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Spider Mites: Everything You Need To Know

Have you ever had a plant which appears to be starting to turn yellow in patches, or which has super-fine webs all over it but you can’t see any spiders?

If so, you may about to be at war with a very common pest both indoors and outdoors – spider mites. Let’s get you brought up to speed on everything you need to know to keep them at bay!

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Spider Mite Overview

Two spotted spider mite
Two spotted spider mite, the most common of them all. source

Spider mites are not an insect, as is commonly believed. Instead, they are a type of arachnid that’s closely related to spiders and ticks. They’re extremely tiny – you’ll need a magnifying glass to get a good look at them. They cause the underside of leaves to appear dusty, but if you look closely, you’ll see that the dust is actually moving.

Types of Spider Mites

The most common spider mite internationally is the two-spotted spider mite (Tetranychus urticae Koch). They are occasionally called red spider mites. Widely found in fields, orchards, greenhouses, yards and gardens, and even in indoor growing environments, they have two dark spots on either side of their bodies. They range in coloration from pale yellow through greens, browns, and red hues.

Other varieties include the tumid spider mite (Tetranychus tumidus Banks), the Pacific spider mite (Tetranychus pacificus MacGregor), the spruce spider mite (Oligonychus ununguius Jacobi), and the southern red mite (Oligonychus illicis MacGregor). Many of these are varieties which focus on particular types of trees or crops.

For a lot more information on the more specialized varieties of mites, read this document from the University of Hawaii’s cooperative extension.

Life Cycle of Spider Mites

Life cycle of a spider mite
Life cycle of a spider mite

Spider mites go through five different stages of life: egg, larva, protonymph, deutonymph, and adult. While there are some slight variables in the life cycle of spider mites, this layout of the twospotted spider mite and its normal life cycle gives a good indication of the various phases.

Egg. Adult spider mites lay their eggs on the underside of leaves. An adult can lay as many as 100 eggs over the course of a three-week period of time. Unfertilized, the spider mite egg will become male. Fertilized eggs become female, and it’s typically about a ratio of 3 males to every female.

Larva. When the eggs hatch, six-legged colorless larvae emerge and immediately begin to feed. They feed on plant tissue and sap, causing paler spots to form on the leaves. As they feed, they begin to change color, and start to develop their two dark spots.

Protonymph. Once a larvae has consumed enough food, they become temporarily inactive and develop into an eight-legged protonymph. Both stages of nymph can form webs.

Deutonymph. After a short period as a protonymph, and once sufficient feeding has occurred, another resting period happens. Shortly thereafter, it goes through a moulting process and becomes an eight-legged deutonymph. Both stages of nymph can form webs.

Adult. After a short period as a deutonymph and the normal feeding and rest cycle, the deutonymph moults again. The eight-legged adult emerges from its prior skin and is now ready to begin laying eggs to restart the cycle. Adults can reproduce sexually or asexually, although most unfertilized eggs hatch male mites. The color of the adult often depends on what crop it has been feeding on. Adult spider mites can form webs.

This process normally takes about 14 days. However, in hotter environments, the process becomes much more rapid, and it can be as short as 7-10 days time.

Common Habitats of Spider Mites

The most rapid growth of spider mites is in hot, dry environments. While they do thrive in humid conditions, they seem to have more egg fertility in dry conditions, and they are commonly known to be problematic in dusty environments as well.

Spider mites prefer areas with plants that have higher levels of nitrogen, phosphorous and carbohydrates in their leaves. This means that the irrigation pattern of many commercial fields in otherwise-dry areas tends to create an ideal breeding ground for spider mites. Similarly, home gardens can be impacted, especially those with drip irrigation systems installed where the mites won’t regularly be washed off of the underside of the leaves.

Spider mites also like indoor hydroponics setups, especially those which use some soil. It’s a slightly more humid environment, but the temperatures tend to be around the right range for moderate to rapid mite population expansion. Since spotting on leaves is commonly viewed as an indication of a possible nutrient deficiency and that is often treated initially, a spider mite infestation can easily be overlooked until the plants start to suffer.

What Do Spider Mites Eat?

Spider mites infesting a leaf
Spider mites infesting a leaf. source

Spider mites feed on plant tissues and sap. They pierce the cell wall of the back of leaves and literally suck the juices out of the leaf, causing it to spot. Over time, this goes from tiny yellow spots through complete yellowing of the leaf, and a large population can decimate a plant quickly.

How To Get Rid Of Spider Mites

Spider mites, and mites in general, can be tricky to get rid of. While most chemical miticides will handle two-spotted spider mites, they might not handle citrus mites, or perhaps they will handle both of those but not broad mites or rust mites. Most organic options focus on broad-spectrum oils to prevent adults from laying eggs, but require very regular use until the mites are gone, so work a bit more gradually.

