Bronze Birch Borers

Bronze Birch Borers (Agrilus anxius)

The bronze birch borer is a North American native. It may be a significant pest of forest and shade trees, mainly birch species. Larval feeding tunnels under the bark encircle the tree’s trunk or branch, causing injury.


Bronze Birch Borer larvae feed on the nutrient and water-conducting vascular tissues beneath the bark. Larvae girdle the tree by creating meandering channels via the phloem, vascular cambium, and xylem. Dieback of the canopy is a sign of Bronze Birch Borer larvae infestation; as the infection advances, more than half of the branches may die back. When a tree’s canopy dies, it replies by sprouting new (epicormic) branches below the damaged tissues. The bark will crack over the dead vascular tissues, and trees can break in as little as two years after symptoms appear.

Life cycle:

The life cycle of a bronze birch borer might span one or two years. The bronze birch borer is a serious insect problem in the United States. The adult is a black beetle with bronzy iridescence on the back that is about 1/2 inch long. It’s a sun-loving bug that may be spotted crawling on the trunk’s sunny side in late May and early June. The female lays her eggs in the bark’s cracks and fissures. The eggs hatch in 2 weeks or less, and the thin larvae tunnel into the phloem tissue to build their galleries. They may molt and overwinter by burrowing into the xylem (wood). In late April or early May, the larvae pupate in the xylem. When the black locust trees blossom, the adults take to the air.


The adult is a slender copper/bronze colored beetle. The larvae that cause the injury feed on the vascular tissue beneath the bark and go unnoticed. The Bronze Birch Borer prefers to feed on trees that are already stressed or dying. A birch afflicted with the Bronze Birch Borer will begin to display dieback in the crown, which will worsen as the infection progresses, eventually leading to the tree’s demise. The trunk may exhibit D-shaped, rust-stained exit holes and swelling extrusions under the bark where the tree tries to grow over larval galleries in later stages of infection.

How to prevent Bronze Birch Borer:

By Keeping Trees Healthy:

Trees that are vigorously growing are less likely to be harmed by borers than trees in poor health. Because birch trees have a limited lifespan, old age, bad weather, or other insect-related pressures can contribute to tree weakness. Most ornamental trees are cultivated in lawns, which are not ideal for healthy, vigorous development. Grass conditions are the polar opposite of the birch tree habitat. As a result, birch trees are frequently weakened, which attracts the bronze birch borer.

By Using Resistant Cultivars:

Borer-resistant species like Japanese birch, river birch, or the cultivar “Heritage” are one strategy to reduce the risk of borer attack. Another option is to water trees to keep them healthy. If rainfall is less than 1 to 1 1/2 inches per week, birches on lawns should be watered every 7-10 days. Allow the garden hose to run for several hours beneath the tree to ensure sufficient moisture reaches the root zone.

How to Controlling Current Infections:


Before the adults appear in early May, remove decaying trees and branches. This can help to limit the amount of borers attacking your trees.

Insecticide Sprays

Infested trees can be sprayed with permethrin (Eight) or Spectracide Bug Stop between mid-May and mid-June to kill adults sprouting from the bark and feeding on the leaves. Astro EC (permethrin) or Onyx can be sprayed by commercial applicators (bifenthrin).

Systemic Insecticide Sprays

Adults that eat on leaves and larvae beneath the bark can be killed with a soil drench of systemic insecticides. Between April 1 and May 15, soil treatments of imidacloprid (Bayer Tree and Shrub Protect I, Xytect, and others) or imidacloprid with clothianidin (Bayer Tree and Shrub Protect II) can be made. Between May 1 and June 15, professionals can administer dinotefuran directly to the trunk or as a soil drench.