Pest alert: Ambrosia beetles

Exotic ambrosia beetles (ABs) established in the U.S. continue to be damaging insects of trees in ornamental tree nurseries and are increasingly being recognized as destructive insects in apple and pecan orchards. ABs are consistently ranked among the most concerning insect pests of horticultural tree crops in the eastern U.S. Rapid branch dieback and tree death on alarming scales are commonly observed following AB infestations due to their wood-boring behavior and association with symbiotic and secondary microorganisms. AB infestations can be a major source of revenue loss. Through a $7.5 million grant from the USDA-NIFA-Specialty Crop Research Initiative, a team of researchers in 10 states in the Eastern U.S. are developing sustainable integrated pest management practices for ABs that maximize tree health and reduce tree vulnerability, optimize monitoring tactics, and identify chemical and biological alternatives to broad-spectrum insecticides. The economic significance of ABs are also being addressed to decrease the negative economic impacts of ABs through prevention of tree loss, increase marketability of nursery trees, sustained orchard production, reduced management inputs by growers and improved profitability.

Identification and distribution

Two exotic species of ABs are especially problematic in ornamental nurseries in the eastern U.S.: the black stem borer Xylosandrus germanus and the granulate ambrosia beetle Xylosandrus crassiusculus (Fig. 1). Both species are native to southeastern Asia. The black stem borer was first reported in 1932 from greenhouse-grown grape vines in New York and the granulate ambrosia beetle was first reported in 1974 from infested peach trees in South Carolina. The black stem borer and granulate ambrosia beetle are currently established in 29 and 32 states, respectively, mainly in the eastern U.S. The black stem borer tends to be more abundant and problematic in ornamental nurseries located in the Midwest and the Northeast while the granulate ambrosia beetle dominates the mid-Atlantic and the South.

These ambrosia beetles are shiny, stout, rounded in front and rear, and with the head hidden under a prominent, rounded shield. The female black stem borer is about 2.0 mm in length and the female granulate ambrosia beetle is about 2.5 mm in length (Fig. 1).

Wood-boring behavior

Adult females tunnel horizontally into the sapwood and heartwood of stems and sometimes branches of trees (Fig. 2). In smaller diameter stems and branches, the tunnel may reach the pith and extend vertically. Tunneling by ambrosia beetles can result in “toothpicks” or “noodles” extending out of the stems and branches (Fig. 2), but these are not always apparent especially during windy, dry or wet conditions. Since the adult females do not feed on the host plant, the “toothpicks” are not frass but instead compacted chewed wood that is pushed out of the tree during the tunneling process. Sap stains on the bark is also a key symptom associated with an AB infestation and is most frequently observed in spring (Fig. 2). Wilting foliage and branch dieback are also symptomatic of an AB infestation and commonly observed in spring.

As these ABs tunnel into a host tree, spores of their nutritional fungal symbiont are transferred from a specialized pouch on the body of the beetle to the tunnel walls inside the tree. Gardens of their fungal symbiont are then established, which serves as the sole source of nourishment for the larvae and adults (Fig. 3). Eggs, larvae and pupae only occur within the galleries created in host trees by the tunneling female beetles (Fig. 3). Males of both BSB and GAB are flightless and unlikely to be seen outside of the gallery.

Check out a timelapse video of a tunneling ambrosia beetle here.

Ambrosiella grosmanniae is the fungal symbiont of the BSB and Ambrosiella roeperi is the symbiont of the GAB. Neither fungal symbiont is considered a plant pathogen, but their presence within host tree tissues might be associated with branch dieback. A variety of secondary bacteria, yeasts and filamentous fungi have been isolated from BSB and GAB galleries and may contribute to branch dieback. Researchers are seeking to understand how ambrosia beetle fungal symbionts and/or secondary microorganisms contribute to branch dieback and tree death.

Seasonal activity. Adult BSB and GAB overwinter as adults within infested trees, predominantly within woodlots adjacent to nurseries and orchards. Ongoing research is seeking to improve our understanding of ambrosia beetle dispersal and infestations in ornamental and orchard cropping systems, including the “who, what, where, when and why” of attacks. One to three generations of BSB and GAB occur in the eastern U.S. depending on latitude. The first flight of the BSB tends to occur in March to April in Illinois, Missouri, North Carolina, Ohio, Tennessee and Virginia. The first flight of the GAB tends to occur in February to March in South Carolina, Mississippi and Florida. Peak flight activity and the initiation of tree infestations tends to occur during the spring months.

Host selection. A wide range of woody species are selected, including horticultural plants and trees growing in ornamental nurseries and orchards. The BSB and GAB can infest a wide variety of 100-200 woody species. BSB and GAB tend to infest thin-barked deciduous species in ornamental nurseries. Some genera or species that have been attacked in ornamental plant nurseries within the eastern U.S. include dogwood (Cornus spp.), honeylocust (Gleditsia triacanthos), Japanese snowbell (Styrax japonicus), magnolia (Magnolia spp.), maple (Acer spp.), oak (Quercus spp.) and redbud (Cercis spp.)

Several studies have demonstrated that BSB and GAB preferentially infest trees in the early stages of physiological stress; healthy trees are rarely attacked or colonized. BSB and GAB have been reported to attack “apparently-healthy” trees showing no visible symptoms of stress, but physiologically stressed trees can visually appear healthy while emitting stress-related volatiles (i.e., ethanol) that attract ambrosia beetles. Ethanol is emitted by trees in response to a variety of stressors and act as a powerful attractant to ambrosia beetles. Research efforts are evaluating portable devices to rapidly detect ethanol and alert growers about the risk of AB infestations. In conjunction with IPM strategies for other insects, maintaining host vigor and plant health is the most important step for reducing tree vulnerability to ambrosia beetles.

