The European struggle to control Ips typographus - past, present and future

The achievement of the Scandinavian campaign is seen in the swift development and unorthodox transfer of an innovative trapping technique into the management of pest populations of the spruce engraver Ips typographus without the use of pesticides. The pheromone traps proved to be far more efficient in the prevention of engraver reproduction and the reduction of beetle numbers than the cumbersome trap trees they replaced. The wholesale application of pheromone traps as an integrated part of engraver pest management is justified alone by the empirically proven benefits that result from a "surveillance with the intent to control", although appraisal of the actual impact of trapping on the population density remains scientifically ambiguous for the lack of reliable methodology. Furthermore, the campaign opens a new era in bark beetle management by stipulating for the commercial availability of pheromones and traps as well as for research on basic and applied aspects. As the years to come may show, behaviour chemistry is likely to bear more promises to engraver management than just trapping techniques.     

Can Norway spruce trees be ‘vaccinated’ against attack by Ips typographus?

1 A field experiment was carried out to test the hypothesis that treatment of Norway spruce trees with the Ips typographus-transmitted blue-stain fungus Ceratocystis polonica enhances tree resistance to later mass attack by this bark beetle. 2 Twenty-five mature trees were pretreated by inoculating a non-lethal dose of the fungus into the bark, while 18 trees served as untreated controls. Three and a half weeks after treatment a bark beetle attack was initiated by attaching dispensers with I. typographus pheromone to the tree trunks. 3 A significantly larger proportion (67%) of the control trees than of the pretreated trees (36%) were killed by the beetle attack. The result is discussed in relation to recent results regarding defence mechanisms in Norway spruce trees.     

Size and shape changes of Ips typographus L. (Coleoptera: Scolytinae) in relation to population level

1 High intraspecific competition is known to occur during Ips typographus outbreaks, and is thought to be the main factor regulating epidemic populations by affecting beetle population productivity. However, little is known about the consequences of intraspecific competition on population quality during outbreaks, although it could have consequences on beetle population dynamics. 2 Ips typographus morphological variations among localities, years and beetle population levels were investigated in 10 Norway spruce (Picea abies) stands having various beetle damage intensities. Beetle size and shape estimators based on wing length, and using isometric size and log‐shape ratios, were employed. Field‐caught beetles were compared with beetles emerging from controlled breeding at different densities, performed in the laboratory. Beetles from this colony were also used to check the influence of breeding densities on the size estimator. 3 Size variations occurred among localities and years and were consistent with the epidemic or latent status of the beetle populations. Controlled breeding confirmed the negative effect of beetle densities encountered in the field on offspring size. Two hypotheses are formulated to explain this increase of intraspecific competition during an outbreak, but our data support the effect of host quality change between latent and epidemic populations. 4 Shape variations also occurred among localities but were unrelated to beetle population levels. No groups consistent with a geographical structure were found, suggesting low genetic variation for I. typographus populations in France.     

Effect of varying monoterpene concentrations on the response of Ips pini (Coleoptera: Scolytidae) to its aggregation pheromone: implications for pest management and ecology of bark beetles

Abstract 1 Host plant terpenes can influence attraction of conifer bark beetles to their aggregation pheromones: both synergistic and inhibitory compounds have been reported. However, we know little about how varying concentrations of individual monoterpenes affect responses. 2 We tested a gradient of ratios of α‐pinene, the predominant monoterpene in host pines in the Great Lakes region of North America, to Ips pini's pheromone, racemic ipsdienol plus lanierone. 3 Ips pini demonstrated a parabolic response, in which low concentrations of α‐pinene had no effect on attraction to its pheromone, intermediate concentrations were synergistic and high concentrations were inhibitory. These results suggest optimal release rates for population monitoring and suppression programmes. 4 Inhibition of bark beetle attraction to pheromones may be an important component of conifer defences. At terpene to pheromone ratios emulating emissions from trees actively responding to a first attack, arrival of flying beetles was low. This may constitute an additional defensive role of terpenes, which are also toxic to bark beetles at high concentrations. 5 Reduced attraction to a low ratio of α‐pinene to pheromone, as occurs when colonization densities become high and the tree's resin is largely depleted, might reflect a mechanism for preventing excessive crowding. 6 Thanasimus dubius, the predominant predator of I. pini, was also attracted to ipsdienol plus lanierone, but its response differed from that of its prey. Attraction increased across all concentrations of α‐pinene. This indicates that separate lures are needed to sample both predators and bark beetles effectively. It also provides an opportunity for maximizing pest removal while reducing adverse effects on beneficial species. This disparity further illustrates the complexity confronting natural enemies that track chemical signals to locate herbivores.     

