Low-Density Releases of Neoseiulus Fallacis Provide for Rapid Dispersal and Control of Tetranychus Urticae (Acari: Phytoseiidae, Tetranychidae) on Apple Seedlings

Releases of Neoseiulus fallacis (Garman) at 1500--6000 per ha when prey were at 0.1-0.3 per leaf provided seasonal control of Tetranychus urticae Koch (all stages) at 1-2 per leaf in an apple seedling rootstock nursery. Predaceous mites (all stages) increased to 0.3-0.4 per leaf after releases and predator prey ratios of < or = 1:3-7 provided pest regulation thereafter. Such low-density releases were thought to be effective because multiple dispersal bouts allowed predators to locate widely distributed spider mites (on 2-6% of leaves). A random-diffusion model simulating predator dispersal (incorporating wind speed and direction parameters) adequately explained movement and pest control patterns. An upright, dense, uniform planting of apple seedlings was an effective producer and recipient for dispersing predators and these attributes seemed to explain why biological control was so effective. Low-density releases of N. fallacis for control of T. urticae are predicted to be less effective on other crops with less prominent profiles and soil coverage.     

Predatory role of Neoseiulus fallacis (Acari: Phytoseiidae): spatial and temporal dynamics in Washington red raspberry fields

The seasonal abundance of spider mites and their predator Neoseiulus fallacis (Garman) (Acari: Phytoseiidae) was determined during three consecutive years in Washington State red raspberry fields. Tetranychus urticae Koch (Acari: Tetranychidae), Eotetranychus carpini borealis (Ewing) (Acari: Tetranychidae), and N. fallacis were commonly found in Skagit and Whatcom Counties. E. carpini borealis colonized the fruiting canes earlier in the season than T. urticae. The two phytophages overlapped in midseason, but T. urticae entered diapause earlier than E. carpini borealis and N. fallacis. Densities of N. fallacis increased with increase in spider mite densities. However, the numerical response of the predator was more evident for T. urticae than for E. carpini borealis. Nevertheless, the predator was spatially associated with the two prey species. The spatial and seasonal distribution of N. fallacis in relationship to host plant phenology and prey distribution may influence the effectiveness of this predator as a biological control agent against spider mites in red raspberry. Densities of the predator increased too late to prevent spider mite damage. The predatory role of N. fallacis could be enhanced by introducing or conserving predators that are more tolerant to climatic factors that prevail in and around the cane canopy in the beginning of the season.     

Effects of nutrients and predators on an old-field food chain: interactions of top-down and bottom-up processes

In theory, selection favours predators that select prey in order to maximise reproductive success. We studied the association between preference and performance of the generalist predator Orius laevigatus with respect to two prey species: spider mites ( Tetranychus urticae ) and western flower thrips ( Frankliniella occidentalis ). Under ample prey supply, the predators had higher maximum reproductive success (measured as intrinsic population growth rate r ) on thrips than on spider mites; hence thrips represent a higher prey quality to the bugs. This was at odds with the observed preference of the predatory bug for plants (patches) with high densities of spider mites to plants with moderate densities of thrips in release-recapture experiments. Thus, prey quality does not suffice to explain the preference of predators for plants with prey. The quality of a patch as an oviposition site (i.e. the number of eggs produced on a patch per bug per day) also did not match preference patterns. Hence, patch preference was not correlated to prey quality or oviposition rate on prey patches. However, patch productivity, i.e. the total number of offspring surviving until adulthood that can be produced by one female on a patch, was correlated with preference. This was further tested by offering the predators a choice between plants with high densities of spider mites and plants with high densities of thrips in an independent set of release-recapture experiments. These two types of prey patches were found equivalent in terms of patch productivity. Indeed, the predators showed no preference for either of the two types of patches, which is in agreement with our predictions. This suggests that the predatory bugs select patches based on patch productivity rather than on prey quality or oviposition rate on a patch.     

