Kairomonal range of generalist predators in specialized habitats: responses to multiple phloeophagous species emitting pheromones vs. host odors

We evaluated responses of the predominant predators of pheromone-producing bark beetles (Coleoptera: Scolytidae) to chemical cues associated with other phloeophagous species that colonize the same trees. This study considered the range of chemical signals exploited by a category of predators that may be viewed either as specialists, because they feed almost exclusively within trees killed by bark beetles, or as generalists, because they feed on a diverse fauna of primary and secondary insects within this habitat. It also evaluated one aspect of a broader model of predator-prey coevolution, that proposes altered semiochemistry as a source of partial escape from predators that exploit kairomones. The predators, Thanasimus dubius (F.) (Coleoptera: Cleridae) and Platysoma cylindrica (Paykull) (Coleoptera: Histeridae), were attracted to cues associated with feeding on bark-phloem disks by two scolytids that produce adult pheromones, Ips pini (Say) and Ips grandicollis (Eichhoff). These predators were not attracted to beetles that feed on lower stems or roots and are not known to produce adult pheromones,Dendroctonus valens LeConte, Hylastes porculus Erickson (Coleoptera: Scolytidae), and Hylobius pales (Herbst) (Coleoptera: Curculionidae). The predator Tenebroides collaris (Sturm) (Coleoptera: Trogositidae) was attracted to I. pini and I. grandicollis, and also to D. valens, H. porculus, and H. pales. Ips pini was attracted to conspecifics only, but I. grandicollis was attracted both to its conspecifics and to volatiles associated with feeding lower stem and root insects. Lower stem and root insects were not or only weakly attracted to cues associated with their conspecifics. These results are consistent with a dynamic coevolved interaction between T. dubius and P. cylindrica and Ips spp.     

Spider cocoons and their suspension systems as barriers to generalist and specialist predators

Summary This study tests hypotheses regarding spider cocoons and their suspension systems as barriers to generalist and specialist predators. Evidence presented here suggests that the suspension systems ofMecynogea lemniscata andArgiope aurantia cocoons are effective barriers against small generalists such as ants, but fail to stop large generalists such as birds. Cocoon covers were found to be generally ineffective against generalist predators. Various component layers of these cocoons are shown, however, to be an effective barrier against the attack modes of specific predator guilds. Cocoon covers function primarily as barriers to specialists that use active “burrowing” larvae to gain entrance into the cocoon, while the flocculent silk layer is shown to be an effective barrier against specialists which use a long ovipositor to attack cocoons. These findings support suggestions that the primary role of the cocoon is to provide protection from predators and parasites. These results also support a close evolutionary relationship between cocoon architecture and specialized predators.     

Artificial nest predation and habitat fragmentation: different trends in bird and mammal predators

Predation on artificial nests was studied in Belgian deciduous forest fragments between 1 and 200 ha Predation rates were compared to fragment size, distance from the forest edge, time period (three replicates), and nest type (ground and tree) Logistic regression analysis showed that overall nest predation did not vary with distance from the edge, forest size, and time period Birds represented over 70% of all predator attacks but their importance decreased in larger areas and away from the forest edge where mammals were responsible for much of the nest predation It is concluded that the effect of habitat fragmentation depends on the composition of the local predator community     

Different hunting strategies of generalist predators result in functional differences

The morphological, physiological, and behavioural traits of organisms are often used as surrogates for actual ecological functions. However, differences in these traits do not necessarily lead to functional differences and/or can be context–dependent. Therefore, it is necessary to explicitly test whether the surrogates have general ecological relevance. To investigate the relationship between the hunting strategies of predators (i.e., how, where, and when they hunt) and their function, we used euryphagous spiders as a model group. We used published data on the diet composition of 76 spider species based on natural prey and laboratory prey acceptance experiments. We computed differences in the position and width of trophic niches among pairs of sympatrically occurring species. Pairs were made at different classification levels, ranked according to the dissimilarity in their hunting strategies: congeners, confamiliars (as phylogenetic proxies for similarity in hunting strategy), species from the same main class of hunting strategy, from the same supra-class, and from different supra-classes. As for niche position computed from the natural prey analyses, species from the same class differed less than species from different classes. A similar pattern was obtained from the laboratory studies, but the congeners differed less than the species from the same classes. Niche widths were most similar among congeners and dissimilar among species from different supra-classes. Functional differences among euryphagous spiders increased continuously with increasing difference in their hunting strategy. The relative frequency of hunting strategies within spider assemblages can, therefore, influence the food webs through hunting strategy-specific predator–prey interactions.     

