EFFECTS OF PREDATION AND COMPETITION ON THE POPULATION-DYNAMICS OF TETRANYCHUS-PACIFICUS ON GRAPEVINES

1, The Pacific spider mite Tetranychus pacificus and the Willamette sp ider mite Eotetranychus willamettei are herbivore pests of grapevines In California, The two spider mite species share a common and often ef fective phytoseiid predator, the Western orchard predatory mite Metase iulus occidentalis. It has been suggested that E. willamettei may be b eneficial in vineyards because it may have a negative impact: an the m ore damaging T, pacificus through their shared predator or through som e form of interspecific competition, We conducted field and greenhouse experiments to determine the relative effects of these interactions b etween the two herbivores on the population dynamics of T, pacificus i n 'Thompson Seedless' grape vineyards. We also used the field data to generate a functional relationship for the combined impact of E. willa mettei and M. occidentalis on T, pacificus. 2, Predation and predator- mediated apparent competition were the only factors affecting T, pacif icus densities in the field experiment, The addition of the predatory mite M, occidentalis alone resulted in a significant reduction in T, p acificus densities, while the addition of E, willamettei alone had lit tle impact on T, pacificus densities. The greatest reductions in T. pa cificus densities occurred in plots where both the predatory mite M, o ccidentalis and E, willamettei were added, The predatory mite occurred earliest and increased at the greatest rate in plots where it was rel eased along with E. willamettei. From April to June, M. occidentalis a bundance was positively correlated with E. willamettei abundance while T. pacificus abundance in July and August was negatively correlated w ith M. occidentalis abundance in the period April to June. 3, Under gr eenhouse conditions, the addition of E. willamettei to T. pacificus vi nes resulted in a marked decrease in T pacificus population growth rat e. In contrast, doubling T. pacificus abundance in the absence E willa mettei did not affect T. pacificus population growth rate, indicating that interspecific competition is more important than intraspecific co mpetition in determining T. pacificus abundance, 4, Contrary to the gr eenhouse data and other studies in 'Zinfandel' vineyards, we found no evidence that E, willamettei competed with T, pacificus in the absence of predation in a Thompson Seedless vineyard. We suggest that the out come of interactions between E, willamettei and T. pacificus may be af fected by grape cultivar, growing region, a combination of cultivar an d region-specific differences, or size and timing of vine infestations with E, willamettei. 5, In Thompson Seedless vineyards, ville infesta tion with E, willamettei would be most useful in increasing the effect iveness of the predatory mite M. occidentalis in controlling T, pacifi cus, primarily through an elevated predatory mite abundance prior to t he development of large T, pacificus populations.