Joirriicrl o/ Effects of predation and competition on the population Applied Ecology 1997, 34, dynamics of Tetvanychus pacificus on grapevines 8 78-8 8 8 R. HANNA, L.T. WILSON*, F.G. ZALOM and D.L. FLAHERTYt Deparlmeii~of Etiromology. Utiiversiry of’ California, Dacis, Californiu, USA Summary 1. The Pacific spider mite Tetranyhus pacificus and the Willamette spider mite Eot- etmnyclzus rvillamettei 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 Metaseiitlus occidentalis. It has been suggested that E. wil- lamettei may be beneficial in vineyards because it may have a negative impact on the more damaging T. pacijicus through their shared predator or through some form of interspecific competition. We conducted field and greenhouse experiments to deter- mine the relative effects of these interactions between the two herbivores on the population dynamics of T.pacijicus 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. pacijicus 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. willanzettei abundance while T. pacijicus abundance in July and August was negatively correlated with rM. occidentalis abundance in the period April to June. 3. Under greenhouse conditions, the addition of E. willamettei to T. paciJicus vines resulted in a marked decrease in T. pacificus population growth rate. In contrast, doubling T.pacificus abundance in the absence E. willametteidid not affect T.pacijicus population growth rate, indicating that interspecific competition is more important than intraspecific competition in determining T. pacijicus 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 inter- actions 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. pacijicus, primarily through an elevated predatory mite abundance prior to the development of large T. pacificus populations. Key-words: Acari, Eotetranychus willamettei, Metaseiultu occidentalis, predator- mediated apparent competition, predator-prey interactions, Vitis tlinijera. Journal of Applied Ecology (1997) 34, 878-888 * Present address: Department of Entomology, Texas A&M University, College Station, TX 77543, USA. 1997 British t Present address: University of California Cooperative Extension, County Civic Centre, Agricultural Building, Visalia. CA Ecological Society 93291, USA. 878 led to speculations that the presence of E. willamettei, 879 Introduction R. Hunnu et al. especially during spring, may prevent outbreaks of T. Direct effects of predation and interspecific competition pacificus, possibly by leading to an early increase in have long been implicated as important factors affect- populations of M. occidmtalis (Flaherty & Huffaker ing the population dynamics of arthropods in natural 1970; Flaherty & Hoy 1971). If this hypothesis is true, systems (e.g. Schoener 1983; Connell 1983; Sih et a/. then predator-mediated apparent competition 1985). More recently, it has been suggested that indirect between T. pacificus and E. willanwttei may be an effects produced by shared predation (i.e. one or more important factor in the dynamics of this acarine pred- predator species sharing more than one prey species) ator-prey system. More recently, results from green- may also cause profound effects on prey population house and field experiments have suggested that nega- dynamics (e.g. Sih et al. 1985; Kerfoot & Sih 1987). tive effects of E. willameftei on T. pac(fictrs may be One class of indirect effects of shared predation results caused by interspecific competition or ‘induced resist- in population and community patterns similar to those ance’ (English-Loeb & Karban 1988; Karban & Engl- produced by interspecificcompetition (Holt 1977,1984; ish-Loeb 1990). However, these alternative responses Holt & Koller 1987; Holt & Lawton 1994; Holt, Grover were cultivar-specific. While they were apparent on & Tilman 1994). Holt (1977) termed this interaction ‘Zinfandel’ and ‘Chardonnay’ cultivars, they were predator-mediated apparent competition, which arises weak or lacking on ‘Thompson Seedless’ grapevines. from strong predator numerical responses to prey den- Also lacking were the relative effects of interspecific sities. Despite the theoretical appeal of apparent com- and intraspecific competition on spider mite densities. petition and the existence of numerous examples of Clearly, several biological processes may be impor- predator-prey interactions that are in line with its pre- tant in determining population abundance and pest- dictions (Holt 1984), there have been few experimental status of spider mites in vineyards. In this paper we demonstrations of its existence (Collyer 1964; Schniitt report the results of a field experiment designed to 1987; Settle & Wilson 1990; Huang & Sih 1990; assess the relative significance of predation, inter- Karban, Hougen-Eitzman & English-Loeb 1994). specific competition, and predator-mediated apparent Although apparent competition theory was developed competition on the short- and long-term dynamics of for prey that do not compete directly (Holt 1977), coni- 7”. pac@ws populations in Thompson Seedless vine- petition for resources and predator-mediated apparent yards in the San Joaquin Valley of California. We competition are not mutually exclusive hypotheses and also report on the results of a greenhouse experiment are likely to be complimentary forces determining sizes designed to compare the relative significance of inter- of animal populations (Holt 1984; Holt et al. 1994; Sih specific competition (concurrent feeding by E. wil- et al. 1985). lumettei) and intraspecific competition on the population In this paper, we explore the relative effects of pre- growth of T. puc$icus. In conducting this study, we dation, interspecific competition, and predator- were also motivated by the practical desire (i) to deter- mediated apparent competition on the population mine the viability of vine infestation with the less dynamics of the Pacific spider mite Tetranychus damaging E. willarnertei to increase the effectiveness pacificus McGregor (Acari: Tetranychidae), which is of M. occidentalis in reducing densities of the more a serious pest of grapevines Vitis uinfera L. in the San damaging T.pacificu.~, and (ii) to generate a functional Joaquin Valley of California. Large populations of relationship for predicting the impact of M. occi- this spider mite species are capable of inflicting con- dentalis densities on T. pac$cu.s populations in the siderable damage to vine growth and grape yield and presence of different levels of E. wil/mne/fei. quality (Flaherty et NI. 1992). The Willamette spider mite Eofetranychus willamettei (McGregor) (Acari: Methods Tetranychidae) is also commonly found on grape- vines, but generally causes considerably less damage LlFE HlSTORY OF THE MlTES than comparable densities of T. pacificus, particularly in vineyards of the Southern San Joaquin Valley, Cal- Both T. pacificus and E. cvillumettri are endemic to ifornia (Flaherty 1969). Both T. pacificus and E. the far western United States. They were first reported willamettei are attacked by the predatory mite on grapes in California vineyards by Lamiman (1 935). Meta&idu.s ( = Typhlodromus = Galendromus) Both species are found throughout California, but T. occidentulis (Nesbitt) (Acari: Phytoseiidae), which is ppc~ficicusis most commonly found in the warm interior their principal biological mortality agent in vineyards TetrFnychus pyc@:us, like (Flaherty et al. 1985). arine’genus Tetr~/?ychus,has That T. pactficus and E. willamettei Co-occur on a wide host range, infesting a variety of perennial and grapevines and share the same predator has led to annual plant species, and is a pest of major crops in speculations that the two species might compete the western United States (Pritchard & Baker 1955). $> 1997 British EcologicalSociety, directly (interspecific Competition) or through the In contrast, E. willumettei infests only a few perennial .iouma/qf’A,lp/ipc/ shared predator (predator-mediated apparent com- plant species including apple Ma/u.s spp., antelope Ecology. 34, 878-888 petition), Observations from several vineyards have brush Purshia tridenratu, box elder Acer negundo cali-