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.     

Susceptibility of willow clones (Salix spp.) to herbivory by Phyllodecta vulgatissima (L.) and Galerucella lineola (Fab.) (Coleoptera, Chrysomelidae)

Levels of damage by mixed natural infestations of the leaf-feeding chrysomelid beetles, Phyllodecta vulgatissima (L.) (the blue willow beetle) and Galerucella lineola (Fab.) (the brown willow beetle), were determined in replicated field plots of 24 Salix clones at Long Ashton (Bristol, UK) during 1993–94. Over the same period, the host plant preferences of both chrysomelids were investigated in a standard multiple-choice laboratory procedure, where beetles were enclosed in Petri dishes with leaf discs cut from young pot-grown trees propagated from shoot cuttings taken from 20 of the 24 willow clones represented in the field study. The laboratory experiments indicated that P. vulgatissima and G. lineola had similar host plant preferences in the range of willows examined (r >0.85). In both field and laboratory, the least preferred Salix clones and hybrids were those of 5. eriocephala, followed by S. purpurea, S. burjatica, S. dasyclados and S. triandra. Clones of S. eriocephala and S. purpurea were frequently rejected altogether in laboratory tests. Most preferred were clones of S. viminalis and several hybrids of S. viminalis, S. aurita, S. caprea and S. cinerea. These results substantiate the reports that P. vulgatissima and G. lineola are deterred from feeding on willows which have relatively high concentrations of phenolic (salicylate) glucosides in the leaves. The least preferred willows, particularly S. eriocephala, S. purpurea and S. burjatica, could be of great potential value in plant breeding for resistance to these willow beetle pests.     

Chlorogenic acid as an antiherbivore defence of willows against leaf beetles

In the leaves of 13 Finnish willow species, the content of a phenolic, chlorogenic acid, was found to vary from 0 up to 18 mg g^–1 D.W. Effects of pure chlorogenic acid on insect feeding behaviour were tested using four common leaf beetle species which are in the field mainly found on willows with low-chlorogenic acid leaves. One species, Lochmaea capreae L., was invariably deterred by pure chlorogenic acid applied in naturally occurring concentrations on the willow leaves. Accordingly, in 2-choice laboratory feeding trials L. capreae was found to prefer low-chlorogenic acid leaves of four willow species over high-chlorogenic acid leaves of Salix pentandra L. and S. myrsinifolia Salisb. When presented on the leaves of S. phylicifolia L, pure chlorogenic acid inhibited also the feeding by Phratora polaris Sp.-Schn. Instead, chlorogenic acid had no significant effect on Ph. polaris when it was presented on the leaves of another willow S. cinerea L. In laboratory, Ph. polaris did not show general preference for willow species with low chlorogenic acid content in their leaves. Thus, the response of Ph. polaris to chlorogenic acid seems to depend on the plant species. Apparently variation in other traits such as leaf hairyness may easily override the potential effect of chlorogenic acid content on Ph. polaris. To two other leaf beetle species, Galerucella lineola F. and Plagiodera versicolora Laich., chlorogenic acid is an ineffective deterrent even at unnaturally high concentrations. In laboratory, G. lineola and P. versicolora did not prefer willows with low chlorogenic acid content in their leaves. Thus, among four studied leaf beetle species, only L. capreae seems to be clearly affected by this phenolic. Therefore, overall importance of chlorogenic acid as a defence against willow-feeding leaf beetles appears to be very limited.     

