by Courtney Llewellyn

Michigan growers are the leading producers of commercial chestnuts in North America, with more than 140 operations encompassing more than 700 acres. Demand for fresh and processed chestnuts is high and likely to increase, in both the Great Lakes State and nationally. However, as chestnut acreage increases and distribution of chestnut orchards expands, the availability of hosts for insect pests also increases.

Discussing the emerging pests of commercial chestnuts in Michigan through their research are Max Ferguson, Daniel Guyer, Louise Labbate and Deborah G. McCullough, all of the Department of Entomology at Michigan State University. They noted a native insect, the native lesser chestnut weevil, and an invasive insect, the Asian chestnut gall wasp (ACGW), have become major problems for Michigan producers. The research team is seeking to better understand the biology and impact of both pests to develop management options to mitigate damage in Michigan orchards.

Adult chestnut weevils were first captured in late May and early June 2021, corresponding to 480 – 550 cumulative growing degree days preceding catkin bloom. The adult weevils fed on pollen-laden catkins from late June to early July (980 – 1,470 GDD). Adult emergence and activity was highest as burs developed in August through October (2,150 – 3,180 GDD). The weevils were mating on burs in September. The females feed on nuts, then lay eggs through the spiky burs into the kernels. Eggs hatch in one to two weeks, and the larvae feed on maturing kernels for three to eight weeks.

The Asian chestnut gall wasp is gaining strength as a chestnut pest. Photo courtesy of Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org

Adult weevil captures were highest in August through October as chestnut burs and kernels were developing. Commercial traps appeared to be less effective than limb tapping for monitoring adult weevils. Baiting traps with attractive compounds to improve trap efficacy will be tested in 2022.

To try to control ACGWs, the Michigan State team introduced a larval parasitoid (Torymus sinensis Kamijo) as a biocontrol. Adult ACGW activity was assessed by placing yellow sticky traps in nine previously uninfested orchards within a 50-mile radius of two orchards first infested in 2018. Visual surveys for galls were also conducted. Galls were dissected to determine if T. sinensis was present. Adult ACGW emergence peaks in mid-July (1,440 – 1,750 GDD).

“We captured and/or found new galls in six of the nine new orchards monitored in 2021,” the team reported. On average, ACGW has spread about 17.5 miles per year since 2015. T. sinensis works by laying eggs inside developing ACGW galls in spring. Parasitoid larvae feed ACGW larvae, then pupate and overwinter in the gall. Unfortunately, it seems the spread of T. sinensis appears to lag behind ACGW by two to three years.

ACGW continues to spread in southwest and west Michigan. As of winter 2021-22, it was established in 18 orchards in 10 counties. The good news is the T. sinensis parasitoid is also spreading. Distribution and spread rates of both are being monitored this year.