Stress can be imposed on ornamental nursery trees in the form of abiotic (non-living) or biotic (living) factors. Field observations and corresponding research have demonstrated that the abiotic stressors of poor drainage/flood stress and low temperature stress induce attacks by BSB and GAB. These ambrosia beetles even preferentially infest trees intolerant of flooding compared to flood tolerant tree species. BSB and GAB have not been found to preferentially attack drought stressed trees, but studies are ongoing. Low temperature stress during the winter months or late spring frosts (especially after a mild winter) have also been determined to precede ambrosia beetle infestations in ornamental nurseries. Herbicide damage has preceded ambrosia beetle infestations, but few studies have been conducted to characterize this relationship. Mechanical damage associated with pruning has not been found to induce ambrosia beetle attacks. Infestations by BSB and GAB can also occur due to biotic stressors such as fireblight (Erwinia amylovora) and Phytophthora infection. Ongoing studies are being conducted to further characterize the impact of abiotic and biotic stressors to aid in developing a management plan. Characterizing the role of plant stress will aid stakeholders in identifying risk factors, modifying production practices and implementing sustainable management plans to minimize the likelihood of future attacks.

Monitoring. Ethanol is the standard attractant used in traps for monitoring BSB and GAB. Traps can be purchased commercially or homemade (Fig. 4) with the use of soapy water as a killing agent. Stem sections (i.e., bolts) can also be soaked for 24-48 hours in ethanol and deployed to induce ambrosia beetle attacks for monitoring. Ethanol-baited traps or bolts should be placed near wooded areas and suspended just above ground level since BSB and GAB disperse from their overwintering sites in woodlots and fly relatively low to the ground. Optimizing monitoring tactics, characterizing AB movement, and seasonal activity within the three cropping agroecosystems is being pursued for precision-based management tactics. Ambrosia beetle activity in the eastern U.S. is available in near real-time by visiting www.StopAB.org.

Management tactics. Minimizing the risk of abiotic and biotic stressors to maintain host vigor is the foundation of an integrated pest management program for BSB and GAB, especially since conventional insecticides are not particularly effective at reducing attacks on stressed trees. Growers that consistently lose crops to ambrosia beetles must consider the role of stress in predisposing trees to attack and attempt to adjust production practices to minimize the risk of future infestations.

Promising studies have demonstrated the potential for entomopathogenic and antagonistic fungi to control ABs and their fungal symbionts. BSB exhibited high mortality after treatment with Beauveria bassiana and Metarhizium brunneum. GAB exhibited poor brood production within stems treated with B. bassiana, M. brunneum, and Trichoderma harzianum. A novel approach to deploying entomopathogenic fungi is to establish the fungi as endophytes in the plant, which can provide “built-in” protection for several seasons. Entomopathogenic nematodes have not been reported as biocontrol agents against ABs, but they are highly virulent to other beetles, and they can disperse into the AB galleries within trees. Ongoing studies are evaluating the efficacy of entomopathogenic fungi and nematodes against BSB and GAB.

Conventional insecticides have limited efficacy in protecting trees against BSB and GAB. Spring emergence of BSB and GAB also coincides with fruit and nut tree bloom, which presents risks to pollinators. Permethrin and bifenthrin are the most effective active ingredients, but their efficacy is variable and highly disruptive to biological control agents. Trunk sprays of pyrethroid insecticides (i.e, bifenthrin, cypermethrin and permethrin) are more effective than other materials but provide inconsistent efficacy against BSB and GAB. No conventional insecticide provides 100% protection, and no rescue treatments exist once a plant has been infested. Preventive insecticide treatment is recommended to occur prior to peak flight activity. Insecticide applications are most effective when closely timed with flight activity. Substrate drenches and trunk sprays of systemic insecticides are not effective, for instance, chlorantraniliprole, cyantraniliprole, tolfenpyrad and dinotefuran. Researchers are conducting ongoing studies to compare the efficacy of conventional and reduced-risk insecticides, including active ingredients, residual longevity, application frequency and surfactants.

Research efforts are seeking to integrate repellents and attractants into a “push-pull” strategy, whereby repellents push beetles away from vulnerable trees and attractants pull beetles into annihilative traps. Studies have demonstrated that verbenone and methyl salicylate can repel or interrupt the attraction of BSB and GAB, but the efficacy is inconsistent and efforts are attempting to improve the push component. Likewise, a perimeter of ethanol-baited traps around trees has shown promise for intercepting beetles to reduce attacks on trees. Ongoing studies are seeking to identify an optimal release rate of ethanol to maximize trap captures. A “push–pull” strategy for ABs might be improved using combinations of repellents and higher ethanol release rates to increase trap captures.

Economics. To ensure that the adoption of improved tactics for managing AB is economically feasible, agricultural economists are seeking to quantify the commodity-specific economic impact of ABs and identify social barriers to facilitate growers adopting the new technology developed. Economic models that estimate the financial losses from ABs, grower input costs, and cost-benefit analyses for management will be made available at www.StopAB.org. Enterprise budget tools will develop to help growers make informed decisions.

For more information, visit www.StopAB.org available through the Southern IPM Center. A toolkit of extension deliverables will be made publicly available via www.StopAB.org to aid in managing ABs, instructional videos, webinar recordings, fact sheets and interactive tools to evaluate risk management.

Funding to address ambrosia beetles infesting trees in nurseries and orchards is being provided by USDA-NIFA-SCRI award #2021-51181-35863, USDA-Floriculture and Nursery Research Initiative, and base funds associated with USDA-ARS National Program 305 Project #5082-21000-018-00D.