Plant phenology-mediated indirect effects: The gall midge opens the phenological window wider for a leaf beetle

We examined whether larvae of the gall midge Rabdophaga rigidae (Diptera: Cecidomyiidae) can modify the seasonal dynamics of the density of a leaf beetle, Plagiodera versicolora (Coleoptera: Chrysomelidae), by modifying the leaf flushing phenology of its host willow species, Salix serissaefolia and Salix eriocarpa (Salicaceae). To test this, we conducted field observations and a laboratory experiment. The field observations demonstrated that the leaf flushing phenology of the willows and the seasonal dynamics of the beetle density differed between shoots with stem galls and shoots without them. On galled shoots of both willow species, secondary shoot growth and secondary leaf production were promoted; consequently, leaf production showed a bimodal pattern and leaf production periods were 1 to 2 months longer than on non‐galled shoots. The adult beetle density on galled shoots was thus enhanced late in the season, and was found to change seasonally, synchronizing with the production of new leaves on the host willow species. From the results of our laboratory experiment, we attributed this synchrony between adult beetle density and willow leaf flush to beetles’ preference to eat new leaves rather than old. Indeed, beetles consumed five times more of the young leaves when they were fed both young and old leaves. These results indicate that stem galls indirectly enhance the adult beetle density by enhancing food quality and quantity late in the beetle‐feeding season. We therefore conclude that midge galls widen the phenological window for leaf beetles by extending the willows’ leaf flush periods.     

Beetle assemblages on willow trees: do phenolic glycosides matter?

Herbivorous beetles were sampled in central Slovakia and in the Tatra Mountains of northern Slovakia from seven different Salix species which are partly characterized by smooth leaves containing phenolic glycosides and partly by hairy leaves containing tannins. The collection included about 8000 individuals representing 129 species. Of the 129 species, 77 species are able to use the willows as hosts; the remaining 52 ‘tourist’ species comprise less than 3% of the collected individuals. The data on species richness and abundance did not generally support the ‘feeding specialization’ hypothesis. The proportion of specialized (= monophagous and oligophagous) beetles feeding on willows of both morphological and biochemical groups was in the same range. Host plants of the two groups could support high diversity and high evenness values, even when leaf characteristics and plant chemicals largely influenced species assemblage. The region in which the willow trees grow had a considerable impact on host plant use. Generalist beetle species predominated in central Slovakia. By contrast, in the Tatra Mountains, specialist feeders which are able to use phenolic glycosides to their advantage were predominant. The number of species and the total density of individuals collected from willows containing phenolic glycosides (S. fragilis and S. purpurea) did not usually vary between the two regions. In contrast, fewer species and individuals were found in the Tatra Mountains when they settled on willow species containing tannins (S. caprea and S. cinerea). Also, the phylogenetic status of host plants affected species assemblages. In central Slovakia willow species of the subgenus Vetrix (S. purpurea, S. caprea and S. cinerea) generally showed a higher beetle diversity (H_s) than species of the subgenus Salix (S. fragilis, S. alba and S. triandra), although both subgenera comprise species of both morphological and biochemical groups. Furthermore, when the analysis was restricted to beetles of central Slovakia, which should be most adapted to their host plants (i.e. catkin feeders and phyllophages in the adult and larval stage), the phylogenetic status was found to be more important than any single leaf character measured.     

Role of olfaction in host plant selection and local adaptation of a polyphagous herbivore,Eucolaspis Sharp

Host plant cues are known to shape insect–host plant association in many insect groups. More pronounced associations are generally manifested in specialist herbivores, but little is known in generalist herbivores. We used a polyphagous native beetle from New Zealand, bronze beetle, Eucolaspis sp. ‘Hawkes Bay’ (Chrysomelidae: Eumolpinae) to explore the role of olfaction in locating host plants and local adaptation. We also tested the role of other cues in the degree of acceptance or rejection of hosts. Adult Eucolaspis beetles were attracted to fresh leaf volatiles from apple and blackberry (Rosaceae). Male and female beetles responded similarly to olfactory cues of host plants. An indication of evolutionary affiliation was observed in olfactory preferences of geographically isolated conspecific populations. We found that geographically isolated populations of the beetles differ in their olfactory responses and exhibit some degree of local adaptation. However, irrespective of geographical and ecological associations, blackberry was preferred over apple as a feeding plant, and another novel plant, bush lawyer (Rubus australis), was readily accepted by 53.25% of the tested beetles. We show that plant volatiles play an important role in host location by Eucolaspis, but the acceptance or rejection of a particular host could also involve visual and contact cues.     