Spatial distribution of predators and prey affect biological control of twospotted spider mites by Phytoseiulus persimilis in greenhouses

We evaluated the effects of predator release pattern and prey distribution on rate of suppression of the twospotted spider mite, Tetranychus urticae Koch (Acari, Tetranychidae) and visual damage to the ornamental plant, Impatiens wallerana Hook.f., in a greenhouse. Sixteen impatiens plants were arranged in a square and infested with the same total number of spider mites distributed either evenly (equal numbers on all plants) or clumped (divided equally among the 4 central plants), simulating a “hot spot.” The predatory mite, Phytoseiulus persimilis Athias-Henriot, was released at a 1:4 predator:prey ratio based on total spider mites in the experimental unit, but the pattern of release was either even or clumped, which simulated broadcast or point-release strategies, respectively. Nine days after predator release, spider mite populations were reduced in all treatments, but only in the clumped pest-clumped predator treatment were spider mites undetectable. Poorest pest suppression occurred in the clumped spider mite-even predator treatment. Eighteen days after predator release, spider mites were eliminated in all treatments, but a reduction in average plant damage occurred only in treatments in which the predator release pattern matched the spider mite distribution (i.e., even-even or clumped-clumped) with the greatest reduction in the even-even treatment. Results suggest that there is an advantage to releasing predators in “hot spots” provided that the recommended predator:prey ratio is maintained within infested patches. If more uniform predator releases are planned, overall predator numbers need to be kept sufficiently high so that the predator:prey ratio of 1:4 shown to prevent damage on impatiens is achieved in higher-density spider mite patches.     

Neoseiulus californicus (McGregor) and Neoseiulus fallacis (Garman): larval responsesto prey and humidity, nymphal feeding drive and nymphal predation on phytoseiid eggs

Are Neoseiulus californicus (McGregor) and Neoseiulus fallacis (Garman) both specialized predators of spider mites? As part of a series of studies made to answer this question, responses by larvae were assessed in treatments either with or without prey (Tetranychus urticae Koch) and with either moderate or high relative humidities (a factor that may distinguish between these two morphologically similar species). Neoseiulus fallacis larvae had more feeding, ambulatory activity and jerking (intra– or interspecific communication) in all treatments than N. californicus. The percent egg hatch was less and development took longer for N. fallacis larvae than for N. californicus larvae without prey at moderate humidity levels. The nymphs of both mites showed similar drives to feed 1–2>h after being held 12–48 h without food and then given eggs of T. urticae. Neoseiulus californicus nymphs fed more on the eggs of either phytoseiid and thereafter became adults than did N. fallacis nymphs. These data indicate that N. californicus may be a less specialized predator of spider mites than is N. fallacis.     

Regional persistence of locally unstable predator/prey populations

Phytoseiid mites are efficient predators capable of completely destroying colonies of spider mites. Thus, coexistence of phytoseiids and their tetranychid prey at a local scale (typically an individual plant) is not likely for more than a single predator/prey cycle. However, the species may coexist at a regional scale, i.e. in a complex environment consisting of many plants, provided local colonisations, extinctions and recolonisations occur asynchronously. This review investigates some of the factors responsible for establishing and maintaining spatial asynchrony between local populations of prey and predators, such as dispersal, environmental heterogeneity and demographic stochasticity. Existing predator/prey models are considered in order to find agreement between theory and empirical data. Based on our current knowledge of spatial processes and their importance for the overall dynamics and persistence of predator/prey interactions, some consequences and aspects for biological control of crop pests by means of natural enemies are outlined.     

Absence of odour-mediated avoidance of heterospecific competitors by the predatory mite Phytoseiulus persimilis

Arthropods use odours associated with the presence of their food, enemies and competitors when searching for patches. Responses to these odours therefore determine the spatial distribution of animals, and are decisive for the occurrence and strength of interactions among species. Therefore, a logical first step in studying food web interactions is the analysis of behaviour of individuals that are searching for patches of food. We followed this approach when studying interactions in an artificial food web occurring on greenhouse cucumber in the Netherlands. In an earlier paper we found that one of the predators of the food web, the predatory mite Phytoseiulus persimilis Athias-Henriot, used to control spider mites, discriminates between odours from plants with spider mites, Tetranychus urticae Koch, and plants with spider mites plus conspecific predators. The odours used for discrimination are produced by adult prey in response to the presence of predators, and probably serve as an alarm pheromone to warn related spider mites. Other predator species may also trigger production of this alarm pheromone, which P. persimilis could use in turn to avoid plants with heterospecific predators. We therefore studied the response of the latter to odours from plants with spider mites and 3 other predator species, i.e. the generalist predatory bug Orius laevigatus (Fieber), the polyphagous thrips Frankliniella occidentalis and the spider-mite predator Neoseiulus californicus (McGregor). Both olfactometer and greenhouse release experiments yielded no evidence that P. persimilis avoids plants with any of the 3 heterospecific predators. This suggests that these predators do not elicit production of alarm pheromones in spider mites, and we argue that this is caused by a lack of coevolutionary history. The consequences of the lack of avoidance of heterospecific predators for interactions in food webs and biological control are discussed.     

