MSU Extension

Michigan State University Extension will be tracking chestnut development during the 2015 season and providing growers with relevant pest management information.

As the weather warms, chestnuts are showing signs of breaking dormancy with green tissue visible in central and southern Michigan, and bud swell in northwest Michigan (Photos 1-2). There is significant variability in growth stages across the state and differences in development are also significant between different cultivars and species. The variability in development stages is reflected in growing degree day (GDD) accumulation differences across the state with the Benton Harbor Enviro-weather station recording 246 GDD50, Clarksville Enviro-weather station at 221 GDD50 and the Northwest Michigan Horticultural Research Center (NWMHRS) Enviro-weather station at just 154 GDD50 as of May 14, 2015.

Most growers have made their first fertilizer and herbicide applications of the year. Dormant oil applications for mite or lecanium scale management should be wrapping up as green tissue emerges and the risk of phytotoxicity increases. At this time of the year, growers should focus their scouting program on mites and potato leafhoppers.

Photo 2. Bud swell of European x Japanese chestnut in northern Michigan. Photo credit: Erin Lizotte, MSU Extension

Potato leafhoppers

Like many plants, chestnuts are sensitive to the saliva of potato leafhoppers, 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 of the injury occurs on new tissue on shoot terminals with potato leafhoppers feeding near the leaf edges 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 (Photo 3).

Photo 3. Potato leafhopper feeding damage to chestnut, causing leaf cupping and necrotic margins. Photo credit: Erin Lizotte, MSU Extension

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 or immature leafhoppers are pale green and have no wings, but are very similar in form to the adults (Photo 4). Potato leafhoppers 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.

Photo 4. Potato leafhopper adults and nymphs at various stages of development on the underside of chestnut leaves. Photo credit: Erin Lizotte, MSU Extension

Scouting should be performed weekly as soon as leaf tissue is present to ensure early detection 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 leafhoppers 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 potato leafhopper treatment, refer to “Pesticides registered for edible chestnuts, 2015” by Michigan State University Extension.

European red mites

Chestnut trees are susceptible to feeding damage from a number of spider mite species including European red mites and twospotted spider mites. 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 chestnuts, but evidence from crops like cherries indicate that some level of feeding is likely tolerable and higher populations can be tolerated as the season progresses through summer. European red mites appear to be the more prevalent mite species for chestnut producers, but growers should keep an eye out for twospotted spider mites as well.

European red mites overwinter as eggs in bark crevices and bud scales. Eggs are small spheres, about the size of the head of a pin, with a single stripe 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 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 European red mites are red and have hairs that give them a spikey appearance (Photo 5). Adult and nymph feeding occurs primarily on the upper surface of the leaves (Photo 6). 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 life cycles much faster with as little as 10 days between generations under ideal conditions. 

 
Photo 5-6. Adult European red mite feeding on leaves (left), and bronzing or “dusty” leaf surface caused by European red mite activity (right). Photo credits: Scott Justis (left), and Erin Lizotte, MSU Extension (right)

Growers should also be looking for twospotted spider mites when scouting (Photo 7). Twospotted spider mites have two distinct spots located on the front half of the body. Males are much smaller than females and have a distinctly pointed abdomen. Twospotted spider mite eggs are spherical and translucent and often found along leaf veins. Nymphs and adults can be a variety of colors depending on the time of the year and development stage. Potential colors include orange, brown, pale yellow or green, so growers are encouraged to use the spots as the key identification characteristic.

Twospotted spider mites overwinter as adults around the base of trees in the duff. As the weeds or grass cover under the trees dries out, twospotted spider mites start to move back up into the canopy in higher numbers from tight cluster through harvest. Like European red mites, twospotted spider mites damage the tree by feeding on the underside of the foliage and give the leaves a dirty appearance when populations become high and leaf surfaces are coated in eggs, discarded exoskeletons from molting and webbing.

Photo 7. Male (mid-sized) and female (largest) twospotted spider mite adults along with smaller nymph and egg. Photo credit: David Cappaert, MSU

As growers are scouting, they are encouraged to 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. The three most important predaceous mites are Amblyseius fallacis (Phytoseiidae), Agistemus fleschneri (Stigmaeidae) and Zetzellia mali (Stigmaeidae). See “A Pocket Guide for IPM Scouting in Michigan Apples” (E2720) for pictures. Predaceous mites are smaller than adult European red mites and twospotted spider mites, but they can be seen with a hand lens and typically move very quickly across leaf surfaces.

Mite control starts with monitoring early in the spring looking for the overwintering European red mite eggs or twospotted spider mite adults 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 and provides for natural control. If pest mite populations are high enough to require control, dormant superior oil applications are an effective method of treatment. Ensuring adequate coverage is critical to attaining optimal control on problem sites as oil alone acts to smother mites and eggs. If you are applying an oil with an additional insecticidal property, such as those containing the neem extract azadirachtin, coverage is still critical as many are insect growth regulators and have anti-feedant properties.

Oil application comes with a risk of phytotoxicity particularly as green tissue emerges. It is recommended that growers do a small test spray if they have not used dormant oils in the past. Keep in mind too that different cultivars may have different sensitivities and proceed with caution. Severely stressed trees should never be sprayed with oil and growers are advised to keep in mind that temperature and humidity also play a role in the risk of phytotoxicity. Temperatures above 80 degrees Fahrenheit and slow drying time (high humidity) increase the chance of damage. Oil also has some potential compatibility issues growers should be aware of including Captan and sulfur.

Lastly, it is not recommended that oil be applied within a 48-hour window prior to and following freezing temperatures due to decreased efficacy and potential plant damage. Read and follow all label directions carefully to minimize the risk of spray damage. For more information on pesticides to control mites, refer to “Pesticides registered for edible chestnuts, 2015.”

This material is based upon work supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2013-41534-21068. 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.