Alterations of the resin canal system of <Emphasis Type="Italic">Pinus pinaster</Emphasis> seedlings after fertilization of a healthy and of a <Emphasis Type="Italic">Hylobius abietis</Emphasis> attacked stand

Changes in resource availability and biotic and abiotic stress may alter the defensive mechanisms of pine trees. The effect of fertilisation on the resin canal structure of Pinus pinaster seedlings established in two trials in NW Spain, one attacked by Hylobius abietis and the other non-attacked, was studied. The leaders of 50 plants were destructively sampled and the resin canal density, the canal area and its relative conductive area in the phloem and xylem were assessed. Experimentally increased nutrient availability significantly decreased resin canal density in the phloem of the seedlings in the two analysed trials, where unfertilised seedlings presented up to 30% more resin canal density than the fertilised seedlings (mean value ± SEM = 0.32 ± 0.02 resin canals mm⁻² in the fertilised plants versus 0.45 ± 0.04 resin canals mm⁻² in the control plants). Fertilisation had no effect on the resin canal system in the xylem, but significantly increased tracheid size. Significant differences of resin canals among sites were observed mainly in the xylem; the resin canal density was 1.7-fold greater in the attacked site than in the non-attacked site. The similar structure of phloem resin canals in both sites supports that phloem resin canals are constitutive mechanisms of defence in P. pinaster, whereas xylem resin canals would be constitutive mechanisms but also inducible mechanisms of resistance following the attack of pine weevils or bark beetles. XM and LS equally contributed to this paper.     

Resistance and response of <Emphasis Type="Italic">Pinus pinaster</Emphasis> seedlings to <Emphasis Type="Italic">Hylobius abietis</Emphasis> after induction with methyl jasmonate

Experimental induction of plant chemical defences with methyl jasmonate (MeJa) is a valuable tool for understanding the ecology of plant defensive responses. However, few studies have examined whether MeJa-induced defences in conifers are effective against insect herbivores. We studied, in 17 half-sib Pinus pinaster families, (i) the effect of MeJa application on plant growth and on the induction of diterpenoid resin in different sections of the stem; (ii) whether MeJa-induced defences increase the resistance of living pine juveniles against the large pine weevil Hylobius abietis in an in vivo bioassay and (iii) the induction of resin content after weevil wounding. Resin concentration was greater in the upper section of the stem compared with basal sections in both MeJa-induced and non-induced seedlings. Sixty days after MeJa application, treated plants showed 40% greater resin content all along the stem, but reduced height growth compared to control plants. MeJa-induction was effective against the pine weevil, as induced seedlings were 21% less damaged than control plants. Wounding activity by H. abietis produced a strong local defensive response after 48 h, where resin concentration was double that observed in the basal and apical sections not exposed to the insects.     

Pine weevil damage to Norway spruce seedlings: effects of nutrient‐loading,soil inversion and physical protection during seedling establishment

• 1 The pine weevil Hylobius abietis (L.) feeds on the bark of young conifer seedlings and is one of the most economically important forest pests in Europe.
• 2 In a field experiment, we examined the combined effects of the treatments: nutrient‐loading of seedlings, planting in scarified plots and protection of seedlings against pine weevil damage for either half a season or a full season.
• 3 Nutrient loading had no significant effect on the amount of pine weevil feeding.
• 4 Seedling mortality was significantly reduced when seedlings were protected from pine weevil feeding during establishment. This occurred even though the debarked area of protected seedlings 5 weeks after the shields had been removed was similar to that of the unprotected seedlings. This indicates that initial protection rendered the seedlings more tolerant to later wounding by pine weevil.
• 5 Planting in soil inversion significantly reduced feeding compared with planting in humus.
• 6 We conclude that nutrient‐loading of seedlings in the autumn before planting would not increase pine weevil feeding after planting. Mortality could be reduced by treatments that postpone the start of pine weevil feeding on seedlings. Such treatments, combined with planting in soil inversion, would result in increased seedling growth, vitality and tolerance to pine weevil attack.

