The effect of multiple natural enemies on a shared herbivore prey

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The effects of plant quality on caterpillar growth and defense against natural enemies

A survey of 85 species of Lepidoptera feeding on 40 hosts on Barro Colorado Island, Panama showed that growth and defensive traits of caterpillars were correlated with the nutritional and defensive traits of their hosts. Growth rates were faster on young than mature leaves, reflecting the higher nitrogen and water content of the former. Growth was also positively correlated with leaf expansion rate, partially because of higher nitrogen and water contents of fast-expanding young leaves. Specialists grew faster than generalists, but both responded positively to nutritional quality. There was no effect of lepidopteran family on growth. In analyses where the effects of nitrogen and water were removed, the residuals for growth rate were greater for young than for mature leaves and were positively correlated with expansion rates of young leaves. This suggests that traits other than nutrition were also important. As young, expanding leaves cannot use toughness as a defense, one possible explanation for the differences in growth is differences in chemical defenses. Growth rate residuals for both specialists and generalists were higher for the more poorly defended fast-expanders, but the effect was greatest for generalists, perhaps because generalists were more sensitive to secondary metabolites. We predicted that slow growth for caterpillars would increase their risk to natural enemies and would select for higher defenses. Generalists had more defensive traits than specialists and were less preferred in feeding trials with ants. Similarly, species feeding on mature leaves were the most defended and those feeding on fast-expanding young leaves were the least defended and most preferred by ants. Thus the effects of plant secondary metabolites and nutrients dictate herbivore growth rates, which in turn influence their susceptibility to the third trophic level and the importance of defenses.     

Plant sex and induced responses independently influence herbivore performance, natural enemies and aphid-tending ants

Sex is an ecologically important form of genetic variation in dioecious plants, with males and females generally differing in constitutive resistance to herbivores. Yet little is known about sexual dimorphism with respect to induced or indirect defense, or whether sex-based differences are underlain by trade-offs among modes of defense. We compared male and female Valeriana edulis plants for constitutive and induced direct resistance to two herbivores, an early-season caterpillar and a late-season aphid, and for constitutive and induced indirect resistance in terms of abundance of natural enemies and aphid-tending ants. No sexual dimorphism was found in constitutive direct plant resistance, yet the sexes differed for constitutive indirect resistance, with 78?% more natural enemies and 117?% more ants present on females than males. Past feeding damage by caterpillars induced direct and indirect resistance in both males and females, increasing caterpillar development time by 26?% and the abundance of natural enemies by 147?%. Caterpillar feeding did not induce direct resistance with respect to caterpillar final mass or aphid performance. In all cases, there were no interactions between the effects of caterpillar damage and plant sex. In summary, plant sexual dimorphism and induced responses to herbivore damage independently influenced herbivore performance and the composition of arthropod communities at higher trophic levels.     

How leaf domatia and induced plant resistance affect herbivores, natural enemies and plant performance

Predators and plant resistance may act together to control herbivorous arthropod populations or antagonistically, which would reduce the control of pest populations. In a field experiment we enhanced predation by adding simulated leaf domatia to plants. Leaf domatia are small structures that often harbor predaceous arthropods that are potentially beneficial to the plant. We also manipulated host plant quality by inducing resistance with controlled, early season exposure of seedlings to spider mite herbivory.
Our manipulations had profound consequences for the natural community of arthropods that inhabited the plants. Leaf domatia had a direct positive effect on abundances of two species of bugs and one species of thrips, all of which are largely predators of herbivores. On leaves with domatia, each of the predators was found inside the domatia two to three times more often than outside the domatia. Eggs of predaceous bugs inside leaf domatia were protected from parasitism compared to eggs outside the domatia. The positive effects of leaf domatia on predator abundances were associated with reduced populations of herbivorous spider mites, aphids, and whiteflies. Plants with experimental leaf domatia showed significantly enhanced reproductive performance.
Induced resistance also affected the community of arthropods. Of the abundant predators, all of which also fed on the plant, only minute pirate bugs were negatively affected by induced resistance. Populations of herbivorous spider mites and whiteflies were directly and negatively affected by induction. In contrast, aphid populations were higher on plants with induced resistance compared to uninduced plants. Effects of induced resistance and domatia were additive for each of the predators and for aphids. However, spider mite and whitefly populations were not suppressed further by employing both induced resistance and domatia compared to each strategy alone. Our manipulations suggest that plant defense strategies can have positive effects on some species and negative effects on others. Negative effects of “resistance traits” on predators and positive effects on some herbivores may reduce the benefits of constitutive expression of resistance traits and may favor inducible defense strategies. Multiple plant strategies such as inducible resistance and morphological traits that aid in the recruitment of predators of herbivores may act together to maximize plant defenses, although they may also be redundant and not act additively.     

