Niklas Björklund

Induced Terpene Accumulation in Norway Spruce Inhibits Bark Beetle Colonization in a Dose-Dependent Manner

Background Tree-killing bark beetles (Coleoptera, Scolytinae) are among the most economically and ecologically important forest pests in the northern hemisphere. Induction of terpenoid-based oleoresin has long been considered important in conifer defense against bark beetles, but it has been difficult to demonstrate a direct correlation between terpene levels and resistance to bark beetle colonization. Methods To test for inhibitory effects of induced terpenes on colonization by the spruce bark beetle Ips typographus (L.) we inoculated 20 mature Norway spruce Picea abies (L.) Karsten trees with a virulent fungus associated with the beetle, Ceratocystis polonica (Siem.) C. Moreau, and investigated induced terpene levels and beetle colonization in the bark. Results Fungal inoculation induced very strong and highly variable terpene accumulation 35 days after inoculation. Trees with high induced terpene levels (n = 7) had only 4.9% as many beetle attacks (5.1 vs. 103.5 attacks m−2) and 2.6% as much gallery length (0.029 m m−2 vs. 1.11 m m−2) as trees with low terpene levels (n = 6). There was a highly significant rank correlation between terpene levels at day 35 and beetle colonization in individual trees. The relationship between induced terpene levels and beetle colonization was not linear but thresholded: above a low threshold concentration of ∼100 mg terpene g−1 dry phloem trees suffered only moderate beetle colonization, and above a high threshold of ∼200 mg terpene g−1 dry phloem trees were virtually unattacked. Conclusion/Significance This is the first study demonstrating a dose-dependent relationship between induced terpenes and tree resistance to bark beetle colonization under field conditions, indicating that terpene induction may be instrumental in tree resistance. This knowledge could be useful for developing management strategies that decrease the impact of tree-killing bark beetles.

Host-plant acceptance on mineral soil and humus by the pine weevil Hylobius abietis (L.)

1 The pine weevil Hylobius abietis (L.) (Coleoptera, Curculionidae) is an economically important pest of conifer forest regeneration in Europe and Asia.

Olfactory and visual stimuli used in orientation to conifer seedlings by the pine weevil, Hylobius abietis

Abstract.  The influence of noncontact plant cues is investigated on the likelihood that individual conifer seedlings will be found by walking adults of the pine weevil, Hylobius abietis, in the field. Traps with solely odour or solely visual stimuli catch significantly more weevils than stimulus‐free traps, and traps with the combination of odour and visual stimuli catch more weevils than traps with odour or visual stimuli alone. There is essentially an additive effect between odour and visual stimuli. The reactions to odour and visual stimuli are similar for three phases of the pine weevils life cycle associated with three ages of clear‐cuttings (i.e. sites where all trees have been harvested). Visual stimuli appear to be at least as important as odour for the pine weevil in finding an undamaged conifer seedling.

Soil type and microtopography influencing feeding above and below ground by the pine weevil Hylobius abietis

Abstract  1 The influence of soil type and microtopography on above and below ground feeding by adult pine weevils Hylobius abietis (L.) (Coleoptera: Curculionidae) was evaluated in a field experiment with enclosed weevil populations of known size.

Seasonal flight patterns of Ips typographus in southern Sweden and thermal sums required for emergence

The spruce bark beetle Ips typographus is the major tree‐killing bark beetle in Eurasia. To increase knowledge about its seasonal flight patterns and about the thermal sums required for emergence, a study was conducted in southern Sweden from 2006 to 2010. Seasonal flight patterns were recorded by pheromone traps and development was recorded by felling trees three times during each season. Flight began, on average, on 27 April [after 47 degree‐days (dd) > 5 °C]. More than 50% of flight activity occurred after mid‐June, and this continued to mid‐August. Re‐emergence of parental beetles after they had produced the first brood started, on average, on 24 May (after 122 dd). Continued flight and oviposition demonstrated that sister broods were frequent. A higher proportion of parental beetles re‐emerged from trees colonized in May (95%) than in July (20%). Beetles that colonized trees late had to accumulate a higher thermal sum before re‐emerging (400 dd). Filial beetles began to emerge, on average, on 29 June (after 437 dd) and started to fly in early July, giving rise to at least a partial second generation in each year. A higher proportion of filial beetles had emerged from trees colonized in May (75%) than in July (15%). Knowledge of these region‐specific flight patterns and the associated thermal sums required for emergence will facilitate efficient pest management by enabling timely removal of fallen and standing weakened host trees. The obtained data will also be useful for improving models that predict the population dynamics in a warmer climate.

