Shading enhances the quality of willow leaves to leaf beetles – but does it matter?

Plant shading is commonly recognised as a factor, which increases susceptibility of plants to attack by herbivorous insects. In this study we experimentally investigated the effect of host plant shading on two willow-feeding leaf beetles, Galerucella lineola feeding upon Salix phylicifolia and Phratora vitellinae feeding upon Salix myrsinifolia . Both beetle species were more abundant on potted willows growing in open habitats than on the same clones placed under the shade of trees. However, in the laboratory the food preference by adults and larval performance showed that the shaded willows are actually better food for both beetle species. On the contrary, when larvae were reared in the field under natural abiotic conditions, we found no difference in larval performance, or if any, even better performance in open habitats. Apparently, higher and more variable daily temperatures in open habitats accelerated the growth of the larvae. When adults were let to emigrate from or immigrate to potted willows, which had been grown in the same conditions but placed either in the open or shady habitats, adults preferred exposed willows. Invertebrate predators were more abundant in open habitats, but we found no differences in leaf beetle mortality by natural enemies between the habitats. Although the larval performance appeared to be approximately equal in the two habitats during the unusually warm study period, we suggest that under suboptimal temperatures the better abiotic conditions of open sites can easily override the better food provided by shaded habitats. The selection of abiotic habitat thus plays a significant role in the adaptive habitat and host plant selection of these beetles within the gradient of shadiness.     

Host-plant effects on Parasyrphus melanderi (Diptera: Syrphidae) feeding on a willow leaf beetle Chrysomela aeneicollis (Coleoptera: Chrysomelidae)

Abstract.

• 1 Generalist predators are repelled by chrysomelid (Chrysomela spp., Phratora vitellinue L.) larval defensive secretions that are obtained from salicin in their host plants. But little is known about the effect of these secretions on specialist predators.
• 2 In this study, we describe the feeding behaviour of a fly, Parasyrphus melanderi Curran (Diptera: Syrphidae), which feeds on Chrysomela aeneicollis Schaeffer (Coleoptera: Chrysomelidae). Parasyrphus melanderi lays its eggs on C.aeneicollis egg clutches, and its larvae consume C.aeneicollis eggs and larvae.
• 3 Chrysomela aeneicollis hatching rates were significantly lower (20%) on clutches with fly eggs than on clutches without them (40%). Half of the clutches with one fly egg had survival rates below 5%, and when two fly eggs were present (four clutches), the entire clutch was consumed.
• 4 In nature, P.melanderi eggs were 3 times more abundant on a salicylaterich willow species S.orestera Schneider, than on the medium-salicylate S.geyeriana Anderss. (1.8 v 0.6 eggs per clutch). On 18% of the S.orestera clones, all the beetle clutches contained fly eggs. In laboratory-choice tests, P.melanderi larvae fed equally rapidly on C.aeneicollis larvae that were chemically defended (feeding on S.orestera) as on larvae that produced no secretion (feeding on the salicylate-poor S.lutea Nutt.). This predator does not appear to be deterred by C.aeneicollis's defensive secretion. We discuss the implications of specialist predators on determining host suitability to herbivorous insects.

     

THE EVOLUTION OF HOST-PLANT USE AND SEQUESTRATION IN THE LEAF BEETLE GENUS PHRATORA (COLEOPTERA: CHRYSOMELIDAE)