Insect feeding preferences and plant phenolic glucosides in the system Gonioctena linnaeana–Salix triandra

The aim of this study was to investigate the preference of a leaf beetle to different Salix clones, and to relate this preference to plant chemistry. The preference of Gonioctena linnaeana Schrank (Coleoptera: Chrysomelidae) was tested in cafeteria experiments using its host Salix triandra L. (Salicaeae) from three Swedish isolated populations and one Russian population from the main distribution range of the host. The leaves from the different host clones were used to analyse the content of phenolic glucosides and the amount of condensed tannins. The larvae did not show any feeding preferences for host clones from the population they originated from, but all Swedish host populations were preferred over the Russian host population. This suggests a preference for regional hosts. We analyzed whether leaf chemistry parameters may explain host plant preferences. Chemical analysis of the leaves showed that the quantities, but not the quality, of the phenolic compounds differed between populations. A Principal Component Analysis of the chemical data also highlighted a difference between the Swedish and Russian host plant populations. The two most important compounds separating Russian from Swedish clones were gallocatechin and salidroside. However, the difference in preference could also be a result of, for example, nutritional quality, water content, or leaf texture. The relative importance of these different factors should be evaluated in future experiments.     

Host-plant effects on larval survival of a salicin-using leaf beetle Chrysomela aeneicollis Schaeffer (Coleoptera: Chrysomelidae)

Several species of willow leaf beetles use hostplant salicin to produce a defensive secretion that consists of salicylaldehyde. Generalist arthropod predators such as ants, ladybird beetles, and spiders are repelled by this secretion. The beetle larvae produce very little secretion when they feed on willows that lack salicylates, and salicin-using beetles prefer salicylate-rich willows over salicylate-poor ones. This preference may exist because the larvae are better defended against natural enemies on salicylate-rich willows. If this is true, the larvae should survive longer on those willows in nature. However, this prediction has not been tested. I determined the larval growth and survival of Chrysomela aeneicollis (Coleoptera: Chrysomelidae) on five willow species (Salix boothi, S. drummondiana, S. geyeriana, S. lutea, and S. orestera). These species differed in their salicylate chemistries and in leaf toughness but not in water content. The water content varied among the individual plants. Larval growth of C. aeneicollis did not differ among the five species in the laboratory, but it varied among the individual plants and it was related to the water content. In the field, C. aeneicollis larvae developed equally rapidly on the salicylate-poor S. lutea and on the salicylate-rich S. orestera. Larval survival was greater on S. orestera than on S. lutea in one year (1986), but there was no difference between them during three succeeding years. In another survivorship experiment, larval survival was low on the medium-salicylate S. geyeriana, but high on the salicylate-poor S. boothi and on S. orestera. Larval survival in the field was related to the larval growth and water content that had been previously measured in the laboratory. These results showed that the predicted relationship between the host plant chemistry and larval survival did not usually exist for C. aeneicollis. One possible reason for this was that the most important natural enemies were specialist predators that were unaffected by the host-derived defensive secretion. One specialist predator, Symmorphus cristatus (Hymenoptera: Eumenidae), probably caused much of the mortality observed in this study. I discuss the importance of other specialist predators to salicin-using leaf beetles.     

Prefeeding Behavior of the Crucifer Flea Beetle, Phyllotreta cruciferae, on Host and Nonhost Crucifers

The prefeeding behaviours of adult crucifer flea beetles, Phyllotreta cruciferae (Coleoptera: Chrysomelidae: Alticinae), were determined on seedlings of the host plant, Brassica napus, and compared to behaviors on seedlings of the nonhost crucifers, Crambe abyssinica, Sinapis alba, and Camelina sativa. Three stages of prefeeding behaviour, i.e., acclimation, stimulation, and initial feeding, were distinguished through observation of filmed beetles. Both antennal and tarsal chemoreceptors are important in determination of host plant quality by the crucifer flea beetle. The results of this study suggest that the sequence of prefeeding behaviors plays a crucial role in the onset of feeding. Differences in time spent on plant tissue and the frequency and duration of prefeeding behaviors provide insight into possible mechanisms of resistance to flea beetles in the non-Brassica crucifers. The nonpreferred hosts C. abyssinica and S. alba contain deterrent phytochemicals that partially inhibit feeding. These deterrent compounds appear to be volatile in nature in S. alba but nonvolatile in C. abyssinica. CFB resistance in the nonhost C. sativa may result from either the presence of repellent or the absence of stimulatory volatile phytochemicals.     

