Michigan chestnut crop report for the week of Aug. 9, 2021
Burs continue to develop around the state. Potato leafhopper and Japanese beetle numbers are high in some areas.
Last week, we experienced cooler than normal temperatures early in the week and above normal temperatures later in the week. Last week through Monday morning, most areas of the state received 0.5-1 inch of precipitation. One exception was southwest Michigan that experienced quite heavy rainfall Aug. 9-10 (3-6-plus inches). Soil moisture levels in northern Michigan are slightly above normal and slightly below normal in some locations across southern Michigan. Much of the Lower Peninsula experienced strong thunderstorms with potentially damaging winds late Tuesday night.
The soupy tropical air will give way to more reasonable, cooler conditions beginning Friday. The weekend should be a nice one. The 6-10 day outlook (Aug. 15-19) suggests above normal temperature, especially across the Upper Peninsula and northern Michigan and less than average precipitation across the entire state.
Watch the most recent agricultural weather forecast from Michigan State University state climatologist Jeff Andresen.
Burs continue to grow and vegetative shoot growth is shutting down. Growers attempting to estimate crop load can refer to the Michigan State University Extension article, “Estimating Crop Load in Edible Chestnuts,” for some ideas on improving accuracy.
Integrated pest management
Growers should be scouting orchards weekly to identify emerging issues. At this time, growers can actively scout for many pests, but the most recent to arrive include Japanese beetle and potentially chestnut weevil.
Asian chestnut gall wasp presence has been confirmed in southwest Michigan and adult emergence began at the end of June this year. Gall wasp adults are very small, about 1/8 inch (3 millimeters) long and have black bodies with yellow legs. Adult wasps lay eggs inside chestnut buds over a four to six week period, typically extending from the last week of June to the second week of August in southwest Michigan. This period corresponds to approximately 1,050 to 2,100 cumulative growing degree days (GDD), using a threshold temperature of 50 F (GDD 50) and a starting date of Jan. 1. Current degree day accumulation in southwest Michigan is 1,100 GDD 50. Eggs hatch after about 40 days, but the tiny larvae remain dormant in the buds throughout the winter until the following spring. Galls and infestations are undetectable until the following spring.
As leaves start expanding the following spring, the Asian chestnut gall wasp larvae begin to feed. Their feeding causes the plant to form small galls, 1/4 to 3/4 inch (5-20 millimeters) in diameter, on current-year shoots and leaves. Larvae feed and develop in chambers inside the galls for about four weeks and then pupate into adults. This new generation of adult wasps begins to emerge in late June, just as catkins begin to senesce. Adults continue to emerge and lay eggs for about six weeks.
While leaf galls usually have little impact, galls that form on current-year shoots can affect vigor and nut production of chestnut trees. Apical galls that form at the tip of shoots may be especially damaging. They reduce shoot elongation and can inhibit flower production, which reduces nut formation. Chestnut producers in Japan, Korea, several European countries and even some U.S. states, have reported yield reductions following Asian chestnut gall wasp invasion. Branch dieback has been observed in China, Japan, Korea, Italy and the U.S. when Asian chestnut gall wasp densities were high.
An array of methods to control Asian chestnut gall wasp have been employed by commercial chestnut growers, with varying levels of success. Integrating strategies for managing Asian chestnut gall wasp should prevent yield loss while minimizing unnecessary pest control costs and impacts on beneficial insects such as pollinators and natural enemies. Effective pest management starts with active scouting. Chestnut growers in western lower Michigan, particularly south of I-96 and west of Highway 127, should be scouting their trees for evidence of Asian chestnut gall wasp during the growing season and in fall or winter, after leaf drop.
If galls are abundant and nut production is low on heavily infested trees, growers may need to control Asian chestnut gall wasp adults with a cover spray of a broad spectrum, conventional insecticide. Generally, pesticide applications are not recommended because Asian chestnut gall wasp populations are adequately regulated by Torymus sinensis, a beneficial parasitic wasp.
