The spatial and temporal distribution of predatory and phytophagous mites in field-grown strawberry in the UK

Extensive sampling of strawberry plants in everbearing and June-bearing strawberry plantations and on potted plants showed that different species of mites were spatially separated. Of the two phytophagous species recorded, Tetranychus urticae was most abundant on old leaves and Phytonemus pallidus on folded leaves and flower/fruit clusters. Predatory phytoseiid mites were found on all plant parts but different species were spatially separated; Neoseiulus cucumeris and N. aurescens were found mostly on folded leaves and clusters, and N. californicus and Phytoseiulus persimilis on old and medium aged leaves. No Typhlodromus pyri were found in the field plantations. These patterns of distribution did not change over sampling dates in summer and early autumn. An understanding of this within-plant zonation of mite species is important when studying predator–prey interactions and when designing sampling strategies for strawberry. A programme to sample the entire mite system on strawberry should be stratified to include all the above mentioned parts of the plant. Different sampling protocols, as appropriate, are required for sampling different pest species and their associated predators.     

Biological control of strawberry tarsonemid mite Phytonemus pallidus and two-spotted spider mite Tetranychus urticae on strawberry in the UK using species of Neoseiulus (Amblyseius) (Acari: Phytoseiidae)

Two species of Neoseiulus, N. californicus and N. cucumeris, showed potential for biocontrol of phytophagous mites on strawberry. N. californicus controlled Tetranychus urticae on potted strawberry plants in a gauze-sided glasshouse at temperatures comparableto early summer in the UK (8–20°C). Both species of phytoseiid reducednumbers of the tarsonemid Phytonemus pallidus on potted strawberry plants under glasshouse conditions (15–23°C). In several experiments reductions in the range of 71–81% in numbers of tarsonemid active stages and eggs, compared to non-release plants, were obtained. The importance of establishing a suitable predator: prey ratio at an earlystage was demonstrated in an experiment where an initial ratio of 1 N. cucumeris: 10 P. pallidus gave a greater degree of controlthan 1:20 or 1:40.     

Life-history of predatory mites Neoseiulus californicus and Phytoseiulus persimilis (Acari: Phytoseiidae) on four spider mite species as prey, with special reference to Tetranychus evansi (Acari: Tetranychidae)

The commercially available strains of Phytoseiulus persimilis Athias-Henriot, the biological control agent of Tetranychus urticae Koch, perform poorly in the Western Mediterranean, probably because they are not well adapted to local climatic conditions. For that reason, efforts are being focused on the development of a biological control programme using native phytoseiid mites. Four species of red spider mites can be found in vegetable crops in eastern Spain: T. urticae, Tetranychus turkestani Ugarov and Nikolski, Tetranychus ludeni Zacher and the recently introduced Tetranychus evansi Baker and Pritchard. To evaluate their potential role as biological control agents, the present study evaluates the life-history of local populations of Neoseiulus californicus (McGregor) and P. persimilis when fed on T. urticae, T. turkestani, T. evansi, and T. ludeni in the laboratory. Results indicate that N. californicus and P. persimilis are able to feed and complete their development on the four tested red spider mite species. The predators may exhibit a particularly high capacity for population increase when fed on T. urticae, T. turkestani, and T. ludeni, thus may be able to provide effective control of these species in the field. When fed T. evansi, however, predator performance was poor; significant increase in development and preoviposition times, and a reduction in oviposition period and fecundity were recorded. The resultant low capacity for population growth suggests poor ability of the two tested predators to suppress T. evansi populations on commercial crops. It is unlikely therefore that P. persimilis and N. californicus, now being widely used to control T. urticae in greenhouse crops in Central Europe, will be able to halt any spread of T. evansi to greenhouse crops in temperate areas.     

