Mite pests of honeylocust
Order: Acari
Family: Tetranychidae, Eriophyidae
Description and life history
Different groups of mites can produce conspicuous feeding injuries on honeylocust (Gleditsia triacanthos) in Colorado. While heavy infestations can reduce tree growth rate and vigor, feeding injuries are usually cosmetic and do not affect the health of the tree.
Honeylocust spider mite (Platytetranychus multidigituli)
The honeylocust spider mite (Acari: Tetranychidae) is a close relative of the twospotted spider mite (Tetranychus urticae). These mites are difficult to spot with the naked eye given their small size and habit of feeding on the underside of leaves. Adults of the honeylocust spider mite are 0.3 mm long and have four pairs of legs. They have pale green or yellow green bodies during warmer weather and orange bodies when overwintering. Immatures are initially transparent until they begin feeding, at which point they become pale green and resemble adults but are smaller.
Honeylocust spider mites do well in hot, dry weather. This species overwinters as adults in clusters on bark or in bud scales of a host plant. Adults become active again in the spring and begin laying eggs along leaf veins. A single female can lay up to 70 eggs, which hatch after budbreak. An individual can progress from egg to adult in as little as four days under favorable environmental conditions. This pest tends to prefer hot, dry weather and can disperse to new hosts via wind. Multiple generations are produced each growing season. In neighboring states like Utah, spider mite populations tend to build through July and decline in August. The honeylocust spider mite is a monophagous pest that only feeds on honeylocust trees.
Eriophyid mites (Aculops spp.)
Certain species of eriophyid mite in the genus Aculops (Acari: Eriophyidae) are commonly referred to as “honeylocust rust mites”. Eriophyid mites measure about 1/10 mm in length and are very unlikely to be seen without a 20x hand lens or greater magnification. These mites have cigar-shaped, white, or translucent bodies, and only two pairs of legs near the head region unlike most mites which have four pairs of legs.
Eriophyid mites overwinter as adult females under the scales of buds or other protect sites. During budbreak the following spring, the overwintered females lay eggs on protected sites of the host plant and feed on new foliage. These mites reproduce continuously and can produce multiple, overlapping generations each growing season.
Adults of honeylocust spider mite. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Overwintering adults of honeylocust spider mite. Note the orange bodies. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Eriophyid mites on a leaf. These mites are extremely small and cannot be seen without 20x or greater magnification. Image credit: Tracy Wootten, University of Delaware, Bugwood.org
Leaf bronzing due to feeding of the honeylocust spider mite. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Quick Facts
- While several mite species can attack honeylocust trees in Colorado, they rarely affect tree health.
- Monitoring approaches include inspecting honeylocust trees for feeding injuries or sampling for mites. Eriophyid mites are very small and can only be seen with a 20x hand lens or greater magnification.
- While the exact type of feeding injury depends on the type of mite, some common feeding injuries include discoloration of foliage and premature leaf drop.
- Practices that preserve natural enemies, such as avoiding the use of broad-spectrum insecticides, often suffice in suppressing pestiferous mite populations. In some instances, applications of horticultural oils or other chemical controls may be warranted.
Injury
Honeylocust spider mite
Infestations of honeylocust spider mites tend to be greater on trees in urban areas prone to dry conditions and drought stress. Light infestations of honeylocust spider mite causes stippling on leaves that may go unnoticed, while heavy infestations cause newly emerging foliage to be distorted. Severe infestations can cause bronzing or browning of the entire canopy, stunted plant growth, and premature leaf drop. Trees can recover partially when defoliation occurs early in the season.
Eriophyid mites
Aculops spp. feed on the undersides of honeylocust leaves which causes noticeable bronzing by late summer. Unlike some species of eriophyid mite, those that attack honeylocust do not form galls. Feeding injuries produced by eriophyid mites are usually cosmetic and do not impact the health of the honeylocust tree. For this reason, chemical control is not typically recommended for managing eriophyid mites on honeylocust.
