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Nicholas J. Mills - One of the best experts on this subject based on the ideXlab platform.
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Small-Scale Intraspecific Life History Variation in Herbivorous Spider Mites (Tetranychus pacificus) Is Associated with Host Plant Cultivar
2016Co-Authors: Katherine Scranton, Nicholas J. Mills, Menelaos Stavrinides, Perry De ValpineAbstract:Life history variation is a general feature of arthropod systems, but is rarely included in models of field or laboratory data. Most studies assume that local processes occur identically across individuals, ignoring any genetic or phenotypic variation in life history traits. In this study, we tested whether field populations of Pacific spider mites (Tetranychus pacificus) on grapevines (Vitis vinifera) display significant intraspecific life history variation associated with host plant cultivar. To address this question we collected individuals from sympatric vineyard populations where either Zinfandel or Chardonnay were grown. We then conducted a ‘‘common garden experiment’ ’ of mites on bean plants (Phaseolus lunatus) in the laboratory. Assay populations were sampled non-destructively with digital photography to quantify development times, survival, and reproductive rates. Two classes of models were fit to the data: standard generalized linear mixed models and a time-to-event model, common in survival analysis, that allowed for interval-censored data and hierarchical random effects. We found a significant effect of cultivar on development time in both GLMM and time-to-event analyses, a slight cultivar effect on juvenile survival, and no effect on reproductive rate. There were shorter development times and a trend towards higher juvenile survival in populations from Zinfandel vineyards compared to those from Chardonnay vineyards. Lines of the same species, originating from field populations on different host plant cultivars, expressed different development times an
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Small-Scale Intraspecific Life History Variation in Herbivorous Spider Mites (Tetranychus pacificus) Is Associated with Host Plant Cultivar
PloS one, 2013Co-Authors: Katherine Scranton, Menelaos C. Stavrinides, Nicholas J. Mills, Perry De ValpineAbstract:Life history variation is a general feature of arthropod systems, but is rarely included in models of field or laboratory data. Most studies assume that local processes occur identically across individuals, ignoring any genetic or phenotypic variation in life history traits. In this study, we tested whether field populations of Pacific spider mites (Tetranychus pacificus) on grapevines (Vitis vinifera) display significant intraspecific life history variation associated with host plant cultivar. To address this question we collected individuals from sympatric vineyard populations where either Zinfandel or Chardonnay were grown. We then conducted a “common garden experiment” of mites on bean plants (Phaseolus lunatus) in the laboratory. Assay populations were sampled non-destructively with digital photography to quantify development times, survival, and reproductive rates. Two classes of models were fit to the data: standard generalized linear mixed models and a time-to-event model, common in survival analysis, that allowed for interval-censored data and hierarchical random effects. We found a significant effect of cultivar on development time in both GLMM and time-to-event analyses, a slight cultivar effect on juvenile survival, and no effect on reproductive rate. There were shorter development times and a trend towards higher juvenile survival in populations from Zinfandel vineyards compared to those from Chardonnay vineyards. Lines of the same species, originating from field populations on different host plant cultivars, expressed different development times and slightly different survival rates when reared on a common host plant in a common environment.
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Influence of temperature on the reproductive and demographic parameters of two spider mite pests of vineyards and their natural predator
BioControl, 2011Co-Authors: Menelaos C. Stavrinides, Nicholas J. MillsAbstract:We evaluated the influence of temperature on demographic parameters of two common vineyard pests, the Pacific spider mite, Tetranychus pacificus McGregor, and the Willamette spider mite, EoTetranychus willamettei (McGregor) (Acari: Tetranychidae). Additionally, we investigated the effects of temperature on their shared predator, the western predatory mite, Galendromus occidentalis (Nesbitt) (Acari: Phytoseiidae). The intrinsic rate of increase ( r _ m ) was higher for T. pacificus than E. willamettei at 15 and 28°C, but similar at 22°C. G. occidentalis achieved a higher r _ m than T. pacificus from 15 to 28°C, but the difference was significant only at 22°C. At 34°C, the r _ m for both T. pacificus and G. occidentalis was negative, while E. willamettei did not develop at this temperature. Prey species did not affect demographic parameters of G. occidentalis . These results suggest that higher temperatures favor T. pacificus over the less damaging E. willamettei , and may also reduce the effectiveness of G. occidentalis .
