Death of vigorous trees benefits bark beetles

Bark beetles (Coleoptera: Scolytidae) are commonly associated with live host trees that are stressed, a relationship that has been attributed to lower host defenses or greater nutritional quality of these trees. However, most bark beetle species commonly inhabit freshly dead trees where induced host defenses are absent. In this study, we investigate the role of tree vigor at the time of death for pine engraver bark beetles, Ips pini (Say), breeding in freshly dead jack pine, Pinus banksiana Lamb. As indices of tree vigor, we considered tree size, phloem thickness, and several measures of recent growth rate (last year's growth increment, mean annual increment and basal area increment in the past 5 and 10 years, and periodic growth ratio). We examined the relationship between these indices in three stands, aged 60, 77, and 126 years, and found that phloem thickness, previously shown to have a strong positive effect on bark beetle reproduction, was only weakly associated with tree growth rate and inconsistently related to tree size among the three stands. To examine the effects of tree vigor on pine engraver reproduction, we felled 20 trees of various sizes from the 77-year-old stand, and experimentally established breeding males and females in 25-cm-long sections. Offspring were collected and characteristics of breeding galleries were measured. Using stepwise regression, we consistently found that indices associated with tree growth rate best explained beetle reproductive performance, as they were positively related to parental male and female establishment on logs, female reproductive success, length of egg galleries, proportion of eggs resulting in emerged offspring, and negatively related to the length of the post-egg gallery. Surprisingly, phloem thickness had no unique effect on pine engraver reproduction, except for a weak negative effect on the establishment success of parental females. The strong effect of tree vigor observed in this study suggests that substantial mortality of vigorous trees, such as caused by windthrow, can contribute to significant increases in bark beetle populations that could trigger outbreaks in living trees. Received: 3 February 1999 / Accepted: 27 April 1999     

Preliminary studies on the efficacy and establishment ofRoptrocerus xylophagorum [Hym.: Torymidae], a parasitoid ofIps grandicollis [Col.: Scolytidae], in Australia

Parasitization ofIps grandicollis Eichhoff byRoptrocerus xylophagorum (Ratzeburg), and intra-specific competition withinIps broods, were studied under laboratory conditions. Largest numbers of immatureR. xylophagorum were found in logs sampled when about half theIps brood were callow adults. Rates of parasitization rarely exceeded 20 %, and were usually much lower. Numbers of parasitoids were not affected by bark thickness. In the absence of parasitism,Ips broods suffered a density-dependent mortality which increased greatly in intensity and continued to act later in development of the broods when initial densities of larvae exceeded a threshold of about 400 per 1 000 cm² of bark. The possible significance of this mortality is discussed in relation to the ability of parasitoids to reduceIps numbers. A field experiment was done to investigate the ability ofR. xylophagorum to establish in a new habitat. Populations were established for 1 or more generations by releasing 50 females at sites prepared by supplying small numbers of logs containing immatureIps.     

Aggregation response of European engraver beetles of the genus Ips mediated by terpenoid pheromones

Upon exposure to vapours of the host tree's monoterpene (–)--pinene, Ips acuminatus produces the terpene alcohol cis-verbenol which, besides ipsdienol and ipsenol, is a component of its aggregation pheromone. I. cembrae transforms the same monoterpene predominantly into myrtenol and trans-verbenol but little into cis-verbenol, which interrupts pheromone response under field conditions.On the other hand, I. cembrae releases, upon feeding in the bark, the terpene alcohol amitinol in major amounts. Amitinol significantly enhances field response to the aggregation pheromone that also includes ipsdienol, ipsenol, and 3-methyl-3-buten-1-ol. However, amitinol reduces pheromone response in I. acuminatus and I. erosus. Also, males of I. sexdentatus release amitinol which appears to increase response to its attractive principle pheromone component, racemic ipsdienol. There is some evidence that present knowledge of the chemical communication systems among European Ips spp. still lacks satisfactory explanation of the naturally occuring aggregation en masse, perhaps with the exception of I. erosus and I. typographus.     

