Functionally different predators break down antipredator defenses of spider mites

Prey that lives with functionally different predators may experience enhanced mortality risk, because of conflicts between the specific defenses against their predators. Because natural communities usually contain combinations of prey and functionally different predators, examining risk enhancement with multiple predators may help to understand prey population dynamics. It is also important in an applied context: risk enhancement with multiple biological control agents could lead to successful suppression of pests. We examined whether risk enhancement occurs in the spider mite Tetranychus kanzawai Kishida (Acari: Tetranychidae) when exposed to two predator species: a generalist ant, Pristomyrmex punctatus Mayr (Hymenoptera: Formicidae), and a specialist predatory mite, Neoseiulus womersleyi Schicha (Acari: Phytoseiidae). We replicated microcosms that consisted of spider mites, ants, and predatory mites. Spider mites avoided generalist ants by staying inside their webs on leaf surfaces. In contrast, spider mites avoided specialist predatory mites that intruded into their webs by exiting the web, which obviously conflicts with the defense against ants. In the presence of both predators, enhanced mortality of spider mites was observed. A conflict occurred between the spider mites’ defenses: they seemed to move out of their webs and be preyed upon by ants. This is the first study to suggest that risk enhancement occurs in web‐spinning spider mites that are exposed to both generalist and specialist predator species, and to provide evidence that ants can have remarkable synergistic effects on the biological control of spider mites using specialist predatory mites.     

Spider mite web mediates anti-predator behaviour

Herbivores suffer significant mortality from predation and are therefore subject to natural selection on traits promoting predator avoidance and resistance. They can employ an array of strategies to reduce predation, for example through changes in behaviour, morphology and life history. So far, the anti-predator response studied most intensively in spider mites has been the avoidance of patches with high predation risk. Less attention has been given to the dense web produced by spider mites, which is a complex structure of silken threads that is thought to hinder predators. Here, we investigate the effects of the web produced by the red spider mite, Tetranychus evansi Baker & Pritchard, on its interactions with the predatory mite, Phytoseiulus longipes Evans. We tested whether female spider mites recognize predator cues and whether these can induce the spider mites to produce denser web. We found that the prey did not produce denser web in response to such cues, but laid more eggs suspended in the web, away from the leaf surface. These suspended eggs suffered less from predation by P. longipes than eggs that were laid on the leaf surface under the web. Thus, by altering their oviposition behaviour in response to predator cues, females of T. evansi protect their offspring.     

The silk road of <Emphasis Type="Italic">Tetranychus urticae</Emphasis>: is it a single or a double lane?

Tetranychus urticae (Acari: Tetranychidae) is a phytophagous mite that forms huge colonies. All active members of a colony (immatures and matures, females and males) spin silken threads. These mites construct a common web that protects the colony from external aggression. The silk coverage is well-known to provide advantages to the colony but very little is known about the characteristics of the threads themselves. Here is the first quantification of the diameter of silken threads spun by two different stages (adult females and larvae) and its relationship with body size of the spinning individuals. Moreover, we observed how silk was deposited on the substrate through their two pedipalps. Threads were observed by means of transmission electron and fluorescence microscopy. Silken threads spun by larvae (0.055 ± 0.018 μm) were significantly thinner than threads spun by adult females (0.111 ± 0.038 μm). In the first step of the silk depositing behaviour, the mite attached the thread to the substrate by putting its pedipalps in contact with the surface (adhesion, double silken threads). When walking, silken threads became detached from the substrate and spitted up (silken threads were free). Finally, silken threads adhered to the surface. The presence of single and double threads makes thread diameter highly variable.     

Generalist-feeding subterranean mites as potential biological control agents of immature corn rootworms

Predatory mites are important components of subterranean food webs and may help regulate densities of agricultural pests, including western corn rootworms (Chrysomelidae: Diabrotica virgifera virgifera). Implementing conservation and/or classical biocontrol tactics could enhance densities of specialist or generalist predatory mites and lead to pest suppression, but first relevant mite species must be identified and their predatory capabilities evaluated. We conducted lab assays to quantify consumption of immature rootworms and oviposition rates of various mite species. Our study indicates that rootworms are a sub-optimal food source for the mite taxa tested. However, all mite species fed upon rootworms to some degree, although consumption by nematophagous Eviphis ostrinus was extremely low. Predators consumed more rootworm larvae than eggs, and mite size was correlated with prey consumption, with larger predators eating more prey. Four mite taxa (Gaeolaelaps sp., S. miles, Gl. americana, and G. aculeifer) had detrimental effects on survival of rootworm larvae, and the latter two species also had negative impacts on densities of pest eggs. Although it is unlikely that any of these mite species by itself has a major impact on rootworm control, the community of generalist soil-dwelling mites may play an important role in regulating immature rootworm populations in the field.     