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Plant Water Stress, Leaf Temperature, and Spider Mite (Acari: Tetranychidae) Outbreaks in California Vineyards
Environmental entomology, 2010Co-Authors: Menelaos C. Stavrinides, Bruce Lampinen, Kent M. Daane, Nicholas J. MillsAbstract:ABSTRACT We evaluated the relationships between plant water status and leaf temperature, and between leaf temperature and spider mite (Acari: Tetranychidae) and predatory mite (Acari: Phytoseiidae) populations in eight vineyards in California in 2006 and 2007. Temperature of south-facing leaves increased significantly by 0.8°C for every 1.0°C increase in ambient air temperature, and by 5.3°C for every one MPa drop in leaf water potential. Peak population densities of Pacific spider mite, Tetranychus pacificus McGregor, increased significantly with increasing frequency of leaf temperatures above 31°C. In contrast, peak population densities of Willamette spider mite, EoTetranychus willamettei (McGregor), showed no relationship with the frequency of leaf temperatures above 31°C. This differential relationship between the two mite species and high leaf temperatures is consistent with their upper thresholds for development, which are 40°C for T. pacificus and 31°C for E. willamettei, as identified in a previou...
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Development of acaricide resistance in Pacific spider mite (Tetranychus pacificus) from California vineyards
Experimental and Applied Acarology, 2010Co-Authors: Menelaos C. Stavrinides, Pieter Nieuwenhuyse, Thomas Leeuwen, Nicholas J. MillsAbstract:In recent years, grape growers in California reported failures of acaricides against Tetranychus pacificus McGregor. We collected T. pacificus populations from four vineyards and tested them for resistance to bifenazate, propargite and pyridaben. In addition, we sequenced part of the cytochrome b gene of bifenazate-resistant and -susceptible T. pacificus to test for the presence of mutations reported to confer resistance to the congeneric T. urticae . None of the mutations conferring resistance to bifenazate in T. urticae were present in resistant T. pacificus . Resistance levels ranged from full susceptibility to statistically significant 11-fold resistance to pyridaben, sevenfold resistance to bifenazate and fourfold resistance to propargite compared to a susceptible population. Despite the relatively low levels of resistance detected, we estimated that under the conditions of our study the highest field rates of bifenazate and pyridaben application would cause less than 58 and 66% mortality of adult females in the most resistant populations, respectively. In contrast, field rates of propargite application would cause close to 100% mortality in the least susceptible population. These results highlight a potential link between resistance development and reduced field effectiveness for bifenazate and pyridaben. Finally, T. pacificus may be more tolerant to bifenazate and propargite than T. urticae , since the LC_50 values for the susceptible population of T. pacificus were several times higher than LC_50’s reported for susceptible T. urticae.
Rachid Hanna - One of the best experts on this subject based on the ideXlab platform.
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'Thompson seedless' grapevine vigour and abundance of Pacific spider mite (Tetranychus pacificus McGregor) (Acari, Tetranychidae)
Journal of Applied Entomology, 1997Co-Authors: Rachid Hanna, Frank G. Zalom, Lloyd T. WilsonAbstract:Spider mite outbreaks have been frequently related to factors that reduce plant vigour. We conducted a field study to determine the relationship between the abundance of Pacific spider mite, Tetranychus pacificus McGregor, and abiotic factors related to the vigour of ‘Thompson Seedless’ grapevines, Vitis vinifera L. Our study supports previous observations on the relationship between Pacific mite abundance and vine vigour, and partially explains these differences in terms of the previously proposed hypotheses of differences in leaf N and leaf temperature on vines of different vigour. Pacific mite densities were 5.5-fold higher on low-vigour compared with high-vigour vines. Total leaf N was weakly correlated with vine vigour and Pacific mite abundance. Average daily and afternoon leaf temperatures were higher on low-vigour vines resulting in 45.3°D greater accumulation on low-vigour vines compared with high-vigour vines during the increasing phase of Pacific mite population growth. This difference represented 37.4% of the °D requirements for the development of 1 Pacific mite generation and could be partially responsible for the observed difference in Pacific mite abundance in relation to vine vigour. Other confounding factors are discussed and an additional hypothesis is proposed to further explain the relationship between vine vigour and Pacific mite abundance.