Mass attack of trees by Ips typographus induced by sex-specific pheromone: a model of attack dynamics

A model of attack dynamics on a single tree under mass attack by the spruce bark beetle Ips typographus is developed from earlier studies of pheromone production and response of the flying population to the pheromone signal. The model is explored for different numbers and sex-ratios of pioneer beetles and responding beetles. Both numbers of pioneers and responders were important for the dynamics, while the sex-ratio of pioneers was less important. A decreased proportion of males among the responding beetles was important for the result of the model. In the early stage of the attack the model predictions of both numbers and sex-ratio of beetles landing were similar to examples from naturally mass-attacked trees, although the attack proceeded more slowly than in the field. Several aspects of the attack dynamics, such as density-regulation of the number of attacks on the tree, as observed in the field, were not predicted by the model. Gaps in the knowledge of the chemical ecology of the tree-insect relation, such as qualitative change in odours from the tree, spacing mechanisms on the bark surface, and mortality due to host resistence, are pointed out with help of the model.     

Host resistance elicited by methyl jasmonate reduces emission of aggregation pheromones by the spruce bark beetle, Ips typographus

We treated Norway spruce (Picea abies) stems with methyl jasmonate (MeJA) to determine possible quantitative and qualitative effects of induced tree defenses on pheromone emission by the spruce bark beetle Ips typographus. We measured the amounts of 2-methyl-3-buten-2-ol and (S)-cis-verbenol, the two main components of the beetle's aggregation pheromone, released from beetle entrance holes, along with phloem terpene content and beetle performance in MeJA-treated and untreated Norway spruce logs. As expected, phloem terpene levels were higher and beetle tunnel length was shorter (an indication of poor performance) in MeJA-treated logs relative to untreated logs. Parallel to the higher phloem terpene content and poorer beetle performance, beetles in MeJA-treated logs released significantly less 2-methyl-3-buten-2-ol and (S)-cis-verbenol, and the ratio between the two pheromone components was significantly altered. These results suggest that host resistance elicited by MeJA application reduces pheromone emission by I. typographus and alters the critical ratio between the two main pheromone components needed to elicit aggregation. The results also provide a mechanistic explanation for the reduced performance and attractivity observed in earlier studies when bark beetles colonize trees with elicited host defenses, and extend our understanding of the ecological functions of conifer resistance against bark beetles.     

Kairomonal Responses of Natural Enemies and Associates of the Southern Ips (Coleoptera: Curculionidae: Scolytinae) to Ipsdienol, Ipsenol and Cis-Verbenol

Bark beetle infested pines are an ephemeral habitat utilized by a diverse assemblage of insects. Although many bark beetle insect associates have little or no measurable impact on bark beetle brood production, some reduce brood production by either competing with brood for the limited phloem tissue or by feeding on brood. Several studies have observed synchrony between the colonization of hosts by bark beetles and the arrival of insect associates. Some insect associates mediate synchrony with bark beetle mass attacks with kairomonal responses to bark beetle aggregation pheromones. The objectives of this study were to document the community of Coleoptera associated with the southern Ips (Ips avulsus, Ips calligraphus and Ips grandicollis) and to test the hypothesis that synchrony of insect associates with the southern Ips is mediated by kairomonal responses to aggregation pheromones. A large community of Coleoptera (109 species) was recorded from traps baited with southern Ips pheromones. A significant treatment effect was observed for the guilds of meristem feeders, natural enemies and woodborers. The southern Ips pheromone ipsenol was broadly attractive to meristem feeders, natural enemies and woodborers and in general blends were more attractive than individual compounds. These results demonstrate that a diverse community of Coleoptera is associated with the southern Ips and that several members of this community facilitate synchrony with kairomonal responses to southern Ips aggregation pheromones.     

