Fredrik Schlyter

Intraspecific competition affecting parents and offspring in the bark beetle ips typographus

In the laboratory we investigated the effect of breeding density on the reemergence of parent spruce bark beetles, Ips typographus (L.), and on the number and quality of their offspring. The parents reemerged sooner at higher densities but the total proportion that reemerged was independent of density. More than 20 offspring per female were produced at the lowest density (0.5 99 /100 cm2)but only 0.6 per female at the highest density (31 99/100 cm2). The offspring from the lowest density were about 50% heavier than those from the highest density and also the fat content (%) increased with decreasing density. Females weighed less and contained less fat than males. Male offspring from lower densities produced larger amounts of the pheromone components cis-verbenol and 2-methyl-3-buten-2-ol than males from the higher densities. Offspring from the highest density produced about half as many progeny as those from the lowest densities, showing an effect of density acting over more than one generation. These findings suggest that density-induced variation of beetle quality might be of importance in the population dynamics of bark beetles.

Olfactory Recognition of Host-Tree Susceptibility by Pine Shoot Beetles

Storm-fallen Scots pines with broken roots and trees with severed tops exude wound oleoresin. These trees are susceptible to bark beetles due to an injured vascular system that can not provide adequate oleoresin in order to resist new attacks by beetles [I]. Once a tree is attacked, most bark beetles use pheromone attractants to locate mates and often to overcome tree resistance through a mass attack [I, 21. It would be clearly advantageous for bark beetles to have evolved sensory systems for efficiently locating their host and in recognizing whether a particular host was less resistant than most healthy trees. However, little is known of how bark beetles select their host from among other plant and tree species [I], or what may attract the first individuals to a susceptible host. We have investigated the semiochemical basis of the mass aggregation of pine shoot beetles, Tomicus piniperda, on storm-injured Scots pine. We found that the beetle can recognize while still in flight a host tree and whether it is susceptible by means of olfaction of three plant monoterpenes evaporating from wound oleoresin. Scots pine logs infested with males and females of T. piniperda were tested in the forest of southern Sweden (Table 1, 1982) to determine their attractiveness to naturally swarming populations in the spring. The resulting catches of beetles indicate that there was a strong attraction to volatiles released from logs

Quantitative variation of pheromone components in the spruce bark beetleIps typographus from different attack phases

Ips typographus beetles were collected in the field, separated into eight attack phases (from beetles walking on the trunk of a tree under attack to those excavating gallery systems with a mother gallery longer than 4 cm), and immediately frozen in liquid nitrogen. 2-Methyl-3-buten-2-ol,cis- andtrans-verbenol, verbenone, myrtenol, trans-myrtanol, ipsenol, ipsdienol, and 2-phenylethanol were quantified from excised hindguts against an internal standard, heptyl acetate, in the extraction solvent. Methylbutenol, the pinene alcohols, and 2-phenylethanol showed the same pattern of variation between attack phases in males, with the largest amounts present before accepting females and then a fast decline. Ipsenol and ipsdienol were not detected in males before the females were accepted, and the amounts increased when the females start their egg laying. Verbenone occurred only in trace amounts. The beetles were sampled from five Norway spruce trees (Picea abies) of differing resin flow. The correlations between the nine pheromone components and five major host monoterpenes in the gut showed that the variation in the amount of methyl-butenol, ipsenol, and ipsdienol could not be explained by the variation in the amounts of host monoterpenes. In contrast over 80% of the quantitative variation ofcis-verbenol,trans-verbenol, and myrtenol was explained by the amount of α-pinene. The nine pheromone components from 36 individual males were also quantified. Both methylbutenol andcis-verbenol showed a large variation in both amounts and proportions. Females containedtrans-verbenol and traces of most other components found in males. When accepted by the male, they also contained a female-specific compound, β-isophorone. Behavioral and biosynthetic implications of the results are discussed.