Potential lethal and non-lethal effects of predators on dispersal of spider mites

Predators can affect prey dispersal lethally by direct consumption or non-lethally by making prey hesitate to disperse. These lethal and non-lethal effects are detectable only in systems where prey can disperse between multiple patches. However, most studies have drawn their conclusions concerning the ability of predatory mites to suppress spider mites based on observations of their interactions on a single patch or on heavily infested host plants where spider mites could hardly disperse toward intact patches. In these systems, specialist predatory mites that penetrate protective webs produced by spider mites quickly suppress the spider mites, whereas generalist predators that cannot penetrate the webs were ineffective. By using a connected patch system, we revealed that a generalist ant, Pristomyrmex punctatus Mayr (Hymenoptera: Formicidae), effectively prevented dispersal of spider mites, Tetranychus kanzawai Kishida (Acari: Tetranychidae), by directly consuming dispersing individuals. We also revealed that a generalist predatory mite, Euseius sojaensis Ehara (Acari: Phytoseiidae), prevented between-patch dispersal of T. kanzawai by making them hesitate to disperse. In contrast, a specialist phytoseiid predatory mite, Neoseiulus womersleyi Schicha, allowed spider mites to escape an initial patch, increasing the number of colonized patches within the system. Our results suggest that ants and generalist predatory mites can effectively suppress Tetranychus species under some conditions, and should receive more attention as agents for conservation biological control in agroecosystems.     

Within‐patch oviposition site shifts by spider mites in response to prior predation risks decrease predator patch exploitation

In egg‐laying animals with no post‐oviposition parental care, between‐ or within‐patch oviposition site selection can determine offspring survival. However, despite the accumulation of evidence supporting the substantial impact predators have on oviposition site selection, few studies have examined whether oviposition site shift within patches (“micro‐oviposition shift”) reduces predation risk to offspring. The benefits of prey micro‐oviposition shift are underestimated in environments where predators cannot disperse from prey patches. In this study, we examined micro‐oviposition shift by the herbivorous mite Tetranychus kanzawai in response to the predatory mite, Neoseiulus womersleyi, by testing its effects on predator patch exploitation in situations where predatory mites were free to disperse from prey patches. Adult T. kanzawai females construct three‐dimensional webs on leaf surfaces and usually lay eggs under the webs; however, females that have experienced predation risks, shift oviposition sites onto the webs even in the absence of current predation risks. We compared the predation of eggs on webs deposited by predator‐experienced females with those on leaf surfaces. Predatory mites left prey patches with more eggs unpredated when higher proportions of prey eggs were located on webs, and egg survival on webs was much higher than that on leaf surfaces. These results indicate that a micro‐oviposition shift by predator‐experienced T. kanzawai protects offspring from predation, suggesting adaptive learning and subsociality in this species. Conversely, fecundity and longevity of predator‐experienced T. kanzawai females were not reduced compared to those of predator‐naïve females; we could not detect any costs associated with the learned micro‐oviposition shift. Moreover, the previously experienced predation risks did not promote between‐patch dispersal of T. kanzawai females against subsequently encountered predators. Based on these results, the relationships of between‐patch oviposition site selection and micro‐oviposition shift are discussed.     

The presence of webbing affects the oviposition rate of two-spotted spider mites, <Emphasis Type="Italic">Tetranychus urticae</Emphasis> (Acari: Tetranychidae)

Several species of tetranychid mites including Tetranychus urticae Koch (Acari: Tetranychidae) construct complicated three-dimensional webs on plant leaves. These webs provide protection against biotic and abiotic stress. As producing web is likely to entail a cost, mites that arrive on a leaf with web are expected to refrain from producing it, because they will gain the benefit of protection from the existing web. Mites that produce less web may then allocate resources that are not spent on web construction to other fitness-enhancing activities, such as laying eggs. To test this, the oviposition rate of T. urticae adult females was examined on leaves with web. As a control, we used leaves where the web had been removed, hence both types of leaves had been exposed to conspecifics previously and were thus damaged. On leaves with web, the oviposition rate of T. urticae females was higher than on leaves where the web had been removed. Therefore, the presence of web constructed by conspecifics enhanced the oviposition rate of T. urticae females. This provides indirect evidence that mites use the web constructed by conspecifics and thereby save resources that can be allocated to other traits that enhance reproductive success.     

