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Bronze Birch Borer

Ravageurs et maladies
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Bronze Birch Borer.
Photo: Jardin botanique de Montréal (Michel Tremblay)

Onglets

Description

Summary

Bronze birch borer has again become a serious problem in urban areas in recent years. This small beetle may attack all birch species. Stressed, weakened and older trees are the most vulnerable. The main damage is caused by the larvae, which carve out tunnels under the bark, preventing the sap from circulating. The symptoms are yellowed leaves that drop prematurely from the crown of the tree, and a few dead branches. Raised welts and "D"-shaped holes can be seen on the trunk and branches. A seriously affected tree may die within a few years or even in the first year of a bronze birch borer infestation.

Signs and symptoms

  • The signs of damage are discoloured leaves that drop prematurely from the crown of the tree, which then appears denuded.
  • In addition, raised welts may be seen on the trunk and infested branches; they are caused by larvae tunnelling under the bark.
  • The tunnels cut off the flow of sap and water, drying out and killing the wood immediately above the infested parts. In subsequent years, the females lay their eggs lower down, killing other parts of the tree.
  • When the adults appear, they leave "D"-or half-moon shaped emergence holes in the bark; brownish sap sometimes leaks from the holes.
  • The adults cause minor damage by feeding on the foliage.
  • Depending on the tree's age, the number of larvae and the location of the tunnels, an affected tree may die within a few years or even in the first year of a bronze birch borer infestation.

 

Latin name (genus)

Agrilus anxius

Host plants

All birch species, but particularly European white birch and its cultivars, paper birch and grey birch.

Development cycle

Description and life cycle

Bronze birch borers are small beetles belonging to the order Coleoptera. They undergo complete metamorphosis (holometabolic insects). Before becoming adults, they pass through the egg, larval and pupal stage.

Eggs: Oval and flattened, about 1.5 mm long; creamy white at first, turning yellowish.

Larvae: White, elongated and flattened; 13 to 35 mm long; legless, with two small pincer-shaped spines at the tip of the abdomen.

Pupae: Similar to adults, but smaller; creamy white, turning olive-green as they mature.

Adults: Elongated body (7 to 12 mm); olive-green to black, with metallic bronze; square head, smooth back and pointed abdomen.

Bronze birch borers overwinter as larvae, hidden deep in the tree. In spring, when the sap begins to flow, the larvae wake up and start carving out tunnels under the bark, where they pupate before emerging as adults.

Adults emerge from early June to August, forming characteristic “D”-or half-moon shaped holes in the bark. Each adult lives for about three weeks. The adults mate shortly after emerging. The females then lay their eggs in crevices in the bark, usually on parts of the tree exposed to the sun.

The eggs hatch after a two-week incubation period. The young larvae burrow through the bark to reach the cambium, where they carve out meandering tunnels.

In fall, they move deeper into the tree, where they spend the winter. In our climate, it usually takes two years for the full life cycle to be completed.

Prevention and control

Favourable conditions

Bronze birch borers generally lay their eggs in trees that are stressed, weakened or old. Some birch species are particularly susceptible to bronze birch borer infestations.

Identification

Keep a close eye on your trees to spot any loss of leaves from the crown and the appearance of raised welts and holes on the trunk and branches. Peel off the bark under the welts to see whether there are any tunnels; sometimes you may find larvae just beneath the bark.

Prevention

  • Plant less-vulnerable birches, including black birch and Japanese white birch.
  • Choose species other than birch, particularly if bronze birch borer is a problem in your area.
  • Keep your trees healthy by fertilizing them properly and watering them during dry spells.

Physical control

  • Remove and destroy all infested branches 30 to 50 cm below all dead wood or below the last "D"-shaped emergence hole. It is best to prune in fall. Avoid pruning in spring, because the abundantly flowing sap at that time of year will attract females in search of a place to lay their eggs.
  • If more than 50% of the volume of the tree is damaged, cut the tree down and destroy it, preferably before the adults emerge in early June.

Biological control

Allow woodpeckers into your yard, as they will feed on larvae under the bark.

Chemical control

The Montréal Botanical Garden does not recommend the use of pesticides to control this insect.

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