Susceptibility of English Oak (Quercus robur) to the Twolined Chestnut Borer, Agrilus bilineatus (Coleoptera: Buprestidae): Observations from Michigan

The twolined chestnut borer, Agrilus bilineatus (Weber), is a native North American buprestid beetle that normally infests oak (Quercus) trees stressed by drought or defoliation. However, during the 1980s and early 1990s, A. bilineatus preferentially infested English oak (Quercus robur L.) trees that initially appeared healthy at two experimental plantings in southern Michigan as well as on the Michigan State University (MSU) campus in East Lansing, Michigan. At both experimental plantings the English oaks were intermixed with native white oaks (Quercus alba L.) and had a border of native northern red oaks (Quercus rubra L.). At both locations, only the English oaks showed evidence of A. bilineatus infestation. Significantly more A. bilineatus adults were captured on sticky band traps on English oaks with low or depleted root starch levels compared to trees with medium to high starch levels. For one of the experimental plantings that was monitored for five consecutive years, 29 English oak trees eventually died that appeared healthy during the first year of observation: 3 (10%) died in one year, 20 (69%) in two years, 4 (14%) in three years, and 2 (7%) in four years. Of 403 English oak trees at MSU that were rated for crown dieback typical of A. bilineatus infestation, 37% showed some level of dieback in 1990, and 60% in 1991. As of 2024, as many as 157 of the original 403 English oak trees are still alive on the MSU campus, with most living trees having been treated with systemic insecticides. These results are discussed in terms of the “naïve host” theory in which plants may have lower resistance to herbivores that they did not coevolve with.