Mechanisms of interspecific competition that result in successful control of Pacific mites following inoculations of Willamette mites on grapevines.

We have successfully reduced populations of economically damaging Pacific mites by introducing another economically unimportant herbivorous mite to grapevines. Although this technique has proven successful in greenhouse and commercial applications, we had a poor understanding of why it worked. Experiments were conducted in a Zinfandel vineyard to assess the relative importance of different forms of competition between these two species and the importance of the timing of the introduction. The early season release of Willamette mites that occurred as shoots were first elongating reduced populations of Pacific mites, indicating that some form of competition occurred in this experiment. However, introductions of Willamette mites 4 weeks later did not reduce populations of Pacific mites. An early season release is essential for control. Control shoots did not receive additions of Willamette mites and had high population densities of Pacific mites. In contrast to this, Pacific mite populations were reduced on treatments with Willamette mites, regardless of whether the two species overlapped spatially or were separated by a barrier. This indicated that physical interference was not responsible, that induced resistance was involved, and that the effect was systemic throughout the shoot.