Population dynamics of interacting predatory mites,Phytoseiulus persimilis and Neoseiulus californicus,held on detached bean leaves

The success of combined release of the predatory mitesPhytoseiulus persimilis and Neoseiulus californicus insuppression of spider mites may be related to the effects of the interactionsbetween the two predators on their population dynamics. We studied populationgrowth and persistence of the specialist P. persimilis andthe generalist N. californicus reared singly versus rearedin combination after simultaneous and successive predator introductions ondetached bean leaf arenas with abundant prey, Tetranychusurticae, and with diminishing prey. When reared singly with abundantprey, either predator population persisted at high densities to the end of theexperiment. In every predator combination system with abundant prey and variousinitial predator:predator ratios N. californicus displacedP. persimilis. When held singly with diminishing prey, thepopulation of P. persimilis grew initially faster than thepopulation of N. californicus but both species reachedsimilar population peaks. Irrespective whether reared singly or in combination,N. californicus persisted three to five times longer afterprey depletion than did P. persimilis. Regarding thecrucial interactions in the predator combination systems, we conclude thatintraguild predation was a stronger force than food competition and finallyresulted in the displacement of P. persimilis. Previousstudies showed that intraguild predation between the specialist P.persimilis and the generalist N. californicusisstrongly asymmetric favoring the generalist. We discuss the implications ofpotential interactions between P. persimilis andN. californicus to biological control of spider mites.     

Comparative life-history traits of three phytoseiid mites associated with Tetranychus urticae (Acari: Tetranychidae) colonies in clementine orchards in eastern Spain: implications for biological control

The management of Tetranychus urticae, a key pest of clementine trees, is mainly based on the use of acaricides. However, more environmentally safe measures, such as biological control, are being encouraged. Life-history traits of the three most abundant phytoseiid mites associated with T. urticae on this crop (Euseius stipulatus, Phytoseiulus persimilis and Neoseiulus californicus) were studied. The experiments were performed under laboratory conditions (25°C, 80 ± 5% RH and 16:8 h (L:D)) on clementine leaves and T. urticae as prey. Euseius stipulatus could not complete its life cycle, whereas P. persimilis and N. californicus completed it satisfactorily. The estimated intrinsic rate of increase (r _m) was significantly higher for P. persimilis (0.344 day⁻¹) than for N. californicus (0.244 day⁻¹) and both were higher than the r _m value of T. urticae on clementine leaves. Implications of these results for the biological control of T. urticae in this crop are discussed.     

Intraguild Predation and Feeding Preferences in Three Species of Phytoseiid Mite Used for Biological Control

The ecological impact of introduced biological control agents on native species of arthropods is a matter of considerable debate. This study investigated the ability of the non-native predatory mite Neoseiulus californicus to feed on the native Typhlodromus pyri and vice versa, as both species now co-occur in UK orchards. Typhlodromips montdorensis is a candidate for introduction into the UK as a glasshouse biological control agent. The ability of T. montdorensis to feed on the widely used N. californicus was investigated to identify possible intraguild predation, which might influence the effectiveness of either or both species as predators of Tetranychus urticae. Both N. californicus and T. pyri consumed larval stages of each other, but in choice experiments both showed a preference for T. urticae. Both N. californicus and T. montdorensis also fed on each other, but whereas N. californicus again showed a preference for T. urticae, T. montdorensis fed equally on T. urticae and N. californicus. Interactions between N. californicus and T. pyri and N. californicus and T. montdorensis are discussed in relation to their effectiveness as biological control agents in the glasshouse and the natural control of spider mite in the field.     

Effects of acaricides,pyrethroids and predator distributions on populations of <Emphasis Type="Italic">Tetranychus urticae</Emphasis> in apple orchards