Glyphosate-induced changes in the attack success and development of the mountain pine beetle and impact of its natural enemies

Roundup (active ingredient glyphosate) administered into the sapwood around the root collar of lodgepole pine trees, Pinus contorta var. latifolia Engelm., was investigated as a tool for creating trap trees for the mountain pine beetle (MPB), Dendroctonus ponderosae Hopk. MPB's attacked semiochemical-baited, glyphosate-treat trees before attacking baited control trees. Bark samples disclosed an increase in survival of MPB eggs and early instar larvae, enhanced larval development and increased attacks by insect parasites and predators in treated trees. The braconid wasp, Coeloides dendroctoni Cush., parasitized MPB larvae found at high densities in treated trees two months following attack. The dipteran predator, Medetera aldrichii Wh., was only found in treated trees. Increased competition by Ips pini (Say) had a negative impact on MPB survivorship. The enhanced impact of insectan agents apparently offset any glyphosate-induced gains in MPB survival or development, but did not cause a significant reduction in MPB emergence.     

Ultrastructure of juvenile hormone-induced degenerating flight muscles in a bark beetle,Ips paraconfusus

Summary Topical application of 5 g of a juvenile hormone analogue (JHA), ZR-615, to female callow adults of Ips paraconfusus induced degeneration of the dorsoventral flight muscles. Within 24h after JHA-treatment the diameter of the myofibrils was reduced to almost half due to the lysis of the peripheral myofilaments. Mitochondria showed conspicuous degenerative changes like swelling, dissolution of the matrix or presence in the matrix of dense filamentous material or myelin-like figures. Degeneration of the mitochondria seemed to take place inside isolation membranes derived from sarcoplasmic reticulum. A number of granular osmiophilic bodies appeared in the sarcoplasm. Three days after JHA-treatment the muscles were very thin and sheath-like. Most of the mitochondria had already degenerated. The dense sarcoplasm contained numerous crystalline bodies. The granular dense bodies were also more frequent. The myofibrils were comprised of only occasional small bundles of myofilaments. The tubules of the T system enclosed an amorphous material. The nuclei and the tracheal system remained intact but they were crowded due to the decreased volume of the muscle. In some specimens degeneration of the myofibrils and mitochondria was completed by the third day. No sign of degeneration was observed in the flight muscles of acetone treated control insects.Supported by the National Research Council of Canada grant A4669.We thank Dr. G.B. Staal, Zoecon Corp., Palo Alto, California, for the generous gift of ZR 615, and Messrs A. Syed and M. Horta for rearing the insects     

Ophiostomatoid fungi associated withIps cembrae in Japan and their pathogenicity of Japanese larch

Six ophiostomatoid fungi, i.e.,Ceratocystiopsis minuta, Ceratocystis laricicola, Ophiostoma brunneo-ciliatum, O. laricis, O. piceae andOphiostoma sp., were isolated from the galleries ofIps cembrae on Japanese Iarch (Larix kaempferi) logs in central Honshu, Japan. Japanese larch trees approximately 30 yr old were inoculated with all six fungi.Ceratocystis laricicola produced the largest lesions on the inner bark around the inoculation point and the largest dry zone in the sapwood. Furthermore,C. laricicola was the only fungus associated withI. cembrae that had the ability to kill Japanese larch, death occurring in 30-yr-old trees within 3.5 mo after inoculation.     

Response of the pine engraver, Ips pini (Say) (Coleoptera: Scolytidae), to conophthorin and other angiosperm bark volatiles in the avoidance of non-hosts

