Interactions among phytophagous mites, and introduced and naturally occurring predatory mites, on strawberry in the UK

In choice test experiments on strawberry leaf disc arenas the phytoseiid mites Neoseiulus californicus and N. cucumeris were more effective than Typhlodromus pyri as predators of the phytophagous mites Tetranychus urticae and Phytonemus pallidus. There were no preferences shown for either prey by any of these predators. In multiple predator leaf disc experiments both Phytoseiulus persimilis and N. cucumeris significantly reduced numbers of T. urticae eggs and active stages; this effect was seen when the two species were present alone or in combination with other predator species. Neoseiulus californicus was less effective at reducing T. urticae numbers, and T. pyri was not effective; no interaction between predator species was detected in these experiments. When T. urticae alone was present as prey on potted plants, P. persimilis and N. californicus were the only phytoseiids to significantly reduce T. urticae numbers. These two predator species provided effective control of T. urticae when P. pallidus was also present; however, none of the predators reduced numbers of P. pallidus. There were no significant negative interactions when different species of predators were present together on these potted plants. In field experiments, releases of both P. persimilis and N. cucumeris significantly reduced T. urticae numbers. However, there was a significant interaction between these predator species, leading to poorer control of T. urticae when both species were released together. These results show the importance of conducting predator/prey feeding tests at different spatial scales.     

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.     

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.     

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.     

Life history of the predatory mite <Emphasis Type="Italic">Phytoseiulus fragariae</Emphasis> on <Emphasis Type="Italic">Tetranychus evansi</Emphasis> and <Emphasis Type="Italic">Tetranychus urticae</Emphasis> (Acari: Phytoseiidae,Tetranychidae) at five temperatures

Tetranychus evansi Baker and Pritchard and Tetranychus urticae Koch (Acari: Tetranychidae) are important pests of Solanaceae in many countries. Several studies have demonstrated that T. urticae is an acceptable prey to many predatory mites, although the suitability of this prey depends on the host plant. T. evansi, has been shown to be an unfavorable prey to most predatory mites that have been tested against it. The predator Phytoseiulus fragariae Denmark and Schicha (Acari: Phytoseiidae) has been found in association with the two species in Brazil. The objective of this work was to compare biological parameters of P. fragariae on T. evansi and on T. urticae as prey. The study was conducted under laboratory conditions at 10, 15, 20, 25 and 30°C. At all temperatures, survivorship was lower on T. evansi than on T. urticae. No predator reached adulthood at 10°C on the former species; even on the latter species, only about 36% of the predators reached adulthood at 10°C. For both prey, in general, duration of each life stage was shorter, total fecundity was lower and intrinsic rate of population increase (r _m ) was higher with increasing temperatures. The slower rate of development of P. fragariae on T. evansi resulted in a slightly higher thermal requirement (103.9 degree-days) on that prey than on T. urticae (97.1 degree-days). The values of net reproduction rate (R ₀), intrinsic rate of increase (r _m ) and finite rate of increase (λ) were significantly higher on T. urticae, indicating faster population increase of the predator on this prey species. The highest value of r _m of the predator was 0.154 and 0.337 female per female per day on T. evansi and on T. urticae, respectively. The results suggested that P. fragariae cannot be considered a good predator of T. evansi.     

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.     

Intraspecific competition in immature Amblyseius fallacis,Amblyseius andersoni,Typhlodromus occidentalis and Typhlodromus pyri (Acari: Phytoseiidae)