Effects of natural enemies on the rate of herbivore adaptation to resistant host plants

The potential of natural enemies to influence the rate of herbivore adaptation to resistance factors in plants is examined using conceptual and mathematical models. Results indicate that natural enemies could increase or decrease the rate of herbivore adaptation. The specific behavioral and physiological effects of a resistance factor on the herbivore, as well as the behavior of the natural enemy, and the population dynamics of the natural enemy/herbivore system are important in assessing the extent to which the natural enemies will affect the rate of herbivore adaptation to a resistance factor. Herbivore adaptation to partial resistance in a host-plant is generally expected to be slower than adaptation to high levels of resistance, even in the presence of natural enemies, if genetic variance is not limiting.

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Résumé Des modèles conceptuels et mathématiques ont servi à examiner l'influence du potentiel d'ennemis naturels sur le taux d'adaptation d'herbivores à des facteurs de résistance des plantes. Les résultats montrent que les ennemis naturels peuvent augmenter ou réduire le taux d'adaptation des herbivores. Les effets spécifiques (comportementaux et physiologiques) d'un facteur de résistance sur l'herbivore, aussi bien que le comportement de l'ennemi naturel et la dynamique de population du système ennemi naturel/herbivore permettent de déterminer le niveau de l'effet des ennemis naturels sur le taux d'adaptation de l'herbivore au facteur de résistance. Une adaptation de l'herbivore à une résistance partielle à une plante est généralement envisagée comme plus lente qu'une adaptation à des hauts niveaux de résistance, même en présence d'ennemis naturels, mais il peut y avoir des exceptions.

     

The role of resources and natural enemies in determining the distribution of an insect herbivore population

1. Both resources and natural enemies can influence the distribution of a herbivore. The ideal free distribution predicts that herbivores distribute themselves to optimise utilisation of resources. There is also evidence of herbivores seeking out refuges that reduce natural enemy attack (enemy‐free space). Which of these theories predominates in a thistle–tephritid Terellia ruficauda (Diptera: Tephritidae)–parasitoid interaction is examined. 2. The plant, Cirsium palustre, had a contagious distribution approximated by the negative binomial distribution. Terellia ruficauda foraged preferentially and oviposited on isolated plants although its larvae gained neither nutritional benefit nor reduced natural enemy pressure from such behaviour. 3. Parasitoids of T. ruficauda foraged and oviposited more frequently on isolated than on crowded T. ruficauda, resulting in inverse density‐dependent parasitoid attack at all spatial scales examined. Neither the herbivore nor natural enemies distributed themselves according to the predictions of the ideal free distribution and the herbivore did not oviposit to reduce natural enemy attack. 4. Extrapolating from the theoretical predictions of the ideal free distribution and enemy‐free space to the field requires considerable caution. Terellia ruficauda and its parasitoids appear to select their oviposition sites to spread the risk of losses through factors (e.g. mammal herbivory) that may damage dense clusters of C. palustre.     

转基因作物对天敌的影响研究进展

随着转基因作物商业化种植的发展,人们越来越关注转基因作物对生态安全的影响,其中的一个焦点就是转基因作物对非靶标昆虫天敌的影响。转基因作物主要通过花粉和取食转基因植物的昆虫两个途径影响天敌的种群。目前国内外转基因作物对天敌影响的研究方法包括田间种群调查,室内转基因作物花粉或取食转基因作物的害虫对天敌影响研究,近年来把高浓度毒蛋白通过人工饲料喂养给天敌的Tier-1法研究也逐渐增多。大多数研究表明,转基因抗虫作物中的杀虫蛋白通过花粉和取食转基因植物的昆虫两个途径对天敌没有产生不利的影响。同样,Tier-1法研究结果,也证明浓度毒蛋白喂养对天敌并没有产生不利的影响。     

