Red spider mite The red spider mite or two spotted spider mite (Tetranychus urticae) is a feared pest on several crops all over the world. At first sight it is not easily distinguished from the carmine spider mite (Tetranychus cinnabarinus) that sometimes occurs on tomato, carnation or other ornamentals.

The female deposits round eggs of about 0.14 mm on the underside of the leaf. Out of the egg a larva with 6 legs hatches that immediately starts sucking plant sap. Subsequently, the larva develops into a protonymph, followed by a deutonymph and an adult stage. The development stages are separated by a quiescent stage, during which the mite settles immobile on the leaf with its legs drawn in.

Once the mite has become adult, it takes another 0.5 to 3 days before the female starts laying eggs (pre-oviposition period). The total deveolopment time varies a lot with temperature, humidity and host plant. In an experiment on rose leaf it appeared to take 7 days at 30°C (86°F), 17 days at 20°C (68°F) and 36 days at 15°C (59°F).

In a population there are about 3 times more females than males. Generally male spider mites can be found in close association with quiescent female deutonymphs, waiting for the latter to complete their development. Unfertilized females only give birth to males. The female lays her eggs during 10 days (at 35°C or 95°F) to 40 days (at 15°C or 59°F). At 20°C (68°F) she lays about 40 eggs in total, but under optimal circumstances this can mount up to 100. Especially at dry and warm weather red spider mites can reproduce very rapidly.

Red spider mites suck plant sap for food. Plant cells turn yellow, which can be seen on the upper surface of the leaf as small yellow spots. This reduces the photosynthetic area of the leaf and the plant gets out of physiological balance. Moreover, the webs made by spider mites reduce the aesthetic value of ornamentals.

In autumn, when temperature and photoperiod drop, fertilised females enter diapause. Such females turn orange-red. They hide in all kinds of cracks in the greenhouse, to appear again early in the following season when circumstances improve.