Growth and nutritional response of Pinus pinaster after a large pine weevil (Hylobius abietis) attack

Hylobius abietis is an important pest of coniferous plantations in Europe, to which high mortality, stem deformities, and growth loss are typically attributed. In pine trees, as in other long-lived organisms, there is uncertainty regarding the long-term costs of short-term resistance against invading organisms. We examined the nutritional status of Pinus pinaster after a 2-year long H. abietis attack, measuring needle and phloem N and P concentrations, and the impact of the damage on subsequent growth, survival, and stem deformities over a period of 5 years. The study sites were a P. pinaster family × fertilization trial, and a neighbouring twin trial with similar climate and soil characteristics that was not attacked. Growth losses after the H. abietis attack were important (up to 40%), but restricted to the first years after the attack. Five years after the attack, the annual height increment of pines in the attacked stand was not related to the initial damage suffered, and plants showed regular stems, normal leader dominance, and regular height after 5 years. These findings suggest strong compensatory growth in P. pinaster and indicate relatively high tolerance to the large pine weevil. Needle nutrient concentrations in the healthy stand were, as expected, significantly greater in experimentally fertilized plants, and they were linearly related to those in phloem showing equilibrated stoichiometry both for nitrogen (r = 0.86; P < 0.01; N = 25) and phosphorus (r = 0.84; P < 0.01; N = 25). However, at the attacked stand, nutrient concentrations in the needles did not follow the experimentally manipulated nutrient availability in soils, and phosphorus concentration in the needles was unexpectedly not related to those in the phloem. The pine seedlings attacked by H. abietis showed altered potential of allocating nutrients to their tissues according to the nutrient availability existing in the soil, as well as altered stoichiometry in N and P concentrations among phloem and leaves. Maritime pine seems to be tolerant to the pine weevil attack, at least in the conditions of this study, where pine weevil damage caused a deep alteration of nutrient allocation and nutritional status. Further research is needed to elucidate to what extent altered nutrient allocation may be part of an induced response to the attack or just derived from the vascular injury caused by the weevil wounding in the phloem.     

Olfactory and visual stimuli used in orientation to conifer seedlings by the pine weevil, Hylobius abietis

Abstract. The influence of noncontact plant cues is investigated on the likelihood that individual conifer seedlings will be found by walking adults of the pine weevil, Hylobius abietis, in the field. Traps with solely odour or solely visual stimuli catch significantly more weevils than stimulus-free traps, and traps with the combination of odour and visual stimuli catch more weevils than traps with odour or visual stimuli alone. There is essentially an additive effect between odour and visual stimuli. The reactions to odour and visual stimuli are similar for three phases of the pine weevil's life cycle associated with three ages of clear-cuttings (i.e. sites where all trees have been harvested). Visual stimuli appear to be at least as important as odour for the pine weevil in finding an undamaged conifer seedling.     

Effects of green tree retention, prescribed burning and soil treatment on pine weevil (Hylobius abietis and Hylobius pinastri) damage to planted Scots pine seedlings

1 Feeding damage and mortality caused to planted Scots pine seedlings by the pine weevils Hylobius abietis and Hylobius pinastri were studied on burned and unburned sites with 0, 10 and 50 m³ per hectare levels of green tree retention from the second to the fourth summer after logging and burning of the sites. 2 The rate of severe feeding damage to pine seedlings caused by pine weevils was higher on burned clearcut sites than on unburned ones, whereas burning did not increase the feeding damage rate on sites with groups of retention trees. The damage rate in the fourth summer was approximately the same on burned and unburned sites. 3 Pine weevil feeding was the major cause of mortality of freshly planted pine seedlings on unburned sites. On burned sites, mortality was higher than the rate of severe feeding damage, particularly in the second summer after burning, possibly owing to fungal attack and abiotic factors. 4 At a retention tree level of 50 m³, feeding damage to the seedlings was lower than on clearcuts and at a 10 m³ retention tree level. Furthermore, on sites with 50 m³ of retention trees, scarification of the soil was found to decrease feeding damage more effectively than on clearcuts and 10 m³ sites. If the seedlings were situated in the centre of scarified patches, scarification alone was as effective as insecticide treatment on unscarified soil for decreasing feeding damage and mortality. 5 The results suggest that when burning is applied as silvicultural treatment after clear‐cuts, retention of trees is recommended to reduce the damages caused by pine weevils on pine seedlings.     

