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Determining the Host Range Prefereences of Asian Longhorned Beetle
on Commonly Planted and Recommended Urban Trees

James C. Sellmer, Silvia Montero, Scott Ludwig, and Kelli Hoover

Pennsylvania State University, Depts. Of Horticulture and Entomology,
University Park, PA 16802

A paper from the Proceedings of the 11th Metropolitan Tree Improvement Alliance (METRIA) Conference held in Gresham, Oregon, August 23-24, 2000, and cosponsored by the Landscape Plant Development Center.

Abstract

The introduction of Asian longhorned beetle (ALB) (Anoplophora glabripennis) into the United States has resulted in the destruction of thousands of street trees in Chicago and New York.  This exotic insect pest has the potential to impact every aspect of the landscape and nursery industry from production scheduling and product mix through landscape plant selection for new and infested sites to maintenance and pest management strategies. Asian longhorned beetles can kill trees and to date the only effective control method for reducing the spread of ALB is by the identification and removal of infested trees.

Presently, our understanding of the susceptibility of trees to feeding and oviposition remains limited to field observations in China, Chicago, and New York and laboratory experiments on excised branches and cut logs. In China, the highly preferred trees include poplar (Populus spp.), willow (Salix spp.), and maple (Acer spp.), whereas chinaberry (Melia spp.), mulberry (Morus spp.), black locust (Robinia pseudoacacia), elm (Ulmus spp.), and some fruit trees (i.e. plums and pears) are occasionally attacked. Observational reports from the New York and Chicago infested sites listed several Acer species (Norway, sugar, silver, sycamore, and boxelder) as primary hosts. In addition, several new larval hosts were identified in these infestations, including Betula species, Aesculus hippocastanum, and Fraxinus pennsylvanica.   Research results from a survey under laboratory conditions using cut logs by USDA/APHIS/PPQ suggest that the potential host range may be larger, although maples remain the preferred target in their studies.

Research initiated this year at Penn State University within an APHIS certified quarantine greenhouse consists of challenging container grown trees that are reportedly susceptible and commonly planted in the landscape to evaluate oviposition preference and larval development.

Keywords:  Anoplophora, Asian longhorned beetle, susceptibility, oviposition, larval development

Introduction

The Asian longhorned beetle (ALB), Anoplophora glabripennis Motsch., a cerambycid species native to Asia, was first discovered in North America in New York in 1996, then in Chicago in 1998 (Haack et al. 1997; USDA-FS 1999). Presently, the only effective control method for reducing the spread of ALB is by the identification and removal of infested trees. To date over 6,100 trees, nearly 80% of them maples, have been destroyed in these two communities at a cost of over $10 million solely for detection and removal of infested trees not including replanting or loss of aesthetic value (USDA-APHIS-PPQ 1998). 

ALB is believed to have colonized North America as a stowaway in solid wood packing materials (SWPM), which includes crating, pallets, dunnage, and stowage originating from China (Haack et al. 1997).  USDA-APHIS-PPQ has mounted a campaign to eradicate this pest from North America by establishing domestic quarantines that prohibit local transportation of potentially infested wood, aggressively scouting for and eliminating infested trees (Haack et al. 1997), and imposing a new and more stringent shipping rule that requires SWPM from China be treated to kill wood borers (18 September 1998 Federal Register 63 FR 50100-50111, Docket No. 98-087-1). 

The USDA has predicted that ALB could have severe economic, social, and ecological impacts on urban and natural forests of North American (Mastro and Cavey, 1996). The USDA estimated the potential national impact if ALB were to infest all urban centers of the United States to be a loss of 26.5% of the canopy cover and 30.7% of the urban trees equaling approximately 1.2 billion trees. This loss would carry a compensatory value of  $522 billion (Nowak et al. 2000). This does not include losses to forest trees, maple sugar sources, dependent industries (e.g. furniture making), recreation, or tourism. With new infestations continuing to be identified near the Chicago and New York sites, the broadening quarantine zones suggest that it is unlikely that ALB will be eradicated from North America.  In fact, the New York Commissioner of Parks & Recreation, Henry Stern, stated in a recent news conference that the chance of eradication remains remote (Lueck 2000). Moreover, new infestations are likely to arise in other parts of the country in areas where ALB adults have emerged from imported wood materials. Over 26 warehouse infestations of ALB have been discovered in 14 states in the past two years.  Given the open nature of these warehouses and the fact that infested crating can sit in warehouses for years, it is possible that some beetles may have already escaped into local tree populations and have yet to be discovered (USDA-Forest Service 1999).  It is notable that ALB was present in both New York and Chicago areas for at least 15 years before being detected (Haack et al. 1997, USDA-APHIS, unpublished data); thus, the species is probably already broadly distributed in both areas, and in other parts of the country, but has escaped detection due to low population densities.  Spread of the pest in North America is unlikely to be inhibited by climate; ALB ranges in China from 21°N to 43°N latitude, corresponding to an area in the Americas from southern Mexico to the Great Lakes (Haack et al. 1997).

