
significant difference between treatments within each trial (PLSD, P<0.05).

Results

These preliminary experiments demonstrate that host suitability for ALB can be evaluated under laboratory and greenhouse conditions. Evaluating early larval survival from insertion of larvae into potted trees provides a means for evaluating host suitability. These tests show that larval insertion can be accomplished without a significant reduction in larval survival. The insertion technique provides an additional tool for determining larval survival especially in living trees and provides a mechanism for testing trees when adult beetles are not readily available.

Insertion experiments have two important limitations: 1) they do not provide information on the behavior of adult female beetles in the process of choosing a tree for oviposition and 2) they can not be used to determine whether a tree or tree species is a suitable or preferred oviposition host for beetles given the opportunity to choose a suitable host. This can only be accomplished by adult preference trials on a variety of live trees.

Results from the oviposition tests on cut wood sections together with the potted tree trials indicate that northern red oak may be an acceptable host for oviposition by ALB females, at least under limited choice conditions, and that larvae are able to develop on northern red oak phloem and wood. The question remains whether live northern red oak would be a preferred host for oviposition under conditions allowing the female ALB to choose among several tree species within the same cage and under the same environment.

3. Oviposition preference on live trees

Recently, oviposition preference trials were initiated at the ALB quarantine greenhouse on the Penn State University campus. In April 2002, enough mating pairs of adult beetles were available to begin preference experiments on northern red oak, sugar maple, green ash, and red maple (Acer rubrum). Two trees of each test species were randomly placed into each of the four trial cages. Each cage received a total of three mating pairs over a two-week period with each cage receiving a pair before a second pair was introduced. Each beetle in a given cage was marked with different color fingernail polish on the elytra or pronotum to aid in identification and to document individual behavior and mating habits. The mating pairs remained in the cages for 30 days. The pairs were observed three times a day and sporadically throughout the night over the 30-day period to document behavior and mating activity.

The experiment remains active at this time. Once all of the beetles have been removed the trees will be thoroughly examined to record oviposition sites, numbers of sites per tree, and location of sites on each tree to determine preference and to allow follow-up on successful egg laying. Trees will be destructively harvested within 90 days of the removal of the adult beetles in order to determine the number of viable larvae and evaluate larval development.

Preliminary behavioral observations include:

Oviposition trials will continue during the next several years. Next on the trial list are callery pear (Pyrus calleryana), panicled goldenraintree (Koelreuteria paniculata), river birch (Betula nigra), London plane tree (Platanus x acerifolia), and littleleaf linden (Tilia cordata).

This highlighted research represents three ways in which we can better understand the biology of ALB and identify both suitable and non-suitable host trees. By characterizing oviposition activities, identifying preferred host trees, and confirming non-preferred trees we can reduce the direct financial and indirect economic impact this pest may have on forest, landscape, and nursery industries in Pennsylvania. Identifying non-susceptible cultivars will accelerate the re-greening of infested sites throughout the U.S., thereby assisting in controlling the spread of the beetle.

Special Thanks

Literature Cited


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Format updated January 12, 2010