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Effects of predation and competition on the population dynamics of Tetranychus pacificus on grapevines.
The Journal of Applied Ecology, 1997Co-Authors: Rachid Hanna, Frank G. Zalom, Lloyd T. Wilson, D. L. FlahertyAbstract:The Pacific spider mite 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. 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. Under greenhouse conditions, the addition of E. willamettei to T. pacificus vines resulted in a marked decrease in T. pacificus population growth rate. In contrast, doubling T. pacificus abundance in the absence 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. 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. 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 to the development of large T. pacificus populations.
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prey preference by metaseiulus occidentalis acari phytoseiidae and the role of prey aggregation
Biological Control, 1991Co-Authors: Rachid Hanna, Theodore L WilsonAbstract:Abstract The phytoseiid predator Metaseiulus occidentalis is an important biological control agent for the Pacific spider mite (PSM) Tetranychus pacificus and the Willamette spider mite (WSM) EoTetranychus willamettei, two economically important spider mite species in California vineyards. Field observations indicated that M. occidentalis is more efficient against PSM than it is against WSM. This paper examines prey preference by M. occidentalis, as it relates to the spatial aggregations of PSM and WSM, to explain differences in M. occidentalis's efficiency on PSM and WSM in vineyards. Prey preference by M. occidentalis was studied in laboratory (individual) and greenhouse (population) experiments. In both experiments, M. occidentalis showed an overall preference for PSM over WSM; however, this preference was dependent on the relative abundance of available prey. An excess of the more abundant prey always was consumed. On the basis of theoretical and experimental consideration of the response of predators to aggregated prey, we conclude that prey preference by M. occidentalis appears to be related to differences in its prey's spatial aggregation. PSM was more aggregated than WSM on grape leaves in the greenhouse, and therefore was generally the more preferred prey. Furthermore, changes in M. occidentalis's prey preference appear to be related also to changes in the relative aggregation of PSM and WSM. Changes in relative aggregation are strongly correlated with changes in prey preference by M. occidentalis. Also discussed is the need for predation experiments to complement experiments on the response of predators to prey cues, especially in preintroductory screening of natural enemies.
Menelaos C. Stavrinides - One of the best experts on this subject based on the ideXlab platform.
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Small-Scale Intraspecific Life History Variation in Herbivorous Spider Mites (Tetranychus pacificus) Is Associated with Host Plant Cultivar
PloS one, 2013Co-Authors: Katherine Scranton, Menelaos C. Stavrinides, Nicholas J. Mills, Perry De ValpineAbstract:Life history variation is a general feature of arthropod systems, but is rarely included in models of field or laboratory data. Most studies assume that local processes occur identically across individuals, ignoring any genetic or phenotypic variation in life history traits. In this study, we tested whether field populations of Pacific spider mites (Tetranychus pacificus) on grapevines (Vitis vinifera) display significant intraspecific life history variation associated with host plant cultivar. To address this question we collected individuals from sympatric vineyard populations where either Zinfandel or Chardonnay were grown. We then conducted a “common garden experiment” of mites on bean plants (Phaseolus lunatus) in the laboratory. Assay populations were sampled non-destructively with digital photography to quantify development times, survival, and reproductive rates. Two classes of models were fit to the data: standard generalized linear mixed models and a time-to-event model, common in survival analysis, that allowed for interval-censored data and hierarchical random effects. We found a significant effect of cultivar on development time in both GLMM and time-to-event analyses, a slight cultivar effect on juvenile survival, and no effect on reproductive rate. There were shorter development times and a trend towards higher juvenile survival in populations from Zinfandel vineyards compared to those from Chardonnay vineyards. Lines of the same species, originating from field populations on different host plant cultivars, expressed different development times and slightly different survival rates when reared on a common host plant in a common environment.
