Mites belong to the most important pests of greenhouse crops.
In addition to the other rarely met species, the common spotted spider mite (Tetranychus urticae) is most frequently observed.
Despite the numerous acaricides available, the chemical control of this pest in practice is very difficult.
The difficulties are due to the following causes:
- All stages of development (adult, larvae, nymphs, dormant stage and eggs) are found in a population of the common spotted spider mite.
Most of the available acaricides are specific and kill either the active stages or the eggs.
Not a single control means kills them in the dormant stage for they are protected by the old skin.
Only preparations of prolonged period of action could give a satisfactory result by a single treatment.
Since such acaricides are lacking in practice, it is necessary the treatments to be more frequent.
Thus for instance, to prevent damages from this pest in one season the cucumber crop should be treated 10–15 times.
- The mode of overwintering of the common spotted spider mite plays a major role in their control.
The climatic conditions prevailing in Western Europe induce a state of diapause in the mite in mid September or early in October.
Then the females should be sought in their places of overwintering.
They hide in crevices, holes, etc., in greenhouse structures.
In these places they are inaccessible to the control preparations.
If the insects have been exposed to low temperatures for a certain period of time, a rise in temperature again activates the females.
If in the period following September there is still a crop attacked by the common spotted spider mite, then the danger is very great that the next spring crop will be infected already in the beginning.
- The polyphase nature of the common spotted spider mite is also of paramount importance in its control.
It attacks not only nearly all cultivated plants but also many weeds.
The quick succession of crops in the greenhouse entails the risk that the common spotted spider mite on the old crops immediately infects the new crops.
That is why it is pointless to apply crop rotation in order to prevent attack.
- The cardinal problem in the chemical control of the common spotted spider mite is the development of resistant strains.
Generally, the possibilities of developing resistance are greatest in those insect species in which the generation follow in rapid succesion and in one year there appear many generations.
The common spotted spider mite multiplies very rapidly, especially at the high greenhouse temperatures and in one year produces at least 10 generations.
Moreover, mutational frequency in the common spotted spider mite is comparatively high (Helle en Van Tzon, 1967).
Therefore, even if a given population has no resistant forms, these