Leaf drop affects herbivory in oaks

Leaf phenology is important to herbivores, but the timing and extent of leaf drop has not played an important role in our understanding of herbivore interactions with deciduous plants. Using phylogenetic general least squares regression, we compared the phenology of leaves of 55 oak species in a common garden with the abundance of leaf miners on those trees. Mine abundance was highest on trees with an intermediate leaf retention index, i.e. trees that lost most, but not all, of their leaves for 2–3 months. The leaves of more evergreen species were more heavily sclerotized, and sclerotized leaves accumulated fewer mines in the summer. Leaves of more deciduous species also accumulated fewer mines in the summer, and this was consistent with the idea that trees reduce overwintering herbivores by shedding leaves. Trees with a later leaf set and slower leaf maturation accumulated fewer herbivores. We propose that both leaf drop and early leaf phenology strongly affect herbivore abundance and select for differences in plant defense. Leaf drop may allow trees to dispose of their herbivores so that the herbivores must recolonize in spring, but trees with the longest leaf retention also have the greatest direct defenses against herbivores.     

Latitudinal variation in resistance and tolerance to herbivory in the perennial herb Lythrum salicaria is related to intensity of herbivory and plant phenology

Both the length of the growing season and the intensity of herbivory often vary along climatic gradients, which may result in divergent selection on plant phenology, and on resistance and tolerance to herbivory. In Sweden, the length of the growing season and the number of insect herbivore species feeding on the perennial herb Lythrum salicaria decrease from south to north. Previous common‐garden experiments have shown that northern L. salicaria populations develop aboveground shoots earlier in the summer and finish growth before southern populations do. We tested the hypotheses that resistance and tolerance to damage vary with latitude in L. salicaria and are positively related to the intensity of herbivory in natural populations. We quantified resistance and tolerance of populations sampled along a latitudinal gradient by scoring damage from natural herbivores and fitness in a common‐garden experiment in the field and by documenting oviposition and feeding preference by specialist leaf beetles in a glasshouse experiment. Plant resistance decreased with latitude of origin, whereas plant tolerance increased. Oviposition and feeding preference in the glasshouse and leaf damage in the common‐garden experiment were negatively related to damage in the source populations. The latitudinal variation in resistance was thus consistent with reduced selection from herbivores towards the northern range margin of L. salicaria. Variation in tolerance may be related to differences in the timing of damage in relation to the seasonal pattern of plant growth, as northern genotypes have developed further than southern have when herbivores emerge in early summer.     

The forgotten season: the impact of autumn phenology on a specialist insect herbivore community on oak

1. Variation in spring phenology – like tree budburst – affects the structure of insect communities, but impacts of autumn phenology have been neglected. Many plant species have recently delayed their autumn phenology, and the timing of leaf senescence may be important for herbivorous insects. 2. This study explored how an insect herbivore community associated with Quercus robur is influenced by variation in autumn phenology. For this, schools were asked to record, across the range of oak in Sweden, the autumn phenology of oaks and to conduct a survey of the insect community. 3. To tease apart the relative impacts of climate from that of tree phenology, regional tree phenology was first modelled as a function of regional climate, and the tree‐specific deviation from this relationship was then used as the metric of relative tree‐specific phenology. 4. At the regional scale, a warmer climate postponed oak leaf senescence. This was also reflected in the insect herbivore community: six out of 15 taxa occurred at a higher incidence and five out of 18 taxa were more abundant, in locations with a warmer climate. Similarly, taxonomic richness and herbivory were higher in warmer locations. 5. Trees with a relatively late autumn phenology had higher abundances of leaf miners (Phyllonorycter spp.). This caused lower community diversity and evenness on trees with later autumn phenology. 6. The findings of the present study illustrate that both regional climate‐driven patterns and local variation in oak autumn phenology contribute to shaping the insect herbivore community. Community patterns may thus shift with a changing climate.     