Leaf beetles in the genus Phratora differ in host plant use and in the chemical composition of their larval defensive secretion. Most species specialize on either poplars or willows (family Salicaceae), but two species feed on birch (family Betulaceae). Phratora vitellinae utilizes salicylates from the host plant to produce its larval secretion, which contains salicylaldehyde, while other Phratora species produce an autogenous secretion. To reconstruct the evolutionary history of host plant use and the larval secretion chemistry in this genus, we sequenced 1383 base pairs of the mt cytochrome oxidase I gene for six European and one North American Phratora species and three outgroup taxa. Bootstrap values of the complete nucleotide sequence were 99-100% for six of eight nodes in the maximum parsimony tree. They were 71% and 77% for the two other nodes. The maximum parsimony tree and the maximum likelihood tree based on nucleotide sequence showed the same relationships as a maximum parsimony tree based on the amino acid sequence. Beetle phylogeny overlapped broadly with host plant taxonomy and chemistry, and it revealed historical constraints influencing host plant use. However, there was one host shift from the willow family (Salicaceae) to the birch family (Betulaceae). The use of host plant phenol glycosides for the larval defensive secretion evolved along the lineage that led to P. vitellinae. Phratora vitellinae feeds on the taxonomically widest range of host plants, which are characterized by moderate to high levels of salicylates. The results support the hypothesis that the use of salicylates for the larval secretion evolved twice independently in chrysomeline leaf beetles.     

The function of turret building behaviour in the larval tiger beetle, Cicindela willistoni (Coleoptera: Cicindelidae)

Abstract. 1. Cicindela willistoni LeConte (Coleoptera: Cicindelidae) adults and larvae occur only on saline and alkali flats with little or no vegetation in southwestern U.S.A. Unlike other larval tiger beetle species, C.willistoni constructs a chimney-like extension (turret) 1–4 cm above its burrow.
2. The function of these unique turrets was tested in relation to several biotic and abiotic factors in the Sulphur Springs Valley, Arizona, U.S.A.
3. The turrets did not prevent flooding of the larval burrows.
4. They did not prevent or reduce the frequency of parasitoids (mainly bombyliid flies of the genus Anthrax ).
5. The turrets did not make it possible for the adult and larval beetles to feed on obviously different types or sizes of prey.
6. However, larvae with turrets were raised sufficiently above the substrate surface and boundary layer to allow them to thermoregulate in the lower temperatures at these heights and remain active throughout the day.
7. The shade produced by the turrets attracted significantly more potential prey items than were available to turretless larvae in the same habitat.     

To be or not to be convergent in salicin-based defence in chrysomeline leaf beetle larvae: evidence from Phratora vitellinae salicyl alcohol oxidase

Glandular chemical defence relying on the action of salicylaldehyde is characteristic for Chrysomela leaf beetle larvae. The salicylaldehyde precursor salicin, sequestered from salicaceous host plants, is deglucosylated and the aglycon further oxidized by a salicyl alcohol oxidase (SAO) to the respective aldehyde. SAOs, key enzymes in salicin-based glandular chemical defence, were previously identified and shown to be of a single evolutionary origin in Chrysomela species. We here identified and characterized SAO of Phratora vitellinae, the only species outside the genus Chrysomela that produce salicylaldehyde as a defensive compound. Although Chrysomela and Phratora are not closest relatives, their SAOs share glucose-methanol-choline oxidoreductase (GMC) affiliation, a specific GMCi subfamily ancestor, glandular tissue-specific expression and almost identical gene architectures. Together, this strongly supports a single origin of SAOs of both Chrysomela and Phratora. Closely related species of Chrysomela and P. vitellinae use iridoids as defensive compounds, which are like salicylaldehyde synthesized by the consecutive action of glucosidase and oxidase. However, we elucidated SAO-like sequences but no SAO proteins in the glandular secretion of iridoid producers. These findings support a different evolutionary history of SAO, related genes and other oxidases involved in chemical defence in the glandular system of salicylaldehyde and iridoid-producing leaf beetle larvae.     