     

Quantitative comparison of chemical, biological and mechanical induction of secondary compounds in Pinus pinaster seedlings

Chemical elicitors and mechanical treatments simulating real insect herbivory have been increasingly used to study induced defensive responses in woody plants. However, simultaneous quantitative comparisons of plant chemical defences elicited by real and simulated herbivory have received little attention. In this paper we compared the effects of real herbivory, simulated herbivory using two chemical elicitors, and mechanical damage treatments on the quantitative secondary chemistry of Pinus pinaster juveniles (namely on non-volatile resin in the stem and total phenolics in the needles). The real herbivory involved phloem wounding by Hylobius abietis and defoliation by Brachyderes lusitanicus (two pine weevils); the chemical elicitors to simulate herbivory induction were 40 mM methyl jasmonate (MJ) and 20 μM benzothiadiazole (BTH); and the mechanical treatments included phloem wounding and needle clipping. We also performed an additional experiment for assessing at what extent insect extracts could increase plant responses over mechanical damage. Chemical induction with MJ, mechanical wounding and real phloem herbivory by H. abietis all produced quantitatively similar results, increasing the concentration of resin in the stem and total phenolics in the needles by equivalent magnitudes. Exogenous application of BTH increased the concentration of phenolic compounds in pine needles, but did not show the same effect on stem resin. Contrastingly, we did not find significant changes in the concentration of resin in the stem or phenolics in the needles after needle clipping and B. lusitanicus feeding. Mechanical damage followed by the application of extracts from the insects B. lusitanicus and H. abietis on the injured tissues did not increase the responses in comparison to mechanical damage alone. The fact that strong induced responses elicited by phloem wounding insects are equally elicited by phloem injuries suggests that defences in pine trees are raised with low specificity regarding biotic enemies. Results from this paper support future methodological approaches using chemical elicitors and mechanical damage as simulated herbivory treatments for the experimental induction of conifer defences.     

The gut microbiota of the pine weevil is similar across Europe and resembles that of other conifer‐feeding beetles

The pine weevil (Hylobius abietis, Coleoptera: Curculionidae) is an important pest of conifer seedlings in Europe. Despite its economic importance, little is known about the composition of its gut microbial community and the role it plays in mediating the weevil's ability to utilize conifers as a food source. Here, we characterized the gut bacterial communities of different populations of H. abietis across Europe and compared them to those of other beetles that occupy similar ecological niches. We demonstrate that the microbial community of H. abietis is similar at higher taxonomic levels (family and genus) across locations in Europe, with Wolbachia as the dominant microbe, followed by Enterobacteria and Firmicutes. Despite this similarity, we observed consistent differences between countries and locations, but not sexes. Our meta‐analysis demonstrates that the gut bacterial community of the pine weevil is very similar to that of bark beetles that also exploit conifers as a food source. The Enterobacteriaceae symbionts of both host taxa are especially closely related phylogenetically. Conversely, the microbiota of H. abietis is distinct from that of closely related weevils feeding on nonconifer food sources, suggesting that the microbial community of the pine weevil is determined by the environment and may be relevant to host ecology. Furthermore, several H. abietis‐associated members of the Enterobacteriaceae family are known to contain genes involved in terpenoid degradation. As such, we hypothesize that the gut microbial community is important for the utilization of conifer seedlings as a food source, either through the detoxification of plant secondary metabolites or through the supplementation of essential nutrients.     

Host finding in the pine weevil Hylobius abietis: effects of conifer volatiles and added limonene

The attraction of Hylobius abietis (L.) (Coleoptera: Curculionidae) to -pinene was recently shown to be inhibited by limonene. This study shows that added limonene also affects the response of H. abietis to odours from natural host material. The experiments included both pieces of Scots pine suspended in pitfall traps and planted pine seedlings that had either been manually wounded or left intact. By adding a limonene dispenser, the catch of pine weevils in pine-baited traps was reduced by about 80–90% in spring and autumn and by about 40–50% in summer. Limonene substantially reduced the rate at which manually wounded seedlings were attacked by weevils in spring and autumn. For intact seedlings, a limonene-induced reduction in attack rate was found only in autumn. In the absence of limonene, a small wound made on the stem of a seedling increased its probability of being attacked by about fivefold. This figure was estimated by comparing data from all experimental periods with a model assuming a constant ratio of attack rates for the two kinds of seedlings.     