Different mechanisms explain feeding type-specific patterns of latitudinal variation in herbivore damage among diverse feeding types of herbivorous insects

To determine whether latitudinal variation in herbivore impact exists, we examined three major herbivorous insect feeding types (chewers, gallers, and miners) on/in leaves of Japanese beech. Herbivores were collected with litter traps deployed in forests across a latitudinal gradient of 10°. Leaf litter analyses demonstrated that chewing herbivory increased with increasing latitude of collection site. However, the densities of miners and gallers decreased with latitude. To test whether latitudinal variation in herbivore damage occurs in the absence of geographically differentiated environmental cueing (e.g., physical stresses or herbivore damage), we measured both genetically determined constitutive leaf traits and herbivore damage in a common-garden experiment. In this experiment, miner density decreased with latitude, but chewing herbivory did not vary latitudinally. Galler density was higher on trees from native provenances than on trees from unrelated provenances likely because of local adaptations. Leaf mass per unit area (LMA), tannin, and phenolics all decreased with latitude of provenance. The latitudinal variation in one constitutive leaf trait (LMA) best explained latitudinal variation in chewing herbivory. Thus, different mechanisms account for feeding type-specific patterns of latitudinal variation in herbivore damage among different herbivore feeding types.     

Defensive role of leaf trichomes in resistance to herbivorous insects in Datura stramonium

This study assessed the role of leaf trichome density as a component of resistance to herbivores, in six populations of Datura stramonium. Phenotypic selection on plant resistance was estimated for each population. A common garden experiment was carried out to determine if population differences in leaf trichome density are genetically based. Among population differences in leaf trichome density, relative resistance and fitness were found. Leaf trichome density was strongly positively correlated to resistance across populations. In 5 out of 6 populations, trichome density was related to resistance, and positive directional selection on resistance to herbivores was detected in three populations. Differences among populations in mean leaf trichome density in the common garden suggest genetic differentiation for this character in Datura stramonium. The results are considered in the light of the adaptive role of leaf trichomes as a component of defence to herbivores, and variable selection among populations.     

Tests of associational defence provided by hairy plants for glabrous plants of Arabidopsis halleri subsp. gemmifera against insect herbivores

1. Trichome‐producing (hairy) and trichomeless (glabrous) plants of Arabidopsis halleri subsp. gemmifera were investigated to test whether plant resistance to herbivory depends on the plants' phenotypes and/or the phenotypes of neighbouring plants (associational effects). 2. A common garden experiment was conducted in which the relative frequency of hairy and glabrous plants was manipulated. Two species of leaf‐chewing insects (larvae of a white butterfly and a cabbage sawfly) were found less often on hairy plants than on glabrous plants. By contrast, the numbers of aphids and flea beetles did not differ significantly between hairy and glabrous plants. For none of these insects did abundance depend on the frequency of the two plant morphs. 3. A field survey was conducted in two natural populations of A. halleri. In the first population, a species of white butterfly was the dominant herbivore, and hairy plants incurred less leaf damage than glabrous plants across 2 years. By contrast, in the other population, where flea beetles were dominant, there were no consistent differences in leaf damage between the two types of plants. In neither of the two populations was any evidence found of associational effects. 4. This study did not provide any conclusive evidence of associational effects of anti‐herbivore resistance, but it was discovered that trichomes can confer resistance to certain herbivores. Given the results of previous work by the authors on associational effects against a flightless leaf beetle, such associational effects of the trichome dimorphism of A. halleri were herbivore‐specific.     

Function of defensive volatiles in pedunculate oak (Quercus robur) is tricked by the moth Tortrix viridana

The indirect defences of plants are comprised of herbivore‐induced plant volatiles (HIPVs) that among other things attract the natural enemies of insects. However, the actual extent of the benefits of HIPV emissions in complex co‐evolved plant‐herbivore systems is only poorly understood. The observation that a few Quercus robur L. trees constantly tolerated (T‐oaks) infestation by a major pest of oaks (Tortrix viridana L.), compared with heavily defoliated trees (susceptible: S‐oaks), lead us to a combined biochemical and behavioural study. We used these evidently different phenotypes to analyse whether the resistance of T‐oaks to the herbivore was dependent on the amount and scent of HIPVs and/or differences in non‐volatile polyphenolic leaf constituents (as quercetin‐, kaempferol‐ and flavonol glycosides). In addition to non‐volatile metabolic differences, typically defensive HIPV emissions differed between S‐oaks and T‐oaks. Female moths were attracted by the blend of HIPVs from S‐oaks, showing significantly higher amounts of (E)‐4,8‐dimethyl‐1,3,7‐nonatriene (DMNT) and (E)‐β‐ocimene and avoid T‐oaks with relative high fraction of the sesquiterpenes α‐farnesene and germacrene D. Hence, the strategy of T‐oaks exhibiting directly herbivore‐repellent HIPV emissions instead of high emissions of predator‐attracting HIPVs of the S‐oaks appears to be the better mechanism for avoiding defoliation.     