Fly parasitoid Megaselia opacicornis uses defensive secretions of the leaf beetle Chrysomela lapponica to locate its host

Larvae of the leaf beetle Chrysomela lapponica derive a defensive secretion from salicyl glucosides found in the host plant Salix borealis. This secretion protects beetle larvae from some natural enemies, but does not appear to repel parasitoids. We tested the hypothesis that the fly parasitoid Megaselia opacicornis (Diptera, Phoridae) uses the larval defensive secretion of Ch. lapponica in its search for prey. In the field, nearly 30 times more M. opacicornis individuals were caught on leaves coated with sticky resin next to a source of secretion than on control leaves. In the laboratory, M. opacicornis females laid six times more eggs next to a cotton ball soaked in secretion than next to one soaked in water. Fly females also lay more eggs on prey rich in larval secretion than on secretion-poor prey. In the field, removal of defensive secretion from beetle prepupae resulted in a 7.5-fold reduction of oviposition by fly females. Parasitoids were nearly twice as likely to lay eggs on prepupae, rich in secretion, as on pupae, which contain little secretion. Fly offspring reared from beetle prepupae reached a 21% larger body mass than those reared from pupae. Finally, M. opacicornis females avoided host prepupae already parasitized by the tachinid fly Cleonice nitidiuscula, which possess little secretion. These experiments indicate that host plant-derived defensive secretions are used by this parasitoid for host location. Adaptation of parasitoids to use defensive secretions of hosts may selectively favor an increase in diet breadth in specialist herbivores.     

Selective predation by larval Agabus (Coleoptera: Dytiscidae) on mosquitoes: support for conservation-based mosquito suppression in constructed wetlands

1. Wetland insect predators can structure aquatic prey communities via selective predation, but receive considerably less attention than vertebrate predators. We conducted laboratory experiments to test selective predation by two species of larval dytiscid beetles ( Agabus ; Coleoptera: Dytiscidae) and the potential contribution of these beetles to suppression of mosquito populations in constructed wetlands.
2.  Agabus consumed copepods, ostracods and mosquito larvae in no-choice tests. When offered a choice, 76% of all prey consumed were mosquito larvae, indicating selective predation. Subsequent experiments revealed this preference was due to ease of capture of mosquito larvae over alternative prey.
3. Cannibalism and intraguild predation were common within and between species of Agabus , which may reduce the overall impact of the observed selective predation.
4.  Agabus larvae selectively preyed on mosquito larvae over alternative prey, which is not characteristic of some fish used as biological control agents for mosquitoes. Predator exclusion or similar experiments in the field could document how these results translate into a natural setting.
5. The findings of this study suggest developing mosquito suppression strategies focused on conservation of native wetland predators. These strategies are preferable to introducing non-native generalist predators, or applying pesticides.     

Oviposition of the dung beetle Aphodius ater in relation to the abundance of yellow dungfly larvae (Scatophaga stercoraria)

1. The dung beetle Aphodius ater and the yellow dungfly Scatophaga stercoraria are temporally co-occurring species in sheep dung, which they use for reproduction and nutrition ( A. ater ) or for reproduction only ( S. stercoraria ) during the spring in northern Germany. Scatophaga stercoraria uses fresh sheep dung pellets a few hours old for oviposition, whereas A. ater lays eggs into 2–10-day-old pellets. In the present study, the egg laying behaviour of A. ater in sheep dung in relation to the presence of larvae of S. stercoraria was investigated experimentally.
2. Choice experiments, based on examining the egg laying behaviour of beetles in 2- and 4-day-old pellets with and without high and low densities of fly larvae, showed the following. In 2-day-old pellets, the beetles did not distinguish between pellets without fly larvae or with fly larvae at low larval density but avoided laying eggs into pellets with a high larval density. In 4-day-old pellets, the beetles always preferred to lay their eggs into pellets without fly larvae, regardless of larval density.
3. The influence of different densities of larvae of S. stercoraria on dung depletion was examined by measuring the dry weight, organic matter content and organic nitrogen content of the remaining dung after larval development. The presence of the larvae led to a reduction in all three parameters.
4. The beetles' behaviour of laying eggs into older pellets, and their awareness of the presence of high densities of fly larvae, enables them to avoid egg laying into pellets that will have been depleted by fly larvae before the beetle larvae have finished their development.     