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Influence of temperature on the reproductive and demographic parameters of two spider mite pests of vineyards and their natural predator
BioControl, 2011Co-Authors: Menelaos C. Stavrinides, Nicholas J. MillsAbstract:We evaluated the influence of temperature on demographic parameters of two common vineyard pests, the Pacific spider mite, Tetranychus pacificus McGregor, and the Willamette spider mite, EoTetranychus willamettei (McGregor) (Acari: Tetranychidae). Additionally, we investigated the effects of temperature on their shared predator, the western predatory mite, Galendromus occidentalis (Nesbitt) (Acari: Phytoseiidae). The intrinsic rate of increase ( r _ m ) was higher for T. pacificus than E. willamettei at 15 and 28°C, but similar at 22°C. G. occidentalis achieved a higher r _ m than T. pacificus from 15 to 28°C, but the difference was significant only at 22°C. At 34°C, the r _ m for both T. pacificus and G. occidentalis was negative, while E. willamettei did not develop at this temperature. Prey species did not affect demographic parameters of G. occidentalis . These results suggest that higher temperatures favor T. pacificus over the less damaging E. willamettei , and may also reduce the effectiveness of G. occidentalis .
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Plant Water Stress, Leaf Temperature, and Spider Mite (Acari: Tetranychidae) Outbreaks in California Vineyards
Environmental entomology, 2010Co-Authors: Menelaos C. Stavrinides, Bruce Lampinen, Kent M. Daane, Nicholas J. MillsAbstract:ABSTRACT We evaluated the relationships between plant water status and leaf temperature, and between leaf temperature and spider mite (Acari: Tetranychidae) and predatory mite (Acari: Phytoseiidae) populations in eight vineyards in California in 2006 and 2007. Temperature of south-facing leaves increased significantly by 0.8°C for every 1.0°C increase in ambient air temperature, and by 5.3°C for every one MPa drop in leaf water potential. Peak population densities of Pacific spider mite, Tetranychus pacificus McGregor, increased significantly with increasing frequency of leaf temperatures above 31°C. In contrast, peak population densities of Willamette spider mite, EoTetranychus willamettei (McGregor), showed no relationship with the frequency of leaf temperatures above 31°C. This differential relationship between the two mite species and high leaf temperatures is consistent with their upper thresholds for development, which are 40°C for T. pacificus and 31°C for E. willamettei, as identified in a previou...
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Development of acaricide resistance in Pacific spider mite (Tetranychus pacificus) from California vineyards
Experimental and Applied Acarology, 2010Co-Authors: Menelaos C. Stavrinides, Pieter Nieuwenhuyse, Thomas Leeuwen, Nicholas J. MillsAbstract:In recent years, grape growers in California reported failures of acaricides against Tetranychus pacificus McGregor. We collected T. pacificus populations from four vineyards and tested them for resistance to bifenazate, propargite and pyridaben. In addition, we sequenced part of the cytochrome b gene of bifenazate-resistant and -susceptible T. pacificus to test for the presence of mutations reported to confer resistance to the congeneric T. urticae . None of the mutations conferring resistance to bifenazate in T. urticae were present in resistant T. pacificus . Resistance levels ranged from full susceptibility to statistically significant 11-fold resistance to pyridaben, sevenfold resistance to bifenazate and fourfold resistance to propargite compared to a susceptible population. Despite the relatively low levels of resistance detected, we estimated that under the conditions of our study the highest field rates of bifenazate and pyridaben application would cause less than 58 and 66% mortality of adult females in the most resistant populations, respectively. In contrast, field rates of propargite application would cause close to 100% mortality in the least susceptible population. These results highlight a potential link between resistance development and reduced field effectiveness for bifenazate and pyridaben. Finally, T. pacificus may be more tolerant to bifenazate and propargite than T. urticae , since the LC_50 values for the susceptible population of T. pacificus were several times higher than LC_50’s reported for susceptible T. urticae.