Gut microbiota of the pine weevil degrades conifer diterpenes and increases insect fitness

The pine weevil (Hylobius abietis), a major pest of conifer forests throughout Europe, feeds on the bark and cambium, tissues rich in terpenoid resins that are toxic to many insect herbivores. Here, we report the ability of the pine weevil gut microbiota to degrade the diterpene acids of Norway spruce. The diterpene acid levels present in ingested bark were substantially reduced on passage through the pine weevil gut. This reduction was significantly less upon antibiotic treatment, and supplementing the diet with gut suspensions from untreated insects restored the ability to degrade diterpenes. In addition, cultured bacteria isolated from pine weevil guts were shown to degrade a Norway spruce diterpene acid. In a metagenomic survey of the insect's bacterial community, we were able to annotate several genes of a previously described diterpene degradation (dit) gene cluster. Antibiotic treatment disrupted the core bacterial community of H. abietis guts and eliminated nearly all dit genes concordant with its reduction in diterpene degradation. Pine weevils reared on an artificial diet spiked with diterpenes, but without antibiotics, were found to lay more eggs with a higher hatching rate than weevils raised on diets with antibiotics or without diterpenes. These results suggest that gut symbionts contribute towards host fitness, but not by detoxification of diterpenes, as these compounds do not show toxic effects with or without antibiotics. Rather the ability to thrive in a terpene‐rich environment appears to allow gut microbes to benefit the weevil in other ways, such as increasing the nutritional properties of their diet.     

Phenotyping tree resistance to a bark‐chewing insect,the pine weevil Hylobius abietis

Breeding for resistance to forest pests and pathogens is emerging as a promising tool for minimising the impact of the increasing biotic threats that our forests are experiencing as a consequence of global change. Efficient phenotyping protocols of resistance are urgently needed. Here we present the results of two experiments aimed to determine whether the variation in resistance to the pine weevil Hylobius abietis, a harmful pest of European conifers, can be inferred by nondestructive bioassays using excised plant material collected in forest genetic trials. Weevil damage and amount of nonvolatile resin induced by weevil feeding were assessed in young trees and in branches of adult trees using several phenotyping procedures (bioassays using either living trees, excised plant material and cut stem twigs) on four pine species (Pinus pinaster, P. radiata, P. sylvestris and P. pinea). Half of the plants were previously induced with methyl jasmonate (MJ), a treatment that is known to affect resistance to the pine weevil. In Experiment 1, living and excised plants showed parallel results: MJ treatment significantly reduced weevil damage, and saplings responded to weevil damage locally increasing the nonvolatile resin (NVR) in the stems proportionally to the damage suffered. This response was, however, slightly lower in excised than in living saplings. On the contrary, patterns of weevil feeding on stem twigs completely departed from those observed in living and excised seedlings. Moreover, cut stem twigs were unable to respond to weevil feeding increasing NVR according to the weevil damage. In Experiment 2, assessment of weevil damage on excised branches explained around 50% of variation in damage on living branches. This relationship became much more pronounced (R² = 0.81) when explored at the mean treatment level; branch manipulation did not alter the patterns of variation in resistance across pine species or MJ treatments. Irrespective of the assessment procedure, MJ consistently decreased weevil damage in all pine species, with larger reduction in weevil damage in stone and maritime pine than in radiata and Scots pine. Radiata pine was the most resistant while Scots pine was the most susceptible to the pine weevil. Overall, results suggest that using excised plant material is an operative alternative for phenotyping weevil resistance whenever care is taken to maintain the functionality of the excised plant material. This will allow taking advantage of multiple available conifer genetic trials to deepen the ecological genetics of resistance to the pine weevil and to screen for resistance without compromising the long‐term utility of those genetic trials.     

Host-plant acceptance on mineral soil and humus by the pine weevil Hylobius abietis (L.)

1 The pine weevil Hylobius abietis (L.) (Coleoptera, Curculionidae) is an economically important pest of conifer forest regeneration in Europe and Asia. 2 Soil scarification, which usually exposes mineral soil, is widely used to protect seedlings from weevil attack. However, the mechanism behind this protective effect is not yet fully understood. 3 Field experiments were conducted to determine the pine weevil's responses to visual and odour stimuli from seedlings when moving on mineral soil and on undisturbed humus surface. 4 One experiment measured the number of pine weevils approaching seedlings, with and without added host odour, on mineral soil and undisturbed humus. Seedlings with added host odour attracted more weevils on both soil types. Unexpectedly, somewhat more weevils approached seedlings surrounded by mineral soil. 5 In a similar experiment, feeding attacks on seedlings planted directly in the soil were recorded. Only half as many seedlings were attacked on mineral soil as on undisturbed humus. 6 In the first experiment, the weevils were trapped 2.5 cm from the bases of the seedlings' stems, whereas they could reach the seedlings in the experiment where seedlings were planted directly in the soil. We conclude that the pine weevils' decision on whether or not to feed on a seedling is strongly influenced by the surrounding soil type and that this decision is taken in the close vicinity of the seedling. The presence of pure mineral soil around the seedling strongly reduces the likelihood that an approaching pine weevil will feed on it.     