Physical barriers and corridors in urban habitats affect colonisation and parasitism rates of a specialist leaf miner

1. Urban environments are fragmented habitats characterised by the presence of physical barriers, which may negatively affect dispersal and colonisation by insect herbivores and their natural enemies. Conversely, plants growing along pavements may function as dispersal corridors, helping to moderate the harmful effects of resource patch isolation on organism movement and population persistence. 2. We experimentally tested the effects of walls as physical barriers to the dispersal of the leaf miner Liriomyza commelinae Frost and colonisation of its host plant, Commelina erecta L., in urban habitats. We also evaluated whether plants along pavements could act as corridors for this species. 3. We exposed experimental host plants to the leaf miner in houses with front gardens and back yards, the latter being completely surrounded by walls. The front gardens had walls but none separating them from the pavement. Previously mined plants were also exposed to parasitoids in the yards to determine parasitoid attack. 4. Liriomyza commelinae took longer to colonise back yards with higher walls, and the abundance of mined plants along pavements reduced the colonisation time. Leaf‐miner abundance was marginally affected by the yard type, and was lower in back yards. Cumulative parasitism rates decreased with increasing distance at which mined plants were placed from pavements. 5. Constructions act as physical barriers, having a negative impact on colonisation of host plants by leaf miners. The function of pavements as corridors seems to depend on the abundance of mined plants. Parasitism may be affected by distance from the corridor rather than physical barriers or other potential hosts.     

GENOTYPIC VARIATION IN LEAF DAMAGE IN PIPER ARIEIANUM (PIPERACEAE) BY A MULTISPECIES ASSEMBLAGE OF HERBIVORES

The shrub Piper arieianum (Piperaceae) has a diverse herbivore fauna (95 species total) in Costa Rican rain forest. The effect of plant genotype on leaf damage by individual herbivore species and total leaf area removed was studied in P. arieianum through a cloning experiment. Damage patterns were measured over 3.5 years for two plots, four genotypes per plot, in the understory of lowland rain forest. In both plots, there were significant differences among genotypes in total leaf area missing throughout the study period. Rankings of genotypes based on overall damage remained constant over time in plot 1 but changed in plot 2. Certain individual herbivore species caused significantly higher damage in some genotypes than in others; the change in genotype rankings in plot 2 was associated with increased damage to particular genotypes by specific herbivore groups. The genotype most heavily damaged by a given insect species varied depending on the herbivore species; thus, resistance to one herbivore species did not necessarily confer resistance against all species. Those herbivore species causing the greatest proportion of damage for a given plant changed over time. Because total damage resulted from the summation of losses to individual herbivore species, whether an individual plant lost more leaf area than its neighbors depended on the relative abundance of the herbivore species at any one time. Finally, for a portion of the study period in each plot, more heavily damaged clones grew less than lesser damaged clones. Together with previous reports that naturally growing plants differ significantly in damage and that these differences are sufficient to cause fitness differences, the results presented here suggest that the herbivores of P. arieianum represent a selective force for changes in resistance but that this selective force changes both in intensity and quality over time.     

Population variation in plant traits associated with ant attraction and herbivory in Chamaecrista fasciculata (Fabaceae)

