Göran Birgersson

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.

Volatiles from Nonhost Birch Trees Inhibit Pheromone Response in Spruce Bark Beetles

557 chelys aprix from the Early Jurassic of North America [11], also possesses four pairs of pores on inframarginal scales (Fig. 2). These pores in K. aprix establish Rathke’s gland as the oldest amniote skin gland. We are unable to document Rathke’s gland pores in several extinct taxa, including the oldest pleurodire, Proterochersis robusta from the Late Triassic of Germany [12], and a variety of cryptodire groups (Fig. 1). Neither do we observe gland pores in the most primitive turtle, Proganochelys quenstedti from the Late Triassic of Germany [13]. Aside from the obscurity of gland pores, determining the presence of Rathke’s gland in some extinct taxa is problematic because the developed skeletal components through which gland ducts would otherwise pass en route to the shell bridge or skin are lacking.

Floral to green: mating switches moth olfactory coding and preference

Mating induces profound physiological changes in a wide range of insects, leading to behavioural adjustments to match the internal state of the animal. Here, we show for the first time, to our knowledge, that a noctuid moth switches its olfactory response from food to egg-laying cues following mating. Unmated females of the cotton leafworm (Spodoptera littoralis) are strongly attracted to lilac flowers (Syringa vulgaris). After mating, attraction to floral odour is abolished and the females fly instead to green-leaf odour of the larval host plant cotton, Gossypium hirsutum. This behavioural switch is owing to a marked change in the olfactory representation of floral and green odours in the primary olfactory centre, the antennal lobe (AL). Calcium imaging, using authentic and synthetic odours, shows that the ensemble of AL glomeruli dedicated to either lilac or cotton odour is selectively up- and downregulated in response to mating. A clear-cut behavioural modulation as a function of mating is a useful substrate for studies of the neural mechanisms underlying behavioural decisions. Modulation of odour-driven behaviour through concerted regulation of odour maps contributes to our understanding of state-dependent choice and host shifts in insect herbivores.

Bark volatiles from nonhost angiosperm trees of spruce bark beetle, Ips typographus (L.) (Coleoptera: Scolytidae): Chemical and electrophysiological analysis

Summary. Volatiles from bark of three nonhost angiosperm trees of Ips typographus (Coleoptera: Scolytidae) (Betula pendula, B. pubescens, and Populus tremula) were collected by headspace sampling and direct solvent extraction in June 1998, and identified and quantified by coupled gas chromatographic-mass spectrometry (GC-MS). Only small amounts of bark volatiles were detected in the aerations in situ from undamaged stems of the nonhost trees. In headspace volatiles from bark chips the two birch species had a similar odour profile, with two sesquiterpene hydrocarbons, α-zingiberene and α-santalene, as the most dominant components. Bark of P. tremula emitted significantly less sesquiterpenes and more green leaf volatiles (GLVs) than the two birch species did. Solvent extraction of fresh bark gave similar volatile composition for the two birches compared to the headspace of bark chips, but large differences were found in P. tremula. Coupled gas chromatographic-electroantennographic detection analysis (GC-EAD) of the headspace volatiles from fresh bark chips of the three nonhost species revealed five compounds that consistently elicited antennal responses by I. typographus. The strongest antennal responses were elicited by trans-conophthorin, (5S,7S)-(–)-7-methyl-1,6-dioxaspiro[4.5]decane with optical purity of ca. 90% (S,S). The other four antennally active bark volatiles were 1-hexanol, (Z)-3-hexen-1-ol, 3-octanol and 1-octen-3-ol, which had similar electroantennogram (EAG) dose-response curves and response thresholds. (±)-trans-Conophthorin showed a different, linear EAG dose-response curve, with a 10 times lower response threshold than the other GC-EAD active compounds, similar to the pheromone component, (−)-(4S)-cis-verbenol. These results suggest that not only the green leaf alcohols from leaves of the angiosperm trees but also the nonhost bark volatiles could be used by I. typographus as long-range olfactory cues to discriminate between its conifer host and the nonhosts, birch and aspen.