Trees wanted--dead or alive! Host selection and population dynamics in tree-killing bark beetles

Bark beetles (Coleoptera: Curculionidae, Scolytinae) feed and breed in dead or severely weakened host trees. When their population densities are high, some species aggregate on healthy host trees so that their defences may be exhausted and the inner bark successfully colonized, killing the tree in the process. Here we investigate under what conditions participating with unrelated conspecifics in risky mass attacks on living trees is an adaptive strategy, and what this can tell us about bark beetle outbreak dynamics. We find that the outcome of individual host selection may deviate from the ideal free distribution in a way that facilitates the emergence of tree-killing (aggressive) behavior, and that any heritability on traits governing aggressiveness seems likely to exist in a state of flux or cycles consistent with variability observed in natural populations. This may have implications for how economically and ecologically important species respond to environmental changes in climate and landscape (forest) structure. The population dynamics emerging from individual behavior are complex, capable of switching between "endemic" and "epidemic" regimes spontaneously or following changes in host availability or resistance. Model predictions are compared to empirical observations, and we identify some factors determining the occurrence and self-limitation of epidemics.     

Shifts in breeding habitat selection behaviour in response to population density

We tested whether mountain pine beetles Dendroctonus ponderosae, an insect herbivore that exhibits outbreak population dynamics, modifies its habitat selection behaviour in response to density‐dependent environmental shifts. Using an individual‐based habitat selection model, we formulated predictions of how beetle population density will influence breeding habitat selectivity. Our model predicted that beetles should be more selective at intermediate densities than at low or high densities. The mechanisms influencing optimal selectivity differed between low and high density populations. In low density populations, breeding site availability was the primary factor affecting selectivity, whereas intraspecific competition and the reliability of habitat quality cues were important in high density populations. We tested our model predictions in natural populations that encompassed a range of beetle population densities. Our empirical findings supported the two key predictions from our model. First, habitat quality was more variable in high density populations. Second, individuals in high density populations were less selective compared to beetles from intermediate density populations. Our results demonstrate that beetles alter their habitat selection behaviour in response to density‐dependent shifts. We propose that the behavioural changes we identified may influence the rate at which beetle populations transition between density states.     

Larval population density affects female weight and fecundity in the dung beetle Aphodius ater

1. Competition for food at high densities during larval development leads to reduced adult weight in the northern temperate dung beetle Aphodius ater. 2. Analysis of female beetles caught in the field showed that numbers of eggs and total egg load per female were correlated positively with beetle size. 3. Female beetles reared at different population densities during larval development in the laboratory were analysed with regard to their lifetime fecundity and reproductive lifespan. 4. High population densities during development had a negative influence on the number of eggs per female and on reproductive lifespan. Lifetime fecundity was correlated positively with female weight. 5. It was concluded that competition during larval development in the first generation of offspring will result in a lower number of offspring in the second generation in Aphodius ater, and thereby reduce parental fitness.     

Specialization of midgut cells for synthesis of male isoprenoid pheromone components in two scolytid beetles,Dendroctonus jeffreyi and Ips pini

Endodermal or midgut cells have only recently been recognized as the site of pheromone synthesis in bark beetles. Midgut cells are not only specialized for digestion, but they have also been recruited to form isoprenoid compounds that function as pheromone components in Ips pini and Dendroctonus jeffreyi. Male bark beetle midgut cells are competent to produce isoprenoid pheromones after feeding or stimulation by juvenile hormone (JH) III. Competent midgut cells share many ultrastructural features with cells that do not secrete isoprenoid pheromone, but they are distinguished from these by abundant and highly ordered arrays of smooth endoplasmic reticula. During secretion, both midgut cells that produce pheromone and cells that do not are characterized by the presence of apical extrusions (apocrine secretion) rather than the presence of vesicles that fuse with the apical membrane and undergo exocytosis (eccrine secretion). Pheromone-producing cells of the midgut do not represent a population of cells that are distinct from cells involved in digestion. All, or most, midgut cells of male I. pini and D. jeffreyi can secrete pheromones as well as digestive enzymes.     