Monitoring
Honeylocust spider mite
Early detection is important for effective management of spider mites. Host plant twigs near bud scars or bark cracks can be inspected for orange, overwintering adults. Using a 10x hand lens can aid in spotting overwintering mites. During budbreak early in the spring, leaves can be monitored for signs of feeding injury and the underside of leaflets can be inspected for mites. Branches can be shaken over a white piece of paper to dislodge active mites, who appear as moving specs on the white surface. Plant-feeding mites tend to produce a green streak when crushed on paper, while beneficial predatory mites are more likely to produce an orange or yellow streak. Management is likely necessary when ten or more spider mites are commonly observed during sampling. Abundant webbing is indicative of a heavy infestation.
Eriophyid mites
The leaves of plants can be examined periodically with a 20x hand lens or greater magnification to determine whether eriophyid mites are present before their population builds to destructive levels. When possible, it is recommended that eriophyid mites be managed without chemicals.
Management
Honeylocust spider mite
Since spider mites are most active during hot, dry weather, adequately watering during warmer months can help reduce mite populations and lessen the severity of feeding injuries. Plants can also be sprayed with a high-pressure stream of water to dislodge mites, which also helps conserve natural enemies. Natural enemies of the honeylocust spider mite include lacewing larvae, minute pirate bugs, spiders, predatory thrips, predatory mites, and certain species of lady beetle. Certain species of predatory mite are commonly sold for spider mite control. However, they tend to be host specific and require specific environmental conditions for optimum performance. Therefore, it is recommended that this information be reviewed before purchasing and releasing predatory mites.
While these natural enemies help suppress the population of spider mites, chemical controls may be needed to prevent injury on susceptible host plants. Dormant oils can be applied to overwintering adults, while active infestations in the summer can be controlled with a horticultural oil, insecticidal soap, or a miticide. Thorough coverage of dormant oils, horticultural oils, and insecticidal soaps on the upper and lower leaf surface is necessary since they work by contact only. However, they should not be applied to flowers, and it is recommended that such products be tested for any phytotoxic effects before application.
Some examples of miticides for controlling honeylocust spider mite include abamectin, bifenzate, hexythiazox, and spiromesifen. If a miticide application does not control the pest population after five to seven days, it is extremely important that a different miticide be used for the next treatment. It is worth noting that conventional pesticides can harm resident populations of natural enemies, which may lead to outbreaks of spider mites. Applying chemical controls after bloom is highly recommended to protect pollinators.
Eriophyid mites
By avoiding pesticide use, resident populations of natural enemies are allowed to persist and can help suppress populations of eriophyid mites. Heavily infested leaves or branches can be pruned and destroyed. If practical, entire plants can be removed to prevent the spread of eriophyid mites. Identifying and removing alternative weed hosts in the area can help reduce the chance of annual reinfestations. One species of predatory mite, the western predatory mite (Galendromus occidentalis), is a predator of eriophyid mites and is commercially available for release in hot, arid climates.
Chemical control may be warranted to help combat severe infestations of eriophyid mites on honeylocust before major aesthetic injuries develop. As with honeylocust spider mite, the use of reduced-risk pesticides such as horticultural soaps and oils is encouraged since they are less toxic toward non-target organisms. Horticultural oils should be applied seven to ten days before bud break and again during bud break.
In addition to horticultural oils, applications of kaolin clay in a liquid suspension can deter feeding of eriophyid mites. Although complete coverage of foliage and repeated applications are necessary, overreliance on kaolin clay can stress plants by interfering with photosynthesis. Sulfur applications can be effective but should not be used during hot periods when temperatures exceed 32°C (90°F), or during periods of high humidity. The botanical insecticide azadirachtin is derived from neem oil and functions as an insect growth regulator.
Broad-spectrum miticides for controlling eriophyid mites include abamectin, bifenthrin, carbaryl, deltamethrin, imidacloprid, malathion, permethrin, and pyrethrin. While broad-spectrum miticides can quickly knock down eriophyid mite populations, they may also kill non-target organisms if used improperly. It is recommended that different miticides be rotated to prevent the development of resistance.
CSU Extension Fact Sheet
Download or view the CSU Extension’s PDF fact sheet for your reference.
References
Cranshaw, W. 2016. Platytetranychus multigituli. BugwoodWiki. Available here
USU. (n.d.). Honeylocust Spider Mite. Utah State University – Extension. Available here.
Shetlar, D. 2011. Spider Mites and Their Control. Ohio State University – Extension. Available here.
Davis, R. 2011. Eriophyid Mites: bud, blister, gall, and rust mites. Utah State University – Extension. Available here.