Effects of green tree retention, prescribed burning and soil treatment on pine weevil (Hylobius abietis and Hylobius pinastri) damage to planted Scots pine seedlings

1 Feeding damage and mortality caused to planted Scots pine seedlings by the pine weevils Hylobius abietis and Hylobius pinastri were studied on burned and unburned sites with 0, 10 and 50 m³ per hectare levels of green tree retention from the second to the fourth summer after logging and burning of the sites. 2 The rate of severe feeding damage to pine seedlings caused by pine weevils was higher on burned clearcut sites than on unburned ones, whereas burning did not increase the feeding damage rate on sites with groups of retention trees. The damage rate in the fourth summer was approximately the same on burned and unburned sites. 3 Pine weevil feeding was the major cause of mortality of freshly planted pine seedlings on unburned sites. On burned sites, mortality was higher than the rate of severe feeding damage, particularly in the second summer after burning, possibly owing to fungal attack and abiotic factors. 4 At a retention tree level of 50 m³, feeding damage to the seedlings was lower than on clearcuts and at a 10 m³ retention tree level. Furthermore, on sites with 50 m³ of retention trees, scarification of the soil was found to decrease feeding damage more effectively than on clearcuts and 10 m³ sites. If the seedlings were situated in the centre of scarified patches, scarification alone was as effective as insecticide treatment on unscarified soil for decreasing feeding damage and mortality. 5 The results suggest that when burning is applied as silvicultural treatment after clear‐cuts, retention of trees is recommended to reduce the damages caused by pine weevils on pine seedlings.     

Reproductive development and maternal effects in the pine weevil Hylobius abietis

1. In a laboratory study of maturation feeding of female pine weevil Hylobius abietis on current and 1‐year‐old stem bark of transplants of Scots and Corsican pine, Norway and Sitka spruce, Douglas fir, and hybrid larch, the length of the pre‐oviposition period was influenced by the species on which weevils fed. The shortest pre‐oviposition period was on hybrid larch (11.8 days) and the longest on Douglas fir (15.5 days). 2. The species on which weevils fed also affected fecundity but there was evidence of a species–year interaction. Over a period of 36 days, most eggs were laid by weevils feeding on current stem of Norway spruce and Corsican and Scots pine and fewest on current stem of Sitka spruce. 3. Significant maternal effects on egg size were observed both in relation to female size and conifer species. The largest eggs were laid on Corsican pine and the smallest on Douglas fir, with no evidence of a trade‐off between number of eggs laid and their size. 4. There was a positive relationship between egg and larval size and between larval size and survival on logs of four conifer species. Residual resistance mechanisms in the bark of recently cut stumps and larval competition are discussed briefly in relation to the importance of the observed maternal effects on weevil population dynamics.     

Olfactory sensilla on the antennae of the pine weevil,Hylobius abietis

Summary Olfactory sensilla on the antennal club of the pine weevil, Hylobius abietis, have been studied by scanning and transmission electron microscopy. Two main types, sensilla basiconica and sensilla trichodea, have been classified. The former are found in two variants, one containing one sensory cell and the other two. Sensilla trichodea have one sensory cell. The two sensillum types are localized in different fields of the club. The numerous sensilla basiconica are confined exclusively to the two constriction bands. Sensilla trichodea, fewer in number, are mainly restricted to the distal part where they also form a third ring. In addition, they are found immediately distal to the two construction bands. The fine structure of the two types suggests an olfactory function which could be proven by electrophysiological studies. The other hairs on the club are shown to belong to mechano- and taste sensilla and uninnervated setae.I am grateful to the Norwegian Research Council for Science and Humanities NAVF and the Norwegian Forest Research Institute for financial support; to Professor D. Schneider for providing laboratory facilities at Seewiesen; to him and his colleagues, particularly Dr. R.A. Steinbrecht, for helpful discussions and to Miss Barbara Müller for electron microscopy assistance. I also wish to thank Dr. T. Braathen and his staff for help with scanning electron microscopy facilities at the University of Oslo.     