The T. sinensis parasitoid and Asian chestnut gall wasp share a long co-evolutionary history in China and the life cycle of the parasitoid is well synchronized with Asian chestnut gall wasp. In early spring, as Asian chestnut gall wasp galls are forming, T. sinensis adult females lay an egg into a gall chamber where an Asian chestnut gall wasp larva is feeding. Each parasitoid larva feeds on an Asian chestnut gall wasp larva within the gall chamber throughout the summer, eventually killing the Asian chestnut gall wasp larva. Parasitoid larvae remain inside the galls throughout the winter. As chestnut buds break and new galls form in spring, adult parasitoid wasps emerge from the dry, previous-year galls to oviposit within the green, succulent, current-year galls.
Fortunately, the T. sinensis parasitoid seems to have arrived in southwest Michigan at about the same time as Asian chestnut gall wasp and this beneficial wasp appears to be spreading. Old galls should not be removed as the beneficial parasitoids may still be developing inside the old galls. Removing those galls will have no effect on Asian chestnut gall wasp density but could reduce the beneficial parasitoid population.
While detection of the invasive Asian chestnut gall wasp in Michigan is not good news, practical and cost-effective management tactics can be used to prevent severe damage. Scouting to assess the abundance of current and previous galls can help identify where Asian chestnut gall wasp densities are relatively high. Monitoring yield is important to evaluate Asian chestnut gall wasp effects on nut production. The T. sinensis parasitoid seems to be spreading with Asian chestnut gall wasp and appears likely to play a major role in regulating Asian chestnut gall wasp populations in the long term.
It will be important to apply insecticide cover sprays only when necessary and to time sprays correctly to avoid affecting the beneficial parasitoid population. When expanding an orchard and planting new trees, consider chestnut cultivars that offer some resistance to Asian chestnut gall wasp . Integrating these options should help minimize impacts of Asian chestnut gall wasp and protect the Michigan chestnut industry over the long term.
Growers should be scouting for chestnut weevil as burs develop. Chestnut weevil lay eggs on kernels and developing larvae feed on and compromise the kernel. If left unchecked, the larvae can infest and destroy the nuts. Larvae can be present at harvest resulting in “wormy” nuts making their way to consumers.
Michigan chestnut producers have reported an increase in the number of larvae in nuts at harvest. It is likely that the observed larvae are immature lesser chestnut weevils (Curculio sayi). As an emerging issue, Michigan producers have had very little experience with chestnut weevil, but formal research on this pest in Michigan is currently underway.
Lesser chestnut weevils have robust bodies, long snouts and are dark brown or tan with brown mottling or stripes. Lesser chestnut weevil is 0.25 inch in length, with a snout of equal or greater length. Scouting for adult weevils should begin just before bloom and continue regularly until harvest.
At this time, effective passive trapping techniques for chestnut weevil have not been identified; you should instead focus on the limb-tapping technique. To use the limb-tapping technique, place a light-colored sheet under the limb you are sampling and tap the branch with a padded pole or stick; tap gently to avoid damage to the tree. Jarring the branch causes the weevils to drop from the tree onto the sheet.
Weevils will “play dead” when disturbed, so don’t be fooled. Chestnut weevils are substantial in size and should be easily visible if present. Sample at least 30 branches per acre. Scouting locations should include both the edges and interior of orchards as well as any known hotspots.
To read more about chestnut weevil, refer to the article, “Managing chestnut weevil in Michigan 2020."
Japanese beetle emergence has been reported in southern and central Michigan. Japanese beetle adults are considered a generalist pest and affect many crops found on or near grassy areas, particularly irrigated turf. Larvae prefer moist soil conditions and do not survive prolonged periods of drought. Adult Japanese beetle typically emerge around early July and feed on hundreds of different plant species. Adult beetles feed on the top surface of leaves skeletonizing the tissue. If populations are high, they can remove all of the green leaf material from the plants.
There are no established treatment thresholds or data on how much Japanese beetle damage a healthy chestnut tree can sustain, but consider that well-established and vigorous orchards will likely not require 100% protection. Younger orchards with limited leaf area will need to be managed more aggressively. Managing Japanese beetle can be a frustrating endeavor as they often re-infest from surrounding areas, especially during peak adult emergence in July. This re-infestation is often misinterpreted as an insecticide failure, but efficacy trials have shown that a number of insecticides remain effective treatment options.