The benefits of ant–plant–herbivore interactions for the plant depend on the abundance of ants and herbivores and the selective pressures these arthropods exert. In plants bearing extrafloral nectaries (EFN), different mean trait values may be selected for by different populations in response to local herbivore pressure, ultimately resulting in the evolution of differences in plant traits that attract ants as defensive agents against herbivory. To determine if variation in traits that mediate ant–plant interactions reflect herbivore selective pressures, we quantified intra- and inter-population variation in plant traits for eight populations of the EFN-bearing annual Chamaecrista fasciculata (Michx.) (Fabaceae). Censuses in rural and urban areas of Missouri and Illinois (USA) showed population differences in ant attendance and herbivore pressure. Seeds were collected from each population, and plants were grown in a common greenhouse environment to measure sugar production, nectar volume and composition, EFN size and time of emergence, leaf pubescence, and leaf quality throughout plant development. Populations varied mainly in terms of nectary size, sugar production, and nectar volume, but to a lesser degree in leaf pubescence. Populations of C. fasciculata within urban areas (low in insect abundance) had small nectaries and the lowest nectar production. There was a positive correlation across populations between herbivore density and leaf damage by those herbivores on the one hand and sugar production and nectar volume on the other. These results, in conjunction with lack of evidence for maternally based environmental effects, suggest that population differences in herbivore damage have promoted differential evolution of EFN-related traits among populations. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Exploiting physical defence traits for crop protection: leaf trichomes of Rubus idaeus have deterrent effects on spider mites but not aphids

Plants possess anti‐herbivore defences that could be exploited for crop protection. The potential for deploying physical defence traits for more sustainable pest management (i.e. reduced pesticide application) has not been fully realised. Using a perennial crop (red raspberry, Rubus idaeus), we take the novel approach of quantifying within‐ and between‐genotype variation in a resistance trait, leaf trichome density, to determine precisely the effect of trichomes on host plant preference and suitability for two shoot‐feeding arthropods, the European large raspberry aphid (Amphorophora idaei) and two‐spotted spider mite (Tetranychus urticae). Additionally, we tested whether this trait influenced searching behaviour of a generalist herbivore predator (lacewing larvae, Chrysoperla carnea). Although there was no consistent genotypic variation in R. idaeus suitability for T. urticae, our hypothesis that T. urticae would avoid high leaf trichome density was supported on certain genotypes. The deterrent effect was mainly on egg deposition rather than leaf selection by adults, with up to sixfold differences in leaf preference depending on the genotypes offered. By contrast, there was significant genotypic variation in R. idaeus suitability for A. idaei (10‐fold variation in aphid abundance), but, contrary to our prediction, aphid preference and infestation levels were unrelated to leaf trichome density. Instead, A. idaei performed best on vigorous genotypes, indicating that plant tolerance traits contributed to R. idaeus suitability for aphids. Leaf trichomes had little effect on the behaviour of the beneficial control agent C. carnea larvae. We conclude that physical anti‐herbivore defences, specifically leaf trichomes, could be deployed to deter particular arthropod pests. However, the mechanistic approach adopted here is necessary to avoid antagonistic effects on other pests or on natural enemies.     

Arboreal Ant Abundance and Leaf Miner Damage in Coffee Agroecosystems in Mexico

Shaded coffee agroecosystems traditionally have few pest problems potentially due to higher abundance and diversity of predators of herbivores. However, with coffee intensification (e.g., shade tree removal or pruning), some pest problems increase. For example, coffee leaf miner outbreaks have been linked to more intensive management and increased use of agrochemicals. Parasitic wasps control the coffee leaf miner, but few studies have examined the role of predators, such as ants, that are abundant and diverse in coffee plantations. Here, we examine linkages between arboreal ant communities and coffee leaf miner incidence in a coffee plantation in Mexico. We examined relationships between incidence and severity of leaf miner attack and: (1) variation in canopy cover, tree density, tree diversity, and relative abundance of Inga spp. shade trees; (2) presence of Azteca instabilis, an arboreal canopy dominant ant; and (3) the number of arboreal twig‐nesting ant species and nests in coffee plants. Differences in vegetation characteristics in study plots did not correlate with leaf miner damage perhaps because environmental factors act on pest populations at a larger spatial scale. Further, presence of A. instabilis did not influence presence or severity of leaf miner damage. The proportion of leaves with leaf miner damage was significantly lower where abundance of twig‐nesting ants was higher but not where twig‐nesting ant richness was higher. These results indicate that abundance of twig‐nesting ants in shaded coffee plantations may contribute to maintenance of low leaf miner populations and that ants provide important ecosystem services in coffee agroecosystems.     

