Spider Mite Prevention & Organic Control

Are spider mites wreaking havoc in your garden? This guide will help you identify spider mites and the damage they cause. You will learn about the conditions that support spider mite growth and what actions to take instead. Most importantly, you will discover effective spider mite prevention and organic controls.

Article Outline:

1. What Are Spider Mites?
2. Conditions That Favor Spider Mites
3. Identifying Spider Mite Damage
4. Preventing Spider Mites
5. Beneficial Insects That Help Control Spider Mites
6. Steps to Treat Spider Mites Organically
7. When to Treat Spider Mites and When to Remove Plants
8. Spider Mite Prevention & Organic Control Conclusion

What Are Spider Mites?

You notice something is off about your bean plants. You may see stippling on the tops of the leaves and tiny moving dots on the undersides of the leaves. There may also be what looks like webbing surrounding some of the leaves. Congratulations – you’ve just met the infamous spider mite!

Spider mites are tiny arachnids related to spiders and ticks. They are small–usually 1/20 of an inch (1mm long), and look like tiny moving dots to the naked eye. Spider mites belong to the Tetranychus genus. These mites are known for their silk webbing on infested leaves, hence the name “spider mites.” The webbing gives mites and their offspring protection from changes in the environment and their natural enemies.

These pests live in colonies on the undersides of leaves. The largest female spider mites can be identified by their oval bodies and two dark blotches on each side. They reproduce rapidly–within 1-3 weeks—especially in favorable conditions, leading to large infestations quickly. One female spider mite can lay hundreds of eggs. Unfortunately, in mild climates, they are active year-round and do not die off. 

Conditions That Favor Spider Mites

Spider mites thrive in hot, dry conditions, making them particularly problematic during summer. Here are some areas and conditions they prefer:

• Preferred Crops: Beans, cucumbers, lettuce, melons, peas, tomatoes, rosemary. Also common on landscape plants like cypress trees.
• Hot Weather: Spider mites reproduce quickly in high temperatures. A single generation can be completed in less than a week.
• Dusty Environments: Spider mites are often found near dusty roadways or garden margins. Dusty conditions create an ideal habitat for these pests.
• Water Stress: Plants under water stress are highly susceptible to spider mite infestations. To reduce the risk, ensure your plants are adequately watered.

Identifying Spider Mite Damage

Spider mites cause significant damage by sucking cell contents from leaves. Here’s how to identify their presence:

• Stippling: White or yellow dots or stippling on leaves are early signs of spider mite feeding.
• Color Changes: Leaves may turn yellowish, reddish, or bronze as feeding continues.
• Webbing: Heavy infestations result in large amounts of webbing covering leaves, twigs, and fruit.
• Dying Plants: Large infestations often kill the host plant. 

High populations of spider mites can weaken plants, leading to leaf drop and reduced yields. For ornamentals, spider mites primarily cause aesthetic damage but can kill annual plants if left unchecked.

Preventing Spider Mites

Effective prevention strategies can save your garden from a spider mite invasion. Here are some tips:

• Regular Monitoring: Check plants regularly, especially during hot, dry periods. If necessary, use a magnifying glass.
• Keep It Clean: Reduce dust in your garden using mulch and regular watering.
• Watering: Proper watering reduces plant stress and makes them less attractive to spider mites. Spray plants with water to dislodge mites and clean leaves.
• Humidity: Higher humidity levels can deter spider mites, especially in greenhouses or indoors. Spray off plants with water occasionally to increase the air’s moisture level.

Read this blog post for more tips for preventing pests in the garden.

Beneficial Insects That Help Control Spider Mites

Nature provides some of the best pest control agents. Certain beneficial insects can help manage spider mite populations:

• Big-eyed bugs, damsel bugs, minute pirate bugs, predatory mites, predatory thrips, and spiders are natural predators of spider mites.
• Lacewings: These insects and their larvae feed on spider mites.
• Ladybugs: Both adult ladybugs and their larvae consume spider mites.

Encouraging these beneficial insects in your garden can significantly reduce spider mite populations without resorting to chemical treatments. Large spider mite populations are more common when pesticides are frequently applied. Pesticides often kill off the many natural predators, allowing them to reproduce unchecked. 

You may choose to introduce beneficial insects into your garden. If you do this, consider a beneficial insect subscription service from Heirloom Roses, which gets its insects from trusted insectories. These introduced insects can help bolster populations throughout the season. Do not purchase ladybugs from local nurseries. Learn why in this blog post.

Steps to Treat Spider Mites Organically

If you find spider mites in your garden, take the following steps. Be conservative in your approach and only proceed to the next more aggressive step if needed.

1. Identify and Isolate: Confirm the presence of spider mites, isolate affected plants if possible, and remove the worst-affected leaves. 
2. Water Spray: Use a knockdown water spray to dislodge mites from plants. Focus on the undersides of the leaves.
3. Insecticidal Soap and Horticultural Oil Treatment: Use treatments sparingly. To protect beneficial insects, target only affected areas.

• Add 1 Tablespoon (19 ml) of Safer Insect Killing Soap and 1 Tablespoon (19 ml) of Monterey Horticultural Oil to a 32-oz (946-ml) spray bottle filled with water.
• Spray affected areas in the morning or evening. Never spray in the middle of the day. Use a pump sprayer and apply a fine mist to all sides of the plant—there is no need to drench it.
• If temperatures are above 90°F (32°C), rinse with water afterward to avoid burning the leaves.
• Repeat application after several days to eliminate newly hatched eggs. If needed, repeat at weekly or bi-weekly intervals up to three times.
• You may want to rinse leaves off after application during hot weather to avoid leaf burn.

When to Treat Spider Mites and When to Remove Plants

Deciding whether to treat or remove infested plants depends on the severity of the infestation and the plant’s condition. Here are some guidelines:

Treat: Regular water sprays and insecticidal treatments often manage early infestations. Strong, healthy plants with minor damage are good candidates for treatment.

Remove: Severely infested plants with significant leaf damage and webbing may be beyond saving. Removing heavily infested plants can prevent the spread to other healthy plants.

Spider Mite Prevention & Organic Control Conclusion

Spider mites can be a persistent pest, but you can protect your home garden with careful monitoring and proactive measures. Regular inspections, proper watering, and encouraging beneficial insects are vital to preventing and managing spider mite infestations.

Targeted treatments using insecticidal soap and horticultural oil can be effective in challenging cases. Before deciding to treat, weigh the severity of the infestation and the plant’s overall health.

Further Reading:

• Organic Pest Control That Really Works
• Garden Hacks: 9 Simple Pest Control Tips
• Squash Bug Prevention and Organic Control

Sources:

• Colorado State Extension Fact Sheet Spider Mites 5.507, by W.S. Cranshaw and D.C. Sclar
• Pest Notes: Spider Mites UC ANR Publication 7405, L. D. Godfrey, Entomology, UC Davis, Produced by UC Statewide Integrated Pest Management Program University of California, Davis, CA 95616
• The Vegetable Garden Pest Handbook, by Susan Mulvihill

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