Intraspecific competition in immature Amblyseius fallacis, Amblyseius andersoni, Typhlodromus occidentalis and Typhlodromus pyri was examined in the laboratory using small cages at five different predator densities (two, four, eight, 16 and 32) in the absence and presence of prey 100 eggs of two-spotted spider mite, Tetranychus urticae (Koch), at 25 ± 1°C, 80% RH and 16L:8D photoperiod. In the absence of spider mite prey, some individuals of immature phytoseiids showed increased development and surival with increasing predator densities up to certain limits, but none survived to the adult stage, except for a single male each of A. andersoni and A. fallacis who completed development by cannibalizing on conspecifics at a density of 32 predators per cage. In the absence of spider mite prey, the mean immature survival time was independent of the initial predator density, but the variance of survival time increased with predator density. In the presence of prey, the proportion of immatures surviving to adulthood generally decreased with initial predator density and dropped sharply to almost none at the predator density of 32 for A. fallacis, eight for A. andersoni, 16 for T. occidentalis and four for T. pyri. The number of prey consumed per predator during the first day generally decreased with predator density in all four species, as prey available per predator decreased and the competition for food increased with predator density. Our data indicate that scramble competition is operating in these four species. Although cannibalism was occasionally observed, especially after the exhaustion of prey and in the generalist predators such as A. andersoni, the immatures of these phytoseiids were less influenced by the interference of conspecifics than by the increasing difficulty of finding food at high predator densities. The implications of this study for understanding phytoseiid population dynamics and their use in biological control are discussed.     

Effects of feeding frequency and sugar concentration on behavior and longevity of the adult aphid parasitoid: Aphidius ervi (Haliday) (Hymenoptera: Braconidae)

The effectiveness of the predatory mite, Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseidae), as a suppressive agent of the twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), was evaluated on greenhouse ivy geraniums at predator:prey release ratios of 1:60, 1:20, and 1:4. Releases at each predator:prey ratio were made at moderate and high T. urticae densities to determine if initial pest population size influenced the suppressive ability of the predator. At ratios of 1:4 and 1:20, P. persimilis significantly reduced T. urticae populations 1 week after release and kept them at low levels thereafter. Plant damage also was significantly reduced at these densities. After 4 weeks, the P. persimilis that were released at a ratio of 1:4 consistently reduced T. urticae populations from densities as high as 30 T. urticae per leaf to fewer than 0.6 per leaf. We found no interaction between release ratio and T. urticae density, indicating that predator effectiveness remains constant, at least within the range of T. urticae densities used. Our work demonstrates the potential of P. persimilis to provide effective control of T. urticae on a greenhouse-grown floricultural crop at a moderately low predator:prey ratio (1:20) and over a range of initial pest densities. However, we recommend that P. persimilis be released at a ratio of 1:4 for greatest reliability and successful control of T. urticae on ivy geraniums.     

Factors affecting biological control of apple mites by mixed populations ofMetaseiulus occidentalis andTyphlodromus pyri

Metaseiulus occidentalis (Nesbitt) andTyphlodromus pyri Scheuten have complementary features/ traits that enable them to control effectively plant-feeding mites on apple. Populations of both predators gave as good or better biological control of the apple rust mite (Aculus schlechtendali Nalepa), European red mite (Panonychus ulmi Koch) and two-spotted spider mite (Tetranychus urticae (Koch)) than single-predator populations. With mixed predators,M. occidentalis provided better control of spider mites the first season after release, butT. pyri gave better control in the second season. Several factors affected the ability of predators to provide biological control: When prey were dense,M. occidentalis rapidly increased during the warm mid-summer, whileT. pyri provided greater predation when it was cool at the start or end of the growing season. When few prey were present, searching byM. occidentalis was more confined on individual apple leaves, but it migrated between leaves and trees more often. Pollen feeding, cannibalism and interspecific predation were more common byT. pyri. In fall, oviposition byM. occidentalis stopped sooner and in the following spring,T. pyri reproduced beforeM. occidentalis. Research needs and management of mixed-predator populations are discussed.     

Studies on the basic components of the predation ofPhytoseiulus persimilis Athias-Henriot (Acarina: Phytoseiidae)

The basic components of the predation of Phytoseiulus persimilisAthias-Henriot feeding upon eggs of Tetranychus urticaeKoch were studied in an open system where the predator could disperse freely. The type of the functional response of the predator to the density of its prey was the same as that studied so far in a closed system, i.e.,Holling's Type 2. The search rate of the predator, however, was much lower in comparison with the result from a closed system. The oviposition of the predator per day was only weakly related to prey densities higher than 10 per leaf disc. But the emigration rate was inversely dependent upon the initial prey density up to 60 per leaf disc. The searching behaviour of the predator was influenced by both the web density spun by T. urticae and the density of the prey: the predator searched for its prey intensively only after it had contact with web. Mutual interference was observed in prey consumption, but not in the emigration rate. The emigration rate was largely dependent upon the prey density available per predator.     