Size class structure and tree dispersion patterns in old-growth cedar-hemlock forests of the northern Rocky Mountains (USA)

Summary The relationship between the food selection of four leaf beetle species (Phratora vitellinae, Plagiodera versicolora, Lochmaea capreae, Galerucella lineola) and the phenolic glycosides of willow (Salix spp.) leaves was tested in laboratory food choice experiments. Four willow species native to the study area (Eastern Finland) and four introduced, cultivated willows were tested.The willow species exhibited profound differences in their phenolic glycoside composition and total concentration. The food selection patterns of the leaf beetles followed closely the phenolic glycoside spectra of the willow species. Both the total amount and the composition of phenolic glycosides affected the feeding by the beetles. Phenolic glycosides apparently have both stimulatory and inhibitory influences on leaf beetle feeding depending on the degree of adaptation of a particular insect. Very rare glycosides or exceptional combination of several glycoside types seem to provide certain willow species with high level of resistance against most herbivorous insects. Analogously the average absolute amount of leaf beetle feeding was lower on the introduced willows than on the native species to which the local herbivores have a good opportunity to become adapted.     

Salicin from host plant as precursor of salicylaldehyde in defensive secretion of Chrysomeline larvae

ABSTRACT. Phratora vitellinae L. and Chrysomela tremulae F. (Chrysomelinae, Coleoptera) feed on Salix or Populus spp. (Salicaceae). Their larvae, as well as the larvae of other chrysomelines feeding on Salicaceae, secrete salicylaldehyde. In this study, we demonstrate that salicylaldehyde is derived from salicin, a phenylglucoside present in the leaves of the host plant. The concentration of salicylaldehyde in the secretion is positively correlated with the amount of salicin in the food of the larvae. The transformation of salicin into salicylaldehyde occurs in the defence glands since the β-glucosidase activity is 4 times higher in their glands than in the gut. The larvae recover most of the glucose that results from the hydrolysis of salicin. For generalist predators, such as ants, salicylaldehyde is a more potent deterrent than saligenin or salicin.     

Predaceous diving beetle, Dytiscus sharpi sharpi (Coleoptera: Dytiscidae) larvae avoid cannibalism by recognizing prey

Larvae of diving beetles such as the various Dytiscus species (Coleoptera: Dytiscidae) are carnivorous and usually prey on other aquatic animals. Cannibalism among larvae of Dytiscus sharpi sharpi (Wehncke) was observed to begin when they were starved for more than two days under artificial breeding conditions. However, the 2-day starved larvae did not show cannibalism in the presence of intact, motionless, frozen tadpoles, or frozen shrimps. The beetle larvae attacked and captured intact tadpoles faster (15 sec) than other motionless and frozen tadpoles (120 sec), indicating that prey movement was an important factor in stimulating feeding behavior in larvae. Prey density does not have an effect on larval cannibalism. In cases in which preys are present at lower densities than that of larvae, a group of beetle larvae frequently fed on single prey. This feeding behavior, therefore, provides direct evidence of self-other recognition at the species level. Using two traps in one aquarium that allows the larvae to detect only prey smell, one containing tadpoles and another empty, the beetle larvae were attracted to the trap with tadpoles at high frequency, but not to the empty trap. In another experiment, the beetle larvae were not attracted to the trap containing a beetle larva. These results suggest that the larvae of D. sharpi sharpi are capable of recognizing prey scent, which enables the promotion of foraging behavior and the prevention of cannibalism.     

The importance of specialist natural enemies for Chrysomela lapponica in pioneering a new host plant