Larval aquatic and terrestrial mites infesting a temperate assemblage of mosquitoes

We collected 22,769 adult female mosquitoes, representing 27 species, from light traps in Norfolk, Virginia (2006–2007) and examined them to assess infestation by larval mites. Mosquitoes were parasitized by two species of aquatic (Acari: Arrenuridae: Arrenurus) and three species of terrestrial mites (Acari: Erythraeidae). The prevalence of infestation varied from 0.55% (2006) to 0.17% (2007). The mean intensity of parasitism ranged from 3.6 mites per host (2006) to 1.8 mites per host (2007). The most common host species for aquatic mites was Culex erraticus, while the most common host for terrestrial mites was Anopheles quadrimaculatus. Relationships between biotic and abiotic factors were investigated in an attempt to provide insight into temporal, spatial, and interspecific variation in mite–mosquito interactions. Scanning electron microscopy was used to examine the mode of attachment for larval mites. While the prevalence of aquatic mite parasitism was correlated for Culex erraticus, the invasive mosquito, Aedes albopictus, was never parasitized through the duration of the study. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Sub-lethal effects of fenbutatin oxide on prey location by the predatory mite <Emphasis Type="Italic">Iphiseiodes zuluagai</Emphasis> (Acari: Phytoseiidae)

We used a Y-tube olfactometer to assess the sub-lethal effects of the acaricide fenbutatin oxide on the olfactory response of the predatory mite Iphiseiodes zuluagai towards odours from: (1) air or undamaged coffee plants; (2) undamaged or red spider mite Oligonychus ilicis-infested coffee plants; (3) undamaged or false spider mite Brevipalpus phoenicis-infested coffee plants. Predatory mite adult females were exposed to residues of fenbutatin oxide or distilled water on leaf discs during a period of 72 h prior experiments. When exposed to distilled water (control treatments), predatory mites significantly preferred undamaged plants over air, O. ilicis-infested plants over undamaged plants, and they did not prefer B. phoenicis-infested plants over undamaged plants. However, predatory mites that had been exposed to residues of fenbutatin oxide were neither attracted towards undamaged plants nor to O. ilicis-infested plants. Thus, fenbutatin oxide affected negatively the olfactory response of I. zuluagai. We conclude that sub-lethal-effect studies should be considered in pesticide selectivity programs since the ability of predatory mites to locate their prey may be negatively affected by non-lethal concentrations of pesticides.     

Behavioral and population consequences of acarodomatia in grapes on phytoseiid mites (Mesostigmata) and implications for plant breeding

We examined the influence of acarodomatia in the riverbank grape Vitis riparia Michaux (Vitaceae) on the distribution and abundance of predatory mites (Phytoseiidae) and their interactions with herbivorous mites. Acarodomatia are tufts of nonglandular trichomes or pits located in major leaf vein axes of many species of woody perennial plants and are often occupied by predatory and mycophagous mites. In common garden plantings of different accessions of V. riparia we found a significant positive relationship between size of domatia and the abundance of naturally occurring predatory mites. Behavior of adult predatory mites may explain this positive association, in part. In separate laboratory experiments, gravid females of Typhlodromus pyri Scheuten and Amblyseius andersoni Chant spent more time and deposited more eggs on half of a V. riparia leaf with accessible domatia versus the other half in which access to domatia was blocked with pruning tar. Domatia also had population consequences. In an outdoor experiment using potted grapevines, population size of T. pyri and A. andersoni mites was greater on V. riparia with open domatia compared to V. riparia in which domatia were blocked with pruning tar. Population size of predatory mites was also greater on V. riparia with domatia than on Vitis vinifera L., whether their axils were blocked or not. Since V. vinifera have very small domatia, these results indicate that the presence of domatia is important, not just access to vein axils. Elevated predatory mite populations in response to domatia, however, did not translate into differences in the abundance of European red mite Panonychus ulmi (Koch), an important pest of grapes.Overall, these results indicate that domatia in uncultivated V. riparia promote higher densities of some species of generalist phytoseiid mites. However, domatia are small in most cultivated grapes. We crossed females and males of V. riparia that varied in domatia size and reared their offspring and found that average domatia size in the parents was highly correlated (r² = 0.77, slope = 0.55) with average domatia size in offspring (high narrow-sense heritability). Given that V. riparia possesses many other desirable agronomic traits, this result suggests it should be practical to breed for well-developed domatia in cultivated accessions.     

