The aetiology and development of damage in young fruit trees infested with fruit tree red spider mite, Panonychus ulmi (Koch)

Infestations of fruit tree red spider mite, Panonychus ulmi (Koch) altered the growth of young plants of plum and apple. At first, damage to the leaves from mite feeding did not affect their photosynthetic rates. The effects on other processes depended on the density of the infestation. Densities of 1–2 mites/cm² of leaf decreased the rate of shoot extension of Brompton plum, but about 0.5 mite/cm² increased it. Less dense infestations apparently caused no damage. The rate of growth of the leaf area of a plant relative to that of the mite population on it determined changes in the mite density, and therefore the effects of infestation. The growth of the root system was decreased before that of the shoots. Later, when some leaves were severely damaged photosynthesis was decreased. The onset and severity of this phase probably depended on the number of mites and days of feeding on individual leaves. The later-formed leaves were smaller, and sometimes fewer on infested plants. Some plants were infested with too low a density of mites to decrease shoot extension, but grew less in dry weight because of decreased photosynthesis later in the season. The initial effects are ascribed to an imbalance in the growth controlling substances caused by feeding. Radioactivity was detected in the growing regions of plants remote from mature leaves on which ¹⁴C-labelled mites were confined.     

Phytoseiulus persimilis response to herbivore-induced plant volatiles as a function of mite-days

The predatory mite, Phytoseiulus persimilis (Acari: Phytoseiidae), uses plant volatiles (i.e., airborne chemicals) triggered by feeding of their herbivorous prey, Tetranychus urticae (Acari: Tetranychidae), to help locate prey patches. The olfactory response of P. persimilis to prey-infested plants varies in direct relation to the population growth pattern of T. urticae on the plant; P. persimilis responds to plants until the spider mite population feeding on a plant collapses, after which infested plants do not attract predators. It has been suggested that this represents an early enemy-free period for T. urticae before the next generation of females is produced. We hypothesize that the mechanism behind the diminished response of predators is due to extensive leaf damage caused by T. urticae feeding, which reduces the production of volatiles irrespective of the collapse of T. urticae population on the plant. To test this hypothesis we investigated how the response of P. persimilis to prey-infested plants is affected by: 1) initial density of T. urticae, 2) duration of infestation, and 3) corresponding leaf damage due to T. urticae feeding. Specifically, we assessed the response of P. persimilis to plants infested with two T. urticae densities (20 or 40 per plant) after 2, 4, 6, 8, 10, 12 or 14 days. We also measured leaf damage on these plants. We found that predator response to T. urticae-infested plants can be quantified as a function of mite-days, which is a cumulative measure of the standing adult female mite population sampled and summed over time. That is, response to volatiles increased with increasing numbers of T. urticae per plant or with the length of time plant was infested by T. urticae, at least as long at the leaves were green. Predatory mites were significantly attracted to plants that were infested for 2 days with only 20 spider mites. This suggests that the enemy-free period might only provide a limited window of opportunity for T. urticae because relatively low numbers of T. urticae per plant can attract predators. Leaf damage also increased as a function of mite-days until the entire leaf was blanched. T. urticae populations decreased at this time, but predator response to volatiles dropped before the entire leaf was blanched and before the T. urticae population decreased. This result supports our hypothesis that predator response to plant volatiles is linked to and limited by the degree of leaf damage, and that the quantitative response to T. urticae populations occurs only within a range when plant quality has not been severely compromised.     

Grape downy mildew Plasmopara viticola, an alternative food for generalist predatory mites occurring in vineyards

Generalist phytoseiids are often observed for long periods on plants in the absence of prey, feeding on alternative foods and reaching high population levels. The persistence of generalist predatory mites on plants with a scarcity or absence of prey is a requirement for successful biocontrol strategies of herbivore mites. The importance of pollen as an alternative food for the support of generalist predatory mite populations is widely recognized. However, on grape the presence of pollen is often limited and thus other food sources should contribute towards generalist predatory mite persistence on perennial plants. Previous field observations reported the relationships between the population increases of generalist phytoseiids with late-season spread of grape downy mildew (GDM) Plasmopara viticola. In this study, we test the hypothesis that GDM could be a suitable food source for the predatory mites Amblyseius andersoni and Typhlodromus pyri. In the laboratory we compared the development times, oviposition rates and life-table parameters of predatory mites feeding on pollen or GDM mycelium and spores. Grape downy mildew supported the survival, development and oviposition of T. pyri and A. andersoni. Life-table parameters showed that GDM was a less suitable food source than pollen for both phytoseiid species and that it was more favorable for A. andersoni than for T. pyri. Implications for predator–prey interactions and conservation biological control in vineyards are discussed.     

