Consider early season mite control on chestnuts in high pressure sites
Chestnut growers who observed high mite populations in 2012 should consider dormant oil applications to reduce damage in the coming season.
Chestnut trees are susceptible to feeding damage from a number of spider mite species including European red mites and two-spotted 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 that higher populations can be tolerated as the season progresses through summer.
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 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 right now (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 1). Adult and nymph feeding occurs on the underside of leaves, so be sure to flip them over when scouting. 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.
Photo 1. Adult European red mite feeding on leaves. Photo credit: Scott Justis
Growers should also be looking for two-spotted spider mites when scouting (Photo 2). Two-spotted 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. Eggs are spherical and translucent and are often found along leaf veins. Nymphs and adults can be a variety of colors depending on the time of year and stage of development. Potential colors include orange, brown, pale yellow or green, so growers are encouraged to use the spots as the key identification characteristic.
Photo 2. Male (mid-sized) and female (largest) two-spotted adult
spider mites along with smaller nymph and egg.
Photo credit: David Cappaert, MSU, Bugwood.org
Two-spotted spider mites overwinter as adults around the base of trees in the duff. As the weed or grass cover under the trees dries out, two-spotted spider mites start to move back up into the canopy in higher numbers from tight cluster through harvest. Like European red mites, two-spotted 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.
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). Predaceous mites are smaller than adult European red mites and two-spotted 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 eggs (European red mites) or adults (two-spotted spider mites) 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 antifeedant 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.
Watch for a follow-up Michigan State University Extension article this season on summer mite control strategies.