The significance of bottom-up effects for host plant specialization in Chrysomela leaf beetles

The leaf beetle species Chrysomela lapponica, which belongs to the so‐called C. interrupta group, forms distinct allopatric populations either on willows (Salicaceae) or birches (Betulaceae). It was recently suggested that, on several occasions, host plant shifts from Salicaceae to Betulaceae occurred independently within the C. interrupta group. Our study aims to elucidate bottom‐up effects of the host plants that might have shaped the evolution of host plant specialization in the populations of C. lapponica, and thus, to shed some light on the driving forces of host shifts within the C. interrupta group, too. We compared the oviposition behaviour and performance of two C. lapponica populations, one of which has adapted to birches and the other to willows. The studies were conducted under laboratory conditions, eliminating the impact of natural enemies. Experiments involving the transfer of individuals of the birch‐specialized population to willows and vice versa with individuals of the willow‐specialized population to birches aimed to examine the plasticity in host plant use. Females of each population almost exclusively chose their natural host plant for oviposition, when offering birch and willow in dual choice experiments. When specimens of the two C. lapponica populations were reared on their natural host plants, the birch specialists suffered higher mortality, needed a longer period of development and produced less larval defensive secretion than the willow specialists. When the birch specialists were fed with willow, these performance parameters decreased even more. Other parameters, such as body weight and fecundity, did not differ between birch and willow specialists when they were fed with their natural host plant. While individuals of the birch‐specialized population could be reared on willow, all neonate larvae from the willow‐specialized population died after being transferred to birch. The significance of these bottom‐up effects for the evolution of host plant specialization in C. lapponica is discussed.     

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.     

Do exotic generalist predators alter host plant preference of a native willow beetle?

1 Selection can favour herbivores that choose host plants benefitting their offspring either by enhancing growth rates or by increasing larval defences against native predators. For exotic predator species that feed on herbivores, their success with invading new habitats may depend upon overcoming defences used by native prey. Whether exotic predators can alter herbivore host choice has remained unexamined. Therefore, we compared the efficacy of larval defence by Chrysomela knabi (a native beetle species) that had fed on two native willow hosts: Salix sericea (a phenolic glycoside (PG)-rich species) and Salix eriocephala (a PG-poor species), when attacked by exotic generalist predators. In addition, the preference and performance of C. knabi on S. sericea and S. eriocephala was examined.
2  Chrysomela knabi preferred and performed better on S. sericea. In a common garden, adult C. knabi were nine-fold more common and oviposited five-fold more frequently on S. sericea than on S. eriocephala . In the laboratory, adult feeding preference on leaf discs and survival rates of larvae were both greater on S. sericea , and time to pupation was shorter.
3  Chrysomela knabi larvae produced significantly more salicylaldehyde when fed S. sericea leaves than when fed S. eriocephala leaves. Additionally, those larvae with greater salicylaldehyde had reduced predation by two exotic generalist predators, Harmonia axyridis larvae and juvenile Tenodera aridifolia sinensis .
4 The results obtained in the present study suggest that selection favoured the preference of C. knabi for PG-rich willow plants because larvae grew and survived better and that selection by common exotic generalist predators would reinforce this preference.     

Conflicting mitochondrial and nuclear phylogeographic signals and evolution of host-plant shifts in the boreo-montane leaf beetle Chrysomela lapponica

