Spider mites in shade trees
Order: Acari
Family: Tetranychidae
Description and life history
Spruce spider mite, Oligonychus ununguis
The spruce spider mite is a native pest of spruce and juniper. Adults are dark green or black and are extremely small at around 0.6 mm in length. When eggs are laid, they are pale yellow but become reddish brown as they age. The spherical eggs are 0.2 mm in diameter. Upon hatching larvae have six legs and are pink, but after feeding turn green. Nymphs have eight legs like adult mites, but are smaller at 0.3 – 0.4 mm.
Spider mites have five life stages: egg, larva, two nymphal stages, and adult. Females can lay up to 50 eggs during their lifetime. After overwintering as eggs, larvae hatch in the spring and begin feeding as they develop into nymphs. Nymphs develop into adults by early summer and feeding continues through the summer and early fall. Other than the egg, all life stages feed on the host plant. Spruce spider mites produce four to seven generations per year.
Honeylocust spider mite, Platytetranychus multidigituli
Adults of the honeylocust spider mite are orange or pale green to yellowish green and extremely small at 0.3 mm – 0.4 mm in length. Nymphs are similar in appearance but smaller than adults.
Adult females overwinter on bark or bud clusters and are orange in color during diapause. In late spring, females begin laying eggs and their color changes from orange to green. After hatching, nymphs begin feeding on leaves and go through two molts. Honeylocust spider mites produce multiple generations per growing season, and their numbers generally increase through July and decrease through August.
Quick Facts
• Spider mites are small arachnids that are difficult to see with the naked eye. Spider mites are highly polyphagous and attack a wide variety of plants.
• Many spider mite species produce webbing for protection from natural enemies, adverse environmental conditions, and to aid in dispersion to new hosts.
• Dispersal of spider mites occurs through wind currents, crawling from tree to tree, and movement of infested nursery stock plants.
Honeylocust spider mite female (left) and male (right). Several species of spider mite are pests of shade trees in Colorado. These include the spruce spider mite (Oligonychus ununguis), honeylocust spider mite (Platytetranychus multidigituli), and less commonly the twospotted spider mite (Tetranychus urticae). Each of these species are quite small and may produce webbing. Spider mite outbreaks are typically limited to drought-stressed trees. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Spruce spider mite adult. Image credit: USDA Forest Service, USDA Forest Service, Bugwood.org
Overwintering form of honeylocust spider mite on bark. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Spruce spider mite eggs. Image credit: USDA Forest Service, USDA Forest Service, Bugwood.org
Spruce spider mite infestation. Note the abundant webbing produced by these mites. Image credit: USDA Forest Service – Region 4 – Intermountain , USDA Forest Service, Bugwood.org
Bronzing of honeylocust leaves due to feeding of spider mites. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Injury
Spruce spider mite
Spruce spider mites attack pine needles with stylet-like mouthparts and remove contents of plant cells resulting in stippling injury. This can cause the foliage to become yellow or brown, and needle drop can occur in severe infestations. The silk webbing spun by these mites spin is frequently visible around needles and twigs. On larger trees, damage is confined to lower branches early on, but can spread to the entire tree as the mite populations increase. Seedlings and saplings can be killed by the spruce spider mites during outbreaks, and larger trees can suffer significant injury and death if severe infestations are sustained over several years.
Honeylocust spider mite
Feeding of honeylocust spider mites is concentrated on the underside of the leaf along the mid-rib. Heavy infestations can cause yellowing or bronzing of leaves and the canopy. Most feeding injury is caused by midsummer, and outbreaks can lead to premature leaf drop in late summer.
Cultural control
Drought-stressed trees are the most susceptible to spider mite outbreaks. Watering trees with low mite populations can help reduce damage during dry periods and prevent an outbreak. Spraying trees with a forceful flow of water through a hose can dislodge mites. When infestations are severe, pesticide applications may be warranted.
Biological control
Research suggests that augmentative release of certain predatory mites in outdoor settings such as nurseries may help prevent outbreaks of spruce spider mite. However, no guidelines are currently in place for incorporating these biological control agents with existing integrated pest management programs, and their use is currently considered to be economically impractical. It is likely that more research is needed to address protocols for developing practical and accurate methods of monitoring spider mite populations, assess the efficacy of releasing predatory mites as a prophylactic measure, and determining the predator: prey release ratios that promote optimal control.
In addition to predatory mites, researchers have observed that conidia of Metarhizium spp. display virulence when directly applied to a species of spider mite (Tetranychus truncatus). Furthermore, research indicates that another entomopathogenic fungus, Beuaveria bassiana, may be a viable candidate for management of the twospotted spider mite (Tetranychus urticae). However, current limitations include lack of suitable formulations for treating mite infestations in the field, and knowledge gaps regarding the effects of combining chemical controls with entomopathogenic fungi. Some evidence suggests that certain insecticides may interfere with the germination and infectivity of fungal conidia.
Chemical control
Chemical control generally involves miticides or acaricides, which are developed specifically for managing spider mites. Often, insecticide applications are ineffective and can even increase the mite population. In addition, insecticide resistance has been well-documented in populations of the twospotted spider mite. For more information on chemical control of spider mites in shade trees, visit the factsheets for spruce spider mite and honeylocust spider mite.
CSU Extension Fact Sheet
Download or view the CSU Extension’s PDF fact sheet on spruce spider mites for your reference.
References
Arcand, T. (n.d.). Spruce Spider Mite, (Oligonychus ununguis). Seskatchewan Ministry of Environment: Forest Pest Fact Sheet. Available http://www.environment.gov.sk.ca/adx/aspx/adxGetMedia.aspx?DocID=3630,184,121,104,81,1,Documents&MediaID=4178&Filename=Spruce+spider+mite.pdf
Cranshaw, W. 2016. Platytetranychus multidigituli (Ewing). Colorado State University – High Plains Integrated Pest Management. Available https://wiki.bugwood.org/HPIPM:Platytetranychus_multidigituli#:~:text=Honeylocust%20spider%20mite%20is%20a,sap%2C%20leaving%20small%20flecked%20wounds.
Irigaray, F., V. Marco-Mancebon, and I. Perez-Moreno. 2003. The entomopathogenic fungus Beauveria bassiana and its compatibility with triflumuron: effects on the twospotted spider mite Tetranychus urticae. 26(2): 168-173. Available https://www.sciencedirect.com/science/article/pii/S1049964402001238
Leeuwen, T., W. Dermauw, M. Grbic, L. Tirry, and R. Feyereisen. 2013. Spider mite control and resistance management: does a genome help? Pest Management Science. 69:156-159. Available https://onlinelibrary.wiley.com/doi/epdf/10.1002/ps.3335
Shrewsbury, P., and M. Hardin. 2003. Evaluation of Predatory Mite (Acari: Phytoseiidae) Releases to Suppress Spruce Spider Mites, Oligonychus ununguis (Acari: Tetranychidae), on Juniper. Journal of Economic Entomology. 96(6): 1675-1684. Available https://academic.oup.com/jee/article/96/6/1675/2217886
Wasuwan, R. et al. 2022. The Fungus Metarhizium sp. BCC 4849 Is an Effective and Safe Mycoinsecticide for the Management of Spider Mites and Other Insect Pests. Insects. 13(42): 1-18. Available https://mdpi-res.com/insects/insects-13-00042/article_deploy/insects-13-00042-v2.pdf?version=1642691791