Effects of soil water stress and twospotted spider mites on net photosynthesis and transpiration of apple leaves

Soil water stress and twospotted spider mites (Tetranychus urticae Koch) were tested for their influence on the content and activity of leaves of greenhouse grown Delicious apple trees. Soil water stress caused reductions in net photosynthesis (Pn), transpiration (Tr), and shoot growth. Leaf water potential was decreased by both water stress and mite feeding. Feeding of 15 adult mites/leaf for 28 days resulted in a 16% reduction in Pn while an initial population of 10 mites leaf/left to develop for 20 days reduced Pn by 27%. Mite feeding reduced leaf nitrogen and non-structural carbohydrate levels when sampled 20 days after placement on the leaf. There was no interaction between the changed physiology of the leaf due to soil water stress and mite feeding.Approved for publication as Journal Article No. 111-80 of the Ohio Agricultural Research and Development Center, Wooster, OH 44691.Associate Professor and Professor, Departments of Horticulture and Entomology, respectively.     

Does a vascular fungus of tomato induce a defence response or a change in host plant quality that also affects the oviposition of spider mites?

It has been suggested that previous infection by a vascular fungus causes induced resistance against two-spotted spider mites. To test the generality of this phenomenon, a series of experiments was carried out using two lines of tomato, differing only in resistance againstFusarium. In addition, tests were done in order to see whether the defense response against the fungus also affects the phytophagous mite directly. Inoculation of tomato plants with a vascular fungus (Fusarium oxysporum f.sp.lycopersici race 1) prior to infestation with spider mites caused a decrease in the rate of oviposition of two-spotted spider mites (Tetranychus urticae) on aFusarium-susceptible line, but only when plants were moderately to severely wilted. Spider mite oviposition did not change significantly of a previously inoculatedFusarium-resistant line.AsFusarium causes vascular occlusion and wilting of the plants, drought stress was experimentally induced to determine its influence on the reduction of oviposition. Drought caused a significant reduction in spider mite oviposition. We conclude that the effect of previousFusarium-inoculation on spider mite oviposition is primarily due to the fungus affecting the quality of the host plant (including the effect it may have on the composition of defensive compounds), rather than due to the stimulation of the defense system of the plant. SinceFusarium seals off the xylem vessels, thereby causing wilting of susceptible plants, the reduction in mite oviposition may well be due to drought stress in the leaves, rather than due to the production of phytoalexins.     

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.     

Pathogen-activated induced resistance of cucumber: response of arthropod herbivores to systemically protected leaves

Summary Restricted (non-systemic) inoculation of cucurbits, green bean, tobacco, and other plants with certain viruses, bacteria, or fungi has been shown to induce persistent, systemic resistance to a wide range of diseases caused by diverse pathogens. The non-specificity of this response has fueled speculation that it may also affect plant suitability for arthropod herbivores, and there is limited evidence, mainly from work with tobacco, which suggests that this may indeed occur. Young cucumber plants were immunized by restricted infection of a lower leaf with tobacco necrosis virus (TNV), and upper leaves were later challenged with anthracnose fungus, Colletotrichum lagenarium, to confirm induction of systemic resistance to a different pathogen. The response of arthropod herbivores was simultaneously measured on non-infected, systemically protected leaves of the same plants. As has been reported before, immunization with TNV gave a high degree of protection from C. lagenarium, reducing the number of lesions and the area of fungal necrosis by 65–93%. However, there was no systemic effect on population growth of twospotted spider mites, Tetranychus urticae Koch, on upper leaves, nor did restricted TNV infection of leaf tissue on one side of the mid-vein systemically affect mite performance on the opposite, virus-free side of the leaf. Similarly, there were no effects on growth rate, pupal weight, or survival when fall armyworm larvae were reared on systemically protected leaves from induced plants. In free-choice tests, greenhouse whiteflies oviposited indiscriminately on induced and control plants. Feeding preference of fall armyworms was variable, but striped cucumber beetles consistently fed more on induced than on control plants. There was no increase in levels of cucurbitacins, however, in systemically-protected foliage of induced plants. These findings indicate that pathogen-activated induced resistance of cucumber is unlikely to provide significant protection from herbivory. The mechanisms and specificity of induced resistance in cucurbits apparently differ in response to induction by pathogens or herbivores.     