ALB is a clear threat to our aesthetic and economically important landscape and urban tree resources.  This exotic insect pest has the potential to impact every aspect of the landscape and nursery industry, from production scheduling and product mix decisions by nurseries and landscapers through plant selection and establishment in new and infested sites, to maintenance and pest management strategies.  However, plant selection, production scheduling, and pest management will be ineffective without information on plant susceptibility. To date minimal information is available on host range specificity in North America.  

Although the adult beetles are visually distinct, feeding damage by adults is difficult to spot and can easily be attributed to other causes. The greatest damage to infested trees occurs through the feeding and development of the larvae. Generally, oviposition occurs in the upper trunk and along major branches, although small branches can be attacked as well (Haack et al., 1997). The eggs are individually oviposited below the bark at the phloem-bark interface. After hatching, early instars feed at this interface before tunneling in through the sapwood to the heartwood. During their development, the larvae tunnel upward for 10 to 30 cm from their oviposition site. Full-grown larvae reach 50 mm in length. Larvae pupate inside the galleries, usually in summer. Newly developed adults emerge from exit holes 6 to 18 mm in diameter. By completing an entire cycle from larval feeding and development to adulthood, ALB causes the death of branches and eventually entire trees. As the crown begins to die, new generations continue laying eggs along the entire trunk and even on exposed roots

The devastating nature of this pest creates a critical need to investigate the susceptibility of commonly planted landscape trees to ALB oviposition and larval development. By proactively addressing the biology of this pest and its hosts we may be able to reduce the destructive potential that this beetle may have on the forest and landscape ornamentals industries. In China, ALB preferentially attacks poplars (Populus spp.), willows (Salix spp.), and maples (Acer spp.), but trees in many other genera are occasional hosts (Haack et al. 1997).  Experiments conducted by USDA/APHIS/PPQ on cut logs suggest that the potential host range may be broader (Table 1).

Table 1. Tree genera identified as susceptible to ALB from cut log screening experiments(USDA-FS, Wang. 1999).

Acer Larix
Abies Liquidambar
Alnus Lirodendron
Arbutus Nyssa
Betula Picea
Carya Pseudotsuga
Catalpa Quercus
Fraxinus Tilia
Fagus Tsuga

Given that the beetle appears to oviposit indiscriminately on cut logs (Vic Mastro, unpublished data), host range experiments using living trees is likely to yield a more accurate picture of the predicted host range in the U.S.

Plant Materials

A first round of screening experiments is currently underway to validate our greenhouse cultural and experimental systems, to characterize the mating and oviposition behavior among ALB adults, and to characterize the lifecycle and growth rate of the developing larvae within greenhouse grown trees (Table 2).

Table 2. Trees undergoing the first round of screening for susceptibility to ALB oviposition activity.

Botanic name Common name Average Caliper (mm)
Acer rubrum Red Maple 20.21
Acer platanoides Norway Maple 19.32
Acer saccharum Sugar Maple 30.11
Fraxinus pennsylvanica Green Ash 44.63
Quercus rubra Red Oak 28.56

We believe that by proactively screening common landscape trees for susceptibility to ALB oviposition and larval development we can provide vital information for field identification of susceptible trees and permit recommendation of non-susceptible trees. By characterizing oviposition activities, identifying susceptible trees, and confirming non-susceptible 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.

Screening experiments will continue with commonly planted urban trees to further verify the results of the cut log experiments conducted by the USDA and to verify that recommended replanting stocks will not be readily infested by ALB (Table 3).

Table 3. Trees to be screened during the next two years.