Attraction to pheromone sources of different quantity, quality, and spacing: Density-regulation mechanisms in bark beetleIps typographus

The density of bark-beetle colonization of a tree could be regulated by a quantitative effect of the pheromone signal from beetles in the tree (cessation of release of attractive pheromone) or by a qualitative effect (production of pheromone components inhibiting attraction). The quantitative hypothesis was tested onIps typographus by varying the release rate of the two known attractive compounds, 2-methyl-3-buten-2-ol (MB) and (4S)-cis-verbenol (cV). The highest number of beetles were captured at traps with the highest release rates. The catch was nearly proportional to the release of MB and cV at a distance between traps of 12 m or more. At 6-, 3-, and 1.5-m distances between traps deployed in a triangular arrangement there was still a good discrimination between release rates, but relatively more beetles, especially males, were caught on the blank. The lower release rates caught an equal sex ratio while the highest release rate caught only about 30% males. The qualitative hypothesis was tested by releasing the suspected inhibitors ipsdienol (Id) and ipsenol (Ie), from traps in the same amounts as cV. Only small effects were noted forI. typographus. However, the competitorI. duplicatus was attracted to Id and inhibited by Ie, while the predatorThanasimus formicarius was attracted to both compounds. On the other hand, when the ratio of Id or Ie to cV was 10∶1 or 0.1∶1 rather than 1∶1, they affected the numbers ofI. typographus attracted. A small amount of Id combined with the attractants increased trap catch, while large amounts of Id or Ie decreased attraction, especially when combined. Attack density regulation is modeled as an effect of both quantitative and qualitative mechanisms acting in sequence.

Behavioural sequence in the attraction of the bark beetle Ips typographus to pheromone sources

ABSTRACT. The functions of the two synergistic pheromone components, (4S)‐cis‐verbenol (cV) and 2‐methyl‐3‐buten‐2‐01 (MB), and the role of ipsdienol in the attraction of Ips typographus (L.) (Scolytidae) to pheromone sources were studied in the field. Absolute and relative beetle catches were compared between several traps placed at and nearby a central pheromone source: a pipe trap containing the source, a surrounding sticky trap, a nearby window trap, and four distant window traps. A higher catch in the outer down‐wind distant traps indicated an up‐wind anemotaxis to the source. Increased MB release, with cV constant, increased the proportion caught in the central pipe trap, indicating MB as a landing stimulus. Release of MB alone gave a very small catch. Ipsdienol could not substitute for cV in the synergism with MB. An increase of cV, with MB constant, increased the number of beetles caught, but not the proportion caught in the pipe trap. The sex ratio was equal in the window traps, but fell to 30% males in both sticky and pipe traps, showing that a large proportion of the males attracted to the source did not land. The proportion of males in the pipe trap was reduced at the highest cV dose. The results support the idea of each pheromone component having a different relative importance in releasing different steps in the behavioural chain.

Field response of spruce bark beetle, Ips typographus, to aggregation pheromone candidates

Six compounds previously identified from hindguts of unmated maleIps typographus (L.) during host colonization: 2-methyl-3-buten-2-ol (MB),cis-verbenol (cV),trans-verbenol (tV), myrtenol (Mt),trans-myrtanol (tM), and 2-phenylethanol (PE), were tested for their attractivity in the field with a subtractive method. The amounts of MB and cV released from a pipe trap were similar to those given off from the commercial bait Ipslure as well as that from a Norway spruce tree,Picea abies (L.) Karst., under mass attack. The blend of the compounds became nonattractive when either MB or cV was subtracted, while subtraction of any of the other four compounds had no effect. Addition of ipsdienol (Id) to the blend did not significantly increase the attraction. In a second comparative test, the addition of three compounds as a group (tV + Mt + PE) to MB + cV again had no effect on the attraction, but the addition of Id increased the catch somewhat. Addition of host logs to a bait releasing MB + cV at a rate lower than in previous experiments did not influence the attraction to pipe traps. Sticky traps containing natural pheromone sources (50 males in a log), which released 1–5 mg/day of MB as determined by aerations with deuterated MB as internal standard, were less attractive than a synthetic source releasing similar amounts of MB.