Mathematical models of predator/prey/plant interactions in a patch environment

Several tritrophic systems are characterized by local over-exploitation of the food source. Interactions between predatory mites, spider mites and their host plants are an example of such systems: either the spider mites over-exploit local patches of host plants or the spider mites are exterminated by predatory mites. It is often stated that modelling the overall population dynamics of such systems in a realistic way would soon lead to an unmanageable edifice. We advocate, however, the use of physiologically structured population models as a both general and formal mathematical framework. The advantage is that analytically tractable models may be obtained from the complex ‘master’ model by time-scale arguments or special choices of model ingredients. In this way a network of models can be derived, each concentrating on a particular aspect, all inadequate to cover the entire spectrum, but together (we hope) providing a coherent set of insights the relative importance of which can be assessed by computer experiments on the ‘master’ model. In this paper a rather realistic model of predator/prey interactions in an ensemble of host-plant patches is presented and, as an example of our approach, some special cases are derived from that model. Their analysis provided some first, useful insights. It is shown that prolonged duration of the prey-dispersal phase and prey dispersal from predator (-invaded prey) patches may result in a stable steady state, whereas a humped plant-production function may — under certain conditions — result in two stable steady states.     

Plant-related factors influence the effectiveness of Neoseiulus fallacis (Acari: Phytoseiidae), a biological control agent of spider mites on landscape ornamental plants

The predatory mite Neoseiulus fallacis (Garman) was evaluated as a biological control agent of herbivorous mites on outdoor-grown ornamental landscape plants. To elucidate factors that may affect predator efficiency, replicated tests were conducted on 30 ornamental plant cultivars that varied in relationship to their generalized morphology (e.g., conifers, shade trees, evergreen shrubs, deciduous shrubs, and herbaceous perennials), production method (potted or field grown), canopy density, and the prey species present on each. Plant morphological grouping and foliar density appeared to be the most influential factors in predicting successful biological control. Among plant morphological groups, N. fallacis was most effective on shrubs and herbaceous perennials and less effective on conifers and shade trees. N. fallacis was equally effective at controlling spider mites on containerized (potted) and field grown plants, and there was no difference in control of mites on plants with Tetranychus spp. versus those with Oligonychus or Schizotetranychus spp. Moderate to unsuccessful control of spider mites by N. fallacis occurred mostly on tall, vertical plants with sparse canopies. Acceptable spider mite control occurred in four large-scale releases of N. fallacis into production plantings of Abies procera, Thuja occidentalis 'Emerald', Malus rootstock, and Viburnum plicatum 'Newport'. These data suggest that N. fallacis can be an effective biological control agent of multiple spider mite species in a range of low-growing and selected higher growing ornamental plants.     

Spider Mites Avoid Plants with Predators

While searching for food, prey can use cues associated with their predators to select patches with a reduced predation risk. In many cases, odours indicate the presence of both food and predators. Spider mites are known to use odours to locate food and mates, but also to avoid interspecific competitors. We studied the response of the two-spotted spider mite, Tetranychus urticae, to cues associated with the presence of their predators, the phytoseiid Phytoseiulus persimilis. We found that the spider mites strongly avoid plants defended by this predator, but do not avoid plants with another predatory mite, Neoseiulus californicus. Since P. persimilis is commonly used in the greenhouse where our strain of T. urticae was collected and strains of this pest are known to adapt to greenhouse environments, we hypothesize that there has been selection on the pest to recognize its enemy. We further hypothesize that there has been no selection to recognize N. californicus, as it has not been used against two-spotted spider mites in the greenhouse where our spider mites were collected. We discuss the implications of avoidance of predation by spider mites and non-lethal effects of predators for biological control of this pest in greenhouses.     

Of mites and movement: the effects of plant connectedness and temperature on movement of Phytoseiulus persimilis

Simulation modelling studies on the biological control of Tetranychus urticae Koch in ornamental crops suggest that the dispersal of the predatory mite Phytoseiulus persimilis Athias-Henriot in the absence of food is important in determining its ability to locate sparsely distributed patches of prey (Skirvin et al., 2002). Experimental work to examine factors influencing dispersal of P. persimilis has shown that ground substrate affects the movement of the predator, and that the greater the number of connections between adjacent plants the greater the number of mites moving. In addition, P. persimilis are able to move across as many as 10 plant–plant connections within 24 h, although the majority of predators tracked moved less than this. Temperature has a significant impact on dispersal of P. persimilis, with more mites leaving release points as temperature increases up to 25 °C, but decreasing above this temperature. This work highlights the importance of understanding how the plant canopy and temperature influence the dispersal of predatory mites. The importance of these results for biological control in ornamental crops is discussed.     