Abstract.  1. The Salicaceae have been suggested as ancestral host plants of Chrysomela species (Coleoptera: Chrysomelidae). In Chrysomela lapponica , some populations are specialised on salicaceous plants, but others have switched to birch. This study aimed to elucidate the significance of natural enemies as possible selective forces for the host plant shift of C. lapponica from willow to birch.
2. Two C. lapponica populations were studied, one specialised on willow Salix borealis in Finland, and another one specialised on birch Betula pubescens in the Czech Republic. Abundances of predators and parasitoids on birches and willows were recorded at both population sites. Furthermore, field and laboratory experiments were conducted.
3. Field data do not support the hypothesis that generalist predators affected the host shift from willow to birch in C. lapponica.
4. Parasitism of C. lapponica (pre)pupae by a specialised phorid fly was significantly stronger in specimens living on willow than in birch-living ones.
5. The predatory syrphid Parasyrphus nigritarsis specialised on Chrysomelinae was only detected on willows. The syrphid preferred to orient towards substrates treated with defensive larval secretion or faeces of the willow-specialised C. lapponica specimens compared with the birch-specialised ones.
6. The data suggest that specialised parasitoids and predators might have been driving forces for C. lapponica to leave willows and to pioneer birches as sites with a lowered risk of predation and parasitism. This hypothesis is discussed with respect to results of earlier studies on the impact of bottom-up effects by the plant.     

Intra- and interspecific effects of larval secretions in some chrysomelids (Coleoptera)

The larval secretions of the Chrysomelinae (Coleoptera, Chrysomelidae) are generally considered to be chemical defense substances against predators and parasites. The experiments presented in this paper provide evidence that the range of activity of these larval secretions also extends to interactions between different conspecific developmental stages and between competing phytophagous species. Four chrysomeline species were tested: Gastrophysa viridula De Geer, Phaedon cochleariae (F.), Plagiodera versicolora (Laich.), and Phratora vitellinae (L.). In the latter species, we did not test the intraspecific effect of the natural larval secretion, but the activity of its purchasable main component, salicylaldehyde (synthetic larval secretion). In G. viridula, Ph. cochleariae, and Ph. vitellinae the (synthetic) larval secretions act as oviposition deterrents against conspecific females. An investigation of the persistence of the oviposition-deterring activity of the larval secretion in G. viridula revealed that the deterrent effect is lost after 20 to 26 hours. In each of the four tested species the (synthetic) larval secretion deters feeding of conspecific adults. Since P. versicolora and Ph. vitellinae may occur on the same host plant (e.g. the willow Salix fragilis L.), the interspecific effect of the larval secretion between competing phytophagous species was examined in these two willow leaf beetles. The larval secretions of both species and salicylaldehyde, respectively, act as interspecific repellent and feeding deterrent against adults of the other species.

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Zusammenfassung Die Sekrete der Larven der Chrysomelinae (Fam. Chrysomelidae) werden im allgemeinen als chemische Abwehrstoffe gegen Prädatoren und Parasiten betrachtet. Die hier dargestellten Untersuchungen zeigen, daß die Wirkung der Larvensekrete nicht nur auf die Feindabwehr begrenzt ist. Das Wirkungsspektrum umfaßt darüber hinaus auch Interaktionen zwischen verschiedenen Entwicklungsstadien einer Art bzw. zwischen konkurrierenden phytophagen Arten. Folgende Chrysomelinenarten wurden untersucht: Gastrophysa viridula De Geer, Phaedon cochleariae (F.), Phratora vitellinae (L.) und Plagiodera versicolora (Laich.). Da das Larvensekret von Ph. vitellinae Salicylaldehyd als Hauptkomponente enthält, wurde hier-wenn nicht anders erwähnt-die Wirkung von synthetischem Salicylaldehyd getestet. Bei G. viridula, Ph. cochleariae und Ph. vitellinae wirkt das Larvensekret bzw. die authentische synthetische Substanz als hochwirksames Eiablage-Deterrens auf konspezifische Weibchen. Die eiablagehemmende Wirkung des Larvensekretes von P. versicolora wurde nicht getestet. Eine Untersuchung der Wirkungsdauer der eiblagehemmenden Aktivität des Larvensekretes von G. viridula ergab, daß die Hemmwirkung nach 20 bis 26 Stunden nicht mehr nachweisbar ist. Bei allen oben genannten vier Arten hemmt das. Larvensekret bzw. dessen authentische synthetische Substanz den Fraß von Adulten derselben Art. Diese fraßhemmende Aktivität ist bei G. viridula, Ph. cochleariae und Ph. vitellinae mindestens 18 Stunden wirksam. Das Larvensekret von P. versicolora zeigte bei 18-stündiger Versuchsdauer keine fraßhemmende Wirkung auf Adulte derselben Art; erst bei Reduktion der Versuchsdauer auf drei Stunden konnte hier eine signifikant fraßhemmende Wirkung nachgewiesen werden. P. versicolora und Ph. vitellinae können als Weidenblattkäfer an derselben Wirtspflanze auftreten. Deshalb wurde auch die interspezifische Wirkung der Larvensekrete dieser beiden Chrysomelinenarten geprüft. Das Larvensekret von Ph. vitellinae sowie auch dessen authentische synthetische Substanz (Salicylaldehyd) wirkt auf adulte P. versicolora als Repellent und Fraßhemmstoff. Ebenso wirkt umgekehrt das Larvensekret von P. versicolora abschreckend und fraßhemmend auf adulte Ph. vitellinae.