Interactions between the willow beetle Phratora vulgatissima and different genotypes of Salix viminalis

Abstract 1 Planting of species mixtures is a strategy for the non‐chemical management of willow beetles in short‐rotation coppice willows. However, the relatively susceptible Salix viminalis genotypes and their hybrids dominate current high‐yielding willows. Interactions between Phratora vulgatissima and different genotypes of S. viminalis were examined under laboratory conditions to determine if these genotypes exhibit genetic differences in susceptibility to willow beetle damage. 2 Seven S. viminalis genotypes and four hybrids were tested for the feeding preference of adult P. vulgatissima, larval performance and within‐season plant response to manual defoliation (50% and 75%). 3 The feeding preference of adult beetles, the growth rate of larvae, and the weight at 33 days of larvae and pupae differed significantly between genotypes. Genotypes also differed significantly in their height and weight responses to mechanical defoliation. Two genotypes were taller, with longer internodes, after defoliation than were undamaged plants. Two hybrids and their S. viminalis parent showed no significant reduction in final dry weight between 0% and 50% defoliation treatments. 4 Susceptibility of genotypes to adult feeding was not correlated with their tolerance to defoliation in terms of weight or height responses; however, larval growth rate on genotypes was negatively correlated both with final height and number of leaves after 75% defoliation and with the susceptibility of genotype to adult feeding. 5 Salix viminalis showed genetic differences for all parameters tested. This suggests that the planting of a mixture of these genotypes would contain genetic differences with respect to host susceptibility to P. vulgatissima. Some parameters showed similarities between a hybrid and its parent, whereas others showed differences between siblings. This offers potential for effective breeding of desirable traits.     

Phenolic compounds of willow bark as deterrents against feeding by mountain hare

Summary Overwintering mountain hares (Lepus timidus) fed selectively on the shoots of a number of northern willow (Salix spp.) species. The hares preferred certain species over others and mature shoots over juvenile ones. There was a negative correlation between the phenolic glycoside concentration in the twigs and the hares' feeding. This correlation was substantiated by feeding experiments in which oat grains treated with purified phenoglycoside and catechin fractions of willow bark were offered along with untreated control oat grains to free-ranging mountain hares. Both fractions in concentrations normally found in willow twigs inhibited hare feeding. The results suggest that these phenolic compounds play a crucial role in the building up of resistance patterns among the willows. The decline in resistance in mature shoots of tall willows indicates that the juvenile resistance can be, perhaps secondarily, an adaptation against mammals browsing from ground level. Accordingly, low willow species retain a high level of resistance also in maturity.     

Defensive larval secretions of leaf beetles attract a specialist predator Parasyrphus nigritarsis

1. Noxious larval secretions of leaf beetles, which repel generalist predators, do not deter specialist syrphid fly predators (genus Parasyrphus ). These flies cause considerable mortality to the beetles, but little is known about their foraging behaviour.
2. Larvae of Parasyrphus nigritarsis were attracted to the volatile larval secretions produced by two prey species Phratora vitellinae and Linaeidea aenea. Parasyrphus nigritarsis feeds on both beetles in nature. Phratora vitellinae feeds on willows and utilizes host plant compounds for secretion production, while the alder-feeding L. aenea produces an autogenous secretion.
3. Fly larvae were strongly attracted to pieces of filter paper treated with larval secretion of the beetles. They attempted to feed on them for up to 7 min, and were equally attracted to the secretions of Ph. vitellinae and L. aenea . Fly larvae were also attracted to pure salicyl aldehyde, the main component of the secretion of Ph. vitellinae .
4. Fly larvae searched extensively for prey on leaves that had been damaged by beetle larvae. They also followed trails made with solutions containing faecal matter of prey larvae. They showed no differential preference for Ph. vitellinae or L. aenea , but always rejected larvae of the non-prey leaf beetle Agelastica alni .
5. Beetle secretions thus play an important, but unexpected, role in the feeding behaviour of P. nigritarsis . This predator uses the beetle secretion to locate its prey. The implications of these results for three trophic level interactions are discussed.     