Evaluating different harvest intensities over understory plant diversity and pine seedlings,in a <Emphasis Type="Italic">Pinus pinaster</Emphasis> Ait. natural stand of Spain

Although modern forestry takes into consideration the analysis of the effects of forest management on plant structure, diversity and seedlings, little is known about how those parameters respond to harvest techniques in the Mediterranean region. We investigated the effect of three different harvest intensities, respect to uncut controls, on understory plant species functional groups, richness, diversity and pine seedlings in a natural Maritime pine stand in Spain, three years after harvesting. The harvest treatments produced a reduction of the number of Pinus pinaster seedlings and woody species cover, and an increase of species richness (total and of annual species) and plant cover of annual species respect to control plots (CO). The Shannon diversity values showed no differences between treatments. These results emphasize that the tree harvest treatments analyzed are not suitable for the management of this P. pinaster stand. Otherwise, the reduction of pine seedling density by harvest treatments and the changes in richness and cover of functional groups would not induce the natural regeneration of this stand maintaining the understory plant layer.     

Understory Invasion by <Emphasis Type="Italic">Acacia longifolia</Emphasis> Alters the Water Balance and Carbon Gain of a Mediterranean Pine Forest

In water-limited ecosystems, where potential evapotranspiration exceeds precipitation, it is often assumed that plant invasions will not increase total ecosystem water use, because all available water is evaporated or transpired regardless of vegetation type. However, invasion by exotic species, with high water use rates, may potentially alter ecosystem water balance by reducing water available to native species, which may in turn impact carbon assimilation and productivity of co-occurring species. Here, we document the impact of invasion by an understory exotic woody species (Acacia longifolia) in a semi-arid Mediterranean dune pine forest. To quantify the effects of this understory leguminous tree on the water use and carbon fixation rates of Pinus pinaster we compare an invaded and a non-invaded stand. A. longifolia significantly altered forest structure by increasing plant density and leaf area index in the mid-stratum of the invaded forest. A. longifolia contributed significantly to transpiration in the invaded forest (up to 42%) resulting in a slight increase in stand transpiration in the invaded relative to non-invaded forest. More importantly, both water use and carbon assimilation rates of P. pinaster were significantly reduced in the invaded relative to non-invaded stand. Therefore, this study shows that exotic plant invasions can have significant impacts on hydrological and carbon cycling even in water-limited semi-arid ecosystems through a repartitioning of water resources between the native and the invasive species.     

Non-pesticide alternatives for reducing feeding damage caused by the large pine weevil (Hylobius abietis L.)

The large pine weevil (Hylobius abietis L.) is an important pest of young forest stands in Europe. Larvae develop under the bark of freshly cut pine and spruce stumps, but maturing weevils feed on the bark of coniferous seedlings. Such seedlings frequently die because of bark consumption near the root collar. We tested the effect of three treatments (the insecticide alpha cypermethrin, a wax coating and a glue coating) on the feeding damage caused by H. abietis on Douglas fir (Pseudotsuga menziesii) and Norway spruce (Picea abies) seedlings under semi-natural conditions. In two experiments (one in 2016 and another in 2017) seedlings in cages were subjected to pine weevil feeding for 16 weeks under shaded outdoor conditions. The experiment in 2016 compared insecticide and wax treatments and an untreated control on Douglas fir and Norway spruce, and the experiment in 2017 compared insecticide, wax and glue treatments and an untreated control on Norway spruce. In both experiments, all treatments significantly reduced H. abietis feeding damage at week 8 at the end of both experiments (week 16); the effect of treatments was significant only on spruce seedlings. The damages on Douglas fir seedlings was less on treated seedlings than on untreated control seedlings but differences were not significant. Coating stems with glue and especially with wax was generally effective at reducing weevil damage and in most cases provided control that was not significantly different from that provided by insecticide treatment. Our results suggest that a wax coating has the potential to replace the protection of seedlings provided by insecticides.     