Reduced Aggressive Behavior: A Benefit of Silk-Spinning in the Parsnip Webworm, Depressaria pastinacella (Lepidoptera: Oecophoridae)

The parsnip webworm, Depressaria pastinacella, spins a silken web within the umbels of its host plant, the wild parsnip Pastinaca sativa, and aggressively defends this web against conspecifics. We first established experimentally that the number of aggressive interactions between caterpillars with their webs removed was significantly higher than for webworms with intact webs. In order to determine whether web-spinning acts to divide food resources and reduce aggressive interactions, we measured relative weight gain and total silk production of parsnip webworms isolated from one another, grouped together with webbing undisturbed, and grouped together with webbing removed daily. Parsnip webworms isolated from one another and therefore unable to engage in aggressive interactions attained the highest pupal weights and spun the smallest amount of silk; caterpillars with webs removed daily and therefore with frequent aggressive interactions until territories were reestablished had the lowest pupal weights and spun the greatest quantity of silk. Our findings indicate that, for the parsnip webworm, constructing a silken web reduces aggressive encounters among conspecifics.     

Bacterial communities in predatory mites are associated with species and diet types

The symbiotic bacterial communities of phytophagous arthropods are affected by host species and feeding habits, but such effects have been poorly studied in natural enemies. Here, we investigated the entire bacterial microbiome of two species of predatory mites, Neoseiulus californicus and Neoseiulus barkeri, feeding on three types of diets (artificial diet, pollen and their natural prey, the spider mite Tetranychus urticae) by high-throughput sequencing of the 16S rRNA gene. We found that the bacterial diversity of predatory mites feeding on artificial diet was significantly different from pollen and spider mite feeding groups in both N. californicus and N. barkeri, while bacterial diversity also differed strikingly between the two species even when feeding on the same artificial diet. This finding suggests that the bacterial community of predatory mites is determined by both species and diet. Alphaproteobacteria and Gammaproteobacteria were the two dominant bacterial classes in both predatory mite species, except for N. californicus feeding on artificial diet. The bacterium Bosea sp. was detected in all samples as the core microbial species in predatory mites. Additionally, we discuss whether Bradyrhizobiaceae and Rhodobacteraceae bacteria could be used as probiotics in the artificial diet of N. californicus for better mass rearing.

     

How predatory mites find plants with whitefly prey

We investigated the searching behaviour of two species of predatory mites, Typhlodromips swirskii (Athias-Henriot) and Euseius scutalis (Athias-Henriot), both known to feed on immature stages of the whitefly Bemisia tabaci Gennadius. When released in a greenhouse inside a circle of cucumber plants that were alternatingly clean or infested with immature whiteflies, the mites took several days to find plants. Both species were recaptured significantly more on plants with whiteflies. This suggests that the mites are able to discriminate between plants with and without whiteflies. The predators may either have been attracted to plants with whiteflies from a distance or arrested on plants with whiteflies. Typhlodromips swirskii that had previously fed on whitefly immatures on cucumber leaves were significantly attracted by volatiles from cucumber plants with whiteflies in a Y-tube olfactometer. This suggests that the mites use volatile cues to discriminate between infested and clean plants. However, this response waned rapidly; if predators, experienced as above, were starved for 3–4 h in absence of cucumber leaves, they no longer preferred volatiles of infested plants to clean plants. Furthermore, T. swirskii that had no experience with immature whiteflies on cucumber plants also did not prefer odours of infested plants to those of clean plants. Because the release experiment with this species in the greenhouse was done with inexperienced predators, this suggests that the aggregation of mites on plants with whiteflies was mainly caused by differential arrestment of mites on plants with prey and clean plants. For T. swirskii, this was in agreement with the finding that the fraction of predators on plants with prey increased with time to levels higher than 70%. A less clear trend was found for E. scutalis, for which the fraction of predators on plants with prey stabilized soon after release to levels from 54–70%. Hence, the predatory mites may find plants with prey by random searching, but they are subsequently arrested on these plants. An earlier study showed that 87% of all whiteflies released in a set-up as used here were recaptured within 1 day. Hence, the effectiveness with which predatory mites locate plants with whiteflies is low compared with that of their prey. We expect this to generate spatial patterns in the dynamics of predator and prey and this may have consequences for biological control of whiteflies with predatory mites.     

Do domatia mediate mutualistic interactions between coffee plants and predatory mites?