Orientation of the pine weevil Hylobius abietis to underground sources of host volatiles

Adults of Hylobius abietis (L.) (Coleoptera: Curculionidae) were found to locate conifer roots suitable for oviposition by utilizing host volatiles diffusing through the soil. Underground sources of host volatiles were presented to weevils in a laboratory bioassay. A cold-trapping condensate of Scots pine, Pinus sylvestris L., and fractions of it were tested. Various fractions containing host terpenes attracted weevils in the bioassay, but the complete pine condensate caused the highest response. Ethanol was also found to be attractive. Weevils caged underground in the absence of host material did not attract weevils on the surface.

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Zusammenfassung Rüsselkäfer (Hylobius abietis) können auf der Erdoberfläche die Lage im Boden verborgenen Brutmaterials feststellen und Wurzeln senkrecht grabend auffinden. Mit einer neu entwickelten Methode für Laborversuche, die sich auf die spezifische Reaktion des Eingrabens zum Brutmaterial gründet, wurden das Orientierungsverhalten der Käfer und die zum Brutmaterial führenden Geruchsstoffe des Wirtes studiert.Beide Geschlechter von H. abietis reagierten gleichartig mit Eingraben auf die Geruchsstoffe des Wirtes. In Wahlversuchen zwischen gleichwertigen Anlockungspunkten kam häufig Aggregation der Käfer an einem Punkt vor. Die stärkere Ansammlung an einzelnen Stellen stand im Zusammenhang mit dem Vorhandensein einer Erdröhre zur Quelle der Geruchsstoffe. In den folgenden Versuchen wurde deshalb den Tieren jeweils nur eine Geruchsstoffquelle angeboten. Weder Weibchen noch Männchen im Boden übten eine Anziehung auf Käfer an der Oberfläche aus.Kiefernstücke und Kondensate flüchtiger Kieferinhaltsstoffe sowie Kondensat nach Passage durch die Kolonne des Gaschromatographen waren stark attraktiv. Alle durch präparative Gaschromatographie hergestellten Fraktionen des Kondensats waren ebenfalls, aber schwächer attraktiv. Zehnfache Verdünnung des Kondensats und der Fraktionen verminderte ihre Attraktivität mit etwa 40–70%. Auch Äthanol, Methanol und Pentan wurden geprüft; nur Äthanol hatte eine mässig anlockende Wirkung.

     

Peptide actions on oviduct contractions in the large pine weevil,Hylobius abietis

Abstract The peptides proctolin, crustacean cardioactive peptide (CCAP) and FMRFamide, which are known to modulate insect muscle contractions, were assayed for their action on oviduct contractions in Hylobius abietis. A video microscopy technique and computer‐based method of data acquisition and analysis were used to investigate the effects of theses peptides on spontaneous contractions of continuously perfused oviducts. All three peptides tested stimulate spontaneous contraction activity of the pine weevil oviduct, increasing the frequency and amplitude of phasic contractions in a dose‐dependent manner. Proctolin is more potent as a stimulator than CCAP. For proctolin a threshold response of oviduct muscles is at concentration of peptide 10^?11–10^?10 mol/L and for CCAP at concentration range 10^?10–10^?9 mol/L. FMRFamide exerts a weak stimulatory effect on the oviduct, and at higher concentrations of the peptide (above 10^?8 mol/L). The peptides exert different responses on oviduct contractions and they may play a role as functional regulators in such processes as egg movement and oviposition.     

Interaction between flight,reproductive development and oviposition in the pine weevil Hylobius abietis

• 1 The development of reproductive and flight capacity of pine weevils Hylobius abietis during the spring and their dispersal to, and subsequent development at, new clearfell oviposition sites comprise key phases in their life cycle in managed forests. At an old clearfell site where autumn‐emerging weevils had overwintered, weevils were trapped as they re‐emerged in the spring and tested for their ability to fly and then dissected to determine the degree of wing muscle and egg development.
• 2 Re‐emerging weevils were most abundant in pine growing at the edge of the clearfell and, over most of the trapping period (April to June), their capacity for flight (proportion flying and wing muscle width) was more advanced than in weevils from the clearfell itself, with a similar trend in the degree of reproductive development (proportion with mature eggs and egg volume).
• 3 In weevils from the clearfell, flight capacity and reproductive development increased concurrently to a peak around mid‐May. In weevils from pine, wing muscles were already well developed at the start of trapping, although few of them flew. Their more advanced development was attributed to the increased opportunities for maturation feeding after emergence in the previous autumn.
• 4 In the spring, weevils reached the canopy of trees for maturation feeding by walking and, to a lesser extent, by flight. Weevils dispersed by flight to oviposition sites in mid‐May when most of them were reproductively mature. After arrival, flight ability and wing muscle size declined rapidly but egg production was maintained until most weevils had stopped flying. When wing muscles reached their minimum size, there was a marked decline in egg size, suggesting that wing muscle breakdown is important in maintaining egg production at oviposition sites. Prospects for further wing muscle and reproductive development are discussed.