Pheromone production in bark beetles

The first aggregation pheromone components from bark beetles were identified in 1966 as a mixture of ipsdienol, ipsenol and verbenol. Since then, a number of additional components have been identified as both aggregation and anti-aggregation pheromones, with many of them being monoterpenoids or derived from monoterpenoids. The structural similarity between the major pheromone components of bark beetles and the monoterpenes found in the host trees, along with the association of monoterpenoid production with plant tissue, led to the paradigm that most if not all bark beetle pheromone components were derived from host tree precursors, often with a simple hydroxylation producing the pheromone. In the 1990s there was a paradigm shift as evidence for de novo biosynthesis of pheromone components began to accumulate, and it is now recognized that most bark beetle monoterpenoid aggregation pheromone components are biosynthesized de novo. The bark beetle aggregation pheromones are released from the frass, which is consistent with the isoprenoid aggregation pheromones, including ipsdienol, ipsenol and frontalin, being produced in midgut tissue. It appears that exo-brevocomin is produced de novo in fat body tissue, and that verbenol, verbenone and verbenene are produced from dietary α-pinene in fat body tissue. Combined biochemical, molecular and functional genomics studies in Ips pini yielded the discovery and characterization of the enzymes that convert mevalonate pathway intermediates to pheromone components, including a novel bifunctional geranyl diphosphate synthase/myrcene synthase, a cytochrome P450 that hydroxylates myrcene to ipsdienol, and an oxidoreductase that interconverts ipsdienol and ipsdienone to achieve the appropriate stereochemistry of ipsdienol for pheromonal activity. Furthermore, the regulation of these genes and their corresponding enzymes proved complex and diverse in different species. Mevalonate pathway genes in pheromone producing male I. pini have much higher basal levels than in females, and feeding induces their expression. In I. duplicatus and I. pini, juvenile hormone III (JH III) induces pheromone production in the absence of feeding, whereas in I. paraconfusus and I. confusus, topically applied JH III does not induce pheromone production. In all four species, feeding induces pheromone production. While many of the details of pheromone production, including the site of synthesis, pathways and knowledge of the enzymes involved are known for Ips, less is known about pheromone production in Dendroctonus. Functional genomics studies are under way in D. ponderosae, which should rapidly increase our understanding of pheromone production in this genus. This chapter presents a historical development of what is known about pheromone production in bark beetles, emphasizes the genomic and post-genomic work in I. pini and points out areas where research is needed to obtain a more complete understanding of pheromone production.     

The stability of the pheromones of xylophagous insects to environmental factors: An evaluation by quantum chemical analysis

The ground and excited states of the pheromone molecules produced by xylophagous insects (the bark beetle Ips typographus L., the black fir sawyer beetle Monochamus urussovi Fisch., and the black pine sawyer M. galloprovincialis Oliv.) were modeled using a quantum chemical method utilizing DFT (density functional theory) with the B3LYP functional. The absorption wavelengths (energies) and dipole moments were calculated; the transitions of electrons from occupied to empty molecular orbitals were considered. The computed data were used to assess the stability of pheromone molecules exposed to environmental factors, such as solar radiation and humidity.     

Olfactometric evidence for aggregation pheromone production by females of the four-eyed fir bark beetle <Emphasis Type="Italic">Polygraphus proximus</Emphasis> Blandf. (Coleoptera,Curculionidae: Scolytinae)

Chemical communication of the four-eyed fir bark beetle Polygraphus proximus Blandf., an aggressive invasive pest of the Siberian fir Abies sibirica Ledeb., was experimentally studied using a four-way olfactometer of a modified design, in which the tested insects were allowed to move on their own from a lightproof plastic container onto the lighted arena. Young hibernated adults of P. proximus were offered four variants of odor: fir log segments infested with (1) 10 males, (2) 10 females, (3) 10 couples of P. proximus, and (4) clean air as the control. The pheromone of P. proximus was shown to be produced by females; a similar response of both sexes characterized it as an aggregation pheromone. All the three variants with infested fir logs were much more attractive than the control variant with clean air. Volatiles from fir tissues not only seem to serve as markers facilitating host selection by the first-to-arrive beetles before direct contact with the tree bark, but also to be used as pheromone precursors or synergists. The absence of difference in response to the logs infested with males and couples of the bark beetle indicated that pheromone synthesis was inhibited after couple formation.     

