This study revealed that prey population attributes, not individual attributes, dictate the efficiency with which Metaseiulus occidentalis (Nesbitt) (Acarina: Phytoseiidae) responds to and controls populations of given spider mite species (Eotetranychus willamettei Ewing and Tetranychus pacificus McGregor) on grapevines.

The study also showed that under vineyard conditions, M. occidentalis has the ability to respond numerically to low or high prey densities. Using individual vines as study units, the length of the lag in predator response to low or high prey densities was shown to be mainly a function of absence or poor distribution of the predator relative to its prey.

Individual vine studies may delineate not only the importance of predation (delayed density-dependent action) but the importance of abiotic factors (density-independent actions), as well. Lumping of sampling data from large groups of vines precludes the appraisal and separation of these two equally important facets of natural control.

Finally, the studies indicate that chemical treatments may disrupt predator and prey distributional patterns, either directly by annihilating the predators or indirectly by annihilating their prey, or a combination of the two.