     

Sub-lethal responses of the large pine weevil, Hylobius abietis, to the pyrethroid insecticide lambda-cyhalothrin

Abstract.  The response of the large pine weevil ( Hylobius abietis ) to the pesticide lambda-cyhalothrin, a pyrethroid, is investigated. Both behavioural (feeding preferences) and nutritional and physiological (lipid content) responses are recorded. Hylobius abietis shows both a significant avoidance of pesticide-treated food sources and a decrease in lipid content after exposure, but a full recovery after feeding on untreated food. It is proposed that the mechanism for the pesticide avoidance and altered lipid levels is due to an anti-feedant effect of the pesticide. Implications for pest management programmes are described.     

Diel behaviour and time budget of the adult pine weevil Hylobius abietis

The pine weevil [Hylobius abietis (L.); Coleoptera: Curculionidae] has a high economic impact on forest regeneration in Europe. The general biology of the pine weevil has received considerable attention, although there is insufficient knowledge about its diel behaviour and time budget. Therefore, in the present study, the feeding and locomotion behaviour of individual adult weevils on Norway spruce seedlings [Picea abies (L.) Karst.] is observed for 24‐h periods in the laboratory. Both girdled and nongirdled seedlings are used to assess how the behaviour of weevils is influenced by the physiological response of plants to the girdling. The locomotion pattern shows a distinct maximum during the beginning of the dark phase, whereas most feeding occurs during the second half of the dark phase and the first hours of the subsequent light phase. The girdling treatment increases the time that weevils spend on the seedlings during the first part of an observation session, although it has no effect on their feeding pattern. The time budgets of weevils on girdled and nongirdled seedlings are similar. On average, weevils spend 34% of their time in locomotion and 6% on feeding. Females spend more time feeding than males (7.1% versus 4.2%), possibly because they have higher food requirements (e.g. for egg production). Females also spend more time in total on the seedlings than males (26.3% versus 7.0%). The present study reveals, in high temporal resolution, the diel feeding and locomotion behaviour and time budget of male and female pine weevils.     

Monoterpenes emitted by the large pine weevil, Hylobius abietis (L.) feeding on Scots pine, Pinus sylvestris L.

Abstract. Attraction of the large pine weevil, Hylobius abietis (L.) (Coleoptera: Curculionidae) to pine logs was enhanced by the presence of weevils already on the logs. We investigated the volatiles emitted by insects feeding on host trees to determine the mechanism of the attraction. Chemical analyses of the collected volatiles showed large amounts of monoterpenes. There were no qualitative differences in the collections, between Scots pine alone and pine plus feeding weevils, nor between males and females, mated or virgin. Strong quantitative differences in monoterpenes existed that were correlated to the surface of cut bark. Electroantennograms (EAGs) were recorded on male and female antennae stimulated by increasing doses of collected volatiles. EAGs for pine alone and pine plus weevils at the same stimulus loading in (a + P)-pinene gave superimposable curves. EAGs of male and female volatiles were also identical. These results suggest an attraction mediated by host plant allelochemicals rather than by an aggregation pheromone.     

Feeding on roots in the humus layer by adult pine weevil, Hylobius abietis

1 The consumption by adult pine weevil, Hylobius abietis, of the bark of roots present in the humus layer was assessed in a field study conducted in southern Sweden during two years (1998 and 2002). The study sites were divided into two areas: (i) a shelterwood where 80–100 mature Scots pine trees per hectare remained after cutting and (ii) a clearcut where no trees were left. 2 On average, 3741 m² per hectare of root bark was present in the humus layer, of which 135 m² was not coniferous but comprised species such as bilberry and broadleaved trees. 3 The mean area debarked by pine weevils was 2.9 m² per hectare; 2.6 m² of conifer roots and 0.3 m² of bilberry roots. Roots of broadleaved trees were almost never consumed. No clear preferences for roots of a specific level of vitality were observed. 4 No consistent difference between the shelterwood and clearcut was found, either in the amount of root bark area available or in the extent of root feeding by pine weevil. 5 A weak negative correlation between debarked areas on roots and seedlings was found, indicating that root feeding may have reduced damage to seedlings. 6 It is concluded that conifer roots in the humus layer constitute a major food source for the pine weevil and can be utilized for a considerable period in both clearcuts and shelterwoods.     