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Comparative influence of temperature on development and biological control of two common vineyard pests (Acari: Tetranychidae)
Biological Control, 2010Co-Authors: Menelaos C. Stavrinides, Jesús R. Lara, Nicholas J. MillsAbstract:Abstract We studied the effects of temperatures from 10 to 40 °C on development of the Pacific spider mite, Tetranychus pacificus (McGregor), and the Willamette spider mite, EoTetranychus willamettei McGregor (Acari: Tetranychidae), feeding on grape foliage. In addition, we investigated the influence of temperatures from 10 to 37 °C on development of the western predatory mite, Galendromus occidentalis (Nesbitt) (Acari: Phytoseiidae), feeding on T. pacificus and evaluated the suitability of E. willamettei as prey for the predatory mite at 28 °C. Using a non-linear development rate model we estimated that the lower threshold for development of the three mites lay around 10 °C. T. pacificus was the most heat-resistant, with its upper threshold for development at 40.3 °C, followed by G. occidentalis at 37.1 °C and E. willamettei at 31.0 °C. T. pacificus developed significantly more rapidly than E. willamettei above 22.8 °C, whereas G. occidentalis developed significantly faster than either spider mite from approximately 11 to 36 °C. G. occidentalis developed 5% faster when feeding on E. willamettei than T. pacificus at 28 °C. These results confirm field observations linking E. willamettei damage to cooler, coastal vineyards and early in the season in inland vineyards, and T. pacificus infestations to hot vineyards in inland and coastal areas. Although T. pacificus seems to be more heat tolerant than G. occidentalis , additional information on temperature effects on adult life history details of the two mites is needed to fully evaluate G. occidentalis performance at high temperatures.
D. L. Flaherty - One of the best experts on this subject based on the ideXlab platform.
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Effects of predation and competition on the population dynamics of Tetranychus pacificus on grapevines.
The Journal of Applied Ecology, 1997Co-Authors: Rachid Hanna, Frank G. Zalom, Lloyd T. Wilson, D. L. FlahertyAbstract:The Pacific spider mite 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. 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. Under greenhouse conditions, the addition of E. willamettei to T. pacificus vines resulted in a marked decrease in T. pacificus population growth rate. In contrast, doubling T. pacificus abundance in the absence 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. 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. 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 to the development of large T. pacificus populations.
Lloyd T. Wilson - One of the best experts on this subject based on the ideXlab platform.
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'Thompson seedless' grapevine vigour and abundance of Pacific spider mite (Tetranychus pacificus McGregor) (Acari, Tetranychidae)
Journal of Applied Entomology, 1997Co-Authors: Rachid Hanna, Frank G. Zalom, Lloyd T. WilsonAbstract:Spider mite outbreaks have been frequently related to factors that reduce plant vigour. We conducted a field study to determine the relationship between the abundance of Pacific spider mite, Tetranychus pacificus McGregor, and abiotic factors related to the vigour of ‘Thompson Seedless’ grapevines, Vitis vinifera L. Our study supports previous observations on the relationship between Pacific mite abundance and vine vigour, and partially explains these differences in terms of the previously proposed hypotheses of differences in leaf N and leaf temperature on vines of different vigour. Pacific mite densities were 5.5-fold higher on low-vigour compared with high-vigour vines. Total leaf N was weakly correlated with vine vigour and Pacific mite abundance. Average daily and afternoon leaf temperatures were higher on low-vigour vines resulting in 45.3°D greater accumulation on low-vigour vines compared with high-vigour vines during the increasing phase of Pacific mite population growth. This difference represented 37.4% of the °D requirements for the development of 1 Pacific mite generation and could be partially responsible for the observed difference in Pacific mite abundance in relation to vine vigour. Other confounding factors are discussed and an additional hypothesis is proposed to further explain the relationship between vine vigour and Pacific mite abundance.
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Effects of predation and competition on the population dynamics of Tetranychus pacificus on grapevines.
The Journal of Applied Ecology, 1997Co-Authors: Rachid Hanna, Frank G. Zalom, Lloyd T. Wilson, D. L. FlahertyAbstract:The Pacific spider mite 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. 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. Under greenhouse conditions, the addition of E. willamettei to T. pacificus vines resulted in a marked decrease in T. pacificus population growth rate. In contrast, doubling T. pacificus abundance in the absence 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. 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. 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 to the development of large T. pacificus populations.