Organic Spider Mite Control

Spider mite eggs
Spider mite eggs. source

Before doing anything else, you need to confirm that you are in fact having a problem with spider mites, and not aphids or other pests. If you tap a leaf onto a sheet of white paper, some mites should fall off onto the paper and will be visible with a magnifying glass. Once you’re sure that they’re mites, it’s time to consider how you wish to control them.

The majority of organic methods use a variety of plant-based oils to smother the mites or their eggs. While it does not necessarily stop all eggs from hatching, it places an oily residue over the surface of the egg which smothers the larva inside. It also disrupts the breathing of the adults, but you may still have adults on your plants. Use a “bug blaster” or other hard-blasting water spray to wash off as many of the mites, nymphs, and larvae as you can before treating the plant.

One of the most common choices is neem oil. It is a broad-spectrum organic herbal pesticide and repellent. If you’re doing hydroponics and don’t want the potential of oil residues building up in your hoses, you’ll want something like AzaMax, which has the same active ingredient that causes neem to work so well (azidirachtin, which occurs naturally in neem oil).

Mite-X uses a mix of cottonseed, clove, and garlic oil to smother the eggs of spider mites and prevent them from hatching. Iit smells mostly of cloves rather than other less-palatable scents. If you’ve been asking “how do I combat spider mites indoors?”, this is one of the most popular options for you.

Sierra Natural Science 217 Mite Control (SNS-217) is slightly different. Made from rosemary extracts, it disrupts the cell structure of the mites, causing them to dehydrate. It also coats the eggs in an oily residue to smother the larvae within.

Take Down Garden Spray is another option. It contains pyrethrins and a canola oil insecticide. The oils work in much the same way as Mite-X to smother the eggs, but the pyrethrins work similarly to SNS-217’s dehydration attributes. As this spray has almost no odor, it’s often used indoors when a more potent scent is not desired. You might also consider a fogger such as Doktor Doom as an alternative that also uses pyrethrins.

Environmental Spider Mite Control

The first thing to do when you suspect spider mites is to spray off your plants thoroughly with a moderately high-powered spray — the “Bug Blaster” works extremely well for this purpose. Keeping your plants regularly cleaned off will help significantly, and should be part of your regular practice if spider mites are common for you.

If you’d like to avoid sprays or foggers entirely, you might want to consider beneficial insects to keep the spider mite population down. Predatory mites are a great option as they’re harmless to plants but will eat a wide variety of spider mites and their eggs. Ladybugs and lacewings are other types of beneficial insects which may help keep the spider mite population low, although they’re also fond of aphids and other insects. There is a type of insect that’s actually named spider mite destroyer, and it’s well-named!

In hydroponics, you can add AzaMax (mentioned above) in limited quantities to your nutrient solution and allow the plants to absorb it, which will discourage spider mites from invading. It’s also advised that once you have a clean room environment for your grow area, keep any new plants you want to add to your hydro garden separate for at least 2-3 weeks to monitor them for pests. The best protection is to disallow any harmful insects from invading your grow room in the first place.

Preventing Spider Mites

Spider mites terrorizing a tomato plant
Spider mites terrorizing a tomato plant. source

The best prevention for any plant problems, whether pest-related or disease-related, is to keep your plants as healthy as is possible. The more healthy your plant is, the better it will resist.

Plants that are stressed due to lower water conditions tend to be prone to a quick death if spider mites manage to take hold. Keeping your plants well-watered may not prevent the mites, but it might buy you enough time to eliminate them before the plant dies. If growing indoors, keeping the soil humid can help to keep the mites at bay too.

If you do find a plant that’s covered with spider mites and it appears as though it’s nearing death, it’s best to remove that plant entirely. Do not compost it – throw it away! This eliminates the existing mites and their eggs from your garden entirely. Similarly, if it’s only a few leaves, remove those leaves and throw them away. It’s better to remove any eggs from your garden than to discover later that you have a much worse infestation.

If you are growing houseplants or have plants inside a greenhouse or grow room, remove any dust from the plant’s leaves at least once a week with a lightly-moistened cloth. That tends to prevent mite infestation since you’re regularly cleaning the leaves.

Frequently Asked Questions

Q: Where do spider mites come from, anyway?

A: Honestly, mites are so tiny that you might be carrying them into your garden or indoor grow room yourself. They can ride in on your skin, your shoes, your clothes. Your dog or cat might have mites on them. But the most common source for spider mites is actually live plants. It’s a good idea to quarantine plants, even ones you purchase at garden centers or are given by friends, for a couple weeks before putting them where you want them to end up. This way, you catch the pests before they can make it onto your established plants.

While demolishing spider mites may take a little bit of time, it can be done — and your plants will be happier for it. Do you have any other techniques you’ve used in the war against mites? Let me know!

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