Host-specific variation in Ieaf miner population dynamics: effects on density, natural enemies and behaviour of Stilbosis quadricustatella (Lepidoptera: Cosmopterigidae)

Abstract. 1. We compared high and low density populations of a leaf miner ( Stilbosis quadricustatella (Cham.)) on two host oaks to ascertain mechanisms influencing abundance. High density miner populations occurred on sand live oak, Quercus geminata (Fagaceae), found in homogeneous stands at inland and coastal sites. Quercus nigra , water oak, a patchily distributed inland species, supported a low density leaf miner population.
2. Average foliar nitrogen of Q.geminata was significantly lower than that of Q.nigra , and lad mining period on Q.geminata was correspondingly longer than on Q.nigra .
3. The average leaf area of Q.nigra was significantly greater than that of Q.geminata .
4. Parasitism was significantly greater in Q.geminata miner populations and predation was significantly higher in Q.nigra populations. However, parasitism and predation rates were roughly reciprocal so that overall larval mortality did not differ significantly between hosts.
5. In a laboratory experiment, pupal overwintering survivorship did not differ significantly between moist and dry treatments of the sand and loam soil types that typify Qgeminata and Q.nipra habitats, respectively.
6. Within-leaf miner density on Q.geminata significantly affected larval survivorship, parasitism and predation. Leaf area and within-leaf miner density were positively correlated.
7. We detected no host-patch area or isolation effect on miner densities on coastal Qgeminata and there was no significant gradient of local coastal conditions affecting larval survivorship or natural enemies.
8. Coastal leaf miner densities were significantly higher on oak patch edges than in interiors.     

Evolutionary response to coexistence with close relatives: increased resistance against specialist herbivores without cost for climatic‐stress resistance

Why can hosts coexist with conspecifics or phylogenetically proximate neighbours despite sharing specialist enemies? Do the hosts evolve increased enemy resistance? If so, does this have costs in terms of climatic‐stress resistance, or in such neighbourhoods, does climatic‐stress select for resistances that are multifunctional against climate and enemies? We studied oak (Quercus petraea) descendants from provenances of contrasting phylogenetic neighbourhoods and climates in a 25‐year‐old common garden. We found that descendants from conspecific or phylogenetically proximate neighbourhoods had the toughest leaves and fewest leaf miners, but no reduction in climatic‐stress resistance. Descendants from such neighbourhoods under cold or dry climates had the highest flavonol and anthocyanin levels and the thickest leaves. Overall, populations facing phylogenetically proximate neighbours can rapidly evolve herbivore resistance, without cost to climatic‐stress resistance, but possibly facilitating resistance against cold and drought via multifunctional traits. Microevolution might hence facilitate ecological coexistence of close relatives and thereby macroevolutionary conservatism of niches.     

Differentiation and adaptation in <Emphasis Type="Italic">Brassica nigra</Emphasis> populations: interactions with related herbivores

Local adaptation and population differentiation of plants are well documented, but studies on interactions with natural enemies are rare. In particular, evidence for plant adaptation to the local biotic environment, such as herbivores remains poor. We used the black mustard Brassica nigra, an annual species of river valley and coastal habitats to (1) analyse population differentiation in plant traits and herbivory in a common garden experiment, (2) examine home versus away differences in a reciprocal transplant experiment and (3) test whether plants are adapted to local herbivores or vice versa under standard greenhouse conditions. In the common garden experiment, we found significant differentiation in plant traits, leaf damage and herbivore number among seven populations of B. nigra from France and Germany (distance 15–1,000 km). Differences were particularly strong among coastal and river valley populations and did not necessarily increase with geographical distance. A herbivore removal treatment did not change population differentiation when compared with the control allowing natural colonisation. The reciprocal transplant experiment at a scale of 15–30 km did not reveal local plant adaptation, whilst one dominant herbivore species (Meligethes aeneus) occurred in significantly higher numbers on local plants. A greenhouse experiment combining three aphid (Brevicoryne brassicae) and plant populations of the same provenance indicated herbivore adaptation to their local plants rather than plant adaptation, but overall contrasts between local and non-local combinations were not significant. The results suggest that herbivores may counteract local plant adaptation to other environmental factors. Our study has important implications for plant translocations in ecological restoration projects.     