Chemical composition and antifeedant activity of some aromatic plants against pine weevil (Hylobius abietis)

The pine weevil Hylobius abietis is an important pest causing severe damage to conifer seedlings in reforestation areas in Europe and Asia. Plants that have no evolutionary history with the pine weevil are of special interest in the search for compounds with a strong antifeedant activity. Thus, the essential oils of nine aromatic plants, viz Amomum subulatum, Cinnamomum tamala, Curcuma longa, Laurus nobilis, Ocimum basilicum, Origanum majorana, Origanum vulgare, Syzygium aromaticum and Trachyspermum ammi were extracted by hydrodistillation. The essential oil constituents were identified by gas chromatography–mass spectrometry, and antifeedant properties towards the pine weevil were assessed using choice feeding bioassay. The essential oils of C. longa, O. majorana, S. aromaticum and T. ammi showed an excellent antifeedant activity towards the pine weevil for 24 hr, whereas the essential oil of other plants showed the activity for 6 hr. There was a positive correlation between the amount of benzenoid compounds and the antifeedant activity of the essential oils. This study suggests that pine weevil non-host plant compounds have potential to be used for the protection of seedlings against pine weevil feeding. However, further study will be needed to explore the antifeedant activity of individual components and oils in the laboratory as well as in the field.     

Mutual benefit interactions between banded pine weevil Pissodes castaneus and blue‐stain fungus Leptographium serpens in maritime pine

• 1 Ecological interactions between banded pine weevil Pissodes castaneus and blue‐stain fungus Leptographium serpens, when simultaneously sharing the same host plant (maritime pine Pinus pinaster) in winter and spring, were investigated. Temporal components of the interaction were taken into account by either introducing the weevils and the pathogen simultaneously or sequentially, with the weevils being introduced 1 month after the fungal inoculation.
• 2 We measured larval mortality, development time, offspring number, sex ratio and body size of P. castaneus. Phloem phosphorus and nitrogen concentrations were also assessed. Furthermore, we tested whether: (i) emerging offspring transported propagules of the fungus; (ii) artificially‐contaminated weevils may transmit the disease to healthy trees; and (iii) field collected P. castaneus carry the fungus.
• 3 The fungus enhanced weevil colonization and brood production in both seasons. During winter and spring, adults from trees where the pathogen was inoculated prior to weevil introduction emerged earlier than weevils from trees where they had been introduced simultaneously with the fungus. During winter, weevils from pre‐inoculated trees were also larger. Sex ratio and larval mortality were not affected. Leptographium serpens did not affect phloem nitrogen content but phosphorus content was greater in plants inoculated with the pathogen, which may explain the findings on weevil growth.
• 4 Sixty‐five percent of the weevils that emerged from inoculated trees carried spores of L. serpens, although no successful isolation was made from field collected weevils. The fungus was recovered from 25% of the trees infested with artificially‐contaminated weevils.
• 5 These results suggest that P. castaneus benefits from the presence of L. serpens and may contribute to its spread.

     

Area‐wide application of verbenone‐releasing flakes reduces mortality of whitebark pine Pinus albicaulis caused by the mountain pine beetle Dendroctonus ponderosae

• 1 DISRUPT Micro‐Flake Verbenone Bark Beetle Anti‐Aggregant flakes (Hercon Environmental, Inc., Emigsville, Pennsylvania) were applied in two large‐scale tests to assess their efficacy for protecting whitebark pine Pinus albicaulis Engelm. from attack by mountain pine beetle Dendroctonus ponderosae Hopkins (Coleoptera: Scolytinae) (MPB). At two locations, five plots of equivalent size and stand structure served as untreated controls. All plots had early‐ to mid‐outbreak beetle populations (i.e. 7.1–29.2 attacked trees/ha). Verbenone was applied at 370 g/ha in both studies. Intercept traps baited with MPB aggregation pheromone were placed near the corners of each plot after the treatment in order to monitor beetle flight within the plots. Trap catches were collected at 7‐ to 14‐day intervals, and assessments were made at the end of the season of stand structure, stand composition and MPB attack rate for the current and previous years.
• 2 Applications of verbenone flakes significantly reduced the numbers of beetles trapped in treated plots compared with controls at both sites by approximately 50% at the first collection date.
• 3 The applications also significantly reduced the proportion of trees attacked in both Wyoming and Washington using the proportion of trees attacked the previous year as a covariate in the model for analysis of current year attack rates; in both sites, the reduction was ≥ 50%.
• 4 The flake formulation of verbenone appears to have promise for area‐wide treatment by aerial application when aiming to control the mountain pine beetle in whitebark pine forests.