Host plant specificity in several species of generalist mite predators

1. Species in the genus Neoseiulus are considered to be generalist predators, with some species used in biological control programmes against phytophagous mites and insects. 2. A general survey of Neoseiulus species in inland Australia indicated that different species are associated with particular tree species. This pattern of host plant use was investigated for four Neoseiulus species (N. buxeus, N. cappari, N. brigarinus, N. eremitus) by means of a sampling programme through time and across space. 3. Each species of Neoseiulus was collected entirely or mostly from one species of tree; little or no overlap was detected despite the tree species growing in well‐mixed stands. Host plant specificity thus appears to be strong in this genus. 4. Species in two other genera (Pholaseius and Australiseiulus), also considered to be predatory, showed a similar association with particular tree species. 5. The implications for the use of these predators in biological control are considerable. In particular, phytoseiid species with specific needs in terms of host plants may not be suitable for use as general purpose predators. Meeting the needs of phytoseiids through the modification of host plant attributes may be a step towards enhancing their efficacy as biological control agents.     

Behavioral and ecophysiological responses of a generalist predator to single- and mixed-species diets of different quality

Prey of different quality can be distinguished by their effects on fitness parameters (e.g., survival, growth, development) of the predator. This paper describes a series of experiments with wolf spiders, Schizocosa sp., designed to analyze the behavioral and physiological mechanisms by which these effects are brought about. Schizocosa responded to prey of inferior quality by developing an aversion. The number of prey encounters needed for the aversion to develop depended on the prey type and the quality of alternative prey available. As expected, fewer low-quality prey were accepted if prey of higher quality were available; however, acceptance was also reduced if alternative prey were of even lower quality. An aversion disappeared within a few hours to about 1 day. Consumption by Schizocosa of even small amounts of some “toxic” prey species reduced growth by inhibiting feeding rate and impairing utilization of food derived from prey of higher quality. These results have implications for understanding the interactions of generalist predators in community food webs. Received: 29 July 1998 / Accepted: 1 February 1999     

Genetic differences in the response to landscape fragmentation by a habitat generalist,the bobcat,and a habitat specialist,the ocelot

The ecology of a species strongly influences genetic variation and population structure. This interaction has important conservation implications because taxa with low dispersal capability and inability to use different habitats are more susceptible to anthropogenic stressors. Ocelots (Leopardus pardalis albescens) and bobcats (Lynx rufus texensis) are sympatric in Texas and northeastern Mexico; however, their ecology and conservation status are markedly different. We used 10 microsatellite loci and a 397-bp segment of the mitochondrial control region to examine how historical and ecological differences in these two species have influenced current patterns of genetic diversity in a landscape heavily altered by anthropogenic activities. Substantially higher genetic diversity (heterozygosity and haplotype diversity) and population connectivity was observed for bobcats in comparison to ocelots. The level of divergence among proximate ocelot populations (<30 km) was greater than between bobcat populations separated by >100 km. Ocelot populations in the US have never recovered from reductions experienced during the twentieth century, and their low genetic variation and substantial isolation are exacerbated by strong preference for dense native thornshrub and avoidance of open habitat. In contrast, despite continued legal harvesting and frequent road-related mortality, bobcats have maintained wide distribution, high abundance, and population connectivity. Our study illustrates that sympatric species with a similar niche can still have sufficient ecological differences to alter their response to anthropogenic change. Sensitive species, such as the ocelot, require additional conservation actions to sustain populations. Ecological differences among species occupying a similar guild are important to consider when developing conservation plans.     

Growth and development rates have different thermal responses

Growth and development rates are fundamental to all living organisms. In a warming world, it is important to determine how these rates will respond to increasing temperatures. It is often assumed that the thermal responses of physiological rates are coupled to metabolic rate and thus have the same temperature dependence. However, the existence of the temperature-size rule suggests that intraspecific growth and development are decoupled. Decoupling of these rates would have important consequences for individual species and ecosystems, yet this has not been tested systematically across a range of species. We conducted an analysis on growth and development rate data compiled from the literature for a well-studied group, marine pelagic copepods, and use an information-theoretic approach to test which equations best describe these rates. Growth and development rates were best characterized by models with significantly different parameters: development has stronger temperature dependence than does growth across all life stages. As such, it is incorrect to assume that these rates have the same temperature dependence. We used the best-fit models for these rates to predict changes in organism mass in response to temperature. These predictions follow a concave relationship, which complicates attempts to model the impacts of increasing global temperatures on species body size.     