Scientific Name Common Name
Betula papyrifera Paperbark Birch
Betula populifolia Gray Birch
Fagus grandifolia American Beech
Liquidambar styraciflua Sweetgum
Liriodendron tulipifera Yellow Poplar
Nysa sylvatica Black Gum
Picea abies Norway Spruce
Picea glauca White Spruce
Tilia americana American Basswood
Betula nigra River birch
Betula populifolia White spire birch
Carpinus betulus European hornbeam
Celtis occidentalis Common Hackberry
Cladrastis kentukea Yellowwood
Corylus colurna* Turkish Filbert
Craetagus crus-galli Hawthorne
Gingko biloba Gingko
Gleditsia triacanthos* Honeylocust
Gymnocladus dioicus* Kentucky coffee tree
Koelreuteria paniculata Golden raintree
Pyrus calleryana Callery pear
Quercus macrocarpa* Bur oak
Quercus palustris Pin oak
Quercus phellos Willow oak
Quercus robur* English oak
Syringa reticulata* Tree lilac
Tilia cordata* Littleleaf Linden
Tilia tomentosa Silverleaf Linden
Zelkova serrata Zelkova

*Trees being recommended and used for replanting in infested areas

Methods

All experiments are conducted in a 1200 square foot quarantine greenhouse in four large cages, 10’ X 9’ X 7’, in which the beetles and potted trees are confined. Each cage holds ten trees in 20-gallon pots (Figure 1).

laboratory facility for screening live trees

Figure 1. Cages used for screening live trees with mating pairs of Asian longhorned beetle.

Asian longhorned beetles for this research are being obtained by rearing adults from: infested logs removed from Chicago or New York ALB-infested sites, infested logs from USDA’s colonies in Newark, DE, Otis, MA, or Hamden, CT, or artificial diet in our own facility. 

Trees are being acquired from several nursery sources across the nation as bare-root, lightly branched liners of approximately 1” caliper. One-inch caliper trees are considered the minimum size to allow larval development. The trees are grown in Fafard 52-bark based nursery soil mix supplemented with a slow release fertilizer within 20-gallon polytainer nursery pots treated with copper hydroxide to promote root development.  Each tree’s canopy and roots are lightly pruned prior to potting to maintain an average height of 6 to 8 feet. Trees will be irrigated weekly for an hour period using a drip irrigation system with an in-line fertilizer injector for supplemental fertilizations. Trees are allowed to equilibrate to the greenhouse and potting conditions for at least eight weeks prior to pruning for fit within the cages of the quarantine greenhouse.

All beetle challenges will be conducted under the same enclosure conditions with three to four replicates of each of five tree species randomized within each cage (Figure 2). A challenge cycle includes one month for the oviposition period and one to two months for

larval development and tree evaluation.  Oviposition by individual females generally peaks at 10-21 days post-adult emergence and begins to decline after 30 days post-adult emergence.

ALB screening cages layout  

Figure 2. A graphic example of the ALB screening cages showing the randomized pattern of potted trees within each cage.

Suspected oviposition sites will be marked and the trees will be cultured for an additional two months within the challenge chamber. During that time, sequential sub-sampling of putative oviposition sites will be carried out to determine the viability of the site and the stage of the egg or larval development. Observations in the field suggest that ALB larvae do not complete development in sycamore. Sequential sub-sampling allows us to efficiently follow larval development in the tested trees.

Non-choice experiments will be conducted with trees that were found to be non-susceptible. This will determine whether ALB will choose and oviposit on non-preferred host trees. In addition, artificial incorporation of eggs into non-susceptible trees will be conducted to determine whether ALB larvae can develop within non-susceptible trees.

The greenhouse challenge system along with the choice based challenge experiments offers the best approach to determine if the trees are susceptible to ALB and allows for the most rapid screening of trees. In nature, the beetles would choose their host trees by preference and would only be constrained by the choice of  trees within a specific locale. Under natural conditions a pattern of preference can be identified. In our effort to define a hierarchy of preference or susceptibility without artificial constraints a similar series of choices must be provided. By testing susceptibility under similar conditions we increase the chance that a preferred tree will be in the mix of available trees as well as less preferred trees. By separating host and non-host trees, we can further characterize the biology of this pest while identifying trees which merit greater testing for resistance either because it is not preferred or the larvae fail to fully develop.

Planned Outcomes

Special Thanks

Literature Cited

Haack, R. A., K. R. Law, et al. (1997). “New York’s Battle with the Asian Long-Horned Beetle.” Journal of Forestry 95 (12): 11-15.

Lueck, T. J. (2000). $10 Million Earmarked to Fight Beetles. New York Times. New York.

Nowak, D. J., J. E. Pasek, et al. (2000). “Potential effect of Asian longhorned beetle on urban trees in the United States.” Journal of Economic Entomology, submitted.

USDA-APHIS-PPQ (1998). Comprehensive strategy for preventing the establishment of the Asian longhorned beetle in the United States, unpublished.

USDA-FS, Wang. (1999). http://willow.ncfes.umn.edu/albpestalert/.


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