1 Seventeen non‐host angiosperm bark volatiles, seven of which are antennally active to Ips pini (Say), the pine engraver (PE), were tested for their ability to disrupt the response of the PE to pheromone‐baited traps. 2 Four green leaf volatiles (GLVs) were tested [1‐hexanol (Z)‐3‐hexen‐1‐ol, hexanal, and (E)‐2‐hexenal]. None had any disruptive effect singly, as a group or in all possible blends based on functional groups, despite the fact that the two aldehydes were antennally active. These compounds may have, in some instances, actually masked the disruptive effect of other compounds. The PE thus differs in its response from other Scolytidae, including other Ips spp. 3 Eight non‐host volatiles that were antennally active to other bark beetles, but not to PEs, had no disruptive effect, validating the use of coupled gas chromatographic‐electroantennographic detection analyses to detect compounds with potential behavioural activity. 4 The bicyclic spiroacetal conophthorin, (E)‐7‐methyl‐1,6‐dioxaspiro[4.5]decane, was disruptive when tested alone. When blends of two aldehydes [salicylaldehyde and nonanal] plus an alcohol and a phenol [benzyl alcohol and guaiacol] were combined with conophthorin, an enhanced disruptive effect was revealed. No single compound, other than conophthorin, disrupted the pheromone‐positive response and no blend that did not contain conophthorin was consistently disruptive to both sexes. Conophthorin seems to be a critical component in the non‐host angiosperm message for I. pini during its host selection phase. 5 Combination of the repellent synomones, verbenone and ipsenol, with the five disruptive non‐host volatiles may provide a potent treatment to protect trees, logs or stands from attack by the PE.     

Inbreeding depression and its effect on intrinsic population dynamics in engraver beetles

Abstract.  1. Phloem-feeding bark beetles (Coleoptera: Curculionidae, Scolytinae) generally disperse before mating, leading to expectations of outbreeding. New York and British Columbia populations of engraver beetles ( Ips pini ) were tested for inbreeding depression using different methods. Among several traits measured, only the number of offspring surviving to adulthood was strongly reduced by inbreeding.
2. There was no evidence of avoidance of inbreeding depression in two possible mechanisms considered: differential male and female emergence times within full sib broods, and early termination of brood construction in forced sib mating.
3. Sib-mated females lay more eggs and have longer galleries than those in outbred crosses, despite a low rate of survival to adulthood for such eggs. This difference may be due to the ability of engraver beetles to assess crowding in broods as larvae begin to feed, and allows partial compensation for the effects of inbreed depression.
4. Population models assuming density-dependent generational effects were modified to account for inbreeding depression. Inbreeding depression makes populations less prone to cyclical behaviour, particularly at lower carrying capacities.
5. Inbreeding depression has not been previously measured in scolytids, nor has inbreeding-related behaviour been explicitly considered outside of exclusively inbreeding tribes.     

Coniferous round wood imports from Russia and Baltic countries to Belgium. A pathway analysis for assessing risks of exotic pest insect introductions

Understanding entry pathways resulting from global trade is critical to assess the risks of introduction of invasive alien species (IAS). From 1996 onward, the import of timber from Russia and the Baltic States to Belgium dramatically increased, with more than one million cubic metres of coniferous round wood imported until 2004. Such a high volume could have served as entry pathway for exotic bark beetles from the East that have the potential to become forest insect pests upon establishment. We collected and cross-checked different data sources (FAOSTAT, Eurostat, National Bank of Belgium, Belgian Customs, the Belgian sawmills industry) regarding the import of timber in an attempt to trace back the spatial and temporal patterns of this trade, and to assess the expected validity of a pest risk analysis based on those data. We found that the timber trade between 1996 and 2004 is particularly dynamic in space and time, and may have allowed several opportunities for exotic bark-beetle introductions. In addition, the patterns of trade change so quickly from year to year that the existing data sources are essentially not adequate for IAS risk assessment in a near real-time fashion. The data are either comprehensive, but then aggregated at a too coarse level (space, time, or category) to be of real use in risk assessment, or available with adequate levels of details, in which case they are mostly partial or incomplete. Better accessibility to data and data exchanges between organizations in charge of trade data collection and plant protection would help better targeting of phytosanitary controls.     

Characterization of microsatellite loci in the spruce bark beetle Ips typographus (Coleoptera: Scolytinae)

Ips typographus is an economically important pest of Norway spruce stands. We developed five polymorphic microsatellite markers using a biotin enrichment protocol. The number of alleles ranged from three to 11 per locus and no strong evidence for null alleles was found. Heterozygosity ranged from 0.3 to 0.97. These markers could be useful tools to study the population structure and genetic consequences of I. typographus population outbreaks.