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Damage by spider mites can often be recognized by the first small yellow spots on leaves. Spider mites damage plants by sucking plant sap from the underside of the leaves, especially in the young parts of the plant. This first creates small yellow spots on the leaves and eventually the leaves even turn completely yellow. This has a negative impact on growth and production, as the plant has less chlorophyll available. In the worst case, this can even cause the plant to die.

In addition, greenhouse spider mite, or tetranychus urticae, also causes cosmetic damage. When spider mites are abundant in a crop, the insects can completely cover the plants with webs. These webs provide a breeding ground for the greenhouse spider mites, causing the problem to spread further and further.

Spider mite damage: 

Spider mites (Tetranychus urticae) pass through the following experimental stages: egg, naiad, protonymph, deutonymph, and grown-up. In the larval and nymphal stages, an active period and a sleeping period of roughly equal duration can be distinguished. Eggs are generally set up on the underpart of leaves. They've an round body that's rounded at the hinder end. Their colour can vary from orange, light unheroic or light green to dark green, red, brown, or nearly black.

Detecting spider mites early is very important because of their rapid reproduction. In practice, this is often difficult because the spider mites are mainly under the leaves and they generally do not move much. You often only notice them when you see webs in the crop, or when the plants are already significantly affected - indicating that the spider mites are already present in large numbers in the crop. 

An adult greenhouse spider mite is about 0.5 millimeters in size with eight legs. It has a yellow-brown color and a brown spot on either side of its body. However, the color depends on the crops on which the spider mite occurs. In cucumber crops, spider mites are often yellow-brown, where as opposed to tomato, the spider mites are red-brown.

Eggs are usually laid around the veins of a leaf or on the underside of the leaf near the attachment of the petiole. Larvae then hatch. These are orange-red in color, 0.14 mm long and they have six legs.

Spider mites have different life stages: egg, larva, nymph and adult. The temperature in the greenhouse determines the duration of a life cycle. If the temperature is higher than 30 degrees, then a life cycle lasts only one week, while at temperatures below 12 degrees, spider mite development comes to a complete halt. A white molting skin often remains during the transition to different stages.

The greenhouse spider mite is often confused with the false spider mite. In fact, false spider mite resembles greenhouse spider mite, but is from a different family, the Tenuipalpidae.  The major difference between spider mite and the false spider mite is the spotting pattern on its body.

In particular, the false spider mite resembles the greenhouse spider mite in color, namely yellow to orange-red. However, the false spider mite has a dark spotting pattern on its body, while a common spider mite has only two spots on its body. 

Spider mite treatment is often done with the use of natural enemies. Spider mites spend much of the day and life cycle on the underside of the leaves because spider mites do not want the sun. Therefore, it is difficult to spot spider mites. If you have spider mites spotted in the crop, it is important to start as soon as possible with spider mite control. If the spider mite infestation is not yet very large, start by removing the leaves on which spider mites are present. Make sure that the spider mites do not come into contact with 'clean' leaves. The advice is to start immediately with biology.

Biological spider mite control can be done using natural enemies. Biological spider mite control can be done with predatory mites, gall midges and sometimes predatory bugs. The following natural enemies are used for biological spider mite control:

• PHYTOcontrol (Phytoseiulus persimilis) contains vermiculite as a carrier. This has the advantage that little residue is left on the leaf. When applying this product, be sure to allow the predatory mite to reach room temperature slowly to avoid shock. The dosage depends on the pest pressure.
• PHYTOcontrol (sawdust) (Phytoseiulus persimilis) contains sawdust as a carrier. This has the advantage that it spreads more easily and stays on the leaf better.
• Feltiline (Feltiella acarisuga) controls spider mite larvae and clears spider hair. 
• ANDERcontrol (Amblyseius andersoni) targets various spider mites. Andercontrol can be used preventively, because the predatory mite survives relatively long without prey.
• AMBLYCAcontrol (Neoseiulus californicus) also targets different spider mites. AMBLYCAcontrol, like ANDERcontrol, can be used prefentively, because the predatory mite survives relatively long without prey.

It is important that you deploy the natural enemies in the wide area around infested plants, even if you do not yet see any infestation on these plants. The spider mite can spread quickly in the crop and this way you prevent this as much as possible. 

For a correction on spider mites, there are several correction agents available. Chemical control of spider mites can be done with acaricides, especially contact agents. Because spider mites are often located on the underside of the leaves, applying the proper spraying technique is important. Sulfur is an appropriate pesticide to control spider mites. However, keep in mind that sulfur interferes with the searching behavior of gall midges. Therefore, it is not recommended to apply gall midges and sulfur at the same time. Apply the space treatment in the evening, as this is when spider mites are most active. In non-illuminated crops, adjust spraying techniques during dark months because spider mites hibernate then. During this overwintering period, spider mites are moderately to not sensitive to pesticides.

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