Interactions among predators and phytophagous mites on apple; possible impact on biocontrol of Panonychus ulmi by Typhlodromus pyri in orchards

Laboratory experiments were conducted to determine the potential impact of the phytoseiid Euseius finlandicus, the mirid Blepharidopterus angulatus and the anthocorid Orius majusculus on the Typhlodromus pyri/Panonychus ulmi predator/prey relationship on apple. Euseius finlandicus consumed more immature spider mites than did T. pyri. When both phytoseiids were present and spider mite prey was abundant, there was no evidence of a negative interaction between the predators. In experiments where each predatory mite was confined with large numbers of the other predator, interspecific predation was exhibited by adults of each species on immatures of the other, but more so by E. finlandicus. In the predatory insect/phytoseiid experiments, when confined with spider mites and large numbers of T. pyri, both B. angulatus and O. majusculus consumed some T. pyri, but spider mites were the preferred prey. In experiments with B. angulatus, O. majusculus and T. pyri feeding on P. ulmi, there was no evidence of negative interactions between the predatory insects and T. pyri.     

Effect of the width of the herbicide strip on mite dynamics in apple orchards

Herbicide strips are used in apple orchards to promote tree growth and survival, to increase yield and to reduce the risk of rodent damage to tree bark. However, herbicide strips, particularly wider ones, may cause problems including soil erosion, reduced organic matter, leaching of nitrates into ground water and increased incidence of plant diseases and pests, including two-spotted spider mites, Tetranychus urticae Koch. In this 2 year study we monitored mite dynamics in apple trees and used sticky bands on tree trunks to determine rates of T. urticae immigration into Nova Spy apple trees in plots with wide (2 m) or narrow (0.5 m) herbicide strips. Use of wider herbicide strips promoted two risk factors that could trigger outbreaks of tetranychid mites. First, concentrations of leaf N in apple trees were higher and those of P and K were lower with the wide strips. Such changes in nutritional quality of leaves would increase the potential for more rapid population growth of T. urticae, and to a lesser extent, the European red mite, Panonychus ulmi (Koch). Second, there were higher rates of T. urticae immigration from the ground cover vegetation into the trees. In 2006, and for most of 2007, densities of T. urticae were higher with wide herbicide strips, whereas densities of P. ulmi were not enhanced. However, by late August to early September in 2007, densities of both tetranychids were lower with wide herbicide strips. This is because both risk factors were counterbalanced, and eventually negated, by the enhanced action of phytoseiid predators, mostly Typhlodromus pyri Scheuten. From July through September 2006, ratios of phytoseiids to tetranychids were always several-fold lower with wide herbicide strips but in 2007, from mid-July onwards, predator–prey ratios were usually several-fold higher with wide strips. However, this numerical response of phytoseiids to prey density can only occur where the pesticide program in orchards is not too harsh on phytoseiids. Hence the impact of width of herbicide strip is contingent on the composition and size of the phytoseiid complex and the impact of pesticides on predation.     

Antipredator responses in Tetranychus urticae differ with predator specialization

The behavioural response of Tetranychus urticae to chemical cues from specialist predatory mites, Phytoseiulus persimilis, or generalist predatory bugs, Orius majusculus, on either bean or strawberry was studied in experimental arenas. Predators were placed on the leaf disc for 24 h and removed before T. urticae females were introduced. After 24 h, prey fecundity (number of eggs laid) and dispersal (number of prey drowned in the water barrier) were assessed. Chemical cues from the specialist predator resulted in reduced prey fecundity, significantly different from the generalist predator and control treatments. No interaction effect was found between plant species and prey fecundity, while significantly more eggs were laid on bean than on strawberry. Predator cues irrespective of predator specialization resulted in more prey dispersal than in the control. Findings emphasize the importance of specialization in the predator species complex for the degree and type of antipredator responses and resulting biological control.     