Individual variation in bark beetle and moth pheromones - a comparison and an evolutionary background

The presently published data of variation in pheromone content in bark beetles (Coleoptera: Scolytidae) and moths (Lepidoptera) are reviewed. In both taxa high coefficients of variation, often up to around 100% for amount of pheromone, seem to be the rule. The contribution to the variation from errors in the chemical methods was small in most studies compared with the intrinsic biological variation. Examples arc given of variation in both absolute amounts, proportions of geometric isomers and in enantiomeric composition. Ratios of isomers often had lower coefficients of variation (25% or less). A lower average variation among sex pheromone in moths could relate to a mating system involving singly calling females, compared with the aggregation pheromone of bark beetles, which often call in large groups. The origin of a resource-based aggregation pheromone system by evolution working via individual selection is outlined.     

Population dynamics in changing environments: the case of an eruptive forest pest species

In recent decades we have seen rapid and co‐occurring changes in landscape structure, species distributions and even climate as consequences of human activity. Such changes affect the dynamics of the interaction between major forest pest species, such as bark beetles (Coleoptera: Curculionidae, Scolytinae), and their host trees. Normally breeding mostly in broken or severely stressed spruce; at high population densities some bark beetle species can colonise and kill healthy trees on scales ranging from single trees in a stand to multi‐annual landscape‐wide outbreaks. In Eurasia, the largest outbreaks are caused by the spruce bark beetle, Ips typographus (Linnaeus), which is common and shares a wide distribution with its main host, Norway spruce (Picea abies Karst.). A large literature is now available, from which this review aims to synthesize research relevant for the population dynamics of I. typographus and co‐occurring species under changing conditions. We find that spruce bark beetle population dynamics tend to be metastable, but that mixed‐species and age‐heterogeneous forests with good site‐matching tend to be less susceptible to large‐scale outbreaks. While large accumulations of logs should be removed and/or debarked before the next swarming period, intensive removal of all coarse dead wood may be counterproductive, as it reduces the diversity of predators that in some areas may play a role in keeping I. typographus populations below the outbreak threshold, and sanitary logging frequently causes edge effects and root damage, reducing the resistance of remaining trees. It is very hard to predict the outcome of interspecific interactions due to invading beetle species or I. typographus establishing outside its current range, as they can be of varying sign and strength and may fluctuate depending on environmental factors and population phase. Most research indicates that beetle outbreaks will increase in frequency and magnitude as temperature, wind speed and precipitation variability increases, and that mitigating forestry practices should be adopted as soon as possible considering the time lags involved.     

Gene pool state and degree of infestation by bark beetle (<Emphasis Type="Italic">Ips tipographus</Emphasis> L.) of Norway spruce (<Emphasis Type="Italic">Picea abies</Emphasis> L. Karst.) natural populations and planted stands in Moscow region

A comparative analysis of the gene pool state in natural populations and planted stands of Norway spruce and the degree of their infestation by the bark beetle in the Moscow region was conducted taking into account the dynamic state of communities (4 populations, 148 samples, 24 isoenzyme loci). The degree of infestation by the bark beetle of conditionally native communities is 0%; for planted stands, it is 90–100%; and for a short-term community, it is 15–20%. The comparison of “healthy” populations and those infested with bark beetle by average values of observed heterozygosity (H _O) detected no significant differences. However, the test on allelic frequency heterogeneity demonstrated the difference of planted stands from conditionally native populations both by three loci (Fe-2, Idh-1, Mdh-3) and by the totality of 18 polymorphic isoenzyme loci; the short-term population differs from conditionally native population only by two loci. The value of the inbreeding coefficient by the Idh-1 locus is significantly higher in both populations infested with the bark beetle than in “healthy” populations. The results of conducted studies demonstrate the necessity of continuation of the study on the gene pool state in Norway spruce populations owing to the degree of their infestation by the bark beetle along with the study on the dynamic state of the communities; this can provide a key to solving the problem of the forest preservation from pests.     

