In warm and dry weather, spider mites quickly develop into a plague that is difficult to get under control, especially since they are often discovered late due to their small size. In the following article we have summarized how you can recognize an infestation with the tiny spiders in good time and how you can prevent and combat it.
Biologically speaking, spider mites are not insects, but are spiders. Adult animals have four pairs of legs, larvae only three. With a size of between 0.2 and 0.6 millimeters, they can hardly be seen with the naked eye, which makes their detection difficult: In many cases, the infestation is already very severe and the first damage has already occurred. Spider mites have stinging bristles with which they suck the sap from the plant leaves. They are also able to weave fine, almost invisible, cobweb-like webs with it. However, this does not apply to all species.
Spider mites – species
The red spider mite, also known as the common spider mite (Tetranychus urticae), is one of the most common species. These are not necessarily red spider mites, because basically the bodies of the animals are always transparent. The coloring varies depending on the species, season and food supply.
- red, green, brown or yellow forms are possible
- red color = little food or cold
For this reason, the females of the common spider mite often have a red body color in autumn, while they are more green in spring.
Another very problematic species is the fruit tree spider mite (Panonychus ulmi), which affects many types of fruit and even strawberries. Here, too, the females have a bright red colour, which is why the two spider mite species mentioned are also collectively referred to as “red spiders”. In contrast to the common spider mite, the fruit tree spider mite does not weave webs, but only separates individual threads. With the help of this, the spider is mobile enough to colonize neighboring plants.
Occurrence and frequently infested plants
Common spider mites mainly attack ornamental and crop plants, especially beans and other legumes such as peas (hence the alternative name “bean spider mite”), potatoes, cucumbers, tomatoes and other vegetables with a high water requirement, the fruit tree spider mites primarily choose fruit trees of all kinds as host. Apple and pear trees as well as plums and currants as well as grapevines are particularly affected. However, plantings in greenhouses, in polytunnels, covered cold beds and propagation beds and indoor plants are particularly at risk.
way of life
The reason for this lies in the microclimate there: Spider mites multiply rapidly in dry and warm climates. Temperatures between 22 and 28 degrees are optimal, at which red spiders feel particularly comfortable. In greenhouses and other protected plant cultures, the conditions for the animals are ideal: They are protected from damp and cold weather, which is why infestation is not just limited to summer.
The spiders also look for sheltered habitats outdoors where they are less exposed to the weather conditions. They are therefore particularly common in sunny, wind and rain-protected south and south-west locations. The typical webs also serve primarily to protect against cold and wet.
Nevertheless, spider mites are very difficult to combat: on the one hand, they multiply extremely quickly under the right conditions. During their lifespan of just a few weeks, the females lay up to 100 eggs, from which the larvae develop within three days. These, in turn, are sexually mature within just one week and in turn lay eggs. In this way, up to seven generations develop in just one summer, the females of which survive the winter even in the deepest sub-zero temperatures. The arachnids hide in leaves, in brittle tree bark or in cracks in walls. The fruit tree mite even lays hardy eggs from which the larvae hatch only in the following spring.
Spider mites are mainly found on the underside of the leaves, where the juvenile and adult animals tap into the cell sap and also lay their eggs. Since the animal cannot be seen with the naked eye, you should look out for the following signs:
- Discoloration of the leaves, these are much lighter than usual
- silvery sheen on the underside of the leaf
- whitish, very small speckles on the leaves (only early stage)
- Discoloration of the leaves, formation of brownish-red spots (advanced stage)
- these roll up later and are thrown off
- fine webs on leaves and shoots (not in the fruit tree mite)
- Stunted growth, generally high susceptibility to other pests and diseases
It is best to subject your plants to regular inspections when the weather is appropriate, using a magnifying glass. Look for the tiny spiders on the underside of the leaves and spray suspicious plants with a fine mist of water from a spray bottle: the droplets get caught in the web and make it visible.
In some years, however, all efforts are of no use and an infestation with annoying spider mites still occurs. The sooner you discover and treat this, the easier it is to get the problem under control: regular checks are therefore essential between April and October when the weather is suitable. Experience has shown that spider mites are best combated with the following means:
use of beneficial insects
The most effective way, especially in closed plantings (greenhouses, etc.), is to use beneficial insects such as the predatory mite (Phytoseiulus persimilis). These eat spider mites in all stages of development with great appetite and can therefore not only counteract an infestation, but even significantly decimate it. Naturally, this works better in greenhouses than outdoors, since the assassin bugs cannot migrate here. Make your garden as beneficial as possible so that useful insects, mites and other animals can settle here – the fewer problems you have with pest control.
Red spiders can also be combated preventively and with a small infestation using sprays you have made yourself. Field horsetail and fermenting nettle brews in particular have proven to be helpful here. These broths also have a plant-strengthening effect, as they have a positive effect on the plants’ defenses and also fertilize. Nettle broth, compost and horn shavings are perfectly adequate fertilizers in many gardens and keep the plants strong and healthy – so spider mites and other pests have fewer chances.
First of all, traditional insecticides won’t help here because red spiders aren’t insects. Instead, in the case of a severe infestation that cannot be handled in any other way, you have to resort to a so-called acaricide, which was specially developed for this group of pests. In the kitchen garden, also make sure to use approved preparations for fruit and other crops. Otherwise, your crops could be in jeopardy as you are no longer allowed to eat the fruits and vegetables from plants sprayed with poisonous agents. Means should also be chosen that do not harm beneficial insects, birds and other peaceful garden dwellers.
Tip: Neem-based preparations have proven to be effective.
Of course, you don’t have to hit the spiders with the chemical club right away, this is only recommended if nothing else helps. Instead, if the arachnids are only slightly infested, first rinse them off with a strong jet of water and possibly nettle or horsetail broth. On the other hand, you should refrain from putting a plastic bag over it, as is sometimes recommended: it can get extremely hot underneath and the plant can therefore suffer irreparable damage. Make sure you rinse or wash off the undersides of the leaves in particular. Repeat these applications at regular intervals.
Since the “red spider” is so difficult to detect and control, you should prevent an infestation as much as possible. Of course, this is easiest to do in closed rooms such as a greenhouse or a cold frame, since you can influence the weather conditions here. These measures have proven to be effective:
- Increasing the humidity, for example by regularly spraying water
- Ventilate greenhouses and other closed systems regularly
- In general, always water plants sufficiently, especially during dry periods
- Mulch plants to keep moisture in the soil longer
- Always clean support rods and similar before each new use
- Fertilize plants in a balanced way, avoiding over-fertilization with nitrogen
- Instead, pay attention to potassium-rich fertilization
- Organic fertilization is optimal, as over-fertilization is less likely here
It also helps not to plant the plants too close together. Always keep to the recommended planting distances so that the plants are not unnecessarily stressed. Weakened plants are particularly susceptible to spider mite infestation, and planting too close together increases the microclimate there positively for the spiders.