Influence of humidity and water availability on the survival ofAmblyseius idaeus andA. anonymus (Acarina: Phytoseiidae)

Amblyseius idaeus (Denmark & Muma) andA. anonymus Chant & Baker are morphologically very similar species of phytoseiids inhabiting areas in South America that have very different levels of humidity. Above 60% RH, nearly all eggs of both species hatch successfully, but below 60% RH the egg hatching rate ofA. anonymus is very poor, whereas eggs ofA. idaeus hatch at humidities as low as 30% RH. The mobile stages ofA. idaeus are better able to survive in absence of food and water than those ofA. anonymus. Water availability promotes survival of both species, but its effect onA. idaeus exceeds that onA. anonymus. These differences in low-humidity tolerance and survival ability are consistent with the climatic origin of these phytoseiid species;Amblyseius ideaus has been reported from very dry and humid areas, andA. anonymus from humid areas only.In comparison with other phytoseiid species, the eggs ofA. idaeus have the highest tolerance to low humidities reported to date, and, among the phytoseiids that are shown to be capable of controllingTetranychus spp., this tolerance appears to be exceptionally high. The impact of this result on the scope of biological control ofTetranychus spp. is discussed.     

Do functional responses of predatory arthropods reach a plateau? A case study of Orius insidiosus with western flower thrips as prey

Parameters determining the functional and numerical response of Orius insidiosus to the density of second instar larvae of the western flower thrips (WFT), Frankliniella occidentalis, were estimated with the aim to be used in models calculating adequate release ratios of predators vs prey. Especially when the prey population must decrease immediately after predator release and the time to suppression must be short, it is of crucial importance to know whether the functional response reaches a plateau or keeps on increasing with prey density within the range of densities that are realistic for the prey. Such plateaus may arise from constraints on the time budget or from constraints on gut-fullness-associated motivation to attack the prey. Estimates of the plateau for O. insidiosus based on prey handling times by far exceed realistic values. In addition, motivation constraints did not apply, because the level of gut fullness above which attack ceases coincides with gut capacity. This implies that the predation rate keeps on increasing linearly with the square root of the density of WFT-larvae and does not reach a plateau within the realistic range of thrips densities (0–20 WFT per cm2). Such plateaus are likely to occur for smaller-sized predators and smaller-sized stages of the same predator and they may also occur when the prey stage offered has better capacities to escape or resist attack. We argue that the presence and level of plateaus in functional responses are of importance for determining initial predator-to-prey ratios.     

How larvae of Thrips tabaci reduce the attack success of phytoseiid predators

Neoseiulus barkeri (= Amblyseius mckenziei) and Amblyseius cucumeris (Acari:Phytoseiidae) are used as control agents of Thrips tabaci (Insecta:Thripidae) in greenhouse crops. Their success in capturing prey larval stages is related to both the feeding state of the predators and to the size of the larvae. When starved, predators are more successful in seizing larvae. Upon contact with a starved predator second stage prey larvae incur a lower death risk than first stage larvae. The larvae of T. tabaci reduce the attack success of their predators by jerking the abdomen and by producing a drop of rectal fluid. When this defensive behaviour is prevented by anaesthetising the larvae with CO₂, predator attack success increases. Anaesthesia does not, however, level out the difference in death risk of the two larval stages. Conceivable causes for this discrepancy are discussed.Availability of suitable prey is dependent on the dynamics of the age structure of the prey population and, hence, may be lower than total thrips density suggests. If so, alternative food sources may be important to maintain the predator population.

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Zusammenfassung Neoseiulus barkeri (= Amblyseius mckenziei) und Amblyseius cucumeris (Acari: Phytoseiidae) werden zur Bekämpfung von Thrips tabaci (Insecta: Thripidae) in Gewächshauskulturen eingesetzt. Sowohl der Ernährungszustand der räuberischen Milben als auch die Grösse der Thripslarven haben Einfluss auf das Ausmass der Beutenahme. Die Prädatoren sind erfolgreicher, wenn sie eine Zeitlang ohne Nahrung gehalten wurden. Beim Zusammentreffen mit einer ausgehungerten Raubmilbe besteht für Thripslarven des zweiten Stadiums ein geringeres Risiko erbeutet und gefressen zu werden als für Larven des ersten Stadiums. T. tabaci Larven mindern den Angriffserfolg der Prädatoren durch kräftiges Hin- und Herschlagen des Abdomens und durch Abgabe eines Tropfens Rektalflüssigkeit. Wird dieses Abwehrverhalten der Larven durch Anaästhesie mit CO₂ verhindert, erhöht sich der Angriffserfolg der Prädatoren. Anästhesie nivelliert jedoch nicht das für beide Larvenstadien unterschiedlich hohe Risiko erbeutet zu werden. Mögliche Ursachen für diesen Unterschied werden diskutiert.Die Verfügbarkeit geeigneter Beutetiere hängt ab von der zeitlichen Entwicklung der Altersstruktur ihrer Population. Das Angebot an wirklich geeigneten Beutetieren kan also unter Umständen geringer sein, als dies die Gesamtthripsdichte zunächst vermuten lässt. Ist das der Fall, dürften alternative Nahrungsquellen für die Ernährung der Prädatorenpopulation wichtig sein.

