Michigan chestnut crop report for the week of Aug. 1, 2022
Bur formation is well underway around the state.
Much of the state has caught up to normal accumulated rainfall this year. The Thumb and isolated areas of the Upper Peninsula remain significantly behind normal. Temperatures last week were above average and continue to contribute to higher degree day accumulation compared to average. The forecast looks hot and dry, with higher than normal temperatures moving in this week and persisting over the long term forecast.
Orchards are currently entering the kernel development stage. Growers should be focused on scouting for pests, estimating the crop and looking ahead to harvest. August drop of non-viable burrs has not yet started.
Asian chestnut gall wasp adult emergence continues. Asian chestnut gall wasp (Dryokosmus kuriphilus Yasumatsu) was discovered in Michigan in 2015. This tiny insect, a native of China, is a major invasive pest of chestnut trees in Japan, Korea, much of Europe, and the United States. At high densities, the spherical galls caused by the ACGW can reduce tree growth and nut production. This invasive pest will continue to spread and could become a serious problem for commercial chestnut producers across the state.
Adult wasps lay eggs inside chestnut buds over a four- to six-week period, typically from the last week of June to the second week in 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 (GDD50F) and a starting date of Jan. 1. Much of the state remains in that egglaying window. Refer to the Cumulative Growing Degree-Day Map below to determine accumulation in your area or visit MSU Enviroweather and select the weather station closest to your orchard.
Growers in or adjacent to the infested region (See map) can hang yellow sticky traps to catch adults and scout for galls season long. Refer to the MSUE Bulletin E3457, Asian Chestnut Gall Wasp, for more information on biology, identification and management.
Growers should be scouting for chestnut weevil as kernelsdevelop. 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 (JB) remain active across the state. Earlier in the season, some growers reported high numbers. JB 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 JB 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 JB damage a healthy chestnut tree can sustain, but growers should 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 JB 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 wing covers 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 JB from other look-alike species. The legs and head are black. Visual observation of adults or feeding damage is an effective scouting technique.
Growers should 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.
Growers continue to report higher levels of European red mite (ERM), often localized to a handful of trees. Chestnut trees are susceptible to feeding damage from a number of spider mite species including European red mite and two-spotted spider mites (TSSM). Affected leaves appear mottled, stippled, or bronzed and become brittle leading to early defoliation and reduced photosynthetic activity. Reduced photosynthesis can lead to reduced nut size and return crop load in subsequent years as well as increased sensitivity to winter injury.
At this time, no treatment thresholds are established for mites in chestnut, but evidence from crops like cherry indicate that some level of feeding is likely tolerable and that higher populations can be tolerated as the season progresses through summer. ERM appears to be the more prevalent mite species for chestnut producers, but growers should keep an eye out for two-spotted spider mites as well.
European red mites overwinter as eggs in bark crevices and bud scales and are the most commonly observed species in Michigan chestnut orchards. Eggs are small spheres, about the size of the head of a pin with a single stipe or hair that protrudes from the top (this is not always visible). Eggs can be viewed with a hand lens or the naked eye once you have established what you are looking for. Growers can scout for overwintering eggs and early nymph activity in the spring to assess population levels in the coming season. As temperatures warm, overwintering eggs hatch and nymphs move onto the emerging leaves and start feeding.
Adult ERM are red and have hairs that give them a spikey appearance. Adult and nymph feeding occurs primarily on the upper surface of the leaves. This first generation is the slowest of the season and typically takes a full three weeks to develop and reproduce. This slow development is due to the direct link between temperature and mite development. Summer generations, favored by the hot and dry weather are able to complete their lifecycles much faster with as little as 10 days between generations under ideal conditions.
Mite control starts with monitoring early in the spring looking for the overwintering eggs (ERM) or adults (TSSM) and assessing the mite pressure. Ideally, growers will be using limited insecticides with miticidal activity in their season long programs as that protects their beneficial mite populations. If pest mite populations are high enough to require control, superior oil applications when the trees are dormant are an effective method of treatment. If issues with mites arrive during the growing season, refer to the 2022 Michigan Chestnut Management Guide for control options.
As you scout, remember that not all mites are bad. Consider documenting the levels of predacious mites in your orchard. If healthy populations of mite predators exist, they will continue to feed on plant parasitic eggs and nymphs and can be an effective component of your mite management program. Predaceous mites are smaller than adult ERM and TSSM, but they can be seen with a hand lens and typically move very quickly across leaf surfaces.
Potato leafhopper (PLH) pressure is low to moderate in most locations. Like many plants, chestnuts are sensitive to the saliva of PLH 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 PLH 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 brown and yellowed edges and eventually drop 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. PLH 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.
The easiest way to scout for PLH 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 2022 Michigan Chestnut Management Guide.
Existing chestnut blight infections (caused by Cryphonectria parasitica) can be observed at this time. There are no commercially available treatments for chestnut blight. Growers may prune out infected branches or cull whole trees as needed to limit disease pressure. Infested material should be burned or buried to further limit inoculum spread. To learn more about chestnut blight, visit the pest management section of the chestnut webpage.
Also, join us for the 2022 MSU Chestnut Growers Chat Series. 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. These Wednesday sessions begin May 11 and run every other week through Aug. 31 from 1:30-2:30 p.m. EST. Registration is free but required. Register today!
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.
This work is supported by the Crop Protection and Pest Management Program [grant no 2021-70006-35450] from the USDA National Institute of Food and Agriculture. 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.