Northern spider mites are common plant pests that feed on sap, causing discoloration and weakening of plants. These tiny arachnids are difficult to see but can cause significant damage when populations grow large.
They thrive in dry conditions and are often found on the undersides of leaves.
The Microscopic Weaver: Northern Spider Mites3
The Northern Spider Mite (Tetranychus turkestani), often discussed alongside its cousin the Two-Spotted Spider Mite, is a prolific plant-feeding arachnid found across the United States. While they are tiny, their ability to reproduce rapidly in warm, dry conditions makes them a major threat to cotton, alfalfa, deciduous fruit trees, and various vegetable crops. They are particularly notorious in the American West and Midwest for their destructive “stippling” feeding habit.
Identification: The “Two-Spot” Look-Alike
Spider mites are barely visible to the naked eye, appearing as moving dust motes. To identify them accurately, a 10x hand lens is usually required. In the U.S., the Northern Spider Mite is distinguished by several subtle features:
- Coloration: Generally pale yellow to green during the growing season. In the fall, as they prepare for diapause (dormancy), the females often turn a bright orange or strawberry red.
- The “Spots”: Like the Two-Spotted variety, they have dark internal food-digesting organs visible through their translucent skin. However, Northern Spider Mites often have several smaller, more diffused spots rather than just two distinct large ones.
- Webbing: As their name suggests, they produce fine, silken webbing on the undersides of leaves. This webbing protects the colony from predators and creates a humid microclimate for their eggs.
The “Stippling” Damage
Spider mites don’t chew leaves; they pierce individual plant cells and suck out the contents (chlorophyll). This results in a very specific damage pattern that can be used for diagnosis:
- Stippling: Small, pale yellow or white dots appear on the upper surface of the leaf. As the infestation grows, these dots coalesce into large bronzed or “scorched” patches.
- Leaf Drop: Heavily infested leaves become brittle, turn brown, and drop prematurely. In fruit trees, this can lead to sunscald on the exposed fruit and a significant reduction in next year’s bud set.
- Dirty Undersides: The bottom of a damaged leaf will often feel “gritty” due to the accumulation of silk, cast-off skins, and microscopic spherical eggs.
U.S. Agricultural and Garden Management
In the United States, spider mite outbreaks are frequently “man-made,” often occurring after the use of broad-spectrum insecticides that kill off their natural predators. Management strategies include:
- The “Water Blast”: For home gardeners, a strong stream of water directed at the undersides of leaves can physically knock mites off and disrupt their silken “cities,” often providing enough control to avoid chemicals.
- Predatory Mites: Releasing beneficial mites like Phytoseiulus persimilis is a common practice in U.S. greenhouses and organic farms. These “good” mites hunt and consume thousands of pest mites.
- Horticultural Oils: Neem oil or paraffin-based oils can smother the mites and eggs. These are preferred over traditional pesticides because mites have developed rapid resistance to many common chemicals.
Identification
Very small, often reddish or greenish mites, with fine webbing on plants.
Damage
Speckled leaves, yellowing, and reduced plant vigor.
Control
Increase humidity, use miticides, and introduce natural predators.