     

Candidate predators for biological control of the poultry red mite Dermanyssus gallinae

The poultry red mite, Dermanyssus gallinae, is currently a significant pest in the poultry industry in Europe. Biological control by the introduction of predatory mites is one of the various options for controlling poultry red mites. Here, we present the first results of an attempt to identify potential predators by surveying the mite fauna of European starling (Sturnus vulgaris) nests, by assessing their ability to feed on poultry red mites and by testing for their inability to extract blood from bird hosts, i.e., newly hatched, young starlings and chickens. Two genuine predators of poultry red mites are identified: Hypoaspis aculeifer and Androlaelaps casalis. A review of the literature shows that some authors suspected the latter species to parasitize on the blood of birds and mammals, but they did not provide experimental evidence for these feeding habits and/or overlooked published evidence showing the reverse. We advocate careful analysis of the trophic structure of arthropods inhabiting bird nests as a basis for identifying candidate predators for control of poultry red mites.     

Biological control of broad mites (<Emphasis Type="Italic">Polyphagotarsonemus latus</Emphasis>) with the generalist predator <Emphasis Type="Italic">Amblyseius swirskii</Emphasis>

The broad mite is a serious pest of a variety of crops worldwide. Several phytoseiid mites have been described to control these mites. However, broad mites are still one of the major pest problems on greenhouse pepper in South-eastern Spain. The generalist predatory mite A. swirskii is widely used against other pests of pepper plants such as thrips and whiteflies, the latter being a vector of broad mites. We assessed the potential of A. swirskii to control broad mites. The oviposition rate of A. swirskii on a diet of broad mites was lower than on a diet of pollen, but higher than oviposition in the absence of food. Population-dynamical experiments with A. swirskii on single sweet pepper plants in a greenhouse compartment showed successful control of broad mites.     

Evolution restricts the coexistence of specialists and generalists: the role of trade-off structure

Environmental variability and adaptive foraging behavior have been shown to favor coexistence of specialists and generalists on an ecological timescale. This leaves unaddressed the question of whether such coexistence can also be expected on an evolutionary timescale. In this article, we study the attainability, through gradual evolution, of specialist-generalist coexistence, as well as the evolutionary stability of such communities when allowing for immigration. Our analysis shows that the potential for specialist-generalist coexistence is much more restricted than originally thought and strongly depends on the trade-off structure assumed. We establish that ecological coexistence is less likely for species facing a trade-off between per capita reproduction in different habitats than when the trade-off acts on carrying capacities alone. We also demonstrate that coexistence is evolutionarily stable whenever it is ecologically stable but that in most cases, such coexistence cannot be reached through gradual evolution. We conclude that an evolutionarily stable community of specialists and generalists may be created only through immigration from elsewhere or through mutations of large effect. Our results highlight that trade-offs in fitness-determining traits can have counterintuitive effects on the evolution of specialization.     

Phytoseiid predator of whitefly feeds on plant tissue

Predatory mites of the family Phytoseiidae feed on herbivorous mites and insects but they also use a variety of non-prey food items, such as pollen and nectar. Plant tissue is another potential food source. We investigated whether plant feeding occurs in the two phytoseiids Euseius scutalis (Athias-Henriot) and Typhlodromips swirskii (Athias-Henriot), which are natural enemies of whiteflies. These predatory mites can suppress populations of Bemisia tabaci (Gennadius) on isolated plants and are candidates for biological control of this pest. Both species can be reared on a diet of pollen, but E. scutalis requires a leaf tissue substrate, suggesting that this species might feed on plant tissue. To test this hypothesis, we applied a systemic insecticide (aldicarb) to cucumber plants and assessed the survival of predatory mites on leaves from insecticide-treated plants and untreated plants, both in presence and absence of pollen. The survival of T. swirskii was not affected by the presence of systemic insecticide in the plant. However, the survival of E. scutalis on leaves from insecticide-treated plants was 10 times lower than on leaves from untreated plants. Since the two species showed similar sensitivity to the insecticide when applied in a slide-dip test, this suggests that E. scutalis ingested insecticide through feeding on the leaf tissue. Mortality on treated leaves was observed both in absence and presence of pollen, suggesting that plant feeding is indispensable for E. scutalis. The extent to which plants are used as food by E. scutalis requires further analysis.     

