USE OF A DAMAGE INDEX TO EVALUATE THE BIOLOGICAL-CONTROL OF THE 2-SPOTTED SPIDER-MITE TETRANYCHUS-URTICAE KOCH (ACARI, TETRANYCHIDAE) ON TOMATO CROPS

In tomato crops protected by the predatory mite Phytoseiulus persimilis against the two-spotted spider mite Tetranychus urticae foliage damage showed an oscillatory pattern with increasing amplitude, indicating an unstable prey-predator equilibrium. Cyclical declines in predator populations were met with subsequent rapid increases in prey populations, and as a consequence, considerable numbers of predatory mites were needed to control the spider mites: 25,000 to 36,300 predators per 100 m2 for a period of 22 to 26 weeks. Only when the spider mites were colonizing was a stable and satisfactory ratio between the number of released predators and foliage damage observed. The instability of the prey-predator equilibrium was attributed to the intensity of the introduction of the predator on each plant and to the unfavourable abiotic environmental conditions.