Host‐plant quality adaptively affects the diapause threshold: evidence from leaf beetles in willow plantations

1. Voltinism of herbivorous insects can vary depending on environmental conditions. The leaf beetle Phratora vulgatissima L. is univoltine in Sweden but will sometimes initiate a second generation in short‐rotation coppice (SRC) willow plantations. 2. The study investigated whether increased voltinism by P. vulgatissima in plantations can be explained by (i) rapid life‐cycle development allowing two generations, or (ii) postponed diapause induction on coppiced willows. 3. In the field, no difference was found in the phenology or development of first‐generation broods between plantations (S. viminalis) and natural willow habitats (S. cinerea). However, the induction of diapause occurred 1–2 weeks later in SRC willow plantations. 4. Laboratory experiments indicated no genetic difference in the critical day‐length for diapause induction between beetles originating from plantations and natural habitats. Development time was unaffected by host‐plant quality but critical day‐length was prolonged by almost an hour when the beetles were reared on a non‐preferred willow species (S. phylicifolia). When reared on new leaves from re‐sprouting shoots of recently coppiced willow plants, diapause incidence was significantly less than when the beetles were reared on mature leaves from uncoppiced plants. 5. The study suggests that P. vulgatissima has a plastic diapause threshold influenced by host‐plant quality. The use of host‐plant quality as a diapause‐inducing stimulus is likely to be adaptive in cases where food resources are unpredictable, such as when new host‐plant tissue is produced after a disturbance. SRC willows may allow two beetle generations due to longer growing seasons of coppiced plants that grow vigorously.     

Sterol glucosylation by the plasma membrane from cotyledons of Phaseolus aureus

Membrane fractions were isolated from dark grown cotyledons of Phaseolus auneus by differential and sucrose density gradient centrifugation. Endoplasmic reticulum-, Golgi apparatus- and plasma membrane-rich fractions were identified by their respective enzymic activities and tested for their ability to transfer glucose from UDP-glucose to endogenous sterols to form steryl glucosides. The glucosyltransferase activity was shown to be located mainly at the plasma membrane.ABBREVIATIONS SG steryl glucoside - ASG acylated steryl glucoside - UDP-glc Uridine diphosphoglucose     

Relevance of visual and olfactory cues for host location in the mustard leaf beetle Phaedon cochleariae

The relevance of visual and olfactory cues for host‐plant location is investigated in males and females of the oligophagous mustard leaf beetle Phaedon cochleariae Fabricius (Coleoptera: Chrysomelidae). Different objects are offered in a walking arena and the behaviour of beetles is observed. Beetles orient toward vertically or horizontally striped black and white pattern independent of stripe orientation. The results suggest that contrast facilitates orientation in the field, whereas the pattern itself may be less important for host location in dense vegetation. The response to green and yellow objects is tested to investigate discrimination abilities between young (green) and mature (yellow) leaves. Beetles prefer green over yellow independent of material (cardboard or leaves of Nasturtium officinale R. Br., Brassicaceae). Preference behaviour tested in a dual‐choice contact assay coincides with visual preferences, where adults prefer young, more nutritious leaves for feeding and oviposition. Furthermore, females discriminate between visual cues of green leaves and green cardboard, whereas males do not, indicating that females are more sensitive in colour discrimination. Differences in colour wavelength influence the choice of beetle behaviour more strongly than differences in intensity. Both sexes of P. cochleariae prefer volatiles of the host plant N. officinale, whereas only females respond to the main volatile compound 2‐phenylethyl isothiocyanate. Given a choice between visual and olfactory cues, males orientate towards the colour cues, whereas females do not show any preferences. In males, visual cues may thus override olfactory cues, whereas, in females, both are equally important, which may reflect different ecological requirements and/or physiological abilities.     