Trophic cascades in bell miner‐associated dieback forests: Quantifying relationships between leaf quality,psyllids and Psyllaephagus parasitoids

Coteries of the meliphagid bird Manorina melanophrys are associated with a form of eucalypt defoliation and recovery called bell miner‐associated dieback (BMAD). Through their defence of cooperative colony boundaries against other insectivorous birds, bell miners may foster greater abundances of lerp‐forming psyllids (Hemiptera: Aphalaridae), some of which reduce the lifespan of leaves. Trophic cascades in BMAD forests need to be understood to have a complete picture of regulatory processes. We studied relationships between leaf quality, psyllid and Psyllaephagus parasitoid/hyperparasitoid abundances within the Gondwana Rainforest World Heritage Area, NSW, Australia; our focal tree species were Eucalyptus propinqua and E. biturbinata. Eucalyptus biturbinata had tougher leaves than E. propinqua; leaf toughness of both species varied with site and tree. We found a statistically significant, negative relationship between toughness (surrogate for leaf age) and foliar nitrogen content; younger leaves had higher nitrogen contents. Both bell miner abundance and foliar nitrogen were positively correlated with psyllid abundance. The abundance of Glycaspis species (the psyllid that produces lerps with the highest sugar content) was more closely correlated with foliar nitrogen content than was the abundance of all five psyllid genera combined. We identified 14 Psyllaephagus spp./morphospecies, comprising 11 primary parasitoids and three hyperparasitoids. The abundance of all Psyllaephagus combined was positively correlated with the abundance of lerps. However, psyllid parasitism was not correlated with the abundance of lerps. The abundance of the three hyperparasitoids was positively correlated with the abundance of Psyllaephagus hosts. The availability of epicormic foliage (young, morphologically juvenile leaves produced following defoliation) is likely to alter the nutritional ecology underpinning the diversity and abundance of psyllid populations. Higher quality epicormic foliage should favour populations of Glycaspis species (by enhancing nymphal survival) creating lerp hotspots that induce residency by opportunistic bell miners. The positive contribution of induced amelioration, interacting with feedbacks from parasitoids and hyperparasitoids, to BMAD requires longitudinal investigation.     

Non‐lethal foraging by bell miners on a herbivorous insect: Potential implications for forest health

Tree health is often negatively linked with the localized abundance of parasitic invertebrates. One group, the sap‐sucking psyllid insects (Homoptera: Psyllidae) are well known for their negative impact upon vegetation, an impact that often culminates in the defoliation and even death of hosts. In Australia, psyllid‐infested forest in poor health is also frequently occupied by a native honeyeater, the bell miner (Manorina melanophrys; Meliphagidae), so much so that the phenomenon has been dubbed ‘bell miner‐associated dieback’ (BMAD). Bell miners are thought to be the causative agent behind BMAD, in part because the species may selectively forage only upon the outer covering (lerp) exuded by psyllid nymphs, leaving the insect underneath to continue parasitizing hosts. As bell miners also aggressively exclude all other avian psyllid predators from occupied areas, these behavioural traits may favour increases in psyllid populations. We examined bell miner foraging behaviour to determine if non‐lethal foraging upon psyllid nymphs occurred more often than in a congener, the noisy miner (M. melanocephala; Meliphagidae). This was indeed the case, with bell miners significantly more likely to remove only the lerp covering during feeding, leaving the insect intact underneath. This arose from bell miners using their tongue to pry off the lerp cases, whereas noisy miners used their mandibles to snap at both the lerp and insect underneath. Furthermore, psyllids left behind following a bell miner foraging event were significantly more likely to be viable and regrow a lerp covering than those exposed by noisy miners. Together, this behaviour supports the theory that non‐lethal foraging behaviour of bell miners may contribute to high psyllid abundance, consistent with the mechanisms by which BMAD is thought to develop.     