Dynamics ofPanonychus ulmi andTyphlodromus pyri: factors contributing to persistence

We addressed the question of persistence of predator and prey in a biological control system by examining temporal patterns ofPanonychus ulmi (Koch) and its predator,Typhlodromus pyri Scheuten at two geographic locations and at two spatial scales. At the scale of an orchard, bothP. ulmi andT. pyri were persistent over the time frame of 6 years. At the scale of an individual tree,T. pyri appeared to be more persistent than its prey,P. ulmi. We used a simulation model of single populations ofP. ulmi andT. pyri to determine which of several aspects of the biology of the two species could contribute to such a pattern. Spatial incongruity between predator and prey was essential for persistence of both species. The generalist food habit ofT. pyri probably contributes to the persistence ofT. pyri on individual trees, and may cause occasional extinction ofP. ulmi at this spatial scale. The presence of alternate food is likely an essential element for successful biological control in this system. Cannibalism byT. pyri results in higher prey densities, that is, it is detrimental to the biological control ofP. ulmi, but has no effect on the relative persistence of the two species.     

Application of Sequential Classification of Prey/Predator Ratio Test to Tetranychus urticae and Phytoseiulus persimilis System in Greenhouse Roses

A sequential sampling procedure for classifying the ratio of prey/predators developed by Nyrop (1988) was examined to implement for a biological control in the greenhouse roses. The procedure was combined with a sequential density classification procedure for use in monitoring a phytophagous mite, Tetranychus urticae Koch, and a phytoseiid predator, Phytoseiulus persimilis Athias-Henriot. This procedure was required four parameters: Means and variances for T. urticae and P. persimilis, correlation coefficient between densities of prey and predator and critical ratio of prey and predator. The parameter values used in this study were 0.725 for the correlation coefficient and 10 for the critical ratio. The variances for each species were estimated using a Taylor's power law model. The procedure is proven to be successfully applicable to T. urticae and P. persimilis system in greenhouse roses at two action threshold levels of 5 and 10 T. urticae per three-leaflet leave. The limitations and implementation of this procedure is also discussed.     

Some effects of pre-release host-plant on the biological control of Panonychus ulmi by the predatory mite Amblyseius fallacis

Amblyseius fallacis Garman has been selected for pyrethroid resistance and mass reared for experimental release as a biological control agent for tetranychid mites on a number of crops in Canada. Several releases of this predator onto apple and peach trees have failed to result in the establishment of A. fallacis, or in the biological control of Panonychus ulmi Koch. Here, we test the hypothesis that the change of host-plant at the time of release is a critical factor in the establishment of A. fallacis for biological control of P. ulmi. Functional and numerical response studies were undertaken on two populations of A. fallacis: a wild strain collected from the canopy foliage of an apple orchard near Vineland, Ontario; and a second strain reared on bean plants in a commercial insectary with Tetranychus urticae as prey. Each population consumed significantly more P. ulmi and produced significantly more eggs when on leaf disks from the plant species they were reared on, than on leaf disks from the novel host plant. A further experiment was conducted to determine if establishment and biological control of mass-reared A. fallacis could be affected by rearing a population for a short term on apple leaves prior to release on apple trees. Three release treatments were made into potted apple trees in a glasshouse, using predators commercially mass-reared on bean and T. urticae: A. fallacis released directly; A. fallacis reared in the laboratory for four weeks on bean and T. urticae; A. fallacis reared on apple leaves and T. urticae for four weeks. They were compared with a control treatment lacking predator release. Contrary to results of the functional and numerical response studies, no difference was observed between release treatments. All release treatments adding A. fallacis resulted in a similar, if limited, degree of biological control of P. ulmi. These results indicated that there may be short-term effects of host plant on the establishment of A. fallacis and biological control of P. ulmi, which in our study were observed as an initial reduction of the predatory response. However, in a test, the predators appeared to overcome these short-term effects and successfully established on the new host-plant to control P. ulmi.     