Pest species diversity enhances control of spider mites and whiteflies by a generalist phytoseiid predator

To test the hypothesis that pest species diversity enhances biological pest control with generalist predators, we studied the dynamics of three major pest species on greenhouse cucumber: Western flower thrips, Frankliniella occidentalis (Pergande), greenhouse whitefly, Trialeurodes vaporariorum (Westwood), and two-spotted spider mites, Tetranychus urticae Koch in combination with the predator species Amblyseius swirskii Athias-Henriot. When spider mites infested plants prior to predator release, predatory mites were not capable of controlling spider mite populations in the absence of other pest species. A laboratory experiment showed that predators were hindered by the webbing of spider mites. In a greenhouse experiment, spider mite leaf damage was lower in the presence of thrips and predators than in the presence of whiteflies and predators, but damage was lowest in the presence of thrips, whiteflies and predators. Whitefly control was also improved in the presence of thrips. The lower levels of spider mite leaf damage probably resulted from (1) a strong numerical response of the predator (up to 50 times higher densities) when a second and third pest species were present in addition to spider mites, and (2) from A. swirskii attacking mobile spider mite stages outside or near the edges of the spider mite webbing. Interactions of spider mites with thrips and whiteflies might also result in suppression of spider mites. However, when predators were released prior to spider mite infestations in the absence of other pest species, but with pollen as food for the predators, we found increased suppression of spider mites with increased numbers of predators released, confirming the role of predators in spider mite control. Thus, our study provides evidence that diversity of pest species can enhance biological control through increased predator densities.     

Field assessment of host plant specificity and potential effectiveness of a prospective biological control agent, Aceria salsolae, of Russian thistle, Salsola tragus

The eriophyid mite, Aceria salsolae de Lillo and Sobhian, is being evaluated as a prospective classical biological control agent of invasive alien tumbleweeds, including Salsola tragus, S. collina, S. paulsenii and S. australis, in North America. Previous laboratory experiments to determine the host specificity of the mite indicated that it could sometimes persist and multiply on some nontarget plants, including Bassia hyssopifolia and B. scoparia. These are both European plants whose geographic range overlaps that of the mite, but the mite has never been observed on them in the field. A field experiment was conducted in Italy to determine if the mite would infest and damage these plants under natural outdoor conditions. The results indicate that this mite does not attain significant populations on these nontarget plants nor does it significantly damage them. Salsola tragus was heavily infested by A. salsolae, and plant size was negatively correlated to the level of infestation. Although S. kali plants were also infested, their size did not appear to be affected by the mites. The other nontarget plants were not as suitable for the mite in the field as in previous laboratory experiments. We conclude that there would be no significant risk to nontarget plants as a result of using A. salsolae as a biological agent to control Salsola species in North America.     

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.     

Influence of sorghum maturity on abundance and damage potential of Banks grass mite,Oligonychus pratensis (Banks)

This research was designed to study the effects of sorghum maturity on abundance and damage potential of Banks grass mite,Oligonychus pratensis (Banks). More mites per plant were recorded on the latest maturing line (M100) and the least on the early maturing lines (60M and CK60). Mite densities on B407 (late maturity) did not differ from any of the lines tested. Mite densities were positively correlated with leaf area. In contrast, mite feeding damage was significantly lower on the late maturing lines, demonstrating that sorghum susceptibility to mite feeding increases as plants reach anthesis and caryopsis.     