Phylogenetic distance can predict susceptibility to attack by natural enemies

If related species share enemies, variation in the damage experienced by species within a community may be predictable based on phylogeny. We examined the hypothesis that plant species more closely related to other community members experience greater herbivory by assessing leaf damage to native and exotic plants in two North American communities: an Eastern hardwood forest and a Rocky Mountain montane community. Pairwise phylogenetic distances between focal species and the hundreds of other native species in each community were calculated. We examined the influence of four measures of relatedness within each community: NND (phylogenetic distance to the nearest native neighbor), MPD (mean phylogenetic distance to the native species in the community), and two new metrics, MIPD (mean inverse phylogenetic distance) and INND (inverse nearest neighbor distance). These new metrics assume a nonlinear increase in interaction strength with relatedness; in the context of natural enemies, they posit that the sharing of enemies between any two species increases nonlinearly with their relatedness. Using regression models, we found that herbivore damage decreased with decreasing phylogenetic similarity of focal species to native species (as measured by MIPD) in both sites, although the pattern was significant only for native focal species in the montane community and exotic focal species in the hardwood forest. Similar decreases in herbivory with decreasing relatedness were detected using INND (montane natives) and MPD (hardwood forest exotics). There was no significant relationship between NND and herbivory for any of the four site by focal plant origin combinations. Our results are the first to support the hypothesis that native species can escape attack as a function of their phylogenetic dissimilarity to the larger community of native species, and to demonstrate that exotic species show these patterns in the wild (as opposed to in common gardens). We suggest that phylogenetic distance metrics assuming a nonlinear increase in interaction strength with relatedness show promise for broader application.     

The relative importance of resources and natural enemies in determining herbivore abundance: thistles, tephritids and parasitoids

1. The relative importance of host-plant resources and natural enemies in influencing the abundance of insect herbivores was investigated in potted plant and natural population experiments, using tephritid (Diptera: Tephritidae) flies, their host plant, creeping thistle Cirsium arvense, and their Hymenoptera parasitoids. 2. Experimental manipulation of host-plant quality (i.e. levels of host-plant nutrients) and resource availability (i.e. the number of buds) increased tephritid abundance. There was no evidence that the seed-feeding tephritid fly Xyphosia miliaria preferentially oviposited on fertilized C. arvense. 3. At low thistle densities, X. miliaria showed a constant rate of resource exploitation. At higher thistle densities, a threshold was detected, above which additional buds were not attacked. 4. Parasitism attack was variable across host (tephritid) densities but levels of parasitism were consistently higher on the fertilized thistles. 5. Experimental manipulation of host-plant quality and resource availability (quantity) not only directly affects the tephritid population but also, indirectly, leads to high rates of parasitism. Both chemical and physical characteristics of host plants affect the performance of natural enemies. 6. Both top-down and bottom-up forces act to influence tephritid abundance, with bottom-up influences appearing to be the most important.     

The tri-trophic interactions hypothesis: interactive effects of host plant quality, diet breadth and natural enemies on herbivores

Several influential hypotheses in plant-herbivore and herbivore-predator interactions consider the interactive effects of plant quality, herbivore diet breadth, and predation on herbivore performance. Yet individually and collectively, these hypotheses fail to address the simultaneous influence of all three factors. Here we review existing hypotheses, and propose the tri-trophic interactions (TTI) hypothesis to consolidate and integrate their predictions. The TTI hypothesis predicts that dietary specialist herbivores (as compared to generalists) should escape predators and be competitively dominant due to faster growth rates, and that such differences should be greater on low quality (as compared to high quality) host plants. To provide a preliminary test of these predictions, we conducted an empirical study comparing the effects of plant (Baccharis salicifolia) quality and predators between a specialist (Uroleucon macolai) and a generalist (Aphis gossypii) aphid herbivore. Consistent with predictions, these three factors interactively determine herbivore performance in ways not addressed by existing hypotheses. Compared to the specialist, the generalist was less fecund, competitively inferior, and more sensitive to low plant quality. Correspondingly, predator effects were contingent upon plant quality only for the generalist. Contrary to predictions, predator effects were weaker for the generalist and on low-quality plants, likely due to density-dependent benefits provided to the generalist by mutualist ants. Because the TTI hypothesis predicts the superior performance of specialists, mutualist ants may be critical to A. gossypii persistence under competition from U. macolai. In summary, the integrative nature of the TTI hypothesis offers novel insight into the determinants of plant-herbivore and herbivore-predator interactions and the coexistence of specialist and generalist herbivores.     