     

Facilitated predation through interaction between life stages in the stinkbug predatorperillus bioculatus (Hemiptera: Pentatomidae)

Competitive interactions in arthropod predators are well-known, but positive interactions have received less attention. The two-spotted stinkbugPerillus bioculatus often feeds gregariously on leaf beetle larvae and caterpillar prey. Consequences of prey sharing amongP. bioculatus conspecifics of dissimilar size (instar) was studied using Colorado potato beetle (CPB) prey. Rearing second-instar (N2) nymphs ofP. bioculatus with an N5 conspecific facilitated early feeding on L4 CPB larvae (a difficult prey to handle by N2 nymphs but not by N5’s), thus increasing survival and accelerating development. One in every 20 cases ofP. bioculatus foraging in the field was accounted for by pairs or small groups of mostly feeding individuals. CPB egg masses and L4’s represented a disproportionate number of cases of aggregated feeding byP. bioculatus, compared to feeding singly. Small CPB larvae decreased in the diet of aggregated stinkbugs compared to L4 larvae and egg masses, suggesting that sharing these prey may be favorable or unavoidable. In a field test measuring residence/survival of N2’s limited to L4 prey, the N2’s rate of residence/survival increased significantly when large nymphs acting as food providers were also present. The function of communal feeding inP. bioculatus is discussed, as well as the potential for greater impact on prey density that may be expected from tolerance to opportunistic feeding by conspecifics in slightly gregarious predators.     

Characterization of an extracellular salicyl alcohol oxidase from larval defensive secretions of Chrysomela populi and Phratora vitellinae (Chrysomelina)

Larvae of a number of chrysomelid leaf beetles sequester phenol glucosides such as salicin from their food plants, i.e. Salix and Populus spp. Salicin is hydrolyzed in the glandular reservoir of the defensive glands. The resulting salicyl alcohol (saligenin) is oxidized by an extracellular oxidase. The product salicylaldehyde accumulates as major defensive compound. The secretions from Chrysomela populi and Phratora vitellinae were preserved in saturated ammonium sulfate solution and subjected to micro-purification of the oxidase by means of electrophoretic methods. The enzyme from P. vitellinae has a native M(r) of 334,000 and a subunit M(r) of 79,000 indicating a tetrameric enzyme. The isoelectric points of the enzymes from C. populi and P. vitellinae are at pH 5.4 and 5.2, respectively. In the oxidation of salicyl alcohol oxygen functions as electron acceptor yielding hydrogen peroxide as product. Hydrogen peroxide does not accumulate in native secretions but appears to be degraded most likely by a catalase. The oxidases from the two species show broad pH optima in the range 5.5 to 6.5, they oxidize salicyl alcohol as main substrate. Minor substrates are several ortho-substituted and to a lesser extent meta- but not para-substituted benzyl alcohols. In the presence of 8-hydroxygeraniol only trace amounts of the respective aldehyde are formed. The Km values of salicyl alcohol are 132 mM (C. populi) and 63 mM (P. vitellinae). The extracellular enzyme, which is functionally related to fungal aryl alcohol oxidase (EC 1.1.3.7) and vanillyl alcohol oxidase (EC 1.1.3.38) was named salicyl alcohol oxidase. The continuous formation of salicylaldehyde in the glandular reservoir can be compared to the operation of an enzyme reactor. Due to its low aqueous solubility the produced aldehyde steadily leaves the aqueous reaction fluid and builds up an organic phase which may account for 15% of the total liquid volume of the secretion.     