Host specificity in phytophagous insects: selection pressure from generalist predators

The narrow host range of insect herbivores is noted, and some of the reasons why specificity has evolved are reviewed. Works indicating the need for new approaches are pointed out including the possibility that generalist predators provide a suitable pressure. Experiments to test the hypothesis that generalists are more vulnerable than specialists to predators are described. They involved a vespid wasp and over thirty species pairs of caterpillars, matched for size and density. Overall, generalists were taken more readily than specialists: some but not all reasons could be detected. The results are discussed in ecological and evolutionary terms.     

麦蚜寄生蜂和捕食性天敌种群对景观复杂性的响应

农业景观结构影响昆虫的物种组成与多样性,本文选择复杂景观和简单景观的麦田景观为研究对象,研究了不同麦田景观结构对麦蚜天敌种群的影响。在简单与复杂两种景观下,分析了麦蚜寄生蜂和捕食性天敌的迁入时间、迁入量、种群增长率及种群密度之间的差异。结果表明:复杂景观中麦蚜寄生蜂和捕食性天敌的物种多样性较高,复杂景观下寄生蜂的迁入量高于简单景观下寄生蜂的迁入量。景观复杂性同样影响捕食性天敌的迁入时间,且捕食性天敌在复杂景观下迁入量均高于简单景观,而且存在显著性差异。两种景观中寄生蜂和捕食性天敌种群增长速率与最大种群密度均存在显著性差异。     

Two thermosensors in Drosophila have different behavioral functions

Insects inhabit extreme temperature environments and have evolved mechanisms to survive there. Small insects are especially susceptible to rapid changes in body temperature. Therefore, the rapid detection of environment and body temperature is important for their survival. Little, however, is known about the thermosensors that detect those temperatures. Using rapid thermosensitivity assays with temperature step gradients and a spatial learning paradigm (the heat-box) in which elevated temperature serves as the negative reinforcer, two thermosensors were identified and their behavioral functions assessed. A low-temperature thermosensor is located on the antenna, detects relatively low temperatures, and can detect spatial temperature gradients directly. Thus, the antennae can be used by Drosophila to quickly orient with respect to temperature cues. A high-temperature thermosensor of unknown location appears to have a roughly similar sensitivity to temperature differences as the low-temperature thermosensor (< or = 3 degrees C) and is both necessary and sufficient for memory formation in the heat-box spatial learning paradigm. Therefore, the high-temperature thermosensor is important for remembering spatial positions in which dangerously high temperatures were encountered.     

Summer and fall ants have different physiological responses to food macronutrient content

Seasonally, long-lived animals exhibit changes in behavior and physiology in response to shifts in environmental conditions, including food abundance and nutritional quality. Ants are long-lived arthropods that, at the colony level, experience such seasonal shifts in their food resources. Previously we reported summer- and fall-collected ants practiced distinct food collection behavior and nutrient intake regulation strategies in response to variable food protein and carbohydrate content, despite being reared in the lab under identical environmental conditions and dietary regimes. Seasonally distinct responses were observed for both no-choice and choice dietary experiments. Using data from these same experiments, our objective here is to examine colony and individual-level physiological traits, colony mortality and growth, food processing, and worker lipid mass, and how these traits change in response to variable food protein–carbohydrate content. For both experiments we found that seasonality per se exerted strong effects on colony and individual level traits. Colonies collected in the summer maintained total worker mass despite high mortality. In contrast, colonies collected in the fall lived longer, and accumulated lipids, including when reared on protein-biased diets. Food macronutrient content had mainly transient effects on physiological responses. Extremes in food carbohydrate content however, elicited a compensatory response in summer worker ants, which processed more protein-biased foods and contained elevated lipid levels. Our study, combined with our previously published work, strongly suggests that underlying physiological phenotypes driving behaviors of summer and fall ants are likely fixed seasonally, and change circannually.     