How predatory mites find plants with whitefly prey

We investigated the searching behaviour of two species of predatory mites, Typhlodromips swirskii (Athias-Henriot) and Euseius scutalis (Athias-Henriot), both known to feed on immature stages of the whitefly Bemisia tabaci Gennadius. When released in a greenhouse inside a circle of cucumber plants that were alternatingly clean or infested with immature whiteflies, the mites took several days to find plants. Both species were recaptured significantly more on plants with whiteflies. This suggests that the mites are able to discriminate between plants with and without whiteflies. The predators may either have been attracted to plants with whiteflies from a distance or arrested on plants with whiteflies. Typhlodromips swirskii that had previously fed on whitefly immatures on cucumber leaves were significantly attracted by volatiles from cucumber plants with whiteflies in a Y-tube olfactometer. This suggests that the mites use volatile cues to discriminate between infested and clean plants. However, this response waned rapidly; if predators, experienced as above, were starved for 3–4 h in absence of cucumber leaves, they no longer preferred volatiles of infested plants to clean plants. Furthermore, T. swirskii that had no experience with immature whiteflies on cucumber plants also did not prefer odours of infested plants to those of clean plants. Because the release experiment with this species in the greenhouse was done with inexperienced predators, this suggests that the aggregation of mites on plants with whiteflies was mainly caused by differential arrestment of mites on plants with prey and clean plants. For T. swirskii, this was in agreement with the finding that the fraction of predators on plants with prey increased with time to levels higher than 70%. A less clear trend was found for E. scutalis, for which the fraction of predators on plants with prey stabilized soon after release to levels from 54–70%. Hence, the predatory mites may find plants with prey by random searching, but they are subsequently arrested on these plants. An earlier study showed that 87% of all whiteflies released in a set-up as used here were recaptured within 1 day. Hence, the effectiveness with which predatory mites locate plants with whiteflies is low compared with that of their prey. We expect this to generate spatial patterns in the dynamics of predator and prey and this may have consequences for biological control of whiteflies with predatory mites.     

Interactions in a tritrophic acarine predator-prey metapopulation system V: Within-plant dynamics of Phytoseiulus persimilis and Tetranychus urticae (Acari: Phytoseiidae, Tetranychidae)

To investigate the relative contributions of bottom-up (plant condition) and top-down (predatory mites) factors on the dynamics of the two-spotted spider mite (Tetranychus urticae), a series of experiments were conducted in which spider mites and predatory mites were released on bean plants. Plants inoculated with 2, 4, 8, 16, and 32 adult female T. urticae were either left untreated or were inoculated with 3 or 5 adult female predators (Phytoseiulus persimilis) one week after the introduction of spider mites. Plant area, densities of T. urticae and P. persimilis, and plant injury were assessed by weekly sampling. Data were analysed by a combination of statistical methods and a tri-trophic mechanistic simulation model partly parameterised from the current experiments and partly from previous data. The results showed a clear effect of predators on the density of spider mites and on the plant injury they cause. Plant injury increased with the initial number of spider mites and decreased with the initial number of predators. Extinction of T. urticae, followed by extinction of P. persimilis, was the most likely outcome for most initial combinations of prey and predators. Eggs constituted a relatively smaller part of the prey population as plant injury increased and of the predator population as prey density decreased. We did not find statistical evidence of P. persimilis having preference for feeding on T. urticae eggs. The simulation model demonstrated that bottom-up and top-down factors interact synergistically to reduce the density of spider mites. This may have important implications for biological control of spider mites by means of predatory mites.     

