Density-mediated responses of bark beetles to host allelochemicals: a link between individual behaviour and population dynamics

Abstract  1. Bark beetles (Coleoptera: Scolytidae) accept or reject host conifers based partly on concentrations of phloem monoterpenes. They colonise trees in aggregations, in response to pheromones that attract flying beetles to trees undergoing colonisation. A series of entry and gallery construction assays was conducted to determine whether responses by individual beetles to monoterpenes are altered by pheromones and/or the presence of other beetles.
2. Entry into the amended media by Ips pini and the length of time until entry were not influenced by the presence of aggregation pheromones.
3. Entry into amended media was influenced by the presence of other beetles on the surface of, or constructing galleries in, the substrate. The effects of alpha-pinene and limonene on host entry behaviour were mediated by the density of beetles on the surface of the assay arena, and by the density of beetles constructing galleries within the medium.
4. The percentage of beetles entering medium amended with higher concentrations of monoterpenes increased with increased density of beetles on the surface of the assay arena, until a threshold density of three or four beetles per assay arena, after which entrance rate declined.
5. The presence of other beetles constructing galleries elicited more rapid entry by the test beetles.
6. Gallery lengths were generally higher in the presence of aggregation pheromones.
7. Gallery lengths increased with increased density of beetles within the assay arena.
8. These results suggest a link between the density of bark beetles and responses of individuals. This linkage may partially explain behavioural changes observed during population eruptions.     

Tree mortality episodes in the intact Picea abies‐dominated taiga in the Arkhangelsk region of northern European Russia

Question: What were the temporal patterns and rates of tree mortality in a recent episodic tree mortality event? Have similar events occurred in the past, and does climatic variability play a role in the disturbance regime? Location: Intact Picea abies‐dominated taiga in the Arkhangelsk region, northwestern Russia. Methods: We reconstructed the past tree mortality and disturbance history by applying dendroecological methods in five forest stands and related these to climatic data. The role of other potential causes of tree mortality was assessed in a field inventory. Results: The recent episode lasted from 1999 to 2004, influenced all stands studied, and killed on average 21% of trees with a diameter of over 10 cm at 1.3‐m height. The annual tree mortality rate in the decades preceding this episode was 0.49%. During the past 200 years, the stands have experienced chronic small‐scale disturbances, with several irregular disturbances of moderate severity. The recent episode was associated with abundant signs of the bark beetle Ips typographus. Furthermore, the timing of both the recent tree mortality episode and the past disturbance events was associated with dry summers. Conclusion: The results indicate a connection between climatic variability and forest dynamics, the likely driving factors being droughts and bark beetles. In the context of the past 200 years, the recent episode was potentially at the higher end of the range of disturbance variability in terms of severity and spatial extent. This has ecological implications in a changing climate, potentially influencing ecosystem structure and long‐term dynamics.     

Prolonged development time of the bark beetle predator Thanasimus formicarius (Col.: Cleridae) in relation to its prey species Tomicus piniperda (L.) and Ips typographus (L.) (Col.: Scolytidae)

1 The generation time of the bark beetle predator Thanasimus formicarius (L.) (Col.: Cleridae) was found to be predominantly two years both in the field and in rearing experiments conducted with two of its main prey species, the pine shoot beetle Tomicus piniperda (L.) and the spruce bark beetle Ips typographus (L.) (Col.: Scolytidae). 2 Emergence of T. formicarius adults in the first summer was only observed in one of the two rearing experiments, and these individuals represented only 6% of that generation. 3 All individuals not emerging as adults in the first summer remained as larvae in their pupal chambers until the second summer. Pupae were found starting around mid-June, and adults (in pupal chambers) were found from late July through to the end of August. 4 Newly emerged adults had to feed in order to survive hibernation. 5 The existence of T. formicarius races, specialized on certain bark beetle species and with phenologies matching their hosts, could not be demonstrated. After hibernation there was no difference in feeding activity, timing of egg-laying or proportion of egg-laying females between the T. formicarius adults reared as larvae on T. piniperda (flight period in April) and those reared as larvae on I. typographus (main flight period generally starting in late May or early June) when exposed to a temperature and day-length typical of the early spring conditions prevailing during the flight period of T. piniperda. 6 T. formicarius was parasitized by Enclisis vindex (Tschek) (Hym.: Ichneumonidae) in the pupal chamber. 7 The importance of these findings for the population dynamics of bark beetles is discussed.     

Escape in space from enemies: a comparison between stands with and without enhanced densities of the spruce bark beetle

1 Populations of the spruce bark beetle, Ips typographus (L.), are known to grow rapidly in storm‐disturbed stands as a result of relaxation from intraspecific competition. In the present study, it was tested whether a second mechanism, escape in space from natural enemies, also contributes to the rapid population increase. 2 The experiment was conducted during the initiation phase of five local outbreaks of I. typographus triggered by a storm‐disturbance in November 1995 in southern Sweden. 3 The impact of natural enemies on the ratio of increase (number of daughters per mother) of I. typographus was compared pairwise between disturbed stands with high numbers of storm‐felled trees and undisturbed stands without wind‐felled trees. 4 Enemy impact was assessed by comparing the ratio of increase in uncaged (exposed to enemies) and caged (protected from enemies) bolts colonized by I. typographus prior to being placed in the stands. The experiment was conducted in the second and third summers after the storm‐felling. 5 Enemy impact was about twice as high in stands without wind‐felled trees compared with in stands with wind‐felled trees in the second summer whereas there was no significant difference between the stand types in the third summer. 6 The result demonstrates that spatial escape from enemies contributes to the rapid population growth of I. typographus after storm‐disturbances.     

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.     

Relationship of Bursaphelenchus xylophilus Population Density to Mortality of Pinus sylvestris

Seven-month-old Scots pine seedlings were inoculated with water or culture filtrate (controls), with 10,000, or 20,000 (experiment 1), and with 2,500 (experiment 2) Bursaphelenchus xylophilus B.C. isolate nematodes and maintained under defined experimental conditions. Controls did not develop pine wilt disease over a 2-month period. In experiment 1, less than 50% of the inoculum was recovered from the nematode-inoculated seedlings in the first 48 hours, after which the nematode population of both treatments increased exponentially resulting in pine death and approximately equal populations at 216 hours after inoculation. In the second experiment, plant mortality, which was always preceded by 2-3 days of chlorosis and associated stem vascular necrosis, first occurred 14 days after inoculation. The nematode population increased until about day 40 after inoculation and declined thereafter. Nematodes extracted from the roots 2 weeks after inoculation accounted for ca.15% of the total number of nematodes per pine. The study indicates that the rate of nematode reproduction is a factor in pine wilt disease. However, the lack of a linear correlation between the number of nematodes and the timing of pine mortality suggests that the timing of pine death may also depend on the location of nematode damage to the host tissue.