Factors affecting the distribution of a predatory mite on greenhouse sweet pepper

The predatory mite Neoseiulus cucumeris is used for biological control of phytophagous mites and thrips on greenhouse cucumber and sweet pepper. In a previous study, N. cucumeris provided effective control of broad mite but was only rarely found on the sampled leaves, raising questions about the factors affecting N. cucumeris distribution. To determine the distribution of N. cucumeris, leaves of pepper plants were sampled three times per day: just after sunrise, at noon and just before sunset for two years and throughout a 24 h period in one year. The presence of other mites and insects was recorded. Biotic (pollen) and abiotic (temperature, humidity) factors were monitored from the three plant levels. The effect of direct and indirect sunlight on the mites was assessed. N. cucumeris was found primarily in flowers; however, the mite’s distribution was affected by other predators (intraguild predation); in the presence of the predatory bug Orius laevigatus virtually no mites occurred in the flowers. Whereas temperature and humidity varied from the top to the lower level of the plants, apparently neither these factors nor the presence of pollen outside the flowers influenced mite distribution. N. cucumeris was found to be negatively phototropic; therefore N. cucumeris were pre-conditioned to light by rearing under light conditions for 4 months before being released. The light-reared mites were initially more numerous during the noon sampling period, however, rearing conditions caused only a temporary and non-significant change in distribution.     

Development of biological methods for the control of Tetranychus urticae on tomatoes using Phytoseiulus persimilis

The biological control of red spider mite using the predatory mite Phytoseiulus persimilis was investigated in 1971, 1972 and 1974. Experiments in small glasshouse compartments showed that the predator should be introduced when the leaf damage index is < 0–3. Uniform and/or patch introductions of P. persimilis at different rates were made into naturally occurring red spider mite infestations on commercial nurseries. In eleven of the seventeen experiments good control was achieved. Introduction of the predator soon after damage appeared on the crop was essential. Poor control was obtained when the predator failed to establish itself, where very large numbers of diapausing mites emerged and built up rapidly or where the predator, introduced into patches, failed to colonize infested plants elsewhere in the crop. When spider mites and predators were introduced on to one-fifth or one-tenth of the plants in a propagating house, a satisfactory interaction was maintained for 4–6 wk after planting out. The predators then died unless red spider mites emerged from diapause or were introduced. Petroleum oil sprays were sometimes used successfully in the presence of the predator to reduce high red spider mite infestations and re-establish the biological equilibrium.     

<Emphasis Type="Italic">Bacillus thuringiensis</Emphasis> var. <Emphasis Type="Italic">tenebrionis</Emphasis> control of synanthropic mites (Acari: Acaridida) under laboratory conditions

Bacillus thuringiensis (Bt) toxins present a potential for control of pest mites. Information concerning the effect of Bt and its possible application to the biocontrol of synathropic mites is rare. The toxic effect of Bacillus thuringiensis var. tenebrionis producing Cry3A toxin was tested on the mites Acarus siro L., Tyrophagus putrescentiae (Schrank), Dermatophagoides farinae Hughes, and Lepidoglyphus destructor (Schrank) via feeding tests. Fifty mites were reared on Bt additive diets in concentrations that ranged from 0 to 100 mg g⁻¹ under optimal conditions for their development. After 21 days, the mites were counted and the final populations were analyzed using a polynomial regression model. The Bt diet suppressed population growth of the four mite species. The fitted doses of Bt for 50% suppression of population growth were diets ranging from 25 to 38 mg g⁻¹. There were no remarkable differences among species. Possible applications of Bt for the control of synanthropic mites are discussed.     

Mycorrhiza modulates aboveground tri‐trophic interactions to the fitness benefit of its host plant

1. Arbuscular mycorrhiza (AM), the association of AM fungi and plant roots, may alter morphological and physiological attributes of aboveground plant parts and thereby influence plant‐associated organisms such as herbivores and their natural enemies, predators and parasitoids. 2. The interactions between AM and the players of aboveground tri‐trophic systems have mainly been considered in isolation from each other. The effects of AM on aboveground herbivore–carnivore population dynamics and the consequences to plant fitness are unknown. 3. We explored AM‐induced compensatory mechanisms for AM‐promoted proliferation of the herbivorous spider mite, Tetranychus urticae Koch, on whole bean plants, Phaseolus vulgaris L. Vegetative and reproductive plant growth, AM fungal colonisation levels, and mite densities were assessed on spider mite‐infested plants colonised or not by the AM fungus Glomus mosseae Nicol. & Gerd, and harbouring the natural enemy of the spider mites, the predatory mite Phytoseiulus persimilis Anthias‐Henriot or not. 4. AM symbiosis modulated the aboveground tri‐trophic system to the fitness benefit of the plant. AM‐increased plant productivity outweighed the fitness decrease due to AM‐promoted herbivory: at similar vegetative growth, mycorrhizal plants produced more seeds than non‐mycorrhizal plants. 5. AM‐increased spider mite population levels were compensated for by enhanced population growth of the predators and increased plant tolerance to herbivory. 6. AM‐enhanced predator performance looped back to the AM fungus and stabilised its root colonisation levels, providing the first experimental evidence of a mutually beneficial interaction between AM and an aboveground third trophic level natural enemy.     

