Sugarberry (Celtis laevigata), commonly called southern hackberry in the southern US, has been declining and dying at elevated rates in South Carolina and Georgia over the past decade (figure 1). While the causal issue has not yet been identified, a woodboring beetle, Agrilus macer, is commonly associated with declining and dying Celtis trees in this region. This beetle is native to the US and develops in stressed Celtis trees in urban, suburban, and forested settings. While the presence of A. macer in any life stage can be an indicator of tree stress and may hasten the death of weakened sugarberry trees, this beetle does not appear to be a primary cause of tree decline or mortality. Under favorable conditions, trees can overcome moderate attacks from this conspicuous secondary pest.1
Agrilus species related to A. macer, such as the two-lined chestnut borer (A. bilineatus) and the bronze birch borer (A. anxius), can cause tree damage and losses within their native range. Other related species are emerging pests highlighting the damaging capabilities of Agrilus species: the emerald ash borer (A. planipennis) is one of the most impactful forest pests in North America, and the two-spotted oak borer (A. biguttatus) is commonly associated with oak decline in Europe.
Description and Life History
Agrilus macer belongs to the family of flatheaded borers (Buprestidae), which are commonly characterized by elongated, “bullet-shaped” bodies (figure 2a). Adult A. macer are metallic black to dark green in color and between 1/3–2/3 in (between 8–16 mm) in length (figure 2b). Adults can be easily distinguished from other native Agrilus species by the raised ridge on each outer wing cover.2
Adults are most active in the summer months and can be found on Celtis foliage or on the trunks of weakened trees. After mating, females will lay eggs in clusters (figure 2a), each with an average of sixteen eggs, on the smooth areas of Celtis trunks and branches. The female then covers the egg clusters with a green substance and smooths the surface with a final tan substance that hardens into a distinctive cap, presumably to protect the egg mass from moisture loss and predators such as ants (figure 2c). The egg masses appear as bumps on the bark of the tree and can be present at incredible densities from the tree base to the ends of branches (figure 3a).
The large egg masses produced by A. macer are unusual, but not unheard of, among buprestid beetles and may be a tactic for overcoming local host-plant defenses. As the larvae inside the egg masses mature, the egg masses turn a dark color. Larvae from each egg mass bore through the bark and begin excavating galleries in the cambium layer (figure 4a, 4b, 4c, 4d). Extensive larval boring can girdle trees and are the primary cause of damage, as the boring limits the movement of sugars and nutrients within the tree. Healthy trees can withstand A. macer larval feeding and boring and will respond by forming callus tissue at the site of the egg mass and hatched larvae (figure 3b and 3c). After larval development, emerging adults will leave D-shaped exit holes along the trunk and main branches of the trees (figure 5).
Distribution and Hosts
Agrilus macer (figure 6a) can be found from California in the West to South Carolina in the East. Six native Celtis species occur across the range of A. macer and are all potential hosts, with sugarberry (C. laevigata) and hackberry (C. occidentalis) being the most widespread species in the eastern US.3 These tree species have smooth bark when young (figure 6b) and are well-known for their warty bark when mature (figure 6c). The mature trees bear round fruit (figure 6d) and have strongly serrated ovate leaves (figure 6e). In addition to these native species, species of non-native Celtis (e.g., Chinese hackberry, C. sinensis) have become naturalized in North America.4
Damage and Impact
Agilus macer larval galleries can be visible beneath the bark and attacked trees may weep black liquid around the egg masses (figure 7). There is no evidence this species transmits highly pathogenic fungi. Discoloration can be seen around the galleries of the larvae, but this color pattern is most likely due to weakly pathogenic fungi that cause a very limited host response.1
A. macer appears to be relatively rare throughout most of its range, but major population increases can occur in areas experiencing high rates of Celtis mortality. Under such situations, attacks by this species are likely to further weaken trees and have the potential to hasten their death. In recent years, parts of South Carolina and Georgia have been experiencing severe sugarberry mortality with associated symptoms of small yellowing leaves, premature defoliation, and branch dieback, and A. macer is active throughout this area.
Prevention and Management
At this time, no effective management strategies are known, although chemical treatments (e.g., emamectin benzoate formulations) are known to be useful in protecting against other species of Agrilus, such as the emerald ash borer. However, because A. macer is not a primary cause of Celtis spp. mortality, trees attacked by this species are likely to be experiencing one or more other stressors that predisposed them to attack by A. macer.1
Thanks to Jennifer Moore-Myers (USDA-Forest Service) and Kevin Chase (Bartlett Tree Experts) for comments on an earlier version of this document.
Poole EM, Ulyshen MD, Horn S, Cram M, Olatinwo R, Fraedrich S. Biology and distribution of Agrilus macer LeConte (Coleoptera: Buprestidae), a species associated with sugarberry (Celtis laevigata Willd.) mortality in the southeastern USA. Ann For Sci. 2019; 76(1):7.
Harpootlian PJ, Bellamy CL. Jewel beetles (Coleoptera: Buprestidae) of South Carolina. Clemson (SC): Clemson University; 2014.
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