Biological, Chemical and Integrated Control of Two-spotted Spider Mite Tetranychus urticae on Dwarf Hops

Chemical, biological and integrated programmes for the control of two-spotted spider mite, Tetranychus urticae, were compared on dwarf hops in 1997 and 1998. In both years integrated control, which consisted of an application of the ovicidal acaricide clofentezine followed by a release of the predatory mite Phytoseiulus persimilis at 10 individuals per plant, was the most effective treatment. Similar numbers of spider mites were recorded on plots that were treated with P. persimilis only or with a single application of the acaricide tebufenpyrad, and plots with either of these treatments had higher levels of infestation than the plots under integrated control. The highest numbers of spider mites were found on the untreated plots. This study indicates that integrated control of T. urticae using clofentezine in conjunction with P. persimilis is likely to be more effective than an approach based on chemical or biological measures only. It is suggested that an integrated system would have the added benefit over a pesticide-only programme of reducing pressure on the pest for the selection of strains resistant to acaricides.     

Diseases of Mites

An overview is given of studies on diseases of mites. Knowledge of diseases of mites is still fragmentary but in recent years more attention has been paid to acaropathogens, often because of the economic importance of many mite species. Most research on mite pathogens concerns studies on fungal pathogens of eriophyoids and spider mites especially. These fungi often play an important role in the regulation of natural mite populations and are sometimes able to decimate populations of phytophagous mites. Studies are being conducted to develop some of these fungi as commercial acaricides.Virus diseases are known in only a few mites, namely, the citrus red mite and the European red mite. In both cases, non-occluded viruses play an important role in the regulation of mite populations in citrus and peach orchards, respectively, but application of these viruses as biological control agents does not seem feasible. A putative iridovirus has been observed in association with Varroa mites in moribund honeybee colonies. The virus is probably also pathogenic for honeybees and may be transmitted to them through this parasitic mite.Few bacteria have been reported as pathogens of the Acari but in recent years research has been concentrated on intracellular organisms such as Wolbachia that may cause distorted sex ratios in offspring and incompatibility between populations. The role of these organisms in natural populations of spider mites is in particular discussed. The effect of Bacillus thuringiensis on mites is also treated in this review, although its mode of action in arthropods is mainly due to the presence of toxins and it is, therefore, not considered to be a pathogen in the true sense of the word.Microsporidia have been observed in several mite species especially in oribatid mites, although other groups of mites may also be affected. In recent years, Microsporidia infections in Phytoseiidae have received considerable attention, as they are often found in mass rearings of beneficial arthropods. They affect the efficacy of these predators as biological control agent of insect and mite pests. Microsporidia do not seem to have potential for biological control of mites.     

Biological Control of Tetranychus Urticae (Acari: Tetranychidae) With Naturally Occurring Predators in Strawberry Plantings in Valenica, Spain

Naturally occurring beneficials, such as the phytoseiid mite Amblyseius californicus McGregor and the insects Stethorus punctillum Weise, Conwentzia psociformis (Curtis) and others, controlled Tetranychus urticae Koch in 11 strawberry plots near Valencia, Spain, during 1989–1992. The population levels of spider mites in 17 subplots under biological control were low or moderate, usually below 3000 mite days and similar to seven subplots with chemical control. In most of the crops A. californicus was the main predator, acting either alone or together with other beneficials. Predaceous insects colonized the crop when tetranychids reached medium to high levels. For levels above one spider mite per leaflet, a ratio of one A. californicus per five to ten T. urticae resulted in a decline of the prey population in the following sample (1–2 weeks later). These results suggest that naturally occurring predators are able to control spider mites and maintain them below damaging levels in strawberry crops from the Valencia area.     

Attraction of Phytoseiulus persimilis (Acari: Phytoseiidae) towards volatiles from various Tetranychus urticae-infested plant species

Plants infested with the spider mite Tetranychus urticae Koch, may indirectly defend themselves by releasing volatiles that attract the predatory mite Phytoseiulus persimilis Athias-Henriot. Several plants from different plant families that varied in the level of spider mite acceptance were tested in an olfactometer. The predatory mites were significantly attracted to the spider mite-infested leaves of all test plant species. No differences in attractiveness of the infested plant leaves were found for predatory mites reared on spider mites on the different test plants or on lima bean. Thus, experience with the spider mite-induced plant volatiles did not affect the predatory mites. Jasmonic acid was applied to ginkgo leaves to induce a mimic of a spider mite-induced volatile blend, because the spider mites did not survive when incubated on ginkgo. The volatile blend induced in ginkgo by jasmonic acid was slightly attractive to predatory mites. Plants with a high degree of direct defence were thought to invest less in indirect defence than plants with a low degree of direct defence. However, plants that had a strong direct defence such as ginkgo and sweet pepper, did emit induced volatiles that attracted the predatory mite. This indicates that a combination of direct and indirect defence is to some extent compatible in plant species.     