We sampled mites in three apple orchards in Nova Scotia, Canada, that had been inoculated with pyrethroid-resistant Typhlodromus pyri and had a history of Tetranychus urticae outbreaks. The objective of this study was to monitor populations of T. urticae and phytoseiid predators on the ground and in trees and to track dispersal between the two habitats. Pesticides were the chief cause of differences in mite dynamics between orchards. In two orchards, application of favourably selective acaricides (abamectin, clofentezine) in 2002, coupled with predation by T. pyri in trees and Neoseiulus fallacis in ground cover, decreased high T. urticae counts and suppressed Panonychus ulmi. By 2003 phytoseiids kept the tetranychids at low levels. In a third orchard, application of pyrethroids (cypermethrin, lambda-cyhalothrin), plus an unfavourably selective acaricide (pyridaben) in 2003, suppressed phytoseiids, allowing exponential increases of T. urticae in the ground cover and in tree canopies. By 2004 however, increasing numbers of T. pyri and application of clofentezine strongly reduced densities of T. urticae in tree canopies despite high numbers crawling up from the ground cover. Another influence on T. urticae dynamics was the distribution of the phytoseiids, T. pyri and N. fallacis. When harsh pesticides were avoided, T. pyri were numerous in tree canopies. Conversely, only a few N. fallacis were found there, even when they were present in the ground cover and on tree trunks. Low numbers were sometimes due to pyrethroid applications or to scarcity of prey. Another factor was likely the abundance of T. pyri, which not only competes with N. fallacis, but also feeds on its larvae and nymphs. The scarcity of a specialist predator of spider mites in trees means that control of T. urticae largely depends on T. pyri, a generalist predator that is not particularly effective in regulating T. urticae. The Canadian Crown's right to retain a non-exclusive, royalty-free licence in and to any copyright is acknowledged.     

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.     

Combined versus Single Species Release of Predaceous Mites: Predator–Predator Interactions and Pest Suppression

Interactions such as competition, intraguild predation (IGP), and cannibalism affect the development and coexistence of predator populations and can have significance for biological control of commonly exploited pest organisms. We studied the consequences of combined versus single release of two predaceous mite species (Phytoseiidae), with differing degrees of diet specialization, on their population dynamics and the suppression of the carmine spider mite, Tetranychus cinnabarinus Boisduval (Tetranychidae), on greenhouse-grown gerbera. Population growth of the specialist predator Phytoseiulus persimilis Athias-Henriot was greater and population decline steeper when released in combination with the generalist Neoseiulus californicus McGregor than when released alone. In contrast, the N. californicus population grew and declined more gradually when released in combination with P. persimilis, compared to the single species release. The differential impact on each other's population dynamics can be primarily attributed to contrasting properties in competition, IGP, and cannibalism. At the same overall predator density and as long as prey was abundant, the specialist P. persimilis was more strongly affected by intraspecific competition than by interspecific competition with the generalist N. californicus. In contrast, interspecific competition with P. persimilis had a greater impact on N. californicus than intraspecific competition. After prey depletion, the generalist predator N. californicus was more likely to engage in IGP than was the specialist predator P. persimilis. Overall, the study demonstrates that prey specificity has significance for the quality and intensity of predator–predator interactions and indicates potential implications for biological control of spider mites. All predator releases (i.e., either species alone and both species in combination) resulted in reduction of the spider mite population to zero density. Individual release of the specialist P. persimilis led to the most rapid spider mite suppression. Nonetheless, in perennial greenhouse-grown crops P. persimilis and N. californicus could have complementary effects and a combination of the two predators could enhance long-term biological control of spider mites. The potential risks and benefits associated with the release of both species are discussed.     

Diet‐dependent intraguild predation between the predatory mites Neoseiulus californicus and Neoseiulus cucumeris

