Potential & Emerging Pests
Asian chestnut gall wasp
Michigan chestnut growers should be aware of the threat of Asian chestnut gall wasp (ACGW) (Dryocosmus kuriphilus), a potentially devastating invasive pest that can infest all species in the genus Castenea including American chestnut, ornamental species and those planted for nut production. ACGW has not been found in Michigan but growers should understand the quarantine affecting chestnut plant material and scout for this pest regularly to ensure early detection and prevent an outbreak in Michigan orchards. The ACGW is native to Asia and was first reported in the U.S. in 1974 when it was observed on Chinese chestnut in Georgia after it was introduced on imported plant material. From Georgia, the geographic range expanded reaching Virginia (2001), Ohio (2002), Kentucky (2003), Maryland and Pennsylvania (2006), Connecticut (2011) and Massachusetts (2012). ACGW will likely continue to spread across eastern North America through natural dispersal and via infested plant material.
The gall wasp reproduces one generation per year via asexual reproduction. Adult females lay eggs inside buds in early summer and eggs hatch shortly. After egg hatch the larvae remain inactive until budbreak the following spring, when they induce the formation of galls. Galls can form on the stem, petiole, or leaf and provides the larvae and pupae protection. Adults emerge from galls in the early summer and locate new chestnut shoots to lay eggs for the next generation. After the wasps emerge, galls become woody and dry out, potentially persisting on the tree for several years. Asian chestnut gall wasp is a potentially devastating exotic insect that causes globular twig, shoot, and leaf galls on actively growing shoots of all Castanea species. Galling reduces fruiting and nut yield, suppresses shoot elongation and twig growth, reduces tree vigor and wood production, and can kill trees. Galling also prevents infested shoots from producing new shoot growth and reproductive ﬂowers, thereby reducing or eliminating future production.
Galls formed by Asian chestnut gall wasp on chestnut. (Photo credit: Dennis Fulbright, Michigan State University)
The easiest way to scout for the ACGW is to visually inspect for galls. Scouting can take place at any time of the year as the galls persist on the tree even after the wasps emerge in early summer. Leaves often remain attached to the galls during the winter making the galls highly visible at that time (Figure N). If you locate what you believe to be ACGW contact MSU Extension Educator Erin Lizotte at 231-779-9480 or firstname.lastname@example.org immediately for further assistance. Chestnut growers should be aware of the quarantine in effect in Michigan and Alabama, Georgia, Kentucky, Maryland, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia and any other state where gall wasp may become established. Regulated materials include living plants and scionwood of all Castanea species including hybrids. For more information contact the Michigan Department of Agriculture, Pesticide and Plant Pest Management Division.
Lesser and large chestnut weevil
The most important insect pest of chestnut trees in the central-eastern U.S. is the lesser chestnut weevil (Curculio sayi). Large chestnut weevil (C. caryatrypes) is also an important pest but is less prevalent. These pests have not yet been a significant problem for Michigan producers, but as acreage expands in the state, growers need to be actively scouting for chestnut weevil. Large and lesser chestnut weevil are native weevils and are host-specific, only infesting tree species in the genus Castanea (American chestnut, Chinese chestnut, European chestnut and chinquapin). Lesser and large chestnut weevil both lay eggs on developing nuts, the larvae feed within the nut, compromising the kernel. If left unchecked, these weevils can infest and destroy the majority of nuts produced in an orchard. The natural range of these pests mirrors the natural range of American chestnut (Castanea dentata) in the Central and Eastern United States. When the American chestnut stands collapsed due to chestnut blight (Cryphonectria parasitica) the populations shrunk to small pockets of the U.S. where chestnuts are present. The prevalence of these pests in Michigan is unknown at this time, but weevil larvae have been found in chestnuts at harvest.
Michigan producers have had very little experience with chestnut weevil and at this time no formal research has been done on the lifecycle of this pest in Michigan. However, based on research out of Kentucky and Missouri as well as the experience of Michigan growers, we can make some educated estimations about the chestnut weevils lifecycles under Michigan conditions.