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.     

Effects of wounding on the terpene content of twigs of maritime pine (Pinus pinaster Ait.)

Summary After wounding the cortical tissues of twigs of Pinus pinaster, we investigated changes in diterpene resin acid content and the ultrastructural modification of secretory structures. There was an increase in total resin acid content in the cortical and woody tissues located near the wound. Not all resin acids responded in the same way, but in wounded tissues the amounts of isopimaric acid and of the group of dehydroabietic, levopimaric and palustric acids significantly increased. The composition of cortical tissues becomes closely related to that of woody tissues. The resin acid enrichment of cortical tissues is correlated with the reactivation of the epithelial cells of the resin ducts and the de novo synthesis of resin acids demonstrated by labelling with ₁₄CO².     

Effects of wounds on the terpene content of twigs of maritime pine (Pinus pinaster Ait.)

Summary The effect of wounds on the volatile terpene composition of the living bark of Pinus pinaster Ait. (maritime pine) twigs was investigated with respect to the processes of mono- and sesquiterpene hydrocarbon biosynthesis. The large increase in the amounts of - and -pinene is a characteristic feature after a mechanical injury, whereas the quantities of the other terpenes are only slightly increased. This is due to the reactivation of the resin duct secretory cells of primary origin located in cortical tissues. The effect of wounding is observed over a long period and the terpene profiles are very different at the end of the experiments as compared with the initial profiles of the same tissues. The traumatic essential oil (obtained after mechanical traumatism) resembles an oleoresin extracted from tissues of secondary origin. Statistical analysis underlines the effects of the between-tree variations and of the dates of application of the wound.     

Feeding by the pine weevil Hylobius abietis in relation to sun exposure and distance to forest edges

Abstract 1 The intensity of feeding by adult pine weevils Hylobius abietis (L.) on the stem bark of Norway spruce Picea abies (L.) Karst. seedlings planted in rows with a north–south orientation across a clear‐cutting, was measured throughout a growth season. The feeding was then correlated to light interception, soil temperature and distance to the nearest forest edge. 2 Feeding was at least twice as intense on seedlings in the central part of the clear‐cutting compared to those at the edges. The decline began approximatety 15 m from the edge and was of similar proportions on both the sun‐exposed and shaded sides. 3 Measures of global radiation and soil temperature correlated well with consumption on the shaded side. However, on the sun‐exposed side, there were no apparent correlations with global radiation or soil temperature that could explain the decline in consumed bark area. 4 We conclude that the decline in feeding towards the forest edges was mainly due to factors other than the microclimate variables we monitored. We suggest that the presence of roots of living trees along the forest edge may reduce damage to seedlings, since they provide an alternative source of food for the weevils. This alternative‐food hypothesis may also explain why seedlings in shelterwoods usually suffer less damage from pine weevils than seedlings in clear‐cuttings.     

Feeding and oviposition rates in the pine weevil Hylobius abietis (Coleoptera: Curculionidae)

Feeding and oviposition in the pine weevil Hylobius abietis (Linnaeus) were monitored under laboratory conditions in two long-term experiments lasting over an extended breeding season. Data were also collected from weevils under semi-natural conditions outdoors. In addition, the effects of crowding and starvation were studied in separate experiments. During the main peak oviposition period, female H. abietis consumed 50% more bark tissue than males. When oviposition ceased, the feeding rate of the females declined to the same level as in the males. The rates and spatial distribution patterns of oviposition and feeding were clearly affected by climatic conditions and the degree of crowding. Females were estimated to lay on average 0.8 eggs per day during the season under outdoor conditions. The realized fecundity of a female weevil during the first season was estimated to be approximately 70 eggs. The estimated average rate of feeding was 23 mm2 of Scots pine bark per weevil per day. This implies that planted seedlings can only constitute a minor part of the food resources needed to sustain H. abietispopulations of the size that usually appear on fresh clear-cuttings in northern Europe.