Elevational turnover in the composition of leaf miners and their interactions with host plants in Australian subtropical rainforest

Leaf miners are specialist herbivorous insects that are potentially vulnerable to environmental change because of their dependency on particular host plants. Little, however, is known about how climate affects the distribution of leaf miner communities and their interactions with host plants. Elevational gradients are useful tools for understanding how ecological communities respond to local clines in climate. Given that plant communities are known to undergo elevational turnover in response to changes in climatic conditions, we expect that leaf miner species will also change with elevation. We repeatedly hand collected leaf miners along three elevational gradients in subtropical rainforest in eastern Australia. Individual leaf miners were counted and identified to species, and their host plants were recorded. We tested if leaf miner species richness and the number of unique interactions among leaf miner and host plant species were affected by elevation. We also tested if the composition of leaf miner species and the composition of interactions between leaf miners and host plants showed a relationship with elevation. The rarefied number of unique leaf miner–host plant interactions significantly decreased with elevation, with a slight peak at approx. 700 m a.s.l., while neither rarefied or observed species richness (species density) of leaf miners nor observed numbers of unique interactions (interaction density) were significantly affected by elevation. The composition of leaf miner species and the composition of leaf miner–host plant interactions (occurrence of pairwise interactions) were significantly related to elevation. Elevational turnover in leaf miner species composition indicated that different species varied in their response to changes in biotic and/or abiotic conditions imposed by increasing elevation. Through our analyses, we identified four leaf miner species that may be locally vulnerable to climate change, as a result of their restricted elevational distribution and level of host specificity.     

Direct and indirect effects of plant genetic variation on enemy impact

Abstract.

• 1 The Tritrophic and Enemy Impact concepts predict that natural enemy impact varies: (a) among plant genotypes and (b) may depend on the abundance of heterospecific herbivores, respectively. I tested these predictions using three herbivore species on potted, cloned genotypes of Salik sericea Marshall in a common garden experiment.
• 2 Densities of the leaf miner (Phyllonorycter salicifoliella (Chambers)) and two leaf galling sawflies (Phyllocolpa nigrita (Marlatt) and Phyllocolpa eleanorae Smith and Fritz) varied significantly among willow clones, indicating genetic variation in resistance.
• 3 Survival and natural enemy impact caused by egg and larval parasitoids and/or unknown predators differed significantly among willow clones for each of the three herbivore species, indicating genetic variation in survival and enemy impact.
• 4 Survival of Phyllonorycter was negatively density-dependent among clones.
• 5 Survival of Phyllonorycter and Phyllocolpa eleanorae were positively correlated with densities of heterospecific herbivores among clones and parasitism of these species were negatively correlated with densities of the same heterospecific herbivores among clones.
• 6 At least for Phyllonorycter this positive correlation may suggest either facilitation of survival between herbivore species, which do not share natural enemies, or an apparent interaction caused by host plant genetic variation.
• 7 Among clones, egg parasitism of Phyllocolpa eleanorae was weakly positively correlated with density of Phyllocolpa nigrita. Since these species share the same Trichogramma egg parasitoid, this interaction could support the hypothesis of apparent competition.

     

Effects of variable phytochemistry and budbreak phenology on defoliation of aspen during a forest tent caterpillar outbreak

1 The present study assessed the relationship between clonally variable rates of defoliation in trembling aspen (Populus tremuloides Michx.) and two potential resistance traits: defensive chemistry and leaf phenology. 2 In 2001, coincident with a major outbreak of the forest tent caterpillar (Malacosoma disstria Hubner) in the northcentral U.S.A., we monitored defoliation rates, phytochemical composition, and foliar development in 30 clones of trembling aspen. Leaf chemistry was also assessed in re‐flushed leaves and 2 years post‐outbreak. 3 Early in the season, differences in defoliation among clones were substantial but, by mid‐June, all clones were completely defoliated. Leaf nitrogen, condensed tannins, and phenolic glycosides varied among clones but did not relate to defoliation levels. Budbreak phenology differed by 3 weeks among clones and clones that broke bud early or late relative to forest tent caterpillar eclosion experienced reduced rates of defoliation. 4 Defoliation led to increased tannins and slight decreases in phenolic glycoside concentrations in damaged leaf remnants, but to moderately decreased tannins and a six‐fold increase in phenolic glycosides in reflushed leaves. This shift in chemical composition may significantly affect late season herbivores. 5 These results suggest that aspen chemical resistance mechanisms are ineffective during intense episodic eruptions of outbreak folivores such as the forest tent caterpillar. Variable budbreak phenology may lead to differential susceptibility during less intense outbreak years and, at peak forest tent caterpillar population densities, mechanisms affording tolerance are probably more important than chemical defences.