Spider Mites

The spider mite family (Tetranychidae) encompasses around 1,200 species. These animals are part of the Prostigmata order within the mite family. Like all mites, they actually aren’t insects – they’re arachnids. Spider mites get their name from the fine webs that some species weave over the host plant’s leaves and shoots. Alongside the red spider mite (Tetranychus urticae), the most common plant pests include the European red mite (Panonychus ulmi) as well as the spruce spider mite (Oligonychus ununguis).

Spider mites tend to grow to between 0.01 and 0.03 inches long. Therefore, most species are not visible without magnification. Like all arachnids, they have eight legs, and their eyes are usually visible as red dots on the front part of the body. Their bodies are elongated ovals, covered with countless microscopic hairs, and not segmented into head, thorax and abdomen like those of insects. On the front of their bodies, spider mites have piercing-sucking mouthparts that they use to pierce the underside of the leaf and drain leaf cells.

The color of any individual animal depends not only on the species but also other factors such as season and host plant. The body varies from pale green and yellow green to orange and red brown.

Spider mites love dry, warm climates and a protected location. These animals may only live for a few weeks but they can reproduce very quickly in the right conditions. In a warm greenhouse or sunny window full of plants, they will even reproduce all year round.

Female spider mites can lay up to one hundred eggs in one lifetime depending on their species and the weather. They are usually placed on the underside of leaves. After around three days, the new generation begins to emerge: within six to fifteen days, the larvae develop into adult animals capable of reproduction.

Spider mites are found outside as well as in greenhouses in summer. Here, they can do a lot of damage to various fruit, vegetable and decorative plants. Houseplants are often infested by spider mites – especially in winter when heating ensures the best possible conditions for them to thrive.

As spider mites are often below 0.04 inches in size, depending on the species, they are usually first spotted by their webs and the damage on the infested plants. Plant lovers can, for example, spot spider mites by fine webs in leaf axils and leaf edges. Unlike most other spider mite species, the European red mite hardly weaves any webs. Another characteristic of a spider mite infestation is a multitude of tiny, light dots on the leaves, which can appear from spring to fall outdoors or all year round on houseplants or greenhouse plants. Spider mites suck cells from leaves, creating a mottled look on the tops and undersides of leaves. The puncture marks take on a slightly silvery shimmer as the light hits them, later often turning light gray to bronze in color. Finally, the leaves begin to roll and die off. Spider mites also damage plants due to their toxic saliva. The small pests can also infect plants with dangerous viruses. The spruce spider mite also causes gray-green discoloration and large webs on the plant’s needles.

Spider mites are not particularly specialized, which gives them a wide selection of host plants. But these pets do prefer infesting weakened plants.

The most common species is the red spider mite. It is found all over the world, and is a major problem in many greenhouses. Beans, cucumbers and roses are among the cultivated plants most commonly affected here. Damage caused by the red spider mite outdoors is comparably minor, as they only venture out on very warm summer days.

In summer, regular airing and increased humidity through regular spraying with water can help reduce spider mite activity. This method is especially helpful in closed rooms, such as greenhouses. A sufficient water supply is also urgently necessary to tackle dryness. A mulch layer can help keep soil retain moisture for longer.

So, air your greenhouse or cold frame regularly. Bean frames, obelisks and trellises should be thoroughly cleaned after use, as they often serve as winter homes for these pests.

As spider mites generally prefer to infest weakened or stressed plants, you can usually prevent infestation through good care. Ensure balanced, potassium-rich fertilization, as this will strengthen the plant’s tissue and increase its resilience against pests. Caution: a very nitrogen-rich fertilizer will soften the leaf tissue, making it more vulnerable. Furthermore, you should ensure each plant is in the ideal location for its needs.

When it comes to fruit trees, trunk care in fall is an important preventative measure: Get rid of any loose bark with a handheld hoe, then apply a whitewash, such as an organic tree paint. This not only prevents frost-related cracks, but also decimates overwintering pests.

Encourage assassin bugs, ground beetles, ladybugs and green lacewings as important natural predators in a natural garden. The preventative use of chemical sprays can damage lots of beneficial insects – so it’s best to avoid these.

Spotting a spider mite infestation early on is the first important step so you can introduce suitable countermeasures. The earlier you tackle these pests, the more effective your countermeasures will be.

As already mentioned, spider mites are incredibly small. Therefore, it is advisable to regularly take a closer look at the underside of your leaves with a magnifying glass. A mister will also help you to spot spider mites: spraying plants with water makes fine webs more easily visible.

Robust and resilient plants with mild infestations can be simply rinsed with a jet of water. You can simply place your houseplants in the shower or bathtub to rinse. If possible, this procedure should be repeated regularly every few days until no more pests can be found on the plants. Placing houseplants in a clear plastic bag afterwards is very effective. High humidity usually means that spider mites die off within one week.

If it’s harder to rid your plants of this pest, you should thoroughly remove all affected leaves and stems from plants that tolerate pruning to prevent any further spread. Don’t put the plant waste into your compost, otherwise the pest will continue to spread here. It’s better to dispose of them in a separate location in the garden or with garden waste.

It is possible to use an organic approach to tackle spider mites, through the use of predatory insects Phytoseiulus persimilis. These beneficial insects are available online, and can be used outdoors as well as in greenhouses. They work best in closed rooms. 
Mesostigmata feed on spider mites, and kill them in any developmental stage - from egg to larva and adult. This means that these beneficial insects prevent mass reproduction. A prerequisite for the use of Mesostigmata is a climate suitable for them. Temperatures around 77 degrees Fahrenheit and 75% humidity are ideal. If these conditions are met, these beneficial insects will spread twice as fast as the pest. Mesostigmata should be used as early on as possible to tackle the spider mites.

As well as the involvement of Mesostigmata, the native gall midge Feltiella acarisuga can also be used. These inconspicuous, nocturnal insects can decimate large numbers of spider mites in a very short time. 
After mating, the females lay up to 100 eggs in spider mite nests. After just a few days, yellow to orange young emerge from the eggs with the ability to destroy up to 30 or 60 spider mite eggs. After going through various larval stages, these beneficial insects pupate on the undersides of leaves. Surrounded by a white web, the cocoon is often positioned near the leaf veins, forming the most visible developmental stage of Feltiella acarisuga.

Optimal conditions for these beneficial insects are temperatures between 20 and 27 degrees Celsius as well as 80% humidity. When used as targeted biocontrol, at least two animals per 10.8 square feet of plants are needed for a successful outcome. You can purchase these insects online from various sellers of beneficial insects.