     

Are the metabolomic responses to folivory of closely related plant species linked to macroevolutionary and plant–folivore coevolutionary processes?

The debate whether the coevolution of plants and insects or macroevolutionary processes (phylogeny) is the main driver determining the arsenal of molecular defensive compounds of plants remains unresolved. Attacks by herbivorous insects affect not only the composition of defensive compounds in plants but also the entire metabolome. Metabolomes are the final products of genotypes and are constrained by macroevolutionary processes, so closely related species should have similar metabolomic compositions and may respond in similar ways to attacks by folivores. We analyzed the elemental compositions and metabolomes of needles from three closely related Pinus species with distant coevolutionary histories with the caterpillar of the processionary moth respond similarly to its attack. All pines had different metabolomes and metabolic responses to herbivorous attack. The metabolomic variation among the species and the responses to folivory reflected their macroevolutionary relationships, with P. pinaster having the most divergent metabolome. The concentrations of terpenes were in the attacked trees supporting the hypothesis that herbivores avoid plant individuals with higher concentrations. Our results suggest that macroevolutionary history plays important roles in the metabolomic responses of these pine species to folivory, but plant–insect coevolution probably constrains those responses. Combinations of different evolutionary factors and trade‐offs are likely responsible for the different responses of each species to folivory, which is not necessarily exclusively linked to plant–insect coevolution.     

Niche‐based relationship between sympatric bark living insect pests and tree vigor decline of Pinus yunnanensis

Forest stands of Pinus yunnanensis Franch in southwestern China are seriously damaged by several bark living insect pests. These pests commonly exist in the same host tree and exploit limited phloem resources. In this study, we hypothesized that sympatric pests would occupy different ecological niches to reduce interspecific competition, and their coordinated attack would aggravate the tree vigor decline of P. yunnanensis. To quantify the ecological niches, we used a sampling method involving three dimensional divisions of tree resource states: (a) sample plot dimension (to evaluate the extent of pest colonization at plot scale), (b) trunk height dimension, and (c) tree vigor dimension. Those attacked pine canopies and colonized trunks were analysed in the field study. The results showed that Tomicus minor and Tomicus yunnanensis both widely and uniformly distributed in lightly, moderately and heavily damaged canopies while they aggregated on particular trunks of dying and withered trees. Lower and middle trunk sections (1–4 m) were mainly occupied by Monochamus alternatus and T. minor, yet T. yunnanensis dominated the upper parts (5–6 m). There was an overlap of the pine shoot beetles in the middle sections (2–4 m), whereas all three species were ecologically segregated in other trunk sections. During the shoot feeding phase, tree vigor declined with pine shoot beetles' increasing shoot feeding density. They coexisted in the same host tree, while M. alternatus only attacked dying and withered tree trunks, causing a greater infestation. Colonization of pine shoot beetle then followed by M. alternatus could be the major causes of tree mortality. This study highlights the resource utilization pattern of sympatric bark living insect pests corresponding with tree vitality. Those findings would help to understand the mechanisms of pest outbreak in P. yunnanensis ecosystem and provide potential guidance for developing an early monitoring pest warning system.     

Egg allocation by Bracon hylobii Ratz., the principal parasitoid of the large pine weevil (Hylobius abietis L.), and implications for host suppression