Individual variation in aggregation pheromone content of the bark beetle, Ips typographus

The total amounts of, and proportions among, components of the aggregation pheromone produced byIps typographus were found to vary considerably among individuals excised from attacks on standing spruce trees. Chemical analyses of 392 individual male beetles were made by GC-MS. Both unmated and mated males had log-normal frequency distributions in their content of the pheromone components 2-methyl-3-buten-2-ol (MB) andcis-verbenol (cV), since a large fraction of males had a low content. The amount of MB in male hindguts varied independently of cV and the other oxygenated monoterpenes, while the amount of cV covaried with the other pinene alcohols and showed a variation between beetles from different spruce trees. Mated males had, on average, lower amounts of MB than unmated, while the average content of cV in mated males varied with the resin content of their host trees. Ipsdienol and ipsenol were only found in mated males, but in less than 40% and 10%, respectively, of these mated males. Even-aged males exposed to α-pinene in the laboratory showed slightly less variation in the amounts of verbenols, and the variations in ratio between cV and tV were similar to those among males attacking the same spruce tree.

Inhibition of attraction to aggregation pheromone by verbenone and ipsenol

The semiochemicals verbenone (Vn), ipsenol (Ie), and ipsdienol (Id), present in late phases of host colonization, have been implicated as qualitative “shut-off” signals regulating attack density. Combinations of the three chemicals were released in pipe traps together with the aggregation pheromone components 2-methyl-3-buten-2-ol (MB) andcis-verbenol (cV) at different levels and in different ratios to MB + cV, and with two spacings of traps to test for possible effects on reducing catch at traps baited with aggregation pheromone. When they were released with the attractants Vn and Ie (alone or together) decreased the mean catch significantly at the higher release rates used (1 mg/day). Id alone or together with Vn at low release rates (0.1 mg/day), with the attractants, increased catch somewhat. A dose-response test of Vn, with the attractants held constant, showed a decline in catches, down to about < 10% of the control, at ratios of Vn to cV between 1∶1 and 150∶ 1. A larger spacing (25 m) of traps gave a stronger response to change in doses of Vn and MB + cV than a smaller (6 m) spacing. The sex ratio was more skewed towards females when two or three inhibitors were present and at higher doses of Vn. It is suggested that Vn could be the most important density-regulating signal in the natural system, as release of Vn from galleries is larger and starts earlier than that of Id and Ie.

Competition and Niche Separation between Two Bark Beetles: Existence and Mechanisms

The existence and effects of competition as well as niche separation were investigated in a system of two bark beetle species which aggregate on Norway spruce, Picea abies, to feed on and reproduce in the phloem. In this and other bark beetle systems, there is evidence for competition and intra- and interspecific density effects at normally occurring densities, in contrast to the concept of «density vagueness». Under natural conditions adults of the two species showed a spatial niche separation, where the smaller species, Ips duplicatus, occupied only the top of trees. Regression analysis of adult density patterns of each species showed effects of habitat variables and of density of the opposite species. When breeding at naturally occurring densities (representing both low, endemic, and high, epidemic, populations phases), in the same substratum in the laboratory, there was a strong, asymmetric larval competition in favour of Ips typographus

E-Myrcenol in Ips duplicatus: an aggregation pheromone component new for bark beetles.

Males of the Eurasian bark beetleIps duplicatus, when feeding in host Norway spruce (Picea abies (L.) Karst.), produced and released ipsdienol andE-myrcenol, which we show to be aggregation pheromone components. Bioassays using walking beetles indicated thatE-myrcenol in synergistic combination with ipsdienol is essential for attraction. Synergism ofE-myrcenol and ipsdienol released at natural rates in the forest was also demonstrated with a new technique using mechanical slow-rotation of sticky traps.