Predation by Pterostichus melanarius (Illiger) (Coleoptera: Carabidae) on immature Rhagoletis mendax Curran (Diptera: Tephritidae) in semi-field and field conditions

Practices that enhance abundance and diversity of generalist predators are often employed with the objective of improving biological control of insect pests. Ground beetles and other predators can prey on blueberry maggot, an important pest of blueberries, when mature larvae pupate in the ground. We conducted mesocosm and field experiments to determine if Pterostichus melanarius, a common predatory ground beetle, lowers maggot numbers in compost mulch or when predator and alternative prey abundances are manipulated. At background (field) densities of alternative prey, increasing densities of P. melanarius did not significantly reduce pest numbers in mesocosms containing compost or soil. When alternative prey were removed from compost, beetles reduced pest numbers by up to 35%. In field experiments, maggot numbers were higher when beetles and other predators were excluded from soil plots, but beetle exclusion had no effect in compost plots where both predator and alternative prey numbers were high. Our results indicate that there can be some reduction of blueberry maggot by P. melanarius and other potential predators when there are few alternative prey. However, despite attracting large numbers of predators compost mulch did not lead to a significant reduction in blueberry maggot; in fact, the high abundance of alternative food associated with compost appeared to interfere with beetle predation on blueberry maggot.     

Can herbivore‐induced plant volatiles inform predatory insect about the most suitable stage of its prey?

The leaf beetle Plagiodera versicolora (Coleoptera: Chrysomelidae) is a specialist herbivore, all of whose mobile stages feed on the leaves of salicaceous plants. Both the larval and adult stages of the ladybird Aiolocaria hexaspilota (Coleoptera: Coccinellidae) are dominant natural enemies of the larvae of the leaf beetle. To clarify the role of plant volatiles in prey‐finding behaviour of A. hexaspilota, the olfactory responses of the ladybird in a Y‐tube olfactometer are studied. The ladybird adults show no preference for willow plants Salix eriocarpa that are infested by leaf beetle adults (nonprey) over that for intact plants but move more to the willow plants infested by leaf beetle larvae (prey) than to intact plants. Moreover, ladybird larvae show no preference for willow plants infested by leaf beetle larvae or adults over intact plants. Using gas chromatography‐mass spectrometry, six volatile compounds are released in larger amounts in the headspace of willow plants infested by leaf beetle larvae than in the headspace of willow plants infested by leaf beetle adults. In addition, the total amount of volatiles emitted from willow plants that are either intact or infested by leaf beetle adults is much smaller than that from willow plants infested by leaf beetle larvae. These results indicate that volatiles from S. eriocarpa infested by P. versicolora inform A. hexaspilota adults about the presence of the most suitable stage of their prey, whereas A. hexaspilota larvae do not use such information.     