Potential lethal and non-lethal effects of predators on dispersal of spider mites

Predators can affect prey dispersal lethally by direct consumption or non-lethally by making prey hesitate to disperse. These lethal and non-lethal effects are detectable only in systems where prey can disperse between multiple patches. However, most studies have drawn their conclusions concerning the ability of predatory mites to suppress spider mites based on observations of their interactions on a single patch or on heavily infested host plants where spider mites could hardly disperse toward intact patches. In these systems, specialist predatory mites that penetrate protective webs produced by spider mites quickly suppress the spider mites, whereas generalist predators that cannot penetrate the webs were ineffective. By using a connected patch system, we revealed that a generalist ant, Pristomyrmex punctatus Mayr (Hymenoptera: Formicidae), effectively prevented dispersal of spider mites, Tetranychus kanzawai Kishida (Acari: Tetranychidae), by directly consuming dispersing individuals. We also revealed that a generalist predatory mite, Euseius sojaensis Ehara (Acari: Phytoseiidae), prevented between-patch dispersal of T. kanzawai by making them hesitate to disperse. In contrast, a specialist phytoseiid predatory mite, Neoseiulus womersleyi Schicha, allowed spider mites to escape an initial patch, increasing the number of colonized patches within the system. Our results suggest that ants and generalist predatory mites can effectively suppress Tetranychus species under some conditions, and should receive more attention as agents for conservation biological control in agroecosystems.     

Plants,mites and mutualism: leaf domatia and the abundance and reproduction of mites on Viburnum tinus (Caprifoliaceae)

Associations between mites and leaf domatia have been widely reported, but little is known about their consequences for either plants or mites. By excising domatia from leaves of the laureltinus, Viburnum tinus L. (Caprifoliaceae), in the garden and laboratory, we showed that domatia alter the abundance, distribution, and reproduction of potential plant mutualists. Over 4 months, leaves with domatia on six garden shrubs had 2–36 times more predatory and microbivorous mites, and more mite eggs than leaves without domatia. However, this effect varied among plants and was weaker on one shrub with few mites on its leaves. Domatia also influenced the distribution of mites on leaves. A significantly higher fraction of mites, representing all life stages, was found in vein axils of leaves with domatia than in vein axils on leaves without domatia. Single-leaf experiments in the laboratory showed that domatia enhanced reproduction by the predatory mite, Metaseiulus occidentalis, especially at low relative humidity (30–38%). When domatia were removed, oviposition was reduced significantly only at low relative humidity, suggesting that domatia provide mites with refuge from environmental extremes on the leaf surface. Moreover, the use of domatia by predatory mites may reduce the impact of some plant enemies. In two experiments where prey consumption was measured, M. occidentalis ate significantly higher percentages of the eggs of the two-spotted spider mite (Tetranychus urticae). Our results are consistent with the viewpoint that mite-domatia associations are mutualistic. By directly aiding and abetting the third trophic level, plants with leaf domatia may increase the efficiency of some predaceous and microbivorous mites in consuming plant enemies.     

Allocating time between feeding, resting and moving by the two-spotted spider mite, Tetranychus urticae and its predator Phytoseiulus persimilis

This study characterizes the timing of feeding, moving and resting for the two-spotted spider mite, Tetranychus urticae Koch and a phytoseiid predator, Phytoseiulus persimilis Athias-Henriot. Feeding is the interaction between T. urticae and plants, and between P. persimilis and T. urticae. Movement plays a key role in locating new food resources. Both activities are closely related to survival and reproduction. We measured the time allocated to these behaviours at four ages of the spider mite (juveniles, adult females immediately after moult and adult females 1 and 3 days after moult) and two ages of the predatory mite (juveniles and adult females). We also examined the effect of previous spider mite-inflicted leaf damage on the spider mite behaviour. Juveniles of both the spider mite and the predatory mite moved around less than their adult counterparts. Newly emerged adult female spider mites spent most of their time moving, stopping only to feed. This represents the teneral phase, during which adult female spider mites are most likely to disperse. With the exception of this age group, spider mites moved more and fed less on previously damaged than on clean leaves. Because of this, the spider mite behaviour was initially more variable on damaged leaves. Phytoseiulus persimilis rested at all stages for a much larger percentage of the time and spent less time feeding than did T. urticae; the predators invariably rested in close proximity to the prey. Compared to adult predators, juveniles spent approximately four times as long handling a prey egg. The predator-prey interaction is dependent upon the local movement of both the predators and prey. These details of individual behaviours in a multispecies environment can provide an understanding of population dynamics.     