We conducted a phylogeographic study on the cold-adapted leaf beetle Chrysomela lapponica, that feeds on willow or birch, by sampling several populations throughout most of the geographic distribution of the species, and by sequencing for each individual one mitochondrial and two nuclear DNA fragments. Patterns of DNA sequence variation from the mitochondrial and nuclear loci, as displayed in the median-joining networks, appear to display contradicting historical signal: a deep genealogical divergence is observed with the mitochondrial genome between the Alpine population and all other populations found in the Euro-Siberian distribution of the species, that is completely absent with both nuclear loci. We use coalescence simulations of DNA sequence evolution to test the hypothesis that this apparent conflict is compatible with a neutral model of sequence evolution (i.e., to check whether the stochastic nature of the coalescence process can explain these patterns). Because the simulations show that this is highly unlikely, we consider two alternative hypotheses: (1) introgression of the mitochondrial genome of another species and (2) the effect of natural selection. Although introgression is the most plausible explanation, we fail to identify the source species of the introgressed mitochondrial genome among all known species closely related to C. lapponica. We therefore suggest that the putative introgression event is ancient and the source species is either extinct or currently outside the geographic range of C. lapponica explored in this study. The observed DNA sequence variation also suggests that a host-plant shift from willow to birch has occurred recently and independently in each of the three birch-feeding populations. This emphasizes further the relative ease with which these beetles can escape their ancestral host-plant specialization on willow, but shows at the same time that host-plant shifts are highly constrained, as they only occur between willow and birch.     

Rapid gall midge adaptation to a resistant willow genotype

Abstract 1 We conducted two experiments to investigate why a basket willow Salix viminalis L. genotype, known to be highly resistant to the leaf-roller gall midge Dasineura marginemtorquens (Bremi), should support very high gall densities in a field plantation at Tälle, south Sweden.
2 The first experiment was a field test of the hypothesis of fine-scale host adaptation in the gall midge/willow system. Support for the hypothesis would be established if midges originating from resistant willows and those originating from nearby susceptible willows differed in their abilities to initiate galls and complete development on resistant plants.
3 The objective of the second experiment was to explore whether there was a genetic basis to the trait for virulence in the midge population and to investigate any potential trade-offs this trait may entail.
4 Our results indicate that there was a fine-scaled microgeographic genetic structure to the midge population at Tälle. Midges originating from resistant plants had a heritable trait that enabled them to establish galls on resistant plants.
5 Midges able to initiate galls on the resistant genotype had longer developmental time on the susceptible genotype. This suggests that there is a physiological cost associated with being adapted to the resistant willow genotype.
6 We suggest that driving forces behind the observed host adaptation are selection imposed on the midge population by very strong willow resistance and restricted gene flow in the midge populations due to the special life history features of D. marginemtorquens .     

Bottom-up cascade in a tri-trophic system: different impacts of host-plant regeneration on performance of a willow leaf beetle and its natural enemy

Abstract.  1. Plant quality can directly and indirectly affect the third trophic level. However, little attention has been paid to how changes in plant quality affect the performance of predators through trophic levels, and which herbivores or predators are affected more strongly by host-plant quality. The present study examined the effects of artificial cutting of willows on the performance of a willow leaf beetle ( Plagiodera versicolora Laicharting) and its predatory ladybird beetle ( Aiolocaria hexaspilota Hope).
2. Laboratory experiments showed that performance (survival rate, developmental time, and adult mass) of the willow leaf beetle was higher when fed with leaves of cut willows than when fed with leaves of uncut willows. Performance (developmental time and adult mass) of the predatory ladybird was also improved when it was fed on the leaf beetle larvae that had been fed on leaves of cut willows, compared with those that had been fed on leaves of uncut willows. This indicates that a bottom-up cascade occurs in the tri-trophic system.
3. In a comparison of improved performance parameters between the leaf beetle and the ladybird, regenerated willows shortened the developmental time of the willow leaf beetle more than that of the ladybird. This indicates that the impacts of willow cutting on insect performance differ between the second and third trophic levels.     

Feeding patterns by roe deer and rabbits on pine, willow and birch in relation to spatial arrangement