Intraspecific variation in induction of feeding preference and performance in a herbivorous mite

Induction of food preference has often been observed in herbivorous insects. The term is used to indicate preference of individuals for the host plant they have already experienced over one they have not experienced. A typical set-up is one where individuals first feed on host X or Y, and are then offered a choice between host X and Y. This set-up – and hence the body of empirical data – has been criticised for lack of a control treatment to untangle the effects of the separate hosts. In this study, we use a design with a third, unrelated host as control to investigate induction of preference in the herbivorous arthropod, Tetranychus urticae. We provide evidence of induced preference, as well as induced performance, and show that there is considerable variation in these two traits among strains. We suggest induced resistance to toxic secondary plant chemicals as one potential explanation for induced performance. This in itself suggests associative learning as the most likely candidate learning mechanism for induction of preference in this species. Phenotypically plastic effects underlying induced performance may be a general aspect of induction of preference in herbivorous arthropods, which warrants closer attention to these phenomena.     

Review Behaviour and indirect interactions in food webs of plant-inhabiting arthropods

With the increased use of biological control agents, artificial food webs are created in agricultural crops and the interactions between plants, herbivores and natural enemies change from simple tritrophic interactions to more complex food web interactions. Therefore, herbivore densities will not only be determined by direct predator–prey interactions and direct and indirect defence of plants against herbivores, but also by other direct and indirect interactions such as apparent competition, intraguild predation, resource competition, etc. Although these interactions have received considerable attention in theory and experiments, little is known about their impact on biological control. In this paper, we first present a review of indirect food web interactions in biological control systems. We propose to distinguish between numerical indirect interactions, which are interactions where one species affects densities of another species through an effect on the numbers of an intermediate species and functional indirect interactions, defined as changes in the way that two species interact through the presence of a third species. It is argued that functional indirect interactions are important in food webs and deserve more attention. Subsequently, we discuss experimental results on interactions in an artificial food web consisting of pests and natural enemies on greenhouse cucumber. The two pest species are the two-spotted spider mite Tetranychus urticae and the western flower thrips, Frankliniella occidentalis. Their natural enemies are the predatory mite Phytoseiulus persimilis, which is commonly used for spider mite control and the predatory mites Neoseiulus cucumeris and Iphiseius degenerans and the predatory bug Orius laevigatus, all natural enemies of thrips. First, we analyse the possible interactions between these seven species and we continue by discussing how functional indirect interactions, particularly the behaviour of arthropods, may change the significance and impact of direct interactions and numerical indirect interactions. It was found that a simple food web of only four species already gives rise to some quite complicated combinations of interactions. Spider mites and thrips interact indirectly through resource competition, but thrips larvae are intraguild predators of spider mites. Some of the natural enemies used for control of the two herbivore species are also intraguild predators. Moreover, spider mites produce a web that is subsequently used by thrips to hide from their predators. We discuss these and other results obtained so far and we conclude with a discussion of the potential impact of functional indirect and direct interactions on food webs and their significance for biological control.     

Predators induce interspecific herbivore competition for food in refuge space

Resource competition among herbivorous arthropods has long been viewed as unimportant because herbivore populations are controlled by predators. Although recently resurrected as an organizing force in arthropod communities on plants, there is still general agreement that resource competition among herbivores is reduced by predators. Here we show the reverse: predators induce interspecific resource competi-tion among herbivores. We found that thrips larvae ( Frankliniella occidentalis ) use the web produced and inhabited by the spider mite Tetranychus urticae as a refuge from predation by the phytoseiid mite Neoseiulus cucumeris . Thrips larvae prefer clean plant parts, but move into the web upon perceiving volatile cues associated with thrips and predators. This behaviour leads to lower predation risk, but also to reduced developmental rate and lower production of thrips larvae due to competition with spider mites. In addition, thrips larvae consume spider-mite eggs. Thus, predators induce interspecific competition and intraguild predation among herbivores within refuge space, even when host plants have an overall green appearance.