Non-additive effects of multiple natural enemies on aphid populations

The question of whether multiple natural enemies often interact to produce lower host mortality than single enemies acting alone has not yet been resolved. We compared the effects of four different combinations of natural enemies-parasitoids, predators, parasitoids plus predators, and no enemies-on caged aphid populations on marsh elder, Iva frutescens, in west-central Florida. Using starting densities of natural enemies commonly found in the field, we showed that parasitoid wasps reduced aphid population densities more than predatory ladybird beetles. The addition of predators to cages containing parasites reduced the ability of parasitoids to decrease aphid population densities. Because the experiments ran only over the course of one generation, such a reduction in the effectiveness of parasites is likely caused by interference of predators with parasitoid behavior. Parasitism in the cages containing both parasitoids and predators was reduced when compared to percent parasitism in parasitoid-only cages, but this could also be due to predation. Our experiments showed that ladybird beetles prey on parasitized aphids. Thus over the long-term, the effectiveness of parasites is impaired by the interference of predators on ovipositing parasitoids and by the predation of parasitized aphids. The effects of natural enemies in this system are clearly non-additive.     

Contrasting effects of natural habitat loss on generalist and specialist aphid natural enemies

The greater susceptibility of higher trophic levels to habitat loss has been demonstrated to disrupt important trophic interactions such as consumer control of prey populations. This pattern is predicted to break down for generalist species that can use matrix habitats, yet empirical studies comparing generalist and specialist enemy pressure in response to natural habitat loss are lacking. Here we examined the effects of landscape simplification resulting from habitat conversion to agriculture on nettles, Urtica dioica , their specialized aphid herbivore, Microlophium carnosum , and associated natural enemies that varied broadly in their degree of specialization. Both nettles and their specialized aphid herbivore were significantly more abundant in complex than simple landscapes. Different enemy groups showed contrasting responses. Aphid specialists (parasitic wasps and cecidomyiid midges) reached higher densities in complex than simple landscapes, and this effect was primarily related to shifts in local resource abundance (i.e. nettle aphid densities). In contrast, densities of generalists (coccinellid beetles and spiders) were significantly higher in simple landscapes, presumably due to spillover of generalists from surrounding cropland habitats. Natural enemy-prey ratios did not differ significantly across landscape types for specialist groups but were significantly higher in simple than complex landscapes for generalist groups, suggesting that enemy pressure on nettle aphids likely increases with landscape simplification. This was supported by our finding that aphid population growth rates were lower in simple than complex landscapes, and declined significantly with increasing coccinellid densities. Thus, in marked contrast to previous work, our results suggest that natural habitat loss may augment rather than disrupt consumer–prey interactions, and this will depend greatly on the degree of specialization of functionally dominant natural enemies.     

两种杀虫剂对稻田捕食性天敌集团捕食功能的影响

将稻田各类捕食性天敌作为一集团,运用功能法评价了甲胺磷和杀虫双对该集团捕食功能的影响,由于各类捕食性天敌对杀虫剂敏感性的不同,甲胺磷和杀虫双造成的各类天敌的死亡率和残存天敌个体的捕食功能减退率均不同。实验表明,隐翅虫类和狼蛛类对甲胺磷较为敏感,而对杀虫双,肖蛸类最为敏感。杀虫双对隐翅虫类的致死作用及捕食功能影响较小。就集团的捕食功能而言,甲胺磷影响明显大于杀虫双,但在总的功能减退率组分中,甲胺磷致     

Latitudinal variation in herbivory: influences of climatic drivers,herbivore identity and natural enemies