The location of a plant and its association with the surrounding vegetation may be a strong determinant of herbivore foraging decisions. The attractiveness of a food plant may be reduced if associated with a less preferred one (repellent–plant hypothesis) or, conversely, it may escape herbivory via association with a more preferred species (attractant–decoy hypothesis). In this study we tested the hypothesis that selection of the same food plants by two herbivore species with different body mass, i.e. roe deer and rabbits, is affected by the spatial disposition of preferred plants in relation to less preferred ones. We planted willows, birches and pines, representing food species of higher, intermediate and lower preference, respectively, in different spatial arrangements, to manipulate patch quality and accessibility to herbivores. Contrasting patches were constructed by planting willows or pines in the midst of birches and single-species arrays of all three species, in a blocked design, replicated six times in an area occupied by both roe deer and rabbits. Preference patterns were studied by recording browsing on current years' shoots and older plant parts. Across all trees in the experiment, we confirmed that roe deer clearly preferred willow over birch, pine was not browsed at all. There was greater herbivory by roe deer on birches occurring peripherally in an array, when the associated central species was willow or pine, as compared to birches in a single-species birch patch. Therefore food choices by roe deer appear to be determined by both patch and individual level selection. The rabbits only showed preference at a species level and did not respond to spatial arrangement of trees. The results from this study indicate that the location and association of trees can strongly affect foraging patterns of roe deer, but not in a manner wholly consistent with the repellent–plant or the attractant–decoy hypotheses.     

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.     

Host phylogeny and nutrient content drive galler diversity and abundance on willows

1. Different groups of specialised herbivores often exhibit highly variable responses to host plant traits and phylogeny. Gall‐forming insects and mites on willows are highly adapted to their hosts and represent one of the richest communities of gallers associated with a single genus of host plants. 2. The present study evaluated the effects of host plant secondary metabolites (salicylates, flavonoids, condensed tannins), physical traits (trichome density), nutrient content (N:C) and phylogeny on the abundance and richness of gall‐forming arthropods associated with eight willow species and Populus tremula. 3. Galler abundance was affected by N:C rather than by willow defensive traits or phylogeny, suggesting that gallers respond differently to host plant traits than to less specialised guilds, such as leaf‐chewing insects. None of the studied defensive traits had a significant effect on gall abundance. Gall morphospecies richness was correlated with the host phylogeny, mainly with the nodes representing the inner division of the willow subgenus Vetrix. This suggests that the radiation of some willow taxa could have been important for the speciation of gallers associated with willows. 4. In conclusion, it is shown that whereas willow traits, such as nutrient content, appeared to affect abundances of gallers, it is probably willow radiation that drives galler speciation.     

A subset of chemosensory genes differs between two populations of a specialized leaf beetle after host plant shift

Due to its fundamental role in shaping host selection behavior, we have analyzed the chemosensory repertoire of Chrysomela lapponica. This specialized leaf beetle evolved distinct populations which shifted from the ancestral host plant, willow (Salix sp., Salicaceae), to birch (Betula rotundifolia, Betulaceae). We identified 114 chemosensory candidate genes in adult C. lapponica: 41 olfactory receptors (ORs), eight gustatory receptors, 17 ionotropic receptors, four sensory neuron membrane proteins, 32 odorant binding proteins (OBPs), and 12 chemosensory proteins (CSP) by RNA‐seq. Differential expression analyses in the antennae revealed significant upregulation of one minus‐C OBP (ClapOBP27) and one CSP (ClapCSP12) in the willow feeders. In contrast, one OR (ClapOR17), four minus‐C OBPs (ClapOBP02, 07, 13, 20), and one plus‐C OBP (ClapOBP32) were significantly upregulated in birch feeders. The differential expression pattern in the legs was more complex. To narrow down putative ligands acting as cues for host discrimination, the relative abundance and diversity of volatiles of the two host plant species were analyzed. In addition to salicylaldehyde (willow‐specific), both plant species differed mainly in their emission rate of terpenoids such as (E,E)‐α‐farnesene (high in willow) or 4,8‐dimethylnona‐1,3,7‐triene (high in birch). Qualitatively, the volatiles were similar between willow and birch leaves constituting an “olfactory bridge” for the beetles. Subsequent structural modeling of the three most differentially expressed OBPs and docking studies using 22 host volatiles indicated that ligands bind with varying affinity. We suggest that the evolution of particularly minus‐C OBPs and ORs in C. lapponica facilitated its host plant shift via chemosensation of the phytochemicals from birch as novel host plant.     

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.     