Based on the hypothesis that matching diets of intraguild (IG) predator and prey indicate strong food competition and thus intensify intraguild predation (IGP) as compared to non‐matching diets, we scrutinized diet‐dependent mutual IGP between the predatory mites Neoseiulus cucumeris and N. californicus. Both are natural enemies of herbivorous mites and insects and used in biological control of spider mites and thrips in various agricultural crops. Both are generalist predators that may also feed on plant‐derived substances such as pollen. Irrespective of diet (pollen or spider mites), N. cucumeris females had higher predation and oviposition rates and shorter attack latencies on IG prey than N. californicus. Predation rates on larvae were unaffected by diet but larvae from pollen‐fed mothers were a more profitable prey than those from spider‐mite fed mothers resulting in higher oviposition rates of IG predator females. Pollen‐fed protonymphs were earlier attacked by IG predator females than spider‐mite fed protonymphs. Spider mite‐fed N. californicus females attacked protonymphs earlier than did pollen‐fed N. californicus females. Overall, our study suggests that predator and prey diet may exert subtle influences on mutual IGP between bio‐control agents. Matching diets did not intensify IGP between N. californicus and N. cucumeris but predator and prey diets proximately influenced IGP through changes in behaviour and/or stoichiometry.     

Effects of <Emphasis Type="Italic">Euseius stipulatus</Emphasis> on establishment and efficacy in spider mite suppression of <Emphasis Type="Italic">Neoseiulus californicus</Emphasis> and <Emphasis Type="Italic">Phytoseiulus persimilis</Emphasis> in clementine

The two-spotted spider mite, Tetranychus urticae, is one of the most problematic phytophagous pests in Spanish clementine orchards. The most abundant predatory mites in this ecosystem are Euseius stipulatus, Phytoseiulus persimilis and Neoseiulus californicus. Euseius stipulatus is dominant but poorly adapted to utilize T. urticae as prey. It mainly persists on pollen and citrus red mite, Panonychus citri. A recent study suggested that the more efficacious T. urticae predators P. persimilis and N. californicus are negatively affected by lethal and non-lethal intraguild interactions with E. stipulatus. Here, we investigated the potential of N. californicus and P. persimilis to colonize and thrive on young clementine trees infested by T. urticae in presence and absence of E. stipulatus. Presence of E. stipulatus interfered with establishment and abundance of P. persimilis and negatively affected the efficacy of N. californicus in T. urticae suppression. In contrast, the abundance of E. stipulatus was not affected by introduction of a second predator. Trait-mediated effects of E. stipulatus changing P. persimilis and N. californicus behavior and/or life history were the most likely explanations for these outcomes. We conclude that superiority of E. stipulatus in intraguild interactions may indeed contribute to the currently observed predator species composition and abundance, rendering natural control of T. urticae in Spanish clementine orchards unsatisfactory. Nonetheless, stronger reduction of T. urticae and/or plant damage in the predator combination treatments as compared to E. stipulatus alone indicates the possibility to improve T. urticae control via repeated releases of N. californicus and/or P. persimilis.     

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.     

Altered host plant preference of Tetranychus urticae and prey preference of its predator Phytoseiulus persimilis (Acari: Tetranychidae, Phytoseiidae) on transgenic Cry3Bb-eggplants

The behavior of the two-spotted spider mite, Tetranychus urticae Koch and the predatory mite Phytoseiulus persimilis A.-H. was investigated in laboratory experiments with transgenic Bt-eggplants, Solanum melongena L., producing the Cry3Bb toxin and corresponding isogenic, non-transformed eggplants. In bitrophic experiments, dual-choice disc tests were conducted to reveal the effects of transgenic eggplants on host plant preference of T. urticae. Adult spider mite females were individually placed on leaf discs (2 cm diameter) and were observed during five days. Females occurred significantly more frequently on transgenic halves on which also significantly more T. urticae eggs were found. The effects of a Cry3Bb-eggplant fed prey on the feeding preference of P. persimilis were investigated in tritrophic experiments. Sixteen spider mite females, eight of which had been taken from transgenic and eight from isogenic eggplants, were offered to well-fed females of P. persimilis and numbers of respective spider mites consumed were registered 12 h later when the predators were offered new spider mites again. This procedure was repeated six times. The results revealed that predatory mites consumed significantly less Bt-fed spider mites than prey that had been raised on control eggplants. These results indicate that eggplants expressing the Cry3Bb toxin for resistance against the Colorado potato beetle are more preferred by spider mites but are less preferred by their predator P. persimilis. Possible consequences of these findings for biological control of spider mites on eggplants are discussed.     

