Spider Mites in Shade Trees
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.
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.
• 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).
Whitney Cranshaw, Colorado State University, Bugwood.org
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 twospotted spider mites (Tetranychus urticae). Each of these species are quite small and may produce webbing. Spider mite outbreaks are typically limited to drought-stressed trees.
Spruce spider mite adult
Overwintering form of honeylocust spider mite on bark.
Spruce spider mite eggs
Spruce spider mite infestation. Note the abundant webbing produced by these mites.
Bronzing of honeylocust leaves due to feeding of spider mites.
Life History and Habits
Spruce spider mite
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
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.
Injury caused by spider mites in shade trees
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.
Management options for spider mites in shade trees
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.
Chemical control generally involves miticides or acaricides, which are developed specifically for managing spider mites. Often, insecticides are ineffective and can even have the opposite effect of increasing the mite population. For more information on chemical control, consult the factsheets for spruce spider mite and honeylocust spider mite.
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
Download or view the CSU Extension’s PDF fact sheet on spruce spider mites for your reference.