The Effects of Prey Patchiness, Predator Aggregation, and Mutual Interference on the Functional Response of Phytoseiulus persimilis Feeding on Tetranychus urticae (Acari: Phytoseiidae, Tetranychidae)

The spatial distributions of two-spotted spider mites Tetranychus urticae and their natural enemy, the phytoseiid predator Phytoseiulus persimilis, were studied on six full-grown cucumber plants. Both mite species were very patchily distributed and P. persimilis tended to aggregate on leaves with abundant prey. The effects of non-homogenous distributions and degree of spatial overlap between prey and predators on the per capita predation rate were studied by means of a stage-specific predation model that averages the predation rates over all the local populations inhabiting the individual leaves. The empirical predation rates were compared with predictions assuming random predator search and/or an even distribution of prey. The analysis clearly shows that the ability of the predators to search non-randomly increases their predation rate. On the other hand, the prey may gain if it adopts a more even distribution when its density is low and a more patchy distribution when density increases. Mutual interference between searching predators reduces the predation rate, but the effect is negligible. The stage-specific functional response model was compared with two simpler models without explicit stage structure. Both unstructured models yielded predictions that were quite similar to those of the stage-structured model.     

Sex-specific developmental plasticity of generalist and specialist predatory mites (Acari: Phytoseiidae) in response to food stress

We studied developmental plasticity under food stress in three female-biased size dimorphic predatory mite species, Phytoseiulus persimilis, Neoseiulus californicus, and Amblyseius andersoni. All three species prey on two-spotted spider mites but differ in the degree of adaptation to this prey. Phytoseiulus persimilis is a specialized spider mite predator, N. californicus is a generalist with a preference for spider mites, and A. andersoni is a broad generalist. Immature predators were offered prey patches of varying density and their survival chances, dispersal tendencies, age and size at maturity measured. Amblyseius andersoni dispersed earlier from and had lower survival chances in low density prey patches than N. californicus and P. persimilis. Age at maturity was not affected by prey density in the generalist A. andersoni, whereas both the specialist P. persimilis and the generalist N. californicus accelerated development at low prey densities. Species-specific plasticity in age at maturity reflects opposite survival strategies when confronted with limited prey: to prematurely leave and search for other food (A. andersoni) or to stay and accelerate development (P. persimilis, N. californicus). In all species, size at maturity was more plastic in females than males, indicating that males incur higher fitness costs from deviations from optimal body size.     

Intraguild Interactions Between the Predatory Mites Neoseiulus californicus and Phytoseiulus persimilis

Species at the same trophic level may interact through competition for food, but can also interact through intraguild predation. Intraguild predation is widespread at the second and third trophic level and the effects may cascade down to the plant level. The effects of intraguild predation can be modified by antipredator behaviour in the intraguild prey. We studied intraguild predation and antipredator behaviour in two species of predatory mite, Neoseiulus californicus and Phytoseiulus persimilis, which are both used for control of the two-spotted spider mite in greenhouse and outdoor crops. Using a Y-tube olfactometer, we assessed in particular whether each of the two predators avoids odours emanating from prey patches occupied by the heterospecific predator. Furthermore, we measured the occurrence and rate of intraguild predation of different developmental stages of P. persimilis and N. californicus on bean leaves in absence or in presence of the shared prey. Neither of the two predator species avoided prey patches with the heterospecific competitor, both when inexperienced with the other predator and when experienced with prey patches occupied by the heterospecific predator. Intraguild experiments showed that N. californicus is a potential intraguild predator of P. persimilis. However, P. persimilis did not suffer much from intraguild predation as long as the shared prey was present. This is probably because N. californicus prefers to feed on two-spotted spider mites rather than on its intraguild prey.     

Plant spatial distribution and predator–prey ratio affect biological control of the twospotted spider mite Tetranychus urticae (Acari: Tetranychidae) by the predatory mite Phytoseiulus persimilis (Acari: Phytoseiidae)

A greenhouse experiment was conducted to determine the effect of plant spacing and predator–prey ratio on dispersal and foraging efficiency of the predatory mite, Phytoseiulus persimilis, on the twospotted spider mite, Tetranychus urticae. When predators were released at the end of spider mite-infested arrays of lima bean plants that had either no spacing or two different patterns of spacing among plant rows, plant damage was uniformly low throughout the experiment at both predator–prey ratios (1:10 and 3:10) in the treatment with no spacing. In contrast, damage was higher in both treatments where plant rows were interrupted by spacing. At the 1:10 ratio, more plants closer to the predator release point experienced moderate damage than at the 3:10 ratio where only the plant rows farthest from the release point had unacceptable damage. Our findings suggest that point releases of P. persimilis at the standard 1:10 predator–prey ratio should be effective within a diameter of at least 65?cm on mite-infested patches of plants where pots are touching. However, if gaps in plant rows exist, even large numbers of predators may not be sufficient to protect parts of the crop unless predators are released at shorter fixed points in the greenhouse crop.