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Chicago Urban Gardening

The day to day experiences of a University of Illinois Extension Urban Horticulture Educator in Chicago, Illinois

Control Bronze Birch Borers

Posted by Ron Wolford -

It is now time to treat for bronze birch borer in southern Illinois. Treat in central Illinois around the middle of the month, and treat in northern Illinois towards the end of this month. The application of imidacloprid (Merit, Imicide, Pointer) as a bark spray or trunk injection provides effective control. Application to the soil is less effective because it takes several weeks to move through the tree, but it will still provide some control this year.

Bronze birch borer is a native species in the same genus as emerald ash borer and attacks the tree in a similar manner, except that it attacks declining birches instead of ash. Bronze birch borer attacks mostly nonnative, white-barked birches as their growth starts to slow down, typically when the trees reach 10 or more years old. It attacks younger trees that are mechanically damaged or planted in poor sites. Native white-barked birches are attacked much later in life, as they decline into old age. Whitespire, a variety of an Asian species, is resistant to the borer, but many other Asian and European varieties and species are very susceptible to attack. River birches are also resistant to attack, and Heritage is a variety of river birch commonly planted because of its light-colored bark.

Adult bronze birch borer beetles lay eggs under loose bark and in bark cracks near the top of the tree. The hatching larvae tunnel through the cambium. If the tunneling circles the stem, this girdling kills the stem beyond that point. Leaves turn brown and fall off. Early attack is recognizable as dead, leafless branches at the top of the tree. In subsequent years, the beetles attack lower and lower on the tree until the entire tree dies.

Bronze birch borer larvae are elongate, white, and flattened, with obvious beadlike segments. Fully-grown larvae are about 1 inch long. The larvae feed through the summer, overwinter as larvae, and pupate in the cambium area in spring. Although the life cycle can be completed in 1 year, a 2-year life cycle is more common. Adult beetles emerge through D-shaped holes during vanhoutte spirea bloom in mid spring. Cross-sections of adult beetles are flattened ventrally and rounded dorsally, that is, D-shaped. They are about 1/2 inch long and appear bullet-shaped from above, being quadrate in front and tapered posteriorly. As the name indicates, they are bronze in direct sunlight but appear blackish in indirect light. The adult beetles feed on the leaves of alder, poplar, and birch, but this feeding is not severe.

Author: Phil Nixon, University of Illinois Extension Entomologist

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