Fungal Biocontrol of Acari

Mites and ticks are susceptible to pathogenic fungi, and there are opportunities to exploit these micro-organisms for biological control. We have collated records of 58 species of fungi infecting at least 73 species of Acari, either naturally or in experiments. Fungal pathogens have been reported to kill representatives of all three orders of the Actinotrichida (the Astigmata, Oribatida and Prostigmata) and the Ixodida and Mesostigmata in the Anactinotrichida. Most reports concern infections in the Prostigmata, particularly in the families Tetranychidae and Eriophyidae. Two species of Acari-specific pathogens - Hirsutella thompsonii and Neozygites floridana - are important natural regulators of pestiferous eriophyoid and tetranychid mites respectively. Research has been done to understand the factors leading to epizootics of these fungi and to conserve and enhance natural pest control. Hirsutella thompsonii was also developed as the commercial product Mycar for the control of eriophyoid mites on citrus, but was withdrawn from sale in the 1980s, despite some promising effects in the field. Beauveria bassiana , Metarhizium anisopliae, Paecilomyces farinosus, Paecilomyces fumosoroseus and Verticillium lecanii infect ixodid ticks in nature, and B. bassiana and M. anisopliae are being studied as biological control agents of cattle ticks in Africa and South America. Beauveria bassiana also has potential as a mycopesticide of the two-spotted spider mite, Tetranychus urticae . There is scope to develop fungal biocontrol agents against a range of acarine pests, both as stand-alone treatments and for use in integrated pest management. Further research is required to clarify the taxonomic status of fungal pathogens of Acari, to study their ecosystem function, and to develop efficient mass production systems for species of Hirsutella and Neozygites .     

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

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

Population‐specific effect of Wolbachia on the cost of fungal infection in spider mites

Many studies have revealed the ability of the endosymbiotic bacterium Wolbachia to protect its arthropod hosts against diverse pathogens. However, as Wolbachia may also increase the susceptibility of its host to infection, predicting the outcome of a particular Wolbachia‐host–pathogen interaction remains elusive. Yet, understanding such interactions and their eco‐evolutionary consequences is crucial for disease and pest control strategies. Moreover, how natural Wolbachia infections affect artificially introduced pathogens for biocontrol has never been studied. Tetranychus urticae spider mites are herbivorous crop pests, causing severe damage on numerous economically important crops. Due to the rapid evolution of pesticide resistance, biological control strategies using entomopathogenic fungi are being developed. However, although spider mites are infected with various Wolbachia strains worldwide, whether this endosymbiont protects them from fungi is as yet unknown. Here, we compared the survival of two populations, treated with antibiotics or naturally harboring different Wolbachia strains, after exposure to the fungal biocontrol agents Metarhizium brunneum and Beauveria bassiana. To control for potential effects of the bacterial community of spider mites, we also compared the susceptibility of two populations naturally uninfected by Wolbachia, treated with antibiotics or not. In one population, Wolbachia‐infected mites had a better survival than uninfected ones in absence of fungi but not in their presence, whereas in the other population Wolbachia increased the mortality induced by B. bassiana. In one naturally Wolbachia‐uninfected population, the antibiotic treatment increased the susceptibility of spider mites to M. brunneum, but it had no effect in the other treatments. These results suggest that natural Wolbachia infections may not hamper and may even improve the success of biological control using entomopathogenic fungi. However, they also draw caution on the generalization of such effects, given the complexity of within‐host–pathogens interaction and the potential eco‐evolutionary consequences of the use of biocontrol agents for Wolbachia‐host associations.     

