Flatheaded appletree borer, Chrysobothris femorata
Order: Coleoptera
Family: Buprestidae
Description
Adults of flatheaded tree borer are green or bronze with zigzag bands on the wing covers. The larvae are large, reaching up to 3.2 cm (1.25 inches) in length. They have a flattened head region and a creamy white body. Given their size and voracious feeding behavior, which involves forming a network of tunnels called a “gallery,” a single larva can cause considerable damage to a host tree, retarding growth or even killing it.
Quick Facts
- The flatheaded appletree borer is a beetle. Beetles are the most diverse insect group with over 350,000 known species worldwide, however only a fraction of these are considered insect pests.
- Damaged and drought stressed trees are more susceptible to flatheaded appletree borer infestations.
- This pest primarily attacks apple trees but can also target other deciduous flowering trees including beech, dogwood, elm, maple, oak, and willow.
- Injury results from larvae burrowing into the bark and feeding on the phloem and sapwood from within the tree.
Flatheaded appletree borer larva inside a tree. The flatheaded appletree borer gets its name from the larval stage, which has a flattened and enlarged thorax just behind the head. The larvae burrow into bark and feed on the phloem and sapwood from within the tree. Although apple trees are the preferred host, this insect pest can attack a variety of deciduous trees. Image credit: James Solomon, USDA Forest Service, Bugwood.org
Flatheaded appletree borer adult. Image credit: James Solomon, USDA Forest Service, Bugwood.org
Life history and habits
Full grown larvae bore deeper into the tree where they overwinter and pupate in early spring. Adults emerge later in the spring by burrowing a D-shaped exit hole through the plant. Females are fertilized within a week after emerging and live about 30 days.
Injury
Flatheaded appletree borer larvae damage the host plant by feeding on the sapwood and phloem, which is important for plant health. Tunnels formed by larvae can be up to 9.5 mm (3/8 inch) in diameter. Trees are more susceptible to flatheaded appletree borer infestation 2-3 years after transplantation, or 1-3 years after grafting because of the exposed wood where the cut was made. Therefore, this pest is especially destructive in plant nurseries and urbanized landscapes. Tree susceptibility declines as tree diameter and age increase.
Some signs of tree damage include wilting or browning of leaves, oozing infections of bacteria or yeast over damaged sections, and splitting of bark. The bark of infested apple trees may also slough off in areas where severe damage has occurred, exposing the heads of borer larvae and their tunnels. This makes infested trees more susceptible to secondary infection by pathogens, which can pose a greater threat than borer damage. In some cases, heavy winds can break younger trees when enough structural damage has been done.
Damage caused by flatheaded appletree borer larvae. A “gallery” is observed, which is the network of tunnels formed by feeding larvae. Image credit: John Ruter, University of Georgia, Bugwood.org
Monitoring
Surveying can be done in spring with purple traps coated with an adhesive paste. This trap targets a wide range of buprestids, so accurate identification depends on knowing which pests are problematic in the area. Trees can also be assessed for injury by flatheaded appletree borer; this is best done in the early spring before adults emerge and begin reproducing. Visual indicators of infestation on the host tree include sap oozing from gaps in the bark, the presence of “galleries” under bark, dieback of twigs and branches, and the presence of D-shaped holes. Sometimes adults can be found on patches of bark exposed to the sun.
Cultural control
The best strategy is to prevent infestations in the first place by keeping trees healthy. Avoid planting new trees near flowering crab apples, hawthorns, or service berries, as these serve as wild host plants for this insect pest. Farmers should consider removing wild host plants within 300 yards of the orchard. Heavily infested trees should be removed and destroyed by burning. Deadwood should also be removed and sanitized.
In the spring, tree trunks can be covered up to the branches. This provides a physical barrier between pregnant females and their preferred sites on bark for egg laying. However, this method can result in unfavorable growth of sprouts when all cut surfaces are covered, giving the tree a knotted appearance.
In addition, the use of trap logs can help mitigate the severity of infestations. Freshly cut posts or fallen logs can be placed in the exposed sun and coated with an adhesive or left alone until after the egg laying period. Trap logs should then be removed and burned.
Lastly, removing larvae by hand is a labor-intensive but effective strategy. Bark should be assessed for pinholes exuding sawdust. Using a knife, cut the bark in this area and use care to not cause further damage to the tree. A hooked wire can be used to impale and remove individual larvae from the tree.
Biological control
Natural enemies of flatheaded appletree borer include ants, woodpeckers, and parasitic wasps. Removing vegetation from the base of trees so the trunk is exposed helps provide these natural enemies with greater access to the pest.
Chemical control
Insecticides can be applied in different ways and target different life stages of the pest. Spraying foliage with contact insecticides targets adult beetles before they have a chance to lay eggs. This should be done after adults emerge and again after three weeks for a repeat application. These insecticides can also be applied shortly after the tree’s blooming period; however, this should be avoided to minimize adverse impacts on pollinators. Lastly, intentional cuts made for grafting and bud growth should be covered with one of these contact insecticides in order to kill egg laying females.
Systemic insecticides can be applied to the soil in the early spring, fall, or after budbreak. This method targets eggs and larvae. The timing of application is important, as the toxins must be absorbed into the tree where they can act on juvenile stages.
References
Mississippi State University. 2020. Flatheaded Appletree Borer, Vol. 6, No. 28. Mississippi State University Extension Service. Available http://extension.msstate.edu/newsletters/bug%E2%80%99s-eye-view/2020/flatheaded-appletree-borer-vol-6-no-28
Texas A&M University (n.d.). Managing the Flatheaded Appletree Borer. Texas A&M University: Extension Entomology. Available https://extensionentomology.tamu.edu/publications/managing-the-flatheaded-appletree-borer/
University of Tennessee. (n.d.). Flatheaded Appletree Borer (Chrysobothris femorata) and Related Species. University of Tennessee: Institute of Agriculture. Available https://extension.tennessee.edu/publications/Documents/W289-Q.pdf