Variation in quality of mountain birch and tea-leaved willow for mammal and insect herbivores: differences among trees, among sites and between tree species

We evaluated palatability of winter dormant tree twigs to the mountain hare Lepus tunidus and the grey-sided vote Clethrionomys rufocanus with captive animals We tested differences among trees within sites, among sites, and between two tree species (the mountain birch Betula pubescens ssp czerepanovii and the tea-leaved willow Salix phylicifolia ) In one of the sites, we also measured growth rates of autumnal moth Epirrita autumnata larvae on the same trees that were used m preference trials with the hares and voles The differences m palatability to hares and voles were greatest at the level of tree Species, both hares and voles preferred birches over willows, but with the hare there was some overlap in palatability between the tree species There were also large and significant differences among sites and among trees within sites Within sites, variation in tree palatability seemed to be larger among willows than among birches Hares and voles may select willows at least partly on the same basis, but there was no correlation between palatability of the trees to the mammals and the growth rate of the larvae of the autumnal moth on the same trees Variation in twig palatability to the hares and voles was so large at all levels (among trees within sites, among sites and between tree species) that we suggest it also has implications for food selection of these mammalian herbivores under natural conditions     

Effects of host-plant shift on immune and other key life-history traits of an eruptive Geometrid, Epirrita autumnata (Borkhausen)

Abstract.  1. Population density of Epirrita autumnata (Lepidoptera: Geometridae) reaches outbreak densities regularly in northernmost Scandinavia. During these outbreak years, the most abundant host species, the mountain birch ( Betula pubescens ssp. czerepanovii ), is regularly exhausted, although larvae may rescue themselves from starvation by using alternative host species.
2. In this paper, the effects of the shift of host species on the immune defence and other life-history traits of E. autumnata were investigated, and possible consequences for population dynamics were briefly discussed. Moth larvae were reared on the leaves of the main host, mountain birch, until larvae reached their third instar. After this, larvae were allocated randomly to five treatments: larvae were either allowed to finish larval stage on the mountain birch or were shifted onto four alternative host species that are typical species for the area.
3. As expected, the host species had a major effect on fitness traits: body weight, development, and survival rate of the moths. The pupal weight was lower and development rates slower on the three alternative host species, Salix myrsinifolia Salisb., Vaccinium uliginosum L., and Betula nana L., than on the main host, mountain birch.
4. The immunity was, however, the same or better on the alternative hosts than on the main host. The immunity and pupal weights were negatively related, suggesting a trade-off between body size and immunocompetence.
5. The decreased body size and fecundity of E. autumnata during outbreak years may be partly due to the shift to alternative host species whereas the host-plant species probably does not affect markedly the rate of parasitism.     

Habitat difference in abundance of willow leaf beetle Phratora vulgatissima (Coleoptera: Chrysomelidae): plant quality or natural enemies?

Herbivorous insects are influenced by both 'bottom-up' forces mediated through host plants and 'top-down' forces from natural enemies. Few studies have tried to evaluate the relative importance of the two forces in determining the abundance of insects. The leaf beetle Phratora vulgatissima Linnaeus sometimes occurs at high densities and severely damages the willow Salix cinerea in forest habitats. For willows growing in open agricultural landscapes (farmland S. cinerea), the leaf beetle generally occurs at low densities and plants receive little damage. The purpose of the present study was to evaluate the relative importance of host plant quality and natural enemies behind the observed difference in P. vulgatissima abundance. Female egg-laying and larval performance (growth and survival) were studied on caged willow branches in the field to investigate if plant quality differs between S. cinerea trees growing in forest and farmland habitats. The survival of eggs exposed to natural enemies was examined to see if predation could explain the low abundance of leaf beetles on farmland willows. The results indicated no difference in plant quality; female egg laying and larval performance did not differ between the forest and the farmland. However, heteropteran predators (true bugs) were more abundant, and the survival of eggs was lower, on plants in the farmland habitat than in the forest habitat. The data suggest that the low abundance of P. vulgatissima on farmland willows could not be explained by a poor quality of plants, but more likely by high predation from heteropterans.     