Impact and management of the eriophyoid mite Trisetacus juniperinus on the evergreen cypress Cupressus sempervirens

Abstract 1 In the Mediterranean region, the eriophyoid mite Trisetacus juniperinus causes considerable damage to the evergreen cypress, Cupressus sempervirens L., particularly in nurseries and young stands, disturbing the apical growth of the tree. 2 The impact of mites on survival and apical growth of two commercial clones of cypress (Agrimed and Bolgheri), as well as the results of differently timed pesticide applications to suppress mite population on newly grafted trees, were evaluated. 3 Mites easily infested clonal scions from rootstocks that were previously infested in the nursery, inducing tip deformation and disturbance of the growth. Apical growth was significantly lower in infested than in control trees 2 years following the graft. 4 Deformed apical buds were left early by mites, which dispersed in the crown and may have incurred high mortality. This is interpreted as a defensive reaction of the cypress to the mite attack, which involves costs resulting in reduced apical growth in both clones. However, mites partly overcame tree defences in the Bolgheri clone. 5 Healthy rootstock and graft material should be used to limit damage and maintain plant growth because natural infestations rarely occur. In case of attack, a pesticide should be applied as soon as possible because precocious mite infestation has a log‐lasting effect on tree growth, with considerable economic damage.     

Antixenotic resistance of subterranean clover cotyledons to redlegged earth mite, Halotydeus destructor

The short-term response of redlegged earth mite, Halotydeus destructor (Tucker) (Acarina: Penthaleidae) to cotyledons of different varieties of subterranean clover (Trifolium subterraneum subsp. subterraneum L.) was assessed by means of paired choice tests, and no-choice tests. H. destructor had lower numbers and fed less on detached cotyledons of resistant than susceptible varieties, yielding a correlation between the numbers of mites and feeding damage to the cotyledons during a three hour test period. For a number of resistant and susceptible varieties, there was a negative correlation between cotyledon deterrence in the three hour choice test and feeding damage to seedling after a two week period. Since the response of the mites to different subclover varieties occurred within three hours, it is concluded that the resistance is based on antixenosis.No evidence was obtained for antifeedant activity in organic solvent extracts from the variety DGI007 (resistant) in comparison with those from the variety Dalkeith (susceptible). Water soluble compounds from DGI007 cotyledons were preferred by mites, in feeding tests in terms of numbers, over those from Dalkeith (susceptible). Squeezed sap from the cotyledons of both varieties showed the same effects on mites as 5% glucose and were more phagostimulatory than water extracts. Mechanically damaged cotyledons of Dalkeith and DGI007 attracted more mites than the undamaged counterparts. The toughness of cotyledons in 17 varieties of T. subterraneum subsp. subterraneum was measured with a manual penetrometer. Results showed a negative correlation between toughness values and mite feeding damage scores (r²=0.752) for all varieties except S3615D (resistant). This implies a likely involvement of epidermal toughness as a contributor in the antixenotic resistance of these varieties. Other mechanisms may be involved in the resistance of S3615D.     

Evaluation of a damage threshold for two-spotted spider mites, Tetranychus urticae Koch (Acari: Tetranychidae), in hop culture

Damage caused by two‐spotted spider mites (Tetranychus urticae) at harvest to yield, quality (measured in percentage α‐acids content) and cone infestation was assessed on hop cvs Hallertauer Magnum, Hallertauer Tradition and Perle. Acaricide‐untreated hop plants with known levels of T. urticae infestation were compared with neighbouring acaricide‐treated plants. Although in 24 of the 36 experimental harvests the untreated hop plants had spider mite infestations of > 100 mites leaf^?1, yields and α‐acids content from the untreated plants were significantly lower than the treated plants in only four instances. However, although mite infestation of cones from untreated hops were significantly higher than acaricide‐treated plants in 27 of the 36 cases, in only one instance did that cause economic loss. Spider mite infestation levels of c. 90 mites leaf^?1 are tolerable at harvest time with little or no risk of causing economic loss to hop growers.     

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.