Occurrence: Spider mites (Tetranychus urticae) is also called greenhouse spider mites or lime spider mites and are among the most common pests in greenhouses. However, they can also attack houseplants, garden plants, wild plants, etc.
FoodSpider mites attack over 200 different plant species and are a major problem for gardeners. Their food includes trees, fruit trees, vegetable plants, berry bushes and flowers.
Combat: Control is all about balancing the ratio of beneficials to arachnids. In practice, this is done by releasing more beneficials - especially spider mites. As a last resort, kill all pests (and beneficial insects) on a plant to "start over" with insect soap.
Besides predatory mites, insect soap is the only effective remedy for spider mites
Spider mite control
Below we go through the most recognized procedure for spider mite control. The most important step in the control is step 3; predator release - i.e. spider mites.
If you have - or think you have - an excessive population of spider mites on your plants, you should start by confirming your suspicions by identifying the mites.
To find spider mites, use a magnifying glass. Examine the undersides of plant leaves for mites, webs and exoskeletons (their "him").
Another very effective method is to hold a white A4 sheet of paper under a leaf and scratch the underside with a ruler, butter knife or similar. This causes the mites to fall onto the paper, making them easier to see than on the leaf.
2. Remove leaves
To reduce the spider mite population quickly, you can remove the worst infested leaves. The plant is not served by these leaves anyway.
Remember to destroy the leaves immediately after removal to reduce the risk of the mites spreading to other plants.
Predators are the most important and effective tool for spider mite control. Predators are also organic and therefore safe for plants, animals and humans.
In practice, other mites - such as predatory mites - should be used to keep the spider mite population down. Examples of predatory mites and other predators that can be used to control spider mites include:
- The tripsrovmid (Amblyseius cucumeris)
- The spider mite (Phytoseiulus persimilis) – You can buy spider mites here
- The beaked tongue/Orius rove tongue (Orius majusculus)
- Netwing species Chrysopa pearl (in the family gold eyes)
The spider mite is the most common predator of spider mites. It can eat 20 spider mite eggs or 5 adult spider mites per day. In practice, spider mites are mostly used against spider mites, as they are the primary predator of spider mites.
Predatory mites can be distinguished from spider mites by their longer legs and the fact that their front legs are often forward-facing. In addition, predatory mites are more active and generally move around faster. They are also often red or orange in color.
4. Temperature and humidity
To reduce the reproduction rate of spider mites, you can try lowering the temperature and increasing the humidity where the infested plants are growing (e.g. the greenhouse). This is also one of the only preventive methods against spider mites. However, it's impossible to be completely safe from them.
Spider mites in greenhouses
Spider mites in greenhouses are often called greenhouse spider mitesalthough these are the same spider mites that infest houseplants, garden plants, etc. Note that citrus mites, fruit tree spider mites and the like are completely different species.
In the greenhouse, spider mites often attack cucumbers, but also tomatoes, peas, green beans and lettuce. In the garden, they are often found on blackcurrants, blueberries and strawberries, but also a number of other plants.
Spider mites in the greenhouse should generally be controlled as described above - i.e. by releasing beneficial insects and, in the worst case, with insecticidal soap.
Spider mites on houseplants
When you have spider mites on houseplants, the first step is to identify them as described above (to make sure they are actually spider mites).
With houseplants, you don't need to be as aggressive as in greenhouses or gardens. Often you can simply spray the plants with water regularly and the mites will fall off. However, it's important to hit the mites directly, so be especially careful when spraying under the leaves, where the concentration of mites and eggs is highest.
If the water doesn't have enough effect, try using insect soap as described above.
Remedies for spider mites
There is practically only one product that works effectively against spider mites; insect soap. You can read more about insect soap here.
Insect soap should only be used for heavy infestations of spider mites and should only be used on infested plants. This is because the soap kills all mites - including the beneficials that help keep the spider mite population down. Therefore, releasing predators (see above) is often the best way to control spider mites.
Besides predatory mites, insect soap is the only effective remedy for spider mites - you can read more about insect soap here
Damage to plants
- 200 plants: Spider mites can attack more than 200 different plants, which includes a variety of flowers, berry bushes, vegetable plants, trees and fruit trees. In greenhouses, spider mites attack cucumber plants in particular.
- Locker: All spider mites - both female and male - spin a fine web on the plants they infest (hence their name). The web holds the eggs and helps the mites move between different plants and plant parts.
- Mouth partsSpider mites have needle-like mouthparts that are used to penetrate plant tissue and extract nutrients.
- Plant damageWhen spider mites attack a plant, its leaves turn grayish or yellowish. When the damage to the plant is advanced, "dead spots" appear on the leaves (i.e. areas where the plant tissue is completely dead). A plant can lose all its leaves if the mites are not controlled.
- Flower damage: When spider mites attack open flowers, it causes a browning and wilting of the flower petals.
The life cycle of a spider mite
There are 4 stages in a spider mite's life:
Under the right conditions (i.e. high temperature and low humidity), a spider mite can develop from egg to adult in 5 - 7 days. At a temperature of 26°C and normal humidity, development takes about 10 days. In Denmark, in practice, development usually lasts around 15-20 days depending on weather conditions.
The individual steps in the life cycle of the spider mite are explained below:
- Egg stage: Female mites lay between 50 and 200 in their lifetime. Unfertilized eggs become males, while fertilized eggs become females. The gender distribution can vary greatly, but females are usually the most common. The eggs are round, shiny, clear and yellowish in color. They are 0.13 mm in diameter and cannot be seen with the naked eye - so you should use a powerful magnifying glass to find them. The eggs are held in place by the spider mite's web, making them even harder to see. The eggs hatch after 3-19 days (depending on the temperature).
- Larval stage: Spider mite larvae have 6 legs and are colorless. The larvae have roughly the same body shape as nymphs and adult mites. They are only slightly larger than the eggs. During the larval stage, the larvae only consume a small amount of food.
- Newborn dietNymphs have 8 legs and resemble adult spider mites, but are smaller and not sexually mature. Nymphs go through two stages (protonymph and deutonymph) before developing into adult mites.
- Adult stageThe adult spider mite has 8 legs and can have different colors; they can be orange, red or brown, but a greenish, yellowish or greenish-yellow color is more common. Overwintering females are orange or orange-red. The female mite is about 4 mm long, while the male mite is only about 3 mm in length. When the mites are viewed from above, you can see two colored spots, which are stomach contents that can be seen through the body. Since the spots are actually waste products, they may be absent in newly developed spider mites.
The adult spider mites overwinter in grassy areas such as lawns, pastures, roadsides, weed beds, etc. Females can also overwinter under foliage or in tree bark. As temperatures rise in the spring, spider mites begin to feed on various grasses, clover, chickweed and the like. Mating and egg laying follows shortly after and continues throughout the rest of the spider mite's life.
The effect of weather on spider mites
Weather influences the life cycle and distribution of spider mites, which ultimately affects the amount of damage they cause.
The warmer the weather, the shorter the generation lifespan, but it's usually around 5 weeks. Dry conditions can drastically increase the spider mite population because:
- low humidity reduces the development time of spider mites, resulting in more generations in less time and thus more adult mites
- they reduce the number of certain naturally occurring fungi that are pathogenic to spider mites (i.e. harmful to their health)
- they reduce the number of spider mite predators, which normally help keep the spider mite population down
The well-being of spider mites is highly dependent on temperature and humidity.