Differences among plant species in acceptance by the spider mite Tetranychus urticae Koch

Abstract:  The spider mite Tetranychus urticae Koch has a broad range of host plants. However, the spider mite does not accept all plants to the same degree because of differences in nutritive and toxic constituents. Other factors, such as the induction of secondary metabolites, the morphology of a leaf surface and the presence of natural enemies, also play an important role in plant acceptance. We compared plants from various families in their degree of acceptance by the spider mite, to get an indication of the plant's direct defence. Glycine max (soybean), Humulus lupulus (hop), Laburnum anagyroides (golden chain) and Nicotiana tabacum (tobacco) were highly accepted by the spider mites. Different glandular hair densities among tobacco cultivars did not affect their suitability towards spider mites significantly. Solanum melalonga (eggplant), Robinia pseudo-acacia (black locust), Vigna unguiculata (cowpea) and Datura stramonium (thorn apple) were accepted by the spider mites to a lesser degree. Vitis vinifera (grapevine) was poorly accepted by the spider mite. It might be that the food quality of the leaves was not high enough to arrest the spider mites. Also, Capsicum annuum (sweet pepper) and especially Ginkgo biloba (ginkgo) were poorly accepted by the spider mite, probably because of the presence and concentration of certain of the secondary metabolites in the leaves. The spider mites accepted all the plants belonging to the Fabaceae for feeding, but those belonging to the Solanaceae showed a larger variance in spider mite acceptance varying from well accepted (tobacco) to poorly accepted (sweet pepper).     

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.     

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.     

Biological control of the two-spotted spider mite Tetranychus urticae on strawberries by the predatory phytoseiid mite Typhlodromus pyri (Acari, Tetranychidae, Phytoseiidae)

The ‘Mikulov’ strain of the predatory mite Typhlodromus pyri Scheuten from south Moravian vineyards was released on cultivated strawberries infested with the two-spotted spider mite, Tetranychus urticae Koch. The strawberries were grown in field plantations and under glass. Typhlodromus pyri on vine shoots were successfully introduced into the field strawberry plantation but they produced no demonstrable control of the spider mites and they eventually declined in density with their prey. In contrast, T. pyri gave good control of spider mites in the glasshouse despite the occurrence of low humidity and water stress of the plants.     

Phytoseiid mites on peppermint and effectiveness of Neoseiulus fallacis to control Tetranychus urticae (Acari: Phytoseiidae, Tetranychidae) in arid growing regions

The humid-adapted species Neoseiulus fallacis (German) was the most common phytoseiid mite collected in either humid (> 100 cm annual rainfall) or arid (20-45 cm annual rainfall) mint growing regions of Washington, Oregon, Montana, Idaho, and California during 1991-1995. In experimental field plots, this predator gave excellent biological control of Tetranychus urticae Koch on mint grown under arid conditions in central Oregon when evaluated by an insecticide check method or by the caging of mites. N. fallacis is effective as a predator in arid areas probably because regular irrigation creates a humid environment in the canopy. The selective miticide propargite, when used in combination with predators, was effective at reducing high spider mite populations to below the treatment threshold faster than did N. fallacis alone.     

Amblyseius andersoni Chant (Acari: Phytoseiidae), a successful predatory mite on Rosa spp

Roses on commercial nurseries commonly suffer from attacks by the two-spotted spider mite, Tetranychus urticae, which have a negative influence on growth and quality. The aim of this project is to find natural enemies that are well adapted to roses, and may improve biological control. At different sites such as a plant collection garden, public parks and field boundaries, leaves were sampled from roses to identify the indigenous species of predatory mites. Amblyseius andersoni was amongst other species frequently found, which suggests that this species thrives well on roses. The possibility for biological control of spider mites with A. andersoni was investigated both in container roses outdoors and in glasshouses. In plots of outdoor roses artificially infested with spider mites, the following treatments were carried out: spider mites alone (untreated plot), Amblyseius andersoni Amblyseius andersoni and ice plants, Neoseiulus californicus, Neoseiulus californicus and ice plants. There were four replications of the treatments. The ice plants, Delosperma cooperi, were added to some treatments to supply pollen as extra food for the predatory mites. Natural enemies such as Chrysoperla spp., Conwentzia sp., Orius sp., Stethorus punctillum, and Feltiella acarisuga occurred naturally and contributed to the control of spider mites. After one month the spider mites were eradicated in all treatments. At the end of the trial, predatory mites were collected from all plots for identification. The ratio of Amblyseius andersoni to Neoseiulus californicus was approximately 9:1. There was no obvious effect of the ice plants on the number of predatory mites. On a nursery, where new roses are bred and selected, Amblyseius andersoni was released in three glasshouses after one early treatment with bifenazate against two-spotted spider mite Tetranychus urticae. In two of these glasshouses Neoseiulus californicus was also released. Samples, which were taken in the summer months showed that the spider mites were kept at a very low level. Amblyseius andersoni was found, even if spider mites were absent. Rose plants infested with spider mites, that were brought in to the glasshouses later developed spider mite 'hotspots'. Phytoseiulus persimilis was introduced in the hot spots and contributed to the control along with Neoseiulus californicus, Amblyseius andersoni and naturally occurring Feltiella acarisuga. These observations showed that Amblyseius andersoni is a good candidate for preventing spider mite outbreaks, as it easily survives without spider mites. This predatory mite is able to survive on other food, including thrips and fungal spores.     