Utilization of prey from the decomposer system by generalist predators of grassland

We investigated the linkage between the detrital subsystem and generalist predators of meadow ecosystems by manipulating prey availability in two different ways: we increased resource availability for the decomposer subsystem and thereby decomposer prey by adding mulch materials (detritus enhancement), and we added fruitflies (Drosophila melanogaster, Diptera; prey enhancement) to fenced plots. Both supplemented materials significantly differed in their ¹³C/¹²C and ¹⁵N/¹⁴N ratios from those of the natural litter. We measured density responses of detritivorous, herbivorous and predaceous arthropods to the increased resource supply. We used ratios of natural stable isotopes of N and C in arthropod tissues to trace the flux from the added resources to consumers and to relate density responses of consumers to changes in resource supply. Effects of resource enhancement propagated through at least two trophic levels, resulting in higher densities of major decomposer and predator taxa. Effects of detritus enhancement were much stronger than those of prey enhancement. Signatures of δ¹³C proved density responses of Collembola taxa to be related to the added mulch materials. Among generalist predators, densities of juvenile wolf spiders (Lycosidae) responded more to detritus-enhancement than to prey-enhancement treatments. In contrast, the density of the web-building linyphiid and the non-web gnaphosid spiders remained unaffected. Each spider taxon, including those which did not respond numerically, was significantly enriched in ¹³C in detritus-enhancement treatments, suggesting that they gain energy from the decomposer system. Numbers of herbivores—cicadellids and aphids—were similar in each of the treatments, indicating that they were unaffected by changes in predator density. Our results indicate that the lack of a numerical response to resource supplementation is not necessarily due to the absence of a trophic linkage, but may be caused by compensatory changes in mortality factors such as cannibalism and intraguild predation.     

Host range in phytophagous insects: the potential role of generalist predators

Summary The potential role of generalist natural enemies is presented as one of the important ecological pressures that select for narrow host range in phytophagous insects, and dominant relative to physiological bases for specialization. Experiments are described in three completely different systems indicating that generalist herbivores are more vulnerable to predation than specialist herbivores. The three predators were (a) the vespid waspMischocyttarus flavitarsus, (b) the Argentine antIridomyrmex humilis and (c) the coccinellid beetleHippodamia convergens. It is concluded the predators may provide strong selection pressure for maintenance and perhaps evolution of narrow host range in insect herbivores.     

Predator–prey interactions: olfactory adaptations of generalist and specialist predators

1 Olfactory responses of the Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae), a generalist predator, Podisus maculiventris (Say) (Hemiptera, Heteroptera: Pentatomidae) (Pm), and a specialist predator, Perillus bioculatus (F.) (Hemiptera, Heteroptera: Pentatomidae) (Pb) were investigated. Volatiles tested included 20 compounds emitted by undamaged potato plants (Solanum tuberosum), plants that had been artificially damaged, or plants damaged by feeding by CPB larvae. 2 Coupled gas chromatography/electroantennogram detector (GC/EAD) recordings revealed five compounds for which reliable responses were recorded from CPB antennae: (E)-2-hexen-1-ol, (Z)-3-hexen-1-ol, (±)-linalool, nonanal, methyl salicylate, and indole. Both Pm and Pb responded selectively to the same compounds as the CPB with exceptions: (1) (Z)-3-hexenyl butyrate elicited reliable responses for both Pm and Pb, and (2) (E)-2-hexen-1-ol and (Z)-3-hexen-1-ol were inactive for Pm and Pb under these conditions. Dose–response curves showed that CPB was at least 100 times more sensitive to (E)-2-hexen-1-ol than were the predators. Both predators were more sensitive to each of the other compounds than were CPB. Both CPB and Pm were attracted to a five component blend comprising (E)-2-hexen-1-ol, (Z)-3-hexen-1-ol, (±)-linalool, nonanal and methyl salicylate. However, attraction of CPB to the blend occurred only with lower doses of (E)-2-hexen-1-ol and (Z)-3-hexen-1-ol. 3 These results show that the herbivore (CPB) has olfactory receptors which are more sensitive to constitutive host plant volatiles, e.g. green leaf volatiles, while both generalist (Pm) and specialist (Pb) predators are more sensitive to systemic volatiles produced in response to prey feeding. Keywords Colorado potato beetle, constitutive compounds, host plant, induced compounds, olfaction, Perillus bioculatus, Podisus maculiventris, predator, prey, tritrophic.