Although a number of investigations have concluded that lower latitudes are associated with increases in herbivore abundance and plant damage, the generality of this pattern is still under debate. Multiple factors may explain the lack of consistency in latitude–herbivory relationships. For instance, latitudinal variation in herbivore pressure may be shaped entirely or not by climatic variables, or vary among herbivore guilds with differing life‐history traits. Additionally, the strength of top–down effects from natural enemies on herbivores might also vary geographically and influence latitude–herbivory patterns. We carried out a field study where we investigated the effects of latitude and climate on herbivory by a seed‐eating caterpillar and leaf chewers, as well as parasitism associated to the former across 30 populations of the perennial herb Ruellia nudiflora (Acanthaceae). These populations were distributed along a 5° latitudinal gradient from northern Yucatan (Mexico) to southern Belize, representing one‐third of the species' latitudinal distribution and the entirety and one‐third of the precipitation and temperature gradient of this species' distribution (respectively). We found opposing latitudinal gradients of seed herbivory and leaf herbivory, and this difference appeared to be mediated by contrasting effects of climate on each guild. Specifically, univariate regressions showed that seed herbivory increased at higher latitudes and with colder temperatures, while leaf herbivory increased toward the equator and with wetter conditions. Multiple regressions including temperature, precipitation and latitude only found significant effects of temperature for seed herbivory and latitude for leaf herbivory. Accordingly, that latitudinal variation in seed herbivory appears to be driven predominantly by variation in temperature whereas latitudinal variation in leaf herbivory was apparently driven by other unexplored correlates of latitude. Parasitism did not exhibit variation with latitude or climatic factors. Overall, these findings underscore that the factors driving latitudinal clines in herbivory might vary even among herbivore species coexisting on the same host plant.     

Root growth of Nicotiana attenuata is decreased immediately after simulated leaf herbivore attack

Image-based non-destructive methods were used to quantify root growth reactions happening within hours following simulated leaf herbivore attack.¹ The induction of wound reactions in leaves of seedlings of Nicotiana attenuata led to transiently decreased root growth rates: Upon application of the oral secretions and regurgitants of the specialist herbivore Manduca sexta, a transient decrease in root growth was observed that was more pronounced than if a mere mechanical wounding was imposed. Root growth reduction was more severe than leaf growth reduction and the timing of the transient growth reduction coincided with endogenous bursts of jasmonate (JA) and ethylene emissions reported in literature. The reaction of root growth was superimposed by a strong diel variation of root growth, which was caused by the fluctuating temperature to which the plants were exposed. Apart from the observed root growth reaction, other defense-related traits such as increased nicotine concentration, trichome length and density were activated within 72 h after wounding. Further experiments indicated that the response was elicited by fatty acid-amino acid conjugates that are contained in the oral secretions and that JA signalling is crucial for root-shoot communication here.Key words: image analysis, plant-insect interactions, Manduca sextaPlants constantly need to acclimate to a suite of fluctuating biotic and abiotic factors. Within the framework of their genetically determined response options, they react towards stress situations by maximizing protection against stress while minimizing deviations from optimal growth and development. Feeding of the larvae of the specialist lepidopteran herbivore Manduca sexta on N. attenuata has become a model case scenario for studying biotic stress.² It has been shown in great detail there, that a diverse set of plant hormones like JA, methyl jasmonate and ethylene are rapidly induced immediately following wounding or herbivore attack.^3,4,5 Acclimation occurs both on the biochemical and morphological level, e.g., by increasing defence compounds,⁶ by synthesizing defence related proteins, by emitting volatiles to attract predators and parasites of herbivores⁷ and by altering plant morphology via increased formation of trichomes, thorns or scleromorphy.^8,9 Many studies demonstrated resource based trade-offs between growth and defence on a large time scale (e.g. refs. ¹⁰ and ¹¹). Yet, it is unclear how fast a trade-off-linked reorganization of plant metabolism, diverting resources away from growth or development towards defence, can occur.Recently, development of growth imaging methods has allowed to study short-term growth responses of above-and belowground sink organs towards fluctuations of environmental factors, such as alterations of light climate¹² or responses towards gravitropic stimuli.^13–15 Clear responses are often seen best in young seedlings as they grow fast and as they can be cultivated in agar-filled, translucent Petri dishes allowing live imaging. Hence, it was the aim of our study to monitor defence reactions in the N. attenuata seedling system while at the same time assessing the dynamics of root growth reaction following such an attack.Herbivory-induced wound reactions were simulated by applying different substances on the mechanically wounded primary leaf of the seedling plant 16 d after germination.¹ Investigated substances were: (i) oral secretions and regurgitants of M. sexta larvae (OS), (ii) methyl jasmonate, (iii) fatty acid-amino acid conjugates, (iv) control solutions for the different test substances (water, buffer, lanolin). All wounding treatments were performed at the same time of the day to make sure that the reaction was not masked by fluctuating responses of the plant to temperature changes (plants were grown in a 14 h/10 h light/ dark regime with temperatures of 26°C and 22°C at day and night, respectively).Root growth was affected strongly by temperature (Fig. 1A) and increased linearly with temperature in the analyzed range. Root growth decreased markedly, if OS was added to the wounds. OS-treated and wounded control plants differed in growth reduction from about 2 h after wounding onwards. To uncouple this reduction from temperature-induced growth reductions, the experiment was repeated under continuous light and temperature, leading to a comparable result (Fig. 1B). When OS was added to wounds, nicotine concentration and trichome length and density increased within 72 h after the wounding reaction. Root growth reaction was comparable between OS-treatment and treatments, in which methyl jasmonate or fatty-acid amino acid conjugates were supplied to the wounds. The strongest difference concerning the distribution of relative elemental growth rate along the root growth zone between control and OS treatment was observed about 1 mm behind the root tip in the beginning of the central elongation zone. Leaf growth was monitored in an additional set of experiments. There, a transient decrease of growth was seen as well, but it was not as pronounced as for root growth.Open in a separate windowFigure 1Root growth in Nicotiana attenuata, grown on agar-filled Petri dishes. (A) Velocity of the root tip (V_Tip) versus temperature of the agar medium. (B) Velocity of the root tip of control plants and of plants with leaves that were wounded at t=0 and treated with oral secretions and regurgitants (OS) of Manduca sexta. Plants were grown in continuous light at T = 26°C (n = 3, mean values and SE).The kinetics of the root growth depression point on a superposition of two physiological effects: During the first hour after wounding, root growth decreases similarly in control and OS-treated plants, indicating a mere hydraulic response due to the loss of water and turgor pressure.¹² Thereafter, a specific response towards OS is observed, which coincides with the time frame of about 2–3 h that was reported for the systemic increase of JA transported from shoot to root. Baldwin et al¹⁶ reported that JA pools in roots are increased systemically (3.5-fold) within 180 minutes following mechanical wounding. Hence, JA might mediate the reduction in root growth and induce the defense machinery, which diverts resources from growth to defense.     