Host race formation in the meadowsweet and strawberry feeding leaf beetle Galerucella tenella

We investigated host race formation in Galerucella tenella, a leaf beetle which feeds preferentially on meadowsweet (Rosaceae) in its natural habitats, but has become an important pest on strawberry (Rosaceae) in agricultural areas. Beetles from two isolated sites (Skeppsvik in Sweden and Solf in Finland) were compared with respect to preference and performance. At Skeppsvik the beetles were found feeding on meadowsweet, while at Solf large populations only develop in strawberry plantations, despite the presence of meadowsweet.In reciprocal field transplantations and laboratory bioassays, beetles from both sites discriminated against their foreign host, using their natal plant to a significantly higher degree for both egg laying and feeding, but with one interesting exception, namely that beetles from Solf increased their use of and even preferred meadowsweet for feeding in the laboratory. This increased use of meadowsweet by adult Solf beetles in the laboratory (without conspecific competitors) may be attributed to a density-dependent strategy, whereby mothers avoid pre-emptying the nutritional resources of the plants that will host their offspring in cases where no competitors are present.Larval fitness did not differ significantly between host plants, although larval survival of Solf beetles was halved when reared on their non-natal host plant (meadowsweet). Although beetles from both sites preferred to oviposit on their local host plant, our results provide little evidence that this presumed adaptation has to do with the nutritional quality of the plants involved. Instead, we suggest that other factors associated with the plants, such as enemy-free space and resistance to drought may be more important selective agents, shaping host preference in the field.     

Host location in Oomyzus gallerucae (Hymenoptera: Eulophidae), an egg parasitoid of the elm leaf beetle Xanthogaleruca luteola (Coleoptera: Chrysomelidae)

Eggs of the elm leaf beetle Xanthogaleruca luteola are often heavily attacked by the chalcidoid wasp Oomyzus gallerucae. We studied the chemical signals mediating interactions between the egg parasitoid, its host, and the plant Ulmus campestris. Olfactometer bioassays with O. gallerucae showed that volatiles of the host-plant complex attract the parasitoid. In order to determine the source of attractive volatiles within this host-plant-complex, we tested separately the effect of odours of eggs, gravid elm leaf beetle females, faeces of the beetles and elm twigs (with undamaged leaves and leaves damaged either mechanically or by feeding of the beetles). Odours of faeces of the elm leaf beetle were attractive, whereas neither volatiles from eggs nor from gravid females acted as attractants. Volatiles from undamaged or damaged plants did not elicit a positive reaction in O. gallerucae, whereas volatiles from feeding-damaged plants onto which host eggs had been deposited were attractive. This latter result suggests that it is not feeding but deposition of host eggs onto elm leaves that induces the production of plant volatiles attractive to the egg parasitoid. Investigations of the search patterns of O. gallerucae within the habitat by laboratory bioassays revealed that the egg parasitoid encounters host eggs by chance. Contact kairomones from faeces were demonstrated to be important in microhabitat acceptance, while contact kairomones isolated from the host eggs are relevant for host recognition. Received: 12 February 1997 / Accepted: 29 April 1997     