Neoseiulus Californicus (Acari: Phytoseiidae) as a Potential Control Agent of Tetranychus Urticae (Acari: Tetranychidae): Effect of Pest/Predator Ratio on Pest Abundance on Strawberry

Neoseiulus californicus (McGregor) is a promising agent for successful Tetranychus urticae Koch control through conservation techniques, in strawberry crops in La Plata (Buenos Aires, Argentina). In prey–predator interaction, initial relative densities have an important effect on system dynamics. The economic threshold level (ETL) used for this pest in the present study was 50 active mites per leaflet. In our laboratory experiments, initial T. urticae to N. californicus ratio had a significant effect on the population abundance of T. urticae at a 7-day period. When pest/predator ratio was 5/1 (at initial pest densities from 5 to 15 females/leaflet) the final number of active T. urticae/leaflet was significantly lower than the ETL, while at 20 females/leaflet this number did not differ from the ETL. At 7.5/1 ratio, the final number of active T. urticae/leaflet, at initial pest densities from 5 to 15 females/leaflet, reached the ETL without surpassing it. At 10/1 and 15/1 ratios, pest densities exceeded the ETL only at 15 initial T. urticae/leaflet. Most greenhouse and field observations were consistent with the predictions of a graphical model based on experimental results. This predator was very effective in limiting pest densities at a 7-day period and within the range of pest–predator ratios and absolute densities used in this study. Conservation of N. californicus promoting favorable pest/predator ratios may result in early control of T. urticae.     

Life tables of the predatory mite Neoseiulus cucumeris (Acari: Phytoseiidae) on two pest mites as prey,Aculops lycopersici and Tetranychus urticae

Abstract

Studies on the life history and life table parameters of Neoseiulus cucumeris Oudemans (Acari: Phytoseiidae) were carried out under laboratory conditions of 25?±?1?°C and 65?±?5% RH; 30?±?1?°C and 60?±?5% RH; 35?±?1?°C and 55?±?5% RH. As prey, immature stages of tetranychid spider mite T. urticae Koch (Acari: Tetranychidae) and the moving stages of the Tomato Russet Mite A. lycopersici (Massee) (Acari: Eriophyideae) were selected. The predatory phytoseiid mite, Neoseiulus cucumeris (Oudemans) was able to develop successfully from egg to adult stage through the entire life history on both preys. The higher of different temperatures and relative humidities shortened the development and increased reproduction and prey consumption and vice versa. The maximum reproduction (3.91, and 3.09 eggs/♀/day) was recorded at 35?°C and 65% RH, while the minimum (2.12, and 1.90 eggs/♀/day) was at 25?±?1?°C and 55?±?5% RH. when N. cucumeris fed on A. lycopersici and T. urticae, respectively. The reproductive rate on eriophyid was significantly higher than previously recorded on tetranychid. Life table parameters indicated that feeding of phytoseiid mite N. cucumeris on tomato russet mite A. lycopersici led to the highest reproduction rate (rm?=?0.268, 0.232 and 0.211 females/female/day), while feeding on T.urticae gave the lowest reproduction rate (rm?=?0.159, 0.143 and 0.131) at 35?°C and 55% RH, 30?°C and 60% RH and 25?°C and 65% RH, respectively. The population of N. cucumeris multiplied (36.81, 28.71 and 20.47) and (24.60, 19.58 and 14.62 times) in a generation time of (20.10, 23.20 and 25.14) and (22.35, 25.36 and 27.79 days) when a predator fed on A. lycopersici and T. urticae at the same temperature above mentioned, respectively. These results suggest that the two mites, particularly A. lycopersici, proved to be suitable prey for N.cucumeris, as a facultative predator.     

Predation,oviposition and conversion rates of the predacious mite,Neoseiulus californicus (McGregor) consuming different densities of Tetranychus urticae Koch,Bemisia tabaci (Genn.) and Thrips tabaci Lind.