Many plant species possess structures on their leaves that often harbour predatory or fungivorous mites. These so‐called domatia are thought to mediate a mutualistic interaction; the plant gains protection because mites decimate plant pathogenic fungi or herbivores, whereas the mites find shelter in the domatia. We tested this hypothesis using two species of coffee (Coffea spp.) plants that posses domatia consisting of small cavities at the underside of the leaves, and which often harbour mites. We assessed densities of domatia, of the predatory mite Iphiseiodes zuluagai Denmark and Muma (Acari: Phytoseiidae) and of herbivorous mites Oligonychus ilicis (McGregor) (Acari: Tetranychidae) and Brevipalpus phoenicis (Geijskes) (Acari: Tenuipalpidae) on Coffea arabica L. (Rubiaceae) and Coffea canephora Pierre in the field. Over a period of 50 days, C. arabica harboured on average 7.5 times more predatory mites and 0.4–0.66 fewer prey mites than C. canephora. Hence, the higher density of predatory mites on C. arabica could not be explained by higher densities of prey. However, the density of domatia on C. arabica was on average 1.65 times higher than on C. canephora, and within each species, leaves with higher densities of domatia also harboured more predators. This suggests a positive effect of domatia on predatory mites. In the laboratory, survival of adult female predatory mites on leaves of C. arabica with open domatia was indeed significantly higher than on leaves with closed domatia. Hence, predatory mites benefited from the domatia. However, plants with higher densities of domatia did not harbour fewer herbivores. Taken together, our study only provides partial evidence for a mutualistic interaction between coffee plants and predatory mites, mediated by domatia.     

Using light as a lure is an efficient predatory strategy in <Emphasis Type="Italic">Arachnocampa flava,</Emphasis> an Australian glowworm

Trap-building, sit-and-wait predators such as spiders, flies and antlions tend to have low standard metabolic rates (SMRs) but potentially high metabolic costs of trap construction. Members of the genus Arachnocampa (glowworms) use an unusual predatory strategy: larvae bioluminesce to lure positively phototropic insects into their adhesive webs. We investigated the metabolic costs associated with bioluminescence and web maintenance in larval Arachnocampa flava. The mean rate of CO₂ production ([(V)\dot] \dot{V}CO₂) during continuous bioluminescence was 4.38 μl h⁻¹ ± 0.78 (SEM). The mean [(V)\dot] \dot{V}CO₂ of inactive, non-bioluminescing larvae was 3.49 ± 0.35 μl h⁻¹. The mean [(V)\dot] \dot{V}CO₂ during web maintenance when not bioluminescencing was 8.95 ± 1.78 μl h⁻¹, a value significantly lower than that measured during trap construction by other predatory arthropods. These results indicate that bioluminescence itself is not energetically expensive, in accordance with our prediction that a high cost of bioluminescence would render the Arachnocampa sit-and-lure predatory strategy inefficient. In laboratory experiments, both elevated feeding rates and daily web removal caused an increase in bioluminescent output. Thus, larvae increase their investment in light output when food is plentiful or when stressed through having to rebuild their webs. As light production is efficient and the cost of web maintenance is relatively low, the energetic returns associated with continuing to glow may outweigh the costs of continuing to attract prey.     

Three‐year surveillance (2016–2018) of chigger mites vector for tsutsugamushi disease in the Hwaseong‐Si area of Gyeonggi‐Do,Republic of Korea

Owing to climate change, the global resurgence of vector‐borne infectious diseases has emerged as a critical public health issue. Orientia tsutsugamushi is the etiological agent of tsutsugamushi disease (scrub typhus) a mite‐borne acute febrile disease occurring in the Asia‐Pacific region. We investigated the prevalence of tsutsugamushi disease transmitted by chigger mite vectors living on rodents. Using sticky‐type chigger traps for three months during 2016–2018, 1,057 chigger mites were collected (chigger mite index, 1.31) from four locations in the Hwaseong‐si area of Gyeonggi‐do, Republic of Korea. Five species distributed among three genera were identified. In addition, 94 rodents were captured (collection rate: 7.83%) using Sherman live traps over the course of three months (April, October, and November) during 2016–2017. Three rodent species were captured and identified and the striped field mouse (Apodemus agrarius) was the dominant rodent host species in the surveyed area. A total of 10,469 ectoparasitic chigger mites were recovered from the 94 rodents, from which 13 species distributed among four genera were identified. Of the 5,250 chigger mites examined, Leptotrombidium pallidum was most abundant (n = 2,558), followed by L. orientale, L. scutellare, L. zetum, Euschoengastia koreaensis, L. subintermedium, and Neotrombicula tamiyai. Of the examined chigger mites, no groups recovered from rodent hosts tested positive for O. tsutsugamushi. This study provides fundamental regional information on vector‐borne disease data collection in the Hwaseong‐si area, Gyeonggi‐do, and will further contribute to formulating disease control and prevention strategies.     

