Spider Mites

Spider mites are arachnids in the Acari (mite) family Tetranychidae and are among the most common pests in North America; there are more than 1,200 known species of the mite feed on hundreds of different tree species in the region. These generally fall within two categories; cool-season spider mites that operate in early spring and fall and warm-season mites active in the summer months. 

Adult spider mites are oval-shaped and can be translucent, green, yellow, red, or brown. Measuring no more than 1mm or 1/25 inch long, these pests may be small and hard to see, but they have gained a pretty destructive reputation. 

They live in colonies on the underside of leaves and survive by sucking up the cell content of the leaves. Their feeding leaves light dots on affected leaves, and in severe cases, the foliage may turn yellow, dry up, and fall off. 

Life Cycle

The life cycle of spider mites is key to understanding how these pests operate. Most species will survive winter as spherical, transparent eggs (usually in a silk webbing) on the leaves and bark of the host plant. These eggs will then hatch in spring into tiny six-legged larvae that molt thrice over a couple of weeks before becoming adults.  

However, note that mite species reproduce at a much faster rate in hot, dry weather. At around 27℃, hatchlings can become sexually mature within as little as five days. Given that a female spider mite continuously produces 20 eggs a day for 2 to 4 weeks, hot, dry conditions are usually accompanied by a massive population explosion of spider mites. 

Tree Species Affected

Spider mites are the least picky of pests, affecting nearly any tree within their reach. This is a major contributor to their success and spread across North America. However, the most common trees affected by these mites include oaks, mountain ash, maples, evergreens, elms, most fruit trees,  pines, Balsam fir, spruces, and honeylocust. 

Note that trees suffer the most during the hottest and driest periods of the year due to accelerated population in the warmer months. 

How to identify spider mite infestation

Individual spider mites are very small and can be quite hard to see. However, the fact that they live and attack in colonies makes it relatively easier to tell whether you have a spider mite infestation. Here are the telltale signs; 

• Their feeding on the bottom of tree leaves usually leaves visible patterns of white or yellow spots on top of the leaves or needles. 
• You may also notice silky webs woven along the petioles of leaves and in between needles. 
• Under heavy infestation, trees will have greyish and unhealthy foliage throughout the growing season. 
• Leaf drop is also common, and there may be a yellow or bronze appearance in some areas of the tree.

Consequences of leaving spider mites untreated

Small colonies of spider mites affect individual leaves and thorns, and it’s rarely serious enough to impact the tree’s overall health. The greatest threat comes when these colonies are left unchecked, especially during hot months, when spider mites reproduce at a much faster rate. 

This is especially the case in conifer tree species, where large colonies will inflict permanent damage on the leaves and needles and are a serious health threat. Other tree species may have it a little easier, but extensive infestation will cause significant damage to the affected trees’ foliage. As the foliage weakens, the trees become more susceptible to attacks from other pests and diseases. 

Preventing/Treating a spider mite infestation

Prevention over cure is a choice many of us will willingly make. In the case of spider mites, keeping your trees watered properly during hot and dry seasons is your first line of defense. Dust on leaves and fruits also provides conditions favorable to spider mites. As such, regular hosing to remove the dust is an excellent preventative measure.  

If this doesn’t work, acting quickly once you notice a few affected leaves or branches is the only way to avoid a full-blown spider mite infestation. Make sure you prune, and trash (avoid composite piles) affected leaves and branches. 

You can also use commercial insecticides while the pest levels are low to medium. However, their effectiveness during hot months is significantly lower compared to other months. This is due to spider mites’ fast reproduction rates, making it easy for the pests to develop immunity to the insecticides. 

Botanical insecticides, neem oil, and biological measures are other ways you can reduce spider mite infestation. 

Final Take 

Spider mites are among the most common pests affecting trees in North America. They may be small enough to go unnoticed, but the damage they cause to trees is far from inconspicuous. Therefore, regular inspection of trees and shrubs by property owners and tree health experts is needed to check their spread.