Rates of prey consumption, egg production and prey conversion by the predacious mite, Neoseiulus californicus (McGregor) were estimated at different densities of Tetranychus urticae Koch, Bemisia tabaci (Genn.) and Thrips tabaci Lind. in the laboratory. N. californicus females functionally and numerically responded to the increasing densities of T. urticae nymphs, B. tabaci nymphs and T. tabaci larvae showing Holling’s type II. The maximum mean predation and oviposition rates by the predator females devouring T. urticae, B. tabaci and T. tabaci occurred at 15, 10 and 10 prey individuals/day, respectively, followed by the plateau levels at higher prey densities. N. californicus females exhibited the highest feeding and oviposition on T. urticae nymphs, followed by T. tabaci larvae and B. tabaci nymphs. The predator females showed the highest efficiency in converting the prey into egg progeny at 5 individuals/day of the previous prey species, respectively. T. urticae was the most favourable for N. californicus females, followed by T. tabaci and B. tabaci.     

Comparison ofTetranychus evansi andT. urticae [Acari: Tetranychidae] as prey for eight species of phytoseiid mites

Eight phytoseiid species were tested to evaluate and compare their potential as predators ofTetranychus evansi Baker & Pritchard andT. urticae (Koch). The study was conducted using arenas of excised nightshade (Solanum douglasii Dunal) and Lima bean (Phaseolus vulgaris L.) leaves infested with eitherTetranychus species. When the prey wasT. evansi, the predatorsAmblyseius californicus (McGregor) andPhytoseiulus persimilis Athias-Henriot from Ventura, California, showed the highest oviposition rates. However, those rates were 4 to 6 times lower than rates for the same predators feeding onT. urticae. OnlyA. californicus and the strain ofP. persimilis from Beni-Mellal, Morocco, had survivorship higher than 50% 8 days after the beginning of the experiment. The results indicated thatT. evansi is an unfavorable prey for all the phytoseiids tested.

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Résumé Huit espèces d'Acariens phytoseiides étaient testées en vue de comparer leurs potentialités prédatrices vis-à-vis deTetranychus evansi Baker & Pritchard et deTetranychus urticae (Koch). L'étude était réalisée sur des feuilles détachées deSolanum douglasii Dunal et dePhaseolus vulgaris L. Les feuilles avaient été infestées avec l'une ou l'autre des espèces deTetranychus. Amblyseius californicus (McGregor) etPhytoseiulus persimilis Athias-Henriot, de Ventura, ont donné les taux d'oviposition les plus élevés avecT. evansi, mais ces taux étaient 4 à 6 fois inférieurs à ceux obtenus lorsque la proie étaitT. urticae. SeulsA. californicus et la souche deP. persimilis de Beni-Mellal avaient une survie supérieure à 50%, 8 jours après le début du test. Les résultats indiquent queT. evansi n'est pas une proie favorable pour toutes les espèces testées.

     

Absorption and cannibalism: do phytoseiids conserve egg resources when prey densities decline rapidly?

The number of eggs oviposited or left in the opisthosomas of dead mites (total eggs) was assessed for Metaseiulus occidentalis (Nesbitt), Neoseiulus fallacis (Garman), Typhlodromus pyri Scheuten or Amblyseius andersoni Chant when each was caged with either (1) no Tetranychus urticae Koch, (2) only odours of T. urticae, (3) ten eggs of M. occidentalis or (4) ten nymphs of M. occidentalis (T. pyri for M. occidentalis). The total eggs for the no prey versus odour tests did not differ within species; the levels were the greatest for N. fallacis > T. pyri > A. andersoni > M. occidentalis. Among treatments, egg means did not differ for M. occidentalis but they did for N. fallacis and T. pyri and similar trends were seen for A. andersoni. Egg means were usually less for mites held with ten predator nymphs than mites held with ten predator eggs or with no prey. Were adult females with nymphs absorbing rather than ovipositing their eggs or dying with them in their opisthosomas? Activity levels (walking) for adult females were no more for mites held with nymphs versus no food. The data indicated that interference by nymphs was not increasing the energy use of females and thus reducing egg levels. However, tests with ten nymphs, one egg and no adult female had egg losses from nymphal predation that could account for fewer eggs in cage tests. Overall, no evidence for absorption was found. If it occurs, it must be among younger eggs or mites exposed to less rapid prey losses than were the mites tested here; in addition, other stimuli may cause absorption. The total eggs in sticky-tape tests were greatest for N. fallacis > M. occidentalis > T. pyri > A. andersoni. Cage versus stick-tape data differed most for M. occidentalis because of cannibalism. All four mites cannibalized eggs but M. occidentalis did most rapidly and extensively. When starved, it laid all of its eggs before the other three species did. Such behaviours may enhance survival of M. occidentalis when prey become scarce.     