The Effects of Timing and Rates of Release of Phytoseiulus persimilis against Two-spotted Spider Mite Tetranychus urticae on Dwarf Hops

Single inoculative releases of the phytoseiid mite Phytoseiulus persimilis were made against the two-spotted spider mite, Tetranychus urticae, on two varieties of dwarf hops in 1996 and 1997 at means of 20, 10, 5, 2.5 and nil per plant, and at up to three timings. The numbers of spider mites recorded on leaves after the predators were released were related inversely to the rates of release. The earliest releases of the predator maintained spider mites at lower population densities than did those made later in the year. In all treatments the numbers of spider mites decreased when the prey:predator ratio reached approximately 10:1.     

Inter‐ and intraspecific variation of spider mite susceptibility to fungal infections: Implications for the long‐term success of biological control

Spider mites are severe pests of several annual and perennial crops worldwide, often causing important economic damages. As rapid evolution of pesticide resistance in this group hampers the efficiency of chemical control, alternative control strategies, such as the use of entomopathogenic fungi, are being developed. However, while several studies have focused on the evaluation of the control potential of different fungal species and/or isolates as well as their compatibility with other control methods (e.g., predators or chemical pesticides), knowledge on the extent of inter‐ and intraspecific variation in spider mite susceptibility to fungal infection is as yet incipient. Here, we measured the mortality induced by two generalist fungi, Beauveria bassiana and Metarhizium brunneum, in 12 spider mite populations belonging to different Tetranychus species: T. evansi, T. ludeni, and T. urticae (green and red form), within a full factorial experiment. We found that spider mite species differed in their susceptibility to infection by both fungal species. Moreover, we also found important intraspecific variation for this trait. These results draw caution on the development of single strains as biocontrol agents. Indeed, the high level of intraspecific variation suggests that (a) the one‐size‐fits‐all strategy may fail to control spider mite populations and (b) hosts resistance to infection may evolve at a rapid pace. Finally, we propose future directions to better understand this system and improve the long‐term success of spider mite control strategies based on entomopathogenic fungi.     

Evaluation of the predatory mite, Neoseiulus californicus, for spider mite control on greenhouse sweet pepper under hot arid field conditions

The efficacy of Neoseiulus californicus (a generalist predatory mite) for the biological control of Tetranychus urticae, was compared to release of Phytoseiulus persimilis (a specialist predatory mite) and an acaricide treatment in sweet pepper plants grown in greenhouse tunnels in a hot and arid climate. To ensure uniform pest populations, spider mites were spread on pepper plants in two seasons; a natural infestation occurred in one season. Predators were released prophylactically and curatively in separate tunnels when plants were artificially infested with spider mites, and at low and moderate spider mite populations when infestations occurred naturally. Although spider mite populations did not establish well the first year, fewer spider mites were recovered with release of N. californicus than with all other treatments. In the second year, spider mites established and the prophylactic release of N. californicus compared favorably with the acaricide-treated plants. In the course of monitoring arthropod populations, we observed a significant reduction in western flower thrips (Frankliniella occidentalis) populations in tunnels treated with N. californicus as compared with non-treated control tunnels. Our field trials validate results obtained from potted-plant experiments and confirm that N. californicus is a superior spider mite predator at high temperatures and low humidities.     

Testing for Non-target Effects of Spinosad on Twospotted Spider Mites and their Predator Phytoseiulus persimilis under Greenhouse Conditions

The compatibility of the selective insecticide spinosad (Conserve SC), at rates recommended for thrips control in greenhouses, with release of the predatory mite Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseiidae) to control spider mites, was investigated in a crop of ivy geranium Pelargonium peltatum, cultivar 'Amethyst 96.' Plants were inoculated with twospotted spider mites, Tetranychus urticae Koch (Acari: Tetranychidae), 2 weeks before treatments were applied. There were three treatment variables, each at two levels: predators (released or not), spray application (water or Conserve SC at 2 ml/3.79 l), and timing of spray (1 day before or after predators were released). Twospotted spider mite populations then were sampled twice each week over a three-week period. The application or timing of spinosad had no effect on the ability of the predator to reduce the population of spider mites. Spider mite populations in the no-predator treatment continued to expand over the course of the experiment, while those in the predator-release treatment declined. We conclude that P. persimilis can be used in conjunction with spinosad on ivy geraniums without causing obvious detrimental effects to this predator or leading to a reduction in biological control.     