Impacts of silvicultural thinning treatments on beetle trap captures and tree attacks during low bark beetle populations in ponderosa pine forests of northern Arizona

Our research used a combination of passive traps, funnel traps with lures, baited trees, and surveys of long-term thinning plots to assess the impacts of different levels of stand basal area (BA) on bark beetle tree attack and on trap captures of Ips spp., Dendroctonus spp., and their predators. The study occurred at two sites in ponderosa pine, Pinus ponderosa Dougl. ex Laws., forests, from 2004 to 2007 during low bark beetle populations. Residual stand BA ranged from 9.0 to 37.0 m2/ha. More predators and bark beetles were collected in passive traps in stands of lower BA than in stands of higher BA; however, significance varied by species and site, and total number of beetles collected was low. Height of the clear panel passive traps affected trap catches for some species at some sites and years. When pheromone lures were used with funnel traps [Ips pini (Say) lure: lanierone, +03/-97 ipsdienol], we found no significant difference in trap catches among basal area treatments for bark beetles and their predators. Similarly, when trees were baited (Dendroctonus brevicomis LeConte lure: myrcene, exo-brevicomin and frontalin), we found no significant difference for days to first bark beetle attack. Surveys of long-term thinning treatments found evidence of bark beetle attacks only in unthinned plots (approximately 37 m2/ha basal area). We discuss our results in terms of management implications for bark beetle trapping and control.     

Effect of Tribolium castaneum (Coleoptera: Tenebrionidae) nutritional environment, sex, and mating status on response to commercial pheromone traps

Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae), the red flour beetle, is an important cosmopolitan pest of stored grains. Commercial traps baited with the synthetic aggregation pheromone 4,8-dimethyldecanal (DMD) are used to monitor T. castaneum population densities in storage facilities. However, trap catches may depend on several intrinsic and extrinsic factors. In this study, we explored the effects of beetle nutritional environment, sex, and mating status on the response of T. castaneum to commercial Storgard Dome traps. Beetles raised on a low-nutrition diet were 1.7 times more likely to enter DMD-baited traps compared with beetles that were raised on a high-nutrition diet. Although no sex difference in trap response was found, unmated beetles of both sexes were more responsive to DMD than were mated beetles, and this effect was especially pronounced for beetles reared on a low-nutrition diet. These results suggest that estimating T. castaneum population densities based on trap catches might be improved by incorporating information about the nutritional quality of infested stored products.     

Attractive responses by Monochamus galloprovincialis (Col., Cerambycidae) to host and bark beetle semiochemicals

Abstract:  The pine sawyer Monochamus galloprovincialis is the European vector of the recently introduced pine wood nematode. This nematode is the causal organism of pine wilt disease, a serious tree killer in East Asia. Efficacious baits and traps to monitor and control this beetle are now required. The effect of bark beetle ( Ips spp.) pheromone components, released individually (ipsenol) or in blends (ipsenol, ipsdienol, cis -verbenol and methyl-butenol), together with host volatiles (turpentine or α -pinene and ethanol) on M. galloprovincialis trap catches has been studied in Spain. A kairomonal response by male and female of M. galloprovincialis to Ips semiochemicals was found. Beetles were more attracted to host blends supplemented with bark beetle pheromones than to host volatiles alone. Ipsenol alone was attractive to pine sawyers, and was synergistic with α -pinene and ethanol. The full blend of the four Ips semiochemicals and the host compounds was highly attractive. Multiple-funnel traps were as effective as black cross-vane traps in capturing this insect when the escape of trapped beetles was prevented. Trapping of non-target bark beetle predators was also evaluated. The trogossitid Temnochila coerulea and clerid Thanasimus formicarius were kairomonally attracted to and killed in traps baited with bark beetle pheromones. These results suggest that effective monitoring of M. galloprovincialis would be possible by baiting any of these traps with host volatiles and Ips semiochemicals, but reduction of the lure components and trap modification to minimize impact on predators should be considered.