Sviluška - Eight-Legged Enemy of Plants

In short: Spider mites, especially the two-spotted spider mite (Tetranychus urticae), suck sap from the underside of leaves, which turn pale, yellow and drop. They thrive in dry, warm air and multiply fastest above 30 °C. Keep humidity above 60% RH, quarantine new plants for at least a week and treat infestations with sprays, oil products or predatory mites.

Spider mites can appear in the garden on their own, but usually we bring them from the garden center along with a plant. They are very dangerous pests, so preventive measures are worthwhile in favorable conditions.

What are spider mites anyway?

Spider mites belong to arachnids, specifically mites. There are more than 100 species, but the most significant for us is the two-spotted spider mite (Tetranychus urticae), which damages both utility and ornamental plants. It is a very common and dangerous parasite especially on houseplants.

The two-spotted spider mite is tiny, measuring only fractions of a millimeter. As adults, they have eight legs and are white with two green spots on their abdomen. In autumn, spider mites turn bright red (be careful not to confuse them with the red velvet mite - it looks similar but is much larger).

These mites feed by sucking on the underside of leaves, causing them to yellow and drop prematurely. They seek dry and warm environments, which allows them to reproduce incredibly quickly. A single fertilized female can, under ideal conditions, cause a population increase of up to 13,000,000 individuals in a month!

The length of the spider mite reproduction cycle depends on humidity and air temperature. At RH above 60%, the reproduction rate significantly decreases. Conversely, with increasing temperature, the reproduction rate grows very rapidly. It reaches its maximum above 30 degrees Celsius, and spider mites begin to overwinter at temperatures below 10 degrees.

Spider mites are not capable of flight, so they only spread on dry surfaces and on foot. They spread fastest when plants touch each other with their leaves. However, they can also use you or even a pet for their spread - they simply hitch a ride to another plant.

Spider mites cause the greatest damage in enclosed spaces - in greenhouses and on houseplants. Outside, there are usually enough antagonists of these pests, i.e., natural predators. Sometimes, however, spider mites can multiply outdoors to such an extent that predators cannot effectively eliminate them.

Symptoms of spider mite infestation

Initially, the infestation manifests inconspicuously and can be mistaken for nutrient deficiency. Leaves pale, later turn yellow and fall off. The infestation progresses from older leaves to younger ones.

In later stages of infestation, spider mites create fine webbing on the plant, which facilitates their movement. We first find it in leaf axils and branches, later it may envelop entire leaves and twigs. At this stage, there are already thousands of spider mites on the plant and getting rid of them will not be easy at all.

Flowers and fruits are usually not attacked by spider mites unless they are extremely overpopulated. However, intense infestation can cause relatively rapid death of the entire plant.

Preventive Measures Against Spider Mites

Prevention is always based on the opposite of what benefits the pest. Spider mites thrive in dry and warm conditions, so the easiest way to deter them is with moisture and coolness. Air humidity should be above 60% RH. If we cannot increase air humidity, we can mist the plants several times a day with a spray bottle. This method is also partially effective for mild spider mite infestations.

Another countermeasure is maintaining isolation distance between plants, which prevents spider mites from crawling directly from plant to plant and spreading quickly.

Keep any plant you bring from a garden center in quarantine for at least a week, but preferably longer. In large commercial greenhouses, spider mites and other pests are very common, and plants are rarely treated adequately.

Active Protection Against Spider Mites

If spider mites have already multiplied in your space, you'll need to use something stronger than water. We can choose between chemical, ecological, or biological products.

Chemical products are usually applied as a spray. They only work on certain developmental stages of spider mites, so spraying must be repeated several times. Chemical products are usually very effective and can save even heavily infested plants. Caution is needed with plants you intend to consume - the pre-harvest interval must be observed and plants should be thoroughly washed before preparation. It's always better to use a product designed specifically against spider mites, as classic insecticides may not work on them at all.

Before applying a product against spider mites, rinse the plant if possible. Water will remove a large portion of the spider mites and the plant will suffer less from spray application.

If you want to use a gentler method, you can try one of the ecologically friendly oil-based products. For example, Biool is quite effective. Oil-based products coat and suffocate the pest. It can be used against a wide spectrum of insect pests, including spider mites. After applying oil-based products, plants need to be rinsed with lukewarm water after two or three days so the plant can breathe.

The biological method involves introducing natural predators. This method is especially effective in greenhouses or enclosed spaces where predators won't escape. Predatory mites Phytoseiulus persimilis and Typhlodromus pyri are used. They feed on spider mites and significantly reduce their numbers. When using these predators, plants cannot be treated chemically anymore, otherwise we would kill them too. Purchasing predators is more expensive, but they remain on plants for quite a long time.

You can now purchase effective sprays against spider mites in our e-shop!