Japanese beetle adults are a substantial insect and measure 3/8 to 1/2 inch long. The thorax is green and wingcovers are copper colored. There are five tufts of white hairs on both sides of the abdomen and a pair of tufts on the end of the abdomen that can help distinguish the Japanese beetle from other look-alike species. The legs and head are black.
Visual observation of adults or feeding damage is an effective scouting technique. Scout along a transect through orchards at least weekly until detection, paying special attention to the tops of trees. Because of their aggregating behavior, they tend to be found in larger groups and are typically relatively easy to spot. Pheromone and floral baited traps are available but are not recommended as visual observation is adequate to determine when beetles emerge and should be managed. For more information on insecticides available for the treatment of Japanese beetle refer to the current Chestnut Management Guide.
Potato leafhopper feeding damage is visible in some orchards. Like many plants, chestnuts are sensitive to the saliva of potato leafhopper, which is injected by the insect while feeding. Damage to leaf tissue can cause reduced photosynthesis which can impact production and quality and damage the tree. Most injury occurs on new tissue on shoot terminals with potato leafhopper feeding near the edges of the leaves using piercing-sucking mouthparts. Symptoms of feeding appear as whitish dots arranged in triangular shapes near the edges. Heavily damaged leaves are cupped with necrotic and chlorotic edges and eventually abscise from the tree. Severely infested shoots produce small, bunched leaves with reduced photosynthetic capacity.
Adult leafhoppers are pale to bright green and about 1/8 inch long. Adults are easily noticeable, jumping, flying or running when agitated. The nymphs (immature leafhoppers) are pale green and have no wings but are very similar in form to the adults. Potato leafhopper move in all directions when disturbed, unlike some leafhoppers which have a distinct pattern of movement. The potato leafhopper can’t survive Michigan’s winter and survives in the Gulf States until adults migrate north in the spring on storm systems.
Scouting should be performed weekly as soon as leaf tissue is present to ensure detection early and prevent injury. More frequent spot checks should be done following rain storms which carry the first populations north. For every acre of orchard, select five trees to examine and inspect the leaves on three shoots per tree (a total of 15 shoots per acre). The easiest way to observe potato leafhopper is by flipping the shoots or leaves over and looking for adults and nymphs on the underside of leaves. Pay special attention to succulent new leaves on the terminals of branches. For more information on insecticides available for the treatment of potato leafhopper refer to the current Chestnut Management Guide.
Existing chestnut blight infections (caused by Cryphonectria parasitica) can be observed at this time. To learn more about chestnut blight, visit the Pest Management section of the MSU Extension Chestnut website.
Grassy weeds are particularly prevalent this season. Weed control looks very good in treated orchards. Most chestnut growers apply directed glyphosate applications in a band down the tree row to eliminate competition for nutrients and water. Glyphosate should be applied carefully to ensure it does not contact any part of the tree when applied.
Growers with wildlife management concerns are encouraged to check out the resources at the Integrated Pest Management Wildlife Management page.
Become a licensed pesticide applicator
All growers utilizing pesticide can benefit from getting their license, even if not legally required. Understanding pesticides and the associated regulations can help growers protect themselves, others, and the environment. Michigan Pesticide Applicator Licenses are administered by the Michigan Department of Agriculture and Rural Development. You can read all about the process by visiting the Pesticide FAQ webpage. Michigan State University offers a number of resources to assist people pursuing their license, including an online study/continuing ed course and study manuals. For more information on becoming licensed, contact Lisa Graves at 517-284-5653.
For more information on chestnut production, visit MSU Extension Chestnuts and sign up to receive our newsletter. Also, join us for the 2021 Chestnut Chat Series every Wednesday at 12 p.m. from May 5 through Sept. 8, 2021. This series of interactive Zoom meetings will allow easy communication between producers and MSU faculty. These informal weekly sessions will include crop and pest updates from Rob Sirrine and Erin Lizotte. In addition, MSU faculty will drop in to address timely issues and provide research project updates. Bring your field notes too! We want to hear what’s going on in your orchard.
This material is based upon work supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2017-70006-27175. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.