A survey of identified kairomones and synomones used by insect parasitoids to locate and accept their hosts

Summary The host utilization process of insect parasitoids can be described by three stages of (1)habitatlocation, (2)host-location, and (3)host-acceptance andoviposition. There are 19 systems in which chemicals used inhabitat-location have been identified, 12 systems in which chemical cues leading tohost-location have been identified, and 16 systems in which chemicals elicitinghost-acceptance andoviposition have been identified. Both the chemical class and the source of the infochemical change with the stage of the host utilization process. Semiochemicals identified in thehabitatlocation stage were predominantly aldehydes, alcohols, sulfur-containing compounds, esters and terpenes, and were equally likely to be from the host-plant of the host, or from the host itself. Semiochemicals identified in thehost-location stage were sugars, alkanes, terpenes and heterocyclic aromatic compounds and 3/4 of them were host-produced cues. In thehost-acceptance andoviposition stage the identified semiochemicals were all produced by the host and were proteins, amino acids, triglycerides and salts. The importance of recognizing specific cues involved in host utilization by parasitoids is discussed, and suggestions for future research are made.     

Aphid alarm pheromone (E)-β-farnesene: A host finding kairomone for the aphid primary parasitoidAphidius uzbekistanicus (Hymenoptera: Aphidiinae)

Summary The present study aimed to test the possible function of the aphid alarm pheromone (E)--farnesene (EBF) as a host finding kairomone for aphid primary parasitoids. Extracts of volatile emissions of undisturbed aphids and of aphids under parasitoid attack were obtained by air entrainment. Extracts of cornicle secretions were gained by disturbing aphids and taking their secretions into solution. Extracts were compared by gas chromatography. Only air entrainments of aphids under attack and solvent extracts of cornicle secretions contained the alarm pheromone. In Y-tube olfactometer bioassays, femaleA. uzbekistanicus were attracted to aphid groups under attack of parasitoids, presumably by released EBF. High concentrations of synthetic EBF (1.4 µg to 5.7 µg) also attractedA. uzbekistanicus females. Females with oviposition experience reacted more readily to lower concentrations of EBF than females without experience. In experiments designed as Petri-dish bioassays, the test animals could contact filter paper discs that were treated with solutions containing EBF. Behavioural effects like antennation or stinging attack were not observed. With computer video analysis of parasitoid movements, some effects onA. uzbekistanicus behaviour were detected, again indicating attraction to EBF.As the volatile aphid alarm pheromone attractedA. uzbekistanicus females, it can be concluded that this chemical stimulus acts as a host finding kairomone for this parasitoid species. However, its effect over long distances seems to be limited due to the relatively high concentrations required for reactions. Of two other parasitoid species examined (P. volucre andL. testaceipes) onlyP. volucre was also significantly attracted to the volatile EBF in the Y-tube olfactometer.     

Why do plants ‘talk’?

Summary Plant defence can be induced by herbivory. This is true for both direct and indirect plant defence. Induced direct defence has been the most studied of the two. However, in most cases induced direct defence does not appear to be a water-tight defence option. In contrast, induced indirect defence through the production of herbivore-induced carnivore attractants can be a decisive factor in the extermination of herbivore populations. In this paper the main characteristics of induced attraction of carnivores by plants are reviewed. This includes the similarities and dissimilarities among tritrophic systems. There are two main patterns of induced carnivore attraction. (1) Through the emission of the same bouquet as that emitted by mechanically damaged plants, but in larger quantities and for a longer period of time after damage. (2) Through emission of large amounts of new volatiles that are synthesizedde novo in response to herbivore feeding andnot in response to mechanical wounding.Herbivore populations may be decimated by carnivores. Therefore it should be realized that herbivoreinduced carnivore attractants are essential in an important step in carnivore foraging,i.e. long-distance herbivore location. Once herbivores have started feeding on a plant and direct defence is not effective, induced indirect defence may be decisive for plant survival. Therefore, it is concluded that indirect defence is an essential aspect of induced plant defence directed at herbivorous arthropods.     

Calling characteristics of parasitized and unparasitized populations of the field cricket Teleogryllus oceanicus

Summary Examination of three populations of the field cricket Teleogryllus oceanicus revealed the presence of an acoustically-orienting parasitoid fly, Ormia ochracea (Tachinidae), in the population of crickets that has been introduced to the Hawaiian Islands. The cricket is native to Australia and the Pacific, and the fly is native to North America but has also been introduced to Hawaii. Up to 27% of males and 7% of females in Hawaii were infested with fly larvae. Song structure in the parasitized Hawaiian population was distinct from that of the other two groups, with the Hawaiian crickets showing several reduced song parameters. In addition, onset and cessation of calling at dusk and dawn were more abrupt in the Hawaiian population. These results are consistent with selective pressure from the phonotactic flies to decrease risky calling. Silent males were present in all three populations, suggesting that these noncallers may not represent a unique adaptation to the parasitoid. Correspondence to: M. Zuk     

Precise sex ratios manifested by several encyrtid parasitoids (Hymenoptera: Encyrtidae) of brown soft scale, <Emphasis Type="Italic">Coccus hesperidum</Emphasis> L. (Hemiptera: Coccidae)