Assessing the Effects of Bt Maize on the Predatory Mite Neoseiulus cucumeris

The investigation of Neoseiulus cucumeris in the context of the ecological risk assessment of insect resistant transgenic plants is of particular interest as this omnivorous predatory mite species is commercially available and considered important for biological control. In a multitrophic feeding experiment we assessed the impact of Bt maize on the performance of N. cucumeris when offered spider mites (Tetranychus urticae) reared on Bt (Bt11, Syngenta) or non-Bt maize (near isogenic line) and Bt or non-Bt maize pollen as a food source. Various parameters including mortality, development time, oviposition rate were measured. Spider mites were used as a prey for N. cucumeris, since these herbivores are known to contain similar levels of Cry1Ab toxin, when reared on Bt maize, as those found in the transgenic leaf material. In contrast, toxin levels in pollen of this transgenic cultivar are very low. No differences in any of the parameters were found when N. cucumeris was fed with spider mites reared on Bt and non-Bt maize. Pollen was shown to be a less suitable food source for this predator as compared to spider mites. Moreover, subtle effects on female N. cucumeris (9% longer development time and 17% reduced fecundity) were measured when fed with pollen originating from Bt maize as compared to non-Bt maize pollen. Our findings indicate that the predatory mite N. cucumeris is not sensitive to the Cry1Ab toxin as no effects could be detected when offered Bt-containing spider mites, and that the effects found when fed with Bt maize pollen can be assigned to differences in nutritional quality of Bt and non-Bt maize pollen. The significance of these findings is discussed with regard to the ecological relevance for risk assessment of transgenic plants.     

Phenotypic plasticity in anti-intraguild predator strategies: mite larvae adjust their behaviours according to vulnerability and predation risk

Interspecific threat-sensitivity allows prey to maximize the net benefit of antipredator strategies by adjusting the type and intensity of their response to the level of predation risk. This is well documented for classical prey-predator interactions but less so for intraguild predation (IGP). We examined threat-sensitivity in antipredator behaviour of larvae in a predatory mite guild sharing spider mites as prey. The guild consisted of the highly vulnerable intraguild (IG) prey and weak IG predator Phytoseiulus persimilis, the moderately vulnerable IG prey and moderate IG predator Neoseiulus californicus and the little vulnerable IG prey and strong IG predator Amblyseius andersoni. We videotaped the behaviour of the IG prey larvae of the three species in presence of either a low- or a high-risk IG predator female or predator absence and analysed time, distance, path shape and interaction parameters of predators and prey. The least vulnerable IG prey A. andersoni was insensitive to differing IGP risks but the moderately vulnerable IG prey N. californicus and the highly vulnerable IG prey P. persimilis responded in a threat-sensitive manner. Predator presence triggered threat-sensitive behavioural changes in one out of ten measured traits in N. californicus larvae but in four traits in P. persimilis larvae. Low-risk IG predator presence induced a typical escape response in P. persimilis larvae, whereas they reduced their activity in the high-risk IG predator presence. We argue that interspecific threat-sensitivity may promote co-existence of IG predators and IG prey and should be common in predator guilds with long co-evolutionary history.