1 The braconid parasitoid Bracon hylobii Ratz. is one of the few specialist natural enemies of the large pine weevil, Hylobius abietis L., a destructive pest of conifer transplants. An assessment of its role as an agent of biological control requires a detailed knowledge of the allocation of its reproductive effort. 2 Parasitoid females were continuously observed in laboratory culture with individually reared host larvae in bark discs. The outcome of sequential parasitoid–host encounters was recorded by subsequent examination of hosts and by rearing all parasitoids. 3 Parasitoids avoided ovipositing on host larvae < 100 mg fresh weight, even though such larvae represented sufficient biomass for complete parasitoid development. All larger larvae were vulnerable to attack, which leaves a window of vulnerability for parasitoids of about 90% of weevil larval life. 4 Parasitoids presented with a range of host sizes showed no preference above 100 mg for the size of host first attacked, but allocated more eggs and a greater total handling time to larger hosts. 5 Most eggs were deposited on the first host attacked, with progressively fewer allocated to subsequent hosts. However, oviposition experience did not affect the time spent on the next host. 6 From these results it is anticipated that when weevil larval size is reduced by less favourable feeding substrates, fewer parasitoid eggs will be allocated to each but more host larvae will ultimately be attacked. 7 Generation time, host finding, oviposition rate, clutch size, life expectancy and diapause induction are strongly affected by temperature. Life expectancy is substantially shorter for parasitoids deprived of non‐host food supplement. At 15 and 20 °C the number of hosts attacked and the number of eggs deposited decreased with female age. 8 Bracon hylobii is inevitably poorly synchronized with a variable life‐cycle host; it is egg‐limited and can enter diapause at a relatively high field temperature. None of these characteristics suggest that it could stabilize the abundance of its host below an economically acceptable threshold density. However, the reproductive potential of the parasitoid suggests that it could make a significant contribution to larval mortality and suppress adult recruitment, thus complementing other control strategies.     

Ants protect conifer seedlings from feeding damage by the pine weevil Hylobius abietis

• 1 Ants that protect food resources on plants may prey on (or deter) herbivores and thereby reduce damage. Red wood ants (of the Formica rufa group) are dominant ants in boreal forests of Eurasia and affect the local abundance of several herbivorous species.
• 2 The pine weevil Hylobius abietis (L.) is a herbivore that causes severe damage by feeding on the bark of coniferous seedlings within areas of forest regeneration.
• 3 We investigated whether ants can protect conifer seedlings from pine weevil feeding. In a manipulative experiment, ants were attracted to sugar baits attached to spruce seedlings and the damage caused by pine weevils was compared with control seedlings without ant‐baits.
• 4 The feeding‐scar area was approximately one‐third lower on the seedlings with ant‐baits compared with the controls. Besides red wood ants, Myrmica ants were also attracted in high numbers to the ant baits and the relative effects of these species are discussed.
• 5 The results obtained in the present study support the trophic cascade hypothesis (i.e. damage to herbivores is suppressed in the presence of predators). The decreased pine weevil feeding on the baited seedlings was probably a result of nonconsumptive interactions [i.e. the presence of (or harassment by) ants distracting pine weevils from feeding].
• 6 Understanding the role of ants may have important implications for future strategies aiming to control pine weevil damage. For example, maintaining suitable conditions for ants after harvesting stands may be an environmentally friendly but currently unexploited method of for decreasing weevil damage.

     

Odors of Norway spruce (Picea abies L.) seedlings: differences due to age and chemotype

Small conifer seedlings (mini-seedlings) are less damaged by the large pine weevil Hylobius abietis (L.) (Coleoptera: Curculionidae) compared to conventional seedlings. Chemical difference between the seedling types is one possible explanation for this phenomenon. In the present paper, the emissions of volatile organic compounds (VOC) of 7- to 43-week-old Norway spruce [Picea abies (L.) Karst.] seedlings were analyzed. Collection and identification of the volatiles was made by solid phase micro-extraction and gas chromatography mass spectrometry (SPME–GC–MS). The enantiomers of α-pinene and limonene were separated in a two-dimensional GC (2D-GC). Most of the seedlings represented either a limonene- or a bornyl acetate-chemotype. Only minor changes in the volatile composition of the two types of seedlings were found during the first year. Age-related changes, however, were found in the volatiles released by wounded phloem where green leaf volatiles (GLVs) and borneol were the dominated VOC for young seedling. The attractive compound for the pine weevil, α-pinene, was first detected in the phloem emissions of 18- to 22-week-old seedlings. Different storage conditions of the seedlings during the winter/early spring-phase influenced the volatile composition in the phloem. High amount of GLVs was characteristic for the 43-week-old seedlings stored in naturally changing outdoor temperature, but not present in the seedlings winter-stored at a constant temperature of ?4 °C. The possible role of these observed differences in odor emissions between seedlings of different age and physiological status for the feeding preferences of the large pine weevil is discussed.