We examined whether several facultatively gregarious encyrtid (Hymenoptera: Encyrtidae) endoparasitoids of brown soft scale, Coccus hesperidum L., manifest precise sex allocation under field conditions. Metaphycus luteolus (Timberlake), Metaphycus angustifrons (Compere), Metaphycus stanleyi (Compere), and Microterys nietneri (Motshulsky) evince brood sex ratios that are female-biased and extremely precise (low variance in the number of sons per host). Typically, this sex allocation pattern is attributed to extreme local mate competition (LMC) in which only one foundress exploits a patch of hosts and mating occurs mostly between her offspring. However, such a pattern of sex allocation was not detected for Metaphycus helvolus (Compere). Also, a large proportion of the broods in all five species contained only daughters; thus, an excess of male-only broods was expected if unmated females (i.e., females that can produce only sons) contribute offspring before mating. All-male broods were rare in our samples. This finding coupled with the life history characteristics of these wasps, such as the exploitation of aggregated hosts and the long life span and mobility of males, suggest that nonlocal mating is frequent. Our empirical work suggests that it is advantageous to allocate precise sex ratios in cases in which mating opportunities for males are not restricted to their natal host and/or when multiple foundresses exploit large patches of hosts. Limited theoretical work also supports this prediction but more detailed studies of this taxon’s mating structure and other life history characteristics are necessary to understand their sex allocation decisions.     

Volatile attractants for three Pteromalid parasitoids attacking concealed spruce bark beetles

Summary. The odour perceptive abilities, and preferences, of three bark beetle parasitoid species; Rhopalicus tutela (Walker), Roptrocerus mirus (Walker), and Roptrocerus xylophagorum (Ratzeburg) (Hymenoptera: Pteromalidae), were investigated to isolate and identify the essential compounds involved in host location. These parasitoids attack several economically important bark beetle species and oviposit preferentially on late larval stages concealed under the bark of conifers. Odours were collected from Norway spruce logs (Picea abies L. Karst.) containing Ips typographus L. (Coleoptera: Scolytidae) larvae. Biologically active compounds were isolated by coupled gas chromatographic-electroantennographic detection (GC-EAD), and identified by GC-mass spectrometry (GC-MS). Based on these analyses, four different synthetic baits were prepared and tested in a Y-tube walking bioassay. In the complex odour samples from spruce logs, only 16 compounds were EAD-active. The tested R. tutela and R. mirus females displayed similar trends in antennal activity to EAD-active compounds, responding mainly to oxygenated monoterpenes that indicate damaged conifers. Consequently, the synthetic baits were exclusively prepared with oxygenated monoterpenes. Parasitoid females (R. tutela and R. mirus) preferred spruce logs containing susceptible hosts over fresh logs, while male parasitoids (R. mirus) did not show any preference. However, when odours from fresh logs were mixed with synthetic baits (mimicking the odour composition of logs containing susceptible hosts), these combinations attracted female parasitoids (R. tutela, R. mirus, and R. xylophagorum). All synthetic baits seemed to be equally attractive to female parasitoids. Received 12 October 2000; accepted 18 January 2001     

Habitat Fragmentation and Species Loss across Three Interacting Trophic Levels: Effects of Life-History and Food-Web Traits

Abstract:  Not all species are likely to be equally affected by habitat fragmentation; thus, we evaluated the effects of size of forest remnants on trophically linked communities of plants, leaf-mining insects, and their parasitoids. We explored the possibility of differential vulnerability to habitat area reduction in relation to species-specific and food-web traits by comparing species–area regression slopes. Moreover, we searched for a synergistic effect of these traits and of trophic level . We collected mined leaves and recorded plant, leaf miner, and parasitoid species interactions in five 100-m² transects in 19 Chaco Serrano woodland remnants in central Argentina. Species were classified into extreme categories according to body size, natural abundance, trophic breadth, and trophic level . Species–area slopes differed between groups with extreme values of natural abundance or trophic specialization. Nevertheless, synergistic effects of life-history and food-web traits were only found for trophic level and trophic breadth: area-related species loss was highest for specialist parasitoids. It has been suggested that species position within interaction webs could determine their vulnerability to extinction. Our results provide evidence that food-web parameters, such as trophic level and trophic breadth, affect species sensitivity to habitat fragmentation .     

The response of Trissolcus basalis to footprint contact kairomones from Nezara viridula females is mediated by leaf epicuticular waxes

Chemical footprints left behind by true bugs are perceived as contact kairomones by scelionid egg parasitoids. Female wasps encountering a contaminated artificial substrate display a characteristic arrestment posture, holding the body motionless and antennating the surface. In the system Nezara viridula (L.) and its egg parasitoid Trissolcus basalis (Wollaston), previous studies have shown that the kairomone mediating such behavior is part of N. viridula’s cuticular hydrocarbons (CHC) and furthermore that the wasp’s ability to discriminate host male and female footprints is mainly based on the presence/absence of nonadecane (nC₁₉). In this study, the effect of epicuticular waxes of leaves of broad bean, Vicia faba, on wasp responses to footprints of N. viridula females were investigated. Approximately 20% of T. basalis females displayed an arrestment posture when released on the adaxial leaf surfaces of broad bean plants with intact wax layer and without host chemical contamination; whereas ∼70% of wasps displayed the arrestment posture when intact leaves were contaminated by host female footprints. Adaxial leaf surfaces of broad bean plants dewaxed with an aqueous solution of gum arabic and afterwards contaminated by N. viridula females induced arrestment responses in about 10% of female wasps; the same percentage of arrestment (10%) was observed when the wasps were released on leaves contaminated by host females and subsequently dewaxed. The side of the polymer film that was appressed to the leaf surface, peeled from the contaminated leaves, induced an arrestment posture in about 95% of observed wasps. Scanning electron microscopy (SEM) revealed that the epicuticular waxes occurred as a film densely crystallized as irregularly shaped platelets with spherical granules randomly distributed. These findings demonstrated that epicuticular waxes of broad bean leaves can mediate the foraging behavior of T. basalis females by absorbing contact kairomones of the host.