Evidence for host race formation in the leaf beetle Galerucella lineola

We examined preference and performance of four Finnish Galerucella lineola F. populations on alder and willow. In standardized two‐choice laboratory feeding trials with alder and willow, only two naturally alder‐associated G. lineola populations accepted alder. Two conspecific willow‐associated populations preferred willow. These preferences seem to be unstable, however, because they can be modified by the beetles’ experience. Thus, there probably is not a complete host preference‐based isolation of alder‐ and willow‐associated G. lineola beetles in nature. In performance experiments, larvae of all four populations survived better on willow than on alder. This may indicate that willows are the ancestral hosts for G. lineola. Nevertheless, larvae of the two alder‐associated G. lineola populations survived better on alder than larvae of the two willow‐associated populations. On the other hand, larvae of the two willow‐associated populations survived better on willow than larvae of the two alder‐associated populations. This performance trade‐off suggests that G. lineola encounters different selective pressures on alders and willows. On both of them, selection probably disfavours those G. lineola genotypes that are the most successful and abundant on alternative hosts. This may reduce the effects of gene flow that is likely to occur as a consequence of incomplete host preference‐based isolation of alder‐ and willow‐associated G. lineola populations. Data from pupal weights support the idea that alder‐ and willow‐associated G. lineola populations may be genetically differentiated. Pupae of the two alder‐associated populations were heavier than those of the willow‐associated populations irrespective of whether larvae had fed on alder or on willow. Overall, our results indicate host race formation in G. lineola. This process may be enforced by the variable abundance of alders and willows in local communities.     

Intrinsic competition between larval parasitoids with different degrees of host specificity

Abstract.  1. Interspecific competition among parasitoids may play a key role in the community dynamics of tritrophic plant–herbivore–parasitoid systems and has important implications for management of herbivorous insect pests.
2. A model system was used to explore the outcome of interspecific competition between parasitoids that differ in host specificity. The system included the lepidopteran pest Heliothis virescens , the generalist parasitoid Cotesia marginiventris , and two specialist parasitoids, Microplitis croceipes and Cardiochiles nigriceps .
3. The generalist, C. marginiventris , dominated intrinsic competition when given an 8-h developmental head start over C. nigriceps or when its oviposition was simultaneous with that of M. croceipes . Microplitis croceipes and especially C. nigriceps larvae prevailed when they were allowed to oviposit prior to C. marginiventris .
4. Rates of host mortality prior to parasitoid emergence varied with parasitoid species composition and with the order of oviposition.
5. Implications for integrated pest management and the adaptive significance of competition as related to host specialisation are discussed.     

Progressive restriction of host plant exploitation along a climatic gradient: the willow psyllid Cacopsylla groenlandica in Greenland

1. Host plant selection by the endemic willow psyllid Cacopsylla groenlandica was studied at eight sites in three locations along a N–S climatic gradient in west Greenland.
2. C. groenlandica oviposited and developed on four willow species: Salix glauca, S. arctophila, S. uva-ursi and S. herbacea. Development took place on female catkins of all species and on leaf shoots of S. glauca .
3. At sites in southern Greenland, C. groenlandica fully utilized all four willow species. However, there was progressive reduction of host plant species and tissues exploited as latitude increased, such that towards the northern limit of its range the psyllid became highly specialized, feeding only on female catkins of S. glauca , despite the presence of alternative hosts. Where plant growth was tightly constrained within a restricted growing season, the demands of synchronizing with more than one species may have become too great, resulting in specialization on a single host.
4. The probable mechanisms producing specialization were between-species differences in host plant phenology, linked to variation in the growing season, set against the partially non-overlapping geographical distribution patterns of the host willows.
5. A small degree of phenological asynchrony between host plants can be advantageous for the insect. Under favourable conditions the reproductive season can be extended by using a series of host plants with staggered phenologies. Some 'early' hosts also create predator-free space. However, as asynchrony increases, the advantages disappear as the insect becomes unable to exploit the full range of host options.
6. The importance of host plant synchrony in the life histories of arctic insects is discussed.     

