Spider mites: how to identify, prevent & get rid of them
Infestation with spider mites is a nightmare for every gardener. We will show how to recognise the damage and how to fight the annoying spider mites naturally and with household remedies.
Every hobby gardener who has ever received unwelcome visits from spider mites knows how persistent the little spider mites can be. Whether roses (Rosa), fruit trees, the oleander (Nerium oleander) in the garden, or orchids (Orchidaceae), ficus (Ficus benjamina) or the citrus tree (Citrus) on the windowsill – spider mites are not picky and can infest a variety of plants. Especially tomatoes (Solanum lycopersicum), cucumbers (Cucumis sativus) and other vegetables cultivated in greenhouses are a popular target for the little mites. If the infestation is on the smaller side, it can go completely unnoticed. In dry and warm or hot weather, however, the number of spider mites increases rapidly, and the tiny pests can quickly become a big problem.
Identifying spider mites
Spider mites (Tetranychidae) are arachnids. Accordingly, they have eight legs. The rear two pairs of legs are rather inconspicuous, whereas the front ones are used more and are clearly visible. Externally, the mites can differ greatly, depending on the species. The colour spectrum of the adult animals ranges from yellow to green and red, which is why some people think these insects are some type of a tiny red spider.
In Europe, the red spider mite (Tetrnychus urticae; known also as the two-spotted spider mite) and the European red mite (Panonychus ulmi) are the most common spider mites that damage plants. These miniscule animals are so small, at well under a millimetre, that they are difficult to recognise with the naked eye. This is why, usually, a spider mite infestation becomes apparent only if severe. In severe infestations, you might be able to find small webs on leaf stalks and branches, that the mites create. The insects themselves can be detected on these webs with the aid of a magnifying glass. To summarise, you can tell a spider mite infestation apart by the webs and the damage caused by the tiny pests.
Tip: By spraying the plant with water using an atomiser, the small droplets of water will stick to them and the webs become more visible.
Spider mites: damage & signs of infestation
In contrast to aphids, which pierce the conductive pathways of leaves, spider mites only suck out individual leaf cells, resulting in characteristic dot-shaped brightening. The brightening on leaves or flowers spreads more and more as the infestation progresses. This can lead to deformed shoot tips or to drying out of infested plant parts. In addition, webs with a large number of the small spider mites can be seen.
How to get rid of spider mites
Do not be fooled by the size and seeming lack of resistance of the spider mites: they are tiny but mighty and are, in fact, extremely persistent! The spider mites not only weaken the plant and make it look extremely unattractive, they can also transmit plant viruses. It is also possible that they infect a large number of different plants, which allows them to spread quickly. It is therefore particularly important with this pest to detect an infestation early on and take immediate control measures to prevent it from spreading.
The first step in an identified infestation should be to isolate the plant if possible. In this way, you can avoid an attack on neighbouring plants.
How to get rid of spider mites naturally
In our opinion, the best way to get rid of spider mites with natural means. We advise against the use of synthetic pesticides, especially in the home garden or in living spaces. Such products can be harmful to various beneficial organisms in the garden. In addition, spider mites are resistant to some synthetic agents and, generally, the use of such agents is not recommended in order not to promote resistance.
Natural preparations are available on the market, but many of them do not have a systemic effect. Several applications are therefore required, which can be harmful to both the environment and the plant. Some plant protection products contain active ingredients from the pyrethrin group, which are of plant origin but can harm beneficial organisms in your garden. We therefore recommend that you do not use these products either. Natural products based on pure rapeseed oil are well tolerated by the environment but require thorough and repeated treatment. Preparations based on potassium soap are also environmentally friendly, but they also need to be used several times to work well.
Natural control of spider mites in summary:
- If possible: isolate the infested plant to prevent the spread of the outbreak
- Synthetic agents have a limited effect due to resistance and are harmful to the environment
- Natural preparations based on rapeseed oil or potassium soap are effective but require repeated application
Household remedies against spider mites
Home remedies against spider mites can only be used to a very limited extent. If it is an outdoor plant, you should remove particularly severely infested shoot tips or parts of the plant if the plant is tolerant to pruning. Since spider mites do not like moisture, a slight infestation can be treated by watering the entire plant daily with a garden hose or in the shower.
Another household remedy against spider mites is the method of putting a plastic bag over affected house plants to increase the humidity. However, the air underneath can heat up so much that the affected plant can also be seriously damaged. We therefore advise to leave the plastic bag over the plant for only four days. After a two-day break, to put the bag over the plant again for four days. You can also place an infested house plant outside in the summer when it rains.
Finally, the use of a water-oil emulsion with rapeseed oil is another good way to combat spider mites. To do this, 70 millilitres of water are mixed with 30 millilitres of oil and should be shaken well to form a white emulsion. This is then applied to the whole plant, including the underside of the leaves. But beware: during the process, keep shaking the spray bottle so that the components of the mixture do not separate.
Unfortunately, many plants do not tolerate treatment with rapeseed oil very well, as it dissolves the protective cuticle of the leaves. It is therefore better to use rapeseed oil as a remedy against spider mites only in the evening, when the sun does not shine as strongly. Repeat the application a maximum of three times in order not to weaken the plant too much. If this measure does not take effect within a week, you should resort to using a natural plant protection product.
Homemade remedies to treat spider mites in summary:
- Isolate infested plants if possible
- Remove infested parts of the plant
- Spray infested plant with water daily
- Put a plastic bag over the infested plant to increase the air humidity underneath
- Put indoor plants out in the rain in summer
- Make a spray mixture from 70 % water and 30 % rapeseed oil; treat plants a maximum of three times
Spider mite control with beneficial insects
Some might be surprised to hear that, but you can use beneficial insects to reduce an infestation of spider mites. Predatory mites such as Phytoseiulus persimilis or gall midges such as Feltiella acarisuga can help you keep the pests under control. It makes sense to use predatory mites mainly in the greenhouse and in the house, where spider mites ultimately cause the most damage. In such conditions, the beneficial insects have a clearer target area and they cannot migrate quickly. And even though their name includes “predatory”, these animals are not dangerous to humans.
Out in the garden or in the field it makes more sense to encourage naturally occurring predatory mites by means of a near-natural garden design. We hope it goes without saying, that while you are treating a spider mite infestation with beneficial insects, you should not use pesticides as these could harm the useful organisms.
Preventing spider mites
Spider mites can be resistant to some pesticides, which is why focusing on preventative measures is practical. In the garden, the risk of a spider mite infestation is particularly high in dry weather and at hot temperatures. These are the conditions under which spider mites thrive. You can prevent spider mites in the following ways:
- Beneficial insects such as gall midges can now help you keep spider mites at bay. You can attract gall midges and other beneficial insects by using natural flower beds. For that, you can use flowering herbs such as chamomile (Matricaria chamomilla). In addition, we recommend that you take care to use plant protection measures that are gentle on beneficial insects. In this way, you won’t harm your little helpers.
- If you apply too much nitrogen fertiliser, your plants can become susceptible to spider mites. We therefore recommend keeping an eye out for how much you fertilise and using mainly organic fertilisers.
- In greenhouses, you can avoid heavy infestation by regulating the temperature and humidity. Make sure that the air humidity is not too low, and the temperature is not too high. Spider mites thrive in dry conditions with temperatures at around 30°C.
- Spider mites can also become indoor pests, especially in winter. Be particularly careful when bringing potted plants into the house for the winter. Carefully examine the plants, that you are moving for winter, for spider mites and other pests. Spider mites can become an issue even in winter, particularly if the air in the house is dry and warm.