Lack of evolution in a leaf beetle that lives on two contrasting host plants

The interactions between plant‐eating insects and their hosts have shaped both the insects and the plants, driving evolution of plant defenses and insect specialization. The leaf beetle Trirhabda eriodictyonis (Chrysomelidae) lives on two shrubs with differing defenses: Eriodictyon crassifolium has hairy leaves, whereas E. trichocalyx has resinous leaves. We tested whether these beetles have differentiated onto the two host plants, and if not, whether the beetles prefer the better host plant and prefer mates who are from that host plant. In feeding tests, adult beetles strongly preferred eating E. trichocalyx regardless of which host they came from. In addition, females laid more eggs if they ate E. trichocalyx than E. crassifolium. So, E. trichocalyx is generally the better host. However, beetle mate preference was not in line with food choice. Males did not prefer to mate with females from E. trichocalyx. Females from E. crassifolium did prefer males from E. trichocalyx over males from E. crassifolium, but did not lay more eggs as a result of these matings. We conclude that the beetle populations we studied have not differentiated based on their host plants and may not have even adapted to the better host. Although to humans these host plant defenses differ dramatically, signs that they have caused evolution in the beetles are lacking. The case of T. eriodictyonis stands counter to many other studies that have seen the differentiation of ecotypes and/or adaptive coordination of an herbivore's life cycle based on host plant differences.     

Host plant effects on parasitoid attack on the leaf beetle<Emphasis Type="Italic"> Chrysomela lapponica</Emphasis>

Larvae of the leaf beetle Chrysomela lapponica obtain salicyl glucosides (SGs) from the host plant to produce a defensive secretion with salicylaldehyde. In northern Russia, larvae and pupae experience high parasitism by the phorid fly Megaselia opacicornis and tachinid fly Cleonice nitidiuscula. We compared the suitability of the SG-rich Salix borealis and SG-poor S. caprea and S. phylicifolia to Ch. lapponica and tested whether enemy pressure on Ch. lapponica varies among host species that differ in SG content. In the laboratory, survival of Ch. lapponica larvae was higher on S. borealis than on S. caprea and S. phylicifolia, while adult body mass was higher on S. borealis and S. caprea than on S. phylicifolia. In the field, parasitism by both M. opacicornis and Cl. nitidiuscula was greater on beetles from S. borealis than from the SG-poor S. caprea or S. phylicifolia. In a laboratory choice test, the pupal parasitoid M. opacicornis laid similar numbers of eggs on beetles reared on SG-rich and SG-poor willows, suggesting that the host plant-derived defence is not effective against this parasitoid. In a field enemy-exclusion experiment, beetle survival was greatly enhanced by the exclusion of enemies, but survival rates did not differ between S. borealis and S. caprea, although larvae developed faster on S. borealis. On the other hand, parasitism and predation were observed more often on S. borealis than on S. caprea. Thus, beetle larvae perform better but also suffer higher predation and parasitism on S. borealis than on SG-poor willows. Ch. lapponica does not appear to obtain enemy-free space by feeding on SG-rich willow species.     

Drivers of host plant shifts in the leaf beetle Chrysomela lapponica: natural enemies or competition?

1. The leaf beetle, Chrysomela lapponica, originally uses the salicyl glucosides (SGs) of its host plants to sequester salicylaldehyde, which serves as a defence against generalist enemies but attracts specialist enemies. However, some populations of C. lapponica have shifted to SG‐poor hosts, and their secretions do not contain salicylaldehyde. 2. In was suggested that beetles shift to SG‐poor hosts to escape from specialist enemies. To test this hypothesis, we compared field mortality between two populations of C. lapponica that were associated with SG‐rich willow, Salix myrsinifolia (Kola Peninsula and Finland) and two populations that fed on SG‐poor willows, S. glauca (Ural) and S. caprea (Belarus). 3. Mortality from generalist enemies was significantly higher in Belarus than in three other populations, whereas mortality from specialists did not differ among populations. A specialist predator (syrphid fly larvae, Parasyrphus nigritarsis) and specialist parasitoids (phorid flies, Megaselia spp.) were attracted to the secretions of larvae reared on both SG‐rich and SG‐poor hosts. 4. Feeding on leaves of S. caprea and S. myrsinifolia both previously damaged by leaf puncturing and by the larvae of potentially competing species Chrysomela vigintipunctata, decreased the weight and prolonged the development of C. lapponica. 5. Thus, populations of C. lapponica that have shifted to SG‐poor willow species did not obtain enemy‐free space because specialist enemies have developed adaptations to herbivores that switched to a novel host plant. We suggest that in some populations host plant shift was favoured by interspecific competition with the early season SG‐using specialist, C. vigintipunctata.