Leaf Volatiles from Nonhost Deciduous Trees: Variation by Tree Species, Season and Temperature, and Electrophysiological Activity in Ips typographus

The leaf volatiles emitted from four nonhost tree species of Ips typographus, i.e. Betula pendula, B. pubescens, Populus tremula, and Sambucus nigra, were collected outdoors by headspace sampling in situ and analyzed by GC-MS. Three major classes of compounds, aliphatics [mainly green-leaf volatiles (GLVs)], monoterpenes, and sesquiterpenes, existed in all the deciduous tree species investigated. In June, when the bark beetles are searching in flight for host trees, GLVs mainly consisting of (Z)-3-hexenyl acetate and (Z)-3-hexen-1-ol were the dominant constituents in B. pendula and S. nigra. In B. pubescens and P. tremula, sesquiterpenes (and their derivatives) and monoterpenes made up the major part of whole volatile blends, respectively. Surprisingly, sesquiterpene alcohols and other oxides released from B. pubescens in considerable amounts were not found in the closely related species, B. pendula. By August, both the total volatiles and individual compounds significantly decreased, mainly due to the maturation of leaves, since the light intensity and temperatures during sampling were the same as in June. There were almost no volatiles detected from P. tremula and S. nigra leaves in August. The total emissions from these deciduous species were significantly different among the species, with B. pubescens releasing 5–10 times more than other species. Under the conditions of constant light intensity and humidity, emissions of both total volatiles and most individual components of severed B. pendula and S. nigra branches (with fresh leaves) increased according to a saturation curve from 16°C to 40°C. Ips typographus antennae responded strongly to green leaf alcohols: (Z)-3-hexen-1-ol, 1-hexanol, and (E)-2-hexen-1-ol, but not to aldehydes or acetates in GC-EAD analyses of B. pendula and B. pubescens leaf volatiles. No antennal responses to monoterpenes, sesquiterpenes, or sesquiterpene oxides were found. These three antennally active GLVs emitted from nonhost tree leaves might be indicators of a wrong habitat in the host selection of conifer bark beetles.

Strategies of a bark beetle, Pityogenes bidentatus, in an olfactory landscape

Abstract Volatiles from leaves or bark of nonhost birch (Betula pendula) and Norway spruce (Picea abies) dramatically reduced the attraction of the bark beetle, Pityogenes bidentatus (Coleoptera: Scolytidae), to their aggregation pheromone components (cis-verbenol and grandisol) in the field. In addition, odors from both the needles and bark of the host Scots pine (Pinus sylvestris) similarly inhibited attraction. Monoterpenes of pine and spruce (α-pinene, β-pinene, terpinolene, and 3-carene) as well as ethanol, chalcogran and some nonhost green leaf alcohols [(Z)-3-hexen-1-ol, (E)-2-hexen-1-ol, and 1-hexanol], also reduced catches. Collections of volatiles from the field-tested plant tissues indicated they released monoterpenes in amounts similar to the synthetics that inhibited responses. The various plant and insect sources of these inhibitory compounds indicate that P. bidentatus bark beetles have evolved several strategies to increase their fitness by avoiding nonhost and unsuitable host trees in a complex olfactory landscape.

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.

Volatiles released from individual spruce bark beetle entrance holes Quantitative variations during the first week of attack

Volatiles released from individual entrance holes of eight spruce bark beetles (Ips typographus) were collected during the first week of attack on a resistant host tree. In order to quantify the release of the highly volatile 2-methyl-3-buten-2-ol (MB) from attacking males, a new method was developed with deuterated quantification standard released at the time of collection. The amounts of collected volatiles, as analyzed by GC and GC-MS, showed a large variation between individual holes and also between subsequent entrainments from the same hole. Most of the quantified compounds on the average have two maxima, with a pronounced intervening depression. The amounts of releasedcis-verbenol (cV) increased five times during the first two days, while the amounts of MB were consistently high. The attacked spruce tree was not taken by the beetles, and the average amounts of the two aggregation pheromone components, MB and cV, increased again after the first maxima. The first peak of oxygenated monoterpene, released in the beginning of the attack containing α-terpineol, terpinen-4-ol, bornyl acetate,trans-pinocarveol, and verbenone, was possibly due to spontaneous oxidation of monoterpene hydrocarbons from the tree. Microorganisms established in the gallery wall phloem probably participated in the production of oxygenated monoterpenes during the second increase.

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.