Borers rarely infest healthy plants growing in their natural environments. However, when trees or shrubs are transplanted into the landscape, stresses such as drought, soil compaction, sun scald, or injuries can weaken them and make them more susceptible to attack. Adult borers may locate suitable egg-laying sites by responding to volatile chemicals that emanate from stressed trees.

Adult borers emerge from infested trees in the spring or summer. After mating, the females fly to a suitable host and lay eggs on the bark, often in crevices or around wounds. Hatching occurs about 10 – 14 days later. The young larvae quickly tunnel beneath the bark where they feed and grow. Once inside the tree, borer larvae are no longer vulnerable to insecticide sprays and are seldom detected until serious damage has been done.

Several species of Clearwing and Flatheaded borers can infest landscape plants. While some are attracted to a range of hosts, most attack only certain kinds of trees and shrubs. It is important to know when the adults of each species are active and which plants are vulnerable in order for treatment to be effective.

Types of Flathead Borers Most Commonly Found in Southeast Idaho

Emerald Ash Borer (Agrillus planipennis)

These adults are distinctive dark metallic green beetles that are about 1/2 long and about 1/8 inches wide. This invasive insect, which has killed hundreds of millions of ash trees in North America, was discovered in Kentucky in 2009. The larvae feed on the inner bark of ash trees from June through October, disrupting the tree's ability to transport water and nutrients. Unless protected, infested trees usually die within 4 to 7 years of attack. All native ash species are susceptible to attack. The insect has been found recently in white fringetree (Chionanthus virginicus).

Ash trees can be protected from attack by this devastating insect by:

1. Systemic insecticide applied by soil injection or soil drench; OR
2. Micro tree injection.

Bronze Birch Borer (Agrilus anxius)

This is a severe pest of white or paper birch trees in the landscape. Symptoms include twig and branch dieback starting in the upper 1/3 of the tree. With successive years of attack, the tree becomes progressively weaker until it dies. Dead or dying limbs will have numerous D-shaped adult exit holes, each about the size of a BB shot. Adult bronze birch borers are slender, olive-bronze beetles about ½-inch long.

The Flatheaded Appletree Borer (Chrysobothris femorata)

is a severe pest of small, stressed landscape trees, especially flowering crab apples, hawthorns, and maples during the first year or two after they are transplanted. This borer may attack almost any hardwood tree that is stressed by defoliation, sun scald, drought, soil compaction, or mechanical injury. This borer makes broad winding tunnels under the bark, destroying the phloem, cambium, and outer xylem. The full-grown larva is about an inch long, legless, and yellow-white. A single borer can girdle and kill a small tree.

Preventive Management for Borers

Stressed trees and shrubs tend to be more susceptible to borer infestation, so plant health care is the first line of defense:

• There are several trees commonly planted in southeast Idaho that are susceptible to bore infestation some of them are Birches, Aspens, Ash, Willow, Lilac, Pines and Spruces. This is not a complete list.
• Select well-adapted cultivars for your region.
• Use proper planting methods. Transplanted trees and shrubs need extra water until they become established.
• Control other pests that contribute to tree stress.
• Be careful not to wound the bark when mowing or using a string trimmer. Fresh trunk wounds are attractive to egg-laying adult borers.
• Avoid tree wraps – they may encourage borer attacks by delaying proper hardening of the bark.
• Inspect susceptible tree species regularly for the telltale signs and symptoms, and apply insecticides only during those periods when borers are vulnerable.
• Apply Deep Root Fertilizer in the spring or fall.