The effects of organic and conventional fertilizers on cereal aphids and their natural enemies

• 1 Aphids are important pests of spring cereals and their abundance and the impact of their natural enemies may be influenced by fertilizer regime.
• 2 We conducted a 2‐year field study investigating the effects of organic slow‐release and conventional fertilizers on cereal aphids, hymenopteran parasitoids and syrphid predators and considered how the effects of fertilizers on barley morphology and colour might influence these species.
• 3 Barley yield was greater in conventionally fertilized pots. Barley morphology was also affected by treatment: vegetative growth was greater under conventional treatments. Barley receiving organic fertilizers or no fertilizer was visually more attractive to aphids compared with plants receiving conventional fertilizers.
• 4 Aphids were more abundant in conventionally fertilized barley but the reason for this increased abundance was species specific. Metopolophium dirhodum was responding to fertilizer effects on plant morphology, whereas Rhopalosiphum padi was sensitive to the temporal availability of nutrients.
• 5 Syrphid eggs were more numerous in conventionally fertilized pots, whereas the response of parasitoids appeared to be dependent on the abundance of aphids, although the number of parasitoid mummies was low in both years.
• 6 This research shows that the fertilizer treatment used can affect numerous characteristics of plant growth and colour, which can then influence higher trophic levels. This knowledge might be used to make more informed fertilizer application choices.

     

The specificity of herbivore-induced plant volatiles in attracting herbivore enemies

Plants respond to herbivore attack by emitting complex mixtures of volatile compounds that attract herbivore enemies, both predators and parasitoids. Here, we explore whether these mixtures provide significant value as information cues in herbivore enemy attraction. Our survey indicates that blends of volatiles released from damaged plants are frequently specific depending on the type of herbivore and its age, abundance and feeding guild. The sensory perception of plant volatiles by herbivore enemies is also specific, according to the latest evidence from studies of insect olfaction. Thus, enemies do exploit the detailed information provided by plant volatile mixtures in searching for their prey or hosts, but this varies with the diet breadth of the enemy.