Lesser chestnut weevil adults likely emerge from the soil during two separate periods in Michigan, once in spring around bloom (May-June) and again in late summer-early fall just before burrs open (September-October). Weevils that emerge in the spring can be observed feeding on catkins. When the catkins decline, the population disappears. It is unknown if these spring weevils return to the soil or move off to feed on other plants. In September-October a second wave of lesser chestnut weevil emerge. As burrs begin to open, the majority of egg laying occurs for both the spring and fall emerging adult weevils. Eggs are typically deposited in the downy lining surrounding the nut and hatch in approximately 10 days at which time the larvae feeds on the kernel and develops within the shell. After 2-3 weeks, larvae chew an exit hole in the nutshell and drop to the soil. The majority of the weevils will overwinter as larvae the first year, pupate in the soil the following fall and overwinter as adults. The total lifecycle is completed in 2-3 years.
Large chestnut weevil adults likely emerge in August or September under Michigan conditions and begin laying eggs in immature burrs almost immediately after emergence (well before lesser chestnut weevil begin laying eggs). Eggs hatch in 5-7 days and the larvae feed and develop within the nut for 2-3 weeks before chewing a small exit hole and leaving the nut. The large chestnut weevil larvae usually exit the chestnut before the nuts drop to the ground and overwinter in the soil. Pupation and adult emergence takes place the following summer, a small population of larva may overwinter a second winter before pupation. The total lifecycle is completed in 1-2 years.
Identification and Detection
Lesser and large chestnut weevil both have robust bodies and are dark brown or tan with brown mottling or stripes. Lesser chestnut weevil is ¼ inch in length, with a snout of equal or greater length. The body of the large chestnut weevil is 3/8 inch long, the snout is 3/8-5/8 inch long.
Scouting for chestnut weevils should begin just before bloom. Passive traps (circle traps on the trunk or pyramid traps, 1 per acre) can be used to capture ascending weevils, these traps should be set well before bloom occurs and checked twice a week. Scouting for weevils using a limb-tapping technique can also be done. Place a light colored sheet under the limb you are sampling and tap the branch with a padded pole or stick. Jarring the branch causes the weevils to drop from the tree onto the sheet. Weevils “play dead” when disturbed so don’t be fooled if they all appear dead, they will reanimate within a few seconds. Chestnut weevils are substantial in size and should be easily visible if present. Growers should sample at least 10 branches per acre. Scouting locations should include both the edges and interior of orchards as well as any hotspots that are identified.
There are chemical, cultural and postharvest treatments available to control chestnut weevils. Ideally, a combination of cultural and chemical management would control the pest and eliminate the need for postharvest treatment which can diminish quality and the marketable yield.
Sanitation is an important part of the management of these pests. Collecting and destroying fallen nuts will remove developing larva from the orchard. Insecticide applications for control should target the two later windows of potential adult activity; August-September (large chestnut weevil adult emergence), and September-October (lesser chestnut weevil fall adult emergence). Growers are cautioned against applying pesticides during adult activity in May-June (lesser chestnut weevil spring adult emergence) as bees are often foraging in the orchard at that time. Carbaryl (Carbaryl 4L, Sevin 4F, Sevin 80S, Sevin XLR Plus, Sevin SL) is the only insecticide labeled against chestnut weevil. The manufacturer recommends making 4 weekly applications beginning in late July to control adults when the burrs are present and vulnerable. However, based on the development of chestnuts under Michigan conditions, growers may want to wait until August to begin applications. Growers will have to carefully budget the 3-4 applications of carbaryl available as the period of burr exposure is long. Applications should only be made in response to positive identification of the weevil with regular scouting.
Thorough and frequent scouting is essential for optimal management, particularly with the lack of information regarding chestnut weevil behavior and prevalence in Michigan. Well timed applications, good sanitation practices and scouting will be the key to successful chestnut weevil management in Michigan.