Effects of predation and competition on the population dynamics of Tetranychus pacificus on grapevines
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1. The Pacific spider mice Tetranychus pacificus and the Willamette spider mite Eotetranychus willamettei are herbivore pests of grapevines in California. The two spider mite species share a common and often effective phytoseiid predator, the Western orchard predatory mite Metaseiulus occidentalis. It has been suggested that E. willamettei may be beneficial in vineyards because it may have a negative impact on the more damaging T. pacificus through their shared predator or through some form of interspecific competition. We conducted field and greenhouse experiments to determine the relative effects of these interactions between the two herbivores on the population dynamics of T. pacificus in 'Thompson Seedless' grape vineyards. We also used the field data to generate a functional relationship for the combined impact of E. willamettei and M. occidentalis on T. pacificus. 2. Predation and predator-mediated apparent competition were the only factors affecting T. pacificus densities in the field experiment. The addition of the predatory mite M. occidentalis alone resulted in a significant reduction in T. pacificus densities, while the addition of E. willamettei alone had little impact on T. pacificus densities. The greatest reductions in T. pacificus densities occurred in plots where both the predatory mite M. occidentalis and E. willamettei were added. The predatory mite occurred earliest and increased at the greatest rate in plots where it was released along with E. willamettei. From April to June, M. occidentalis abundance was positively correlated with E. willamettei abundance while T. pacificus abundance in July and August was negatively correlated with M. occidentalis abundance in the period April to June. 3. Under greenhouse conditions, the addition of E. willamettei to T. pacificus vines resulted in a marked decrease T. pacificus population growth rate. In contrast, doubling T. pacificus abundance in the absence of E. willamettei did not affect T. pacificus population growth rate, indicating that interspecific competition is more important than intraspecific competition in determining T. pacificus abundance. 4. Contrary to the greenhouse 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 outcome of interactions between E. willamettei and T. pacificus may be affected by grape cultivar, growing region, a combination of cultivar and region-specific differences, or size and timing of vine infestations with E. willamettei. 5. In Thompson Seedless vineyards, vine infestation with E. willamettei would be most useful in increasing the effectiveness of the predatory mite M. occidentalis in controlling T. pacificus, primarily through an elevated predatory mite abundance prior the development of large T. pacificus populations.