Host plant variation in plant-mediated indirect effects: moth boring-induced susceptibility of willows to a specialist leaf beetle

Abstract.  1. We examined the plant-mediated indirect effects of the stem-boring moth Endoclita excrescens (Lepidoptera: Hepialidae) on the leaf beetle Plagiodera versicolora (Coleoptera: Chrysomelidae) in three willow species, Salix gilgiana , S. eriocarpa , and S. serissaefolia.
2. When the stem-boring moth larvae damaged stems in the previous year, willows were stimulated to produce vigorously growing lateral shoots on these stems. These new lateral shoots were significantly longer and the upper leaves had significantly higher nitrogen and water content than current-year shoots on unbored stems, although the carbon content and leaf dry mass were not different between lateral and current-year shoots.
3. In the field, leaf beetle larvae and adults had significantly greater densities on lateral shoots of bored stems than on current-year shoots of unbored stems. A laboratory experiment showed that female beetles had significantly greater mass and fecundity when fed on leaves of newly-emerged lateral shoots. Thus, the stem-boring moth had a positive effect on the temporally and spatially separated leaf beetle by increasing resource availability by inducing compensatory regrowth.
4. The strength of the indirect effects on the density and performance of the leaf beetle differed among willow species, because there was interspecific variation in host quality and herbivore-induced changes in plant traits. In particular, we suggest that the differences in magnitude of the changes among willow species in shoot length and leaf nitrogen content greatly affected the strength of the plant-regrowth mediated indirect effect, coupled with host-plant preference of the leaf beetle.     

Avoidance of the host immune response by a generalist parasitoid, Compsilura concinnata Meigen

Abstract.  1. Ecological interactions between parasitoids and their hosts are extremely strong as parasitoid offspring rely entirely on an individual host to complete development. The ability of a parasitoid to use a host is influenced directly by the degree to which the parasitoid can overcome host defences and grow within the host.
2. Hymenopteran parasitoids have evolved different host-specific strategies to defeat the host immune system, such as the use of venom, endosymbiont virus, or mimicking the host tissue. Dipteran parasitoids from the Tachinidae family do not use these subterfuges and rely mainly on avoiding the host immune system by hiding in specific tissues.
3. Little is known of the effect of this strategy on the host immune system, the absorption of nutrients by the parasitoid larvae, or the implications for parasitoid host range.
4. In this study, the impact of a polyphagous tachinid parasitoid Compsilura concinnata Meigen on a pest lepidopteran Trichoplusia ni Hübner are assessed. Phenoloxidase levels and haemolymph proteins were measured in parasitised T. ni as a function of host immune response.
5. Haemolymph phenoloxidase in the host did not vary with parasitisation but was triggered when a piece of monofilament was implanted in the haemocoel. Haemolymph proteins were depleted in heavily parasitised T. ni .
6. These results indicate that C. concinnata has a strategy that avoids the host immune system, and accesses the necessary nutrients for larval growth. This strategy could explain the success of this tachinid and its wide host range.     

Effects of leaf pubescence in Salix borealis on host- plant choice and feeding behaviour of the leaf beetle, Melasoma lapponica

Density of leaf trichomes in Salix borealis affected both the choice of individual host plants and feeding behaviour of adults and last instar larvae of the willow feeding leaf beetle, Melasoma lapponica. Beetles clearly preferred shaved disks to unshaved ones taken from the same leaf; this preference was highest in leaves of the most pubescent plants. High leaf pubescence explained the low preference for willow clones from the high density site in among-site preference trials; shaving significantly increased the consumption of these pubescent willow clones. In no-choice experiments, the food consumption by both adults and last instar larvae decreased with an increase in leaf pubescence. The time budget of adults did not depend on leaf pubescence of the host plants, however adults compelled to feed on highly pubescent plants changed their feeding sites twice as often as on less pubescent willow clones. Larvae feeding on highly pubescent plants spend moving three times as much time as larvae feeding on less pubescent plants. Combined with our earlier observations on the increase in leaf pubescence in the year(s) following defoliation, these data suggest that leaf hairiness may have contributed to the delayed induced resistance in S. borealis by disturbing the feeding behaviour of M. lapponica.