Influence of leaf trichome density on the efficiency of two polyphagous insect predators

Plant characteristics, such as leaf structure or hairiness, are important for the movement and attachment of insects. It has been suggested that increased trichome density on new Salix cinerea L. (Salicaceae) leaves, produced after grazing by the willow leaf beetle Phratora vulgatissima L. (Coleoptera: Chrysomelidae), function as an inducible defence against the beetle and especially its larvae. Here we studied whether and how two of the main natural enemies of P. vulgatissima, viz., Anthocoris nemorum L. (Heteroptera: Anthocoridae) and Ortothylus marginalis L. (Heteroptera: Miridae), were influenced by trichome density on S. cinerea leaves. The effect of trichome density on these two predators was studied on plants with different trichome densities, comparing natural enemy efficiency, measured as number of P. vulgatissima eggs consumed or larvae missing and/or killed. To obtain different trichome densities, cuttings of several different clones of S. cinerea were used. In the experiment using eggs as prey, an increase in trichome density was, in addition, induced through leaf beetle defoliation on half of the plants of each willow clone. Furthermore, a field study was performed to investigate whether trichome density was correlated with natural enemy abundance. The results indicate that neither the efficiency of these two natural enemies in the greenhouse, nor their abundance in the field was influenced by trichome density. A well‐known behavioural difference between the two predator species could probably account for the higher disappearance of larvae after exposure to the more active predator. These findings are relevant for the development of pest management programs, not least because the enemies are polyphagous predators. It is concluded that an induced increase in leaf hairiness in willows in response to leaf beetle grazing could be a plant resistance trait worthy of further study in this system, because no negative effects on the main natural enemies were observed.     

Prey perception of predation risk: volatile chemical cues mediate non-consumptive effects of a predator on a herbivorous insect

Predators can affect prey in two ways—by reducing their density (consumptive effects) or by changing their behavior, physiology or other phenotypic traits (non-consumptive effects). Understanding the cues and sensory modalities prey use to detect predators is critical for predicting the strength of non-consumptive effects and the outcome of predator–prey encounters. While predator-associated cues have been well studied in aquatic systems, less is known about how terrestrial prey, particularly insect larvae, detect their predators. We evaluated how Colorado potato beetle, Leptinotarsa decemlineata, larvae perceive predation risk by isolating cues from its stink bug predator, the spined soldier bug, Podisus maculiventris. When exposed to male “risk” predators that were surgically manipulated so they could hunt but not kill, beetles reduced feeding 29 % compared to controls. Exposure to risk females caused an intermediate response. Beetles ate 24 % less on leaves pre-exposed to predators compared to leaves never exposed to predators, indicating that tactile and visual cues are not required for the prey’s response. Volatile odor cues from predators reduced beetle feeding by 10 % overall, although male predators caused a stronger reduction than females. Finally, visual cues from the predator had a weak effect on beetle feeding. Because multiple cues appear to be involved in prey perception of risk, and because male and female predators have differential effects, beetle larvae likely experience tremendous variation in the information about risk from their local environment.     

Interspecific differences in movement and hunting success in damselfly larvae (Zygoptera: Insecta): responses to prey availability and predation threat

SUMMARY. 1. Freshwater invertebrate predators can alter their hunting behaviour in response to prey availability and threat from their own enemies. Movement patterns and hunting success of the larvae of two species of damselfly (Zygoptera: Insecta), Enallagma cyathigerum (L.) and L. sponsa (Hansemann), were monitored in four combinations of prey availability and enemy threat. Aeshna juncea (L.) (Anisoptera: Insecta) larvae acted as predator and Daphnia magna Straus (Cladocera: Crustacea) as prey in the combinations: no predator/no prey; one Aeshna present/no prey; no predator/thirty Daphnia present; one Aeshna present/ thirty Daphnia present.
2. E. cyathigerum showed significantly reduced movement in the +Aeshna/no prey treatment but hunting success was not significantly affected. No other treatment effects were noted. L. sponsa movement patterns differed significantly across all four treatments and hunting success was significantly reduced in the presence of Aeshna .
Interspecifically, movement patterns of the two species differed markedly in all four treatments. L. sponsa larvae were much more active, and caught many more prey. Despite their activity L. sponsa larvae did not appear markedly more vulnerable than the immobile E. cyathigerum .
3. The interspecific differences between the two damselflies reflect predictions based on larval life-history. Activity patterns and ability to capture adequate prey under varying levels of predation may be important in the ecology of damselfly species.