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.     

Distribution and influence of grazing on wheat curl mites (Aceria tosichella Keifer) within a wheat field

Wheat streak mosaic virus (WSMV) is a serious disease of wheat and is primarily transmitted from infected to healthy plants by the wheat curl mite, Aceria tosichella Keifer. Although wheat is the primary plant host of A. tosichella, wheat curl mites have been recorded on more than 60 different plant hosts; this broad host range allows mites to survive outside the wheat‐growing season by providing a ‘green bridge’. Despite the fact that A. tosichella can only crawl short distances, the mites can disperse via wind and thus have the capacity to readily infest wheat crops from neighbouring refuges. In this study, we undertook field trials to investigate the temporal movement of A. tosichella, as well as the importance of wind and livestock grazing on mite dispersal late in the cropping season. We demonstrate there is a window in spring when A. tosichella undergo significant movement in south‐eastern Australia, and this is likely related to the development stage of wheat plants, and may also be influenced by wind direction. We found that grazing wheat crops reduced mite numbers, suggesting that any increase in WSMV issues in ‘grain and graze’ crops is likely due to the longer season wheat varieties used in these systems rather than the direct effects of grazing. These results emphasize the importance of crop management strategies in the control of A. tosichella.     

Building a better insect trap; An experimental investigation of prey capture in a variety of spider webs

Summary Web-building spiders (Araneae; Theridiidae, Linyphiidae, Araneidae) are catagorized as searchers because they devote a large amount of energy to the construction of the web which constitutes the search phase in the foraging sequence. In this study search energy is equated with the density of threads in a web and the effectiveness of a variety of webs in three broad catagories (tangle webs, sheet webs & orb webs) is tested in the light of current foraging theory. Within each web type there is a distinct thread density at which the number of approaching Drosophila (Diptera; Drosophilidae) that are captured is maximized (Figs. 1, 2, 3). That maximum results from a combination of factors that are a function of the density of threads in the web. The visibility of the web to an approaching Drosophila increases which acts to decrease the number of flies that enter the web (Tables 2, 3, 4). The ability of the web to detain a Drosophila that contacts it (capture efficiency) increases to an asymptote as a function of thread density (Fig. 4). These data support an assumption of many optimal foraging models that with increasing investment in search the predator receives a diminishing return.More Drosophila intercept orb webs than intercept sheet or tangle webs. In addition orb webs detain a greater proportion of the flies that contact them (Fig. 4). Sheet webs are intermediate between orb and tangle webs in their relative abilities to contact and detain Drosophila.     

Plants,mites and mutualism: leaf domatia and the abundance and reproduction of mites on Viburnum tinus (Caprifoliaceae)

Associations between mites and leaf domatia have been widely reported, but little is known about their consequences for either plants or mites. By excising domatia from leaves of the laureltinus, Viburnum tinus L. (Caprifoliaceae), in the garden and laboratory, we showed that domatia alter the abundance, distribution, and reproduction of potential plant mutualists. Over 4 months, leaves with domatia on six garden shrubs had 2–36 times more predatory and microbivorous mites, and more mite eggs than leaves without domatia. However, this effect varied among plants and was weaker on one shrub with few mites on its leaves. Domatia also influenced the distribution of mites on leaves. A significantly higher fraction of mites, representing all life stages, was found in vein axils of leaves with domatia than in vein axils on leaves without domatia. Single-leaf experiments in the laboratory showed that domatia enhanced reproduction by the predatory mite, Metaseiulus occidentalis, especially at low relative humidity (30–38%). When domatia were removed, oviposition was reduced significantly only at low relative humidity, suggesting that domatia provide mites with refuge from environmental extremes on the leaf surface. Moreover, the use of domatia by predatory mites may reduce the impact of some plant enemies. In two experiments where prey consumption was measured, M. occidentalis ate significantly higher percentages of the eggs of the two-spotted spider mite (Tetranychus urticae). Our results are consistent with the viewpoint that mite-domatia associations are mutualistic. By directly aiding and abetting the third trophic level, plants with leaf domatia may increase the efficiency of some predaceous and microbivorous mites in consuming plant enemies.