Prey preference of the phytoseiid miteTyphlodromus pyri 1. Response to volatile kairomones

Using a Y-tube olfactometer, a study has been made of the response of females of the predatory miteTyphlodromus pyri Scheuten (Acarina: Phytoseiidae) to volatile kairomones of three prey species: the European red spider mite (Panonychus ulmi (Koch)), the two-spotted spider mite (Tetranychus urticae Koch) and the apple rust mite (Aculus schlechtendali (Nalepa)).Predators that had been reared onT. urticae responded only to the volatile kairomone ofP. ulmi. In contrast, when reared onVicia faba L. pollen, they responded to the kairomones of all three prey species. Pollen-reared predators, offered a choice between kairomones of two different prey species, prefer theP. ulmi kairomone to those ofA. schlechtendali orT. urticae.The difference in response between predators reared onV. faba pollen andT. urticae seems to be caused by the low carotenoid content ofV. faba pollen. Predators that had been reared onV. faba pollen mixed with crystalline -carotene behaved in a way similar to conspecific that had been reared on the carotenoid-rich prey miteT. urticae. Obviously, pollen-rearedT. pyri females are in need of carotenoids, which can be obtained from, e.g.,P. ulmi, T. urticae orA. schlechtendali. This may explain why pollen-reared predators respond to more prey species thanT. urticae-reared predators.WhyT. pyri females need carotenoids has not been established. The only known function of carotenoids in mites is involvement in diapause induction. However, as pollen-rearedT. pyri enter reproductive diapause under short-day conditions, they either extract sufficient amounts of carotenoids fromV. faba pollen, or do not need carotenoids for diapause induction.Apart from the effect of dietary requirements on prey selection, food deprivation also affects the predator's response to kairomones. All the data mentioned above have been obtained for predators that had been starved for 20 h. Predators that had been reared onT. urticae and starved for 48 h before the experiment did respond to the volatile kairomone ofT. urticae in contrast to predators from the same culture that had been starved for 20 h. Thus foraging decisions byT. pyri are affected by both starvation time and specific hunger for carotenoids.     

Characteristics of development and reproduction in Typhlodromus pyri on Tetranychus urticae and Cecidophyopsis ribis. I. Overwintered females

The results of a study of bionomic parameters in overwintered females of Typhlodromus pyri Scheuten under constant laboratory conditions (18±1°C, RH 75%, 18L:6D) are described. The predatory mites were fed two prey species, the two-spotted spider mite, Tetranychus urticae Koch and the eriophyid gall mite, Cecidophyopsis ribis (Westwood). The results indicate that T. pyri females survive and reproduce on C. ribis as well as on T. urticae. However, the average fecundity of females feeding on C. ribis was lower (13.79 eggs) compared with that of females fed T. urticae (29 eggs). The length of preoviposition period varied greatly (8 to 96 days), the number of ovipositing females increasing at a statistically significant rate on T. urticae. Differences in the mean duration of postoviposition period (35.27 and 52.50 days) and longevity in the laboratory (97.82 and 116.04 days) were not statistically significant. Accumulation of metabolites in the alimentary canal was noted in some females of T. pyri.     

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.     

Distribution of the Predatory Mite Typhlodromus pyri (Acari: Phytoseiidae) on Different Apple Cultivars

The effect of apple cultivar on the distribution of the predatory mite Typhlodromus pyri was studied in an experimental orchard where spider mites occurred at negligible densities. Seven apple scab-resistant cultivars, showing some differences in their leaf morphology, were considered. In particular, their leaf blade was classified according to four levels of pubescence. The distribution of T. pyri along the shoots was also studied. In the first experimental year the colonization of different cultivars by T. pyri showed definite patterns, sometimes influenced by the occurrence of eriophyids. One year later, T. pyri abundance again showed some differences among cultivars in conditions of prey scarcity. In both years large phytoseiid populations were recorded on the cultivar N.Y. 18491, despite the low eriophyid occurrence, probably because of its highly pubescent leaf undersurfaces. In contrast, Prima and TSR 29T219, characterized by slightly pubescent leaf undersurfaces, supported low phytoseiid densities independently of prey availability.