Within-plant distribution of twospotted spider mites (Acari: Tetranychidae) on impatiens: development of a presence-absence sampling plan

The twospotted spider mite, Tetranychus urticae Koch, is an important pest of impatiens, a floricultural crop of increasing economic importance in the United States. The large amount of foliage on individual impatiens plants, the small size of mites, and their ability to quickly build high populations make a reliable sampling method essential when developing a pest management program. In our study, we were particularly interested in using spider mite counts as a basis for releasing biological control agents. The within-plant distribution of mites was established in greenhouse experiments and these data were used to identify the sampling unit. Leaves were divided into three zones according to location on the plant: inner, intermediate, and other. On average, 40, 33, and 27% of the leaves belonged to the inner, intermediate, and other leaf zones, respectively. However, because 60% of the mites consistently were found on the intermediate leaves, intermediate leaves were chosen as the sampling unit. These results lead to the development of a presence-absence sampling method for T. urticae by using Taylor coefficients generic for this pest. The accuracy of this method was verified against an independent data set. By determining numerical or binomial sample sizes for consistently estimating twospotted spider mite populations, growers will now be able to determine the number of predatory mites that should be released to control twospotted spider mites on impatiens.     

Resistance status of the carmine spider mite,Tetranychus cinnabarinus and the twospotted spider mite,Tetranychus urticae to selected acaricides on strawberries

The carmine spider mite, Tetranychus cinnabarinus (Boisduval) and the twospotted spider mite, Tetranychus urticae Koch, are serious pests of strawberries and many other horticultural crops. Control of these pests has been heavily dependent upon chemical acaricides. Objectives of this study were to determine the resistance status of these two pest species to commonly used acaricides on strawberries in a year‐round intensive horticultural production region. LC₉₀ of abamectin for adult carmine spider mites was 4% whereas that for adult twospotted spider mites was 24% of the top label rate. LC_90s of spiromesifen, etoxazole, hexythiazox and bifenazate were 0.5%, 0.5%, 1.4% and 83% of their respective highest label rates for carmine spider mite eggs, 0.7%, 2.7%, 12.1% and 347% of their respective highest label rates for the nymphs. LC_90s of spiromesifen, etoxazole, hexythiazox and bifenazate were 4.6%, 11.1%, 310% and 62% of their respective highest label rates for twospotted spider mite eggs, 3%, 13%, 432,214% and 15% of their respective highest label rates for the nymphs. Our results suggest that T. cinnabarinus have developed resistance to bifenazate and that the T. urticae have developed resistance to hexythiazox. These results strongly emphasize the need to develop resistance management strategies in the region.     

Volatile spider-mite pheromone and host-plant kairomone, involved in spaced-out gregariousness in the spider mite Tetranychus urticae

ABSTRACT. From the host plant-spider mite complex Phaseolus lunatus—Tetranychus urticae Koch a volatile chemical is emitted that acts as a kairomone for the predatory mite Phytoseiulus persimilis Athias-Henriot (Sabelis et al. , 1984a). This kairomone is apparently a byproduct of a vital physiological process and/or it has a function in the biology of the spider mite as well.
The spider mite—host plant complex also emits a volatile spider-mite dispersing pheromone. This is shown in the present study where spider mites were introduced into an odour patch on a horizontal screen in a vertical airflow olfactometer. When spider-mite infested leaves of Lima bean are offered, the spider mites walk mainly straight and soon reach the edge of the screen. On the other hand, when clean Lima bean leaves are offered, the mites walk tortuously most of the time and reach the edge of the screen much later. Artificially damaged plants elicit the same response as undamaged plants. Differences in spider-mite behaviour are observed in the vertical airflow olfactometer when odour of either clean or spider-mite infested leaves is offered. A comparison of the behaviour in these two situations with that when no odour was offered suggests that Lima bean leaves emit a volatile kairomone that activates T. urticae and makes them return after losing the stimulus. A Y-tube olfactometer experiment confirms the existence of this kairomone.
At a low ratio of dispersing pheromone to plant kairomone, the spider mites behave as if only kairomone is present, walking mainly tortuously. At a high ratio they disperse. No aggregation-pheromonal effect is observed.
The possibility that the spider-mite dispersing pheromone acts as a kairomone for P. persimilis is discussed.