Cues For Shelter Use in a Phytophagous Insect

Many insects spend a large proportion of their life inactive, often hiding in shelters. The presence of shelters may, therefore, influence where insects feed. This study examines stimuli affecting the use of shelters by adults of the pine weevil, Hylobius abietis (L.) (Coleoptera, Curculionidae). This species is an economically important forest pest in Europe since the adults feed on the stem bark of newly planted conifer seedlings. When there are hiding or burrowing places present in close proximity to a seedling, pine weevils may hide there and repeatedly return to feed on the same seedling. Experiments were conducted in a laboratory arena with above-ground or below-ground shelters and in the presence or absence of wind. Pine weevils were highly attracted to shelters both above and below ground. Weevils in shelters were often observed assuming a characteristic “resting” posture. Experiments with opaque and transparent shelters showed that visual stimuli are used for orientation towards shelters and also increase the probability of an individual remaining in a shelter. The presence of wind increased the weevils’ propensity to use shelters both above and below ground. The present study indicates that shelters have a major influence on the behavior of the pine weevil and possible implications of the results are discussed.

Effects of jasmonate-induced resistance in conifer plants on the feeding behaviour of a bark-chewing insect, Hylobius abietis

Conifer defences can be induced by exogenous chemical elicitors, thereby reducing damage caused by bark-feeding insects. However, the insect behavioural mechanisms involved are poorly understood. Thus, effects of artificially induced plant defences on feeding behaviour of the pine weevil (Hylobius abietis), a serious forest pest, were examined to explore mechanisms involved in decision-making processes connected to feeding. To induce plant defences, we applied methyl jasmonate (MeJA), a naturally occurring plant hormone, to young Norway spruce (Picea abies) plants. The weevils’ feeding behaviour on plants with and without MeJA treatment was studied in both a no-choice and a choice laboratory experiment. MeJA treatment did not affect the initiation of feeding, but it affected the weevils’ subsequent feeding patterns. In the no-choice experiment, the only observed effect of its treatment was that it reduced the size of the initial feeding scars. In the choice experiment, it reduced both the numbers and sizes of the feeding scars and hence the total debarked area. Thus, the MeJA-induced resistance did not deter the pine weevils from attacking the spruce plants, but reduced the amounts they consumed at one place, which would reduce risks of girdling and plant death. This may be the behavioural mechanism behind the previously recorded increases in survival rates of MeJA-treated plants in the field.

Diel behaviour and time budget of the adult pine weevil Hylobius abietis

The pine weevil [Hylobius abietis (L.); Coleoptera: Curculionidae] has a high economic impact on forest regeneration in Europe. The general biology of the pine weevil has received considerable attention, although there is insufficient knowledge about its diel behaviour and time budget. Therefore, in the present study, the feeding and locomotion behaviour of individual adult weevils on Norway spruce seedlings [Picea abies (L.) Karst.] is observed for 24‐h periods in the laboratory. Both girdled and nongirdled seedlings are used to assess how the behaviour of weevils is influenced by the physiological response of plants to the girdling. The locomotion pattern shows a distinct maximum during the beginning of the dark phase, whereas most feeding occurs during the second half of the dark phase and the first hours of the subsequent light phase. The girdling treatment increases the time that weevils spend on the seedlings during the first part of an observation session, although it has no effect on their feeding pattern. The time budgets of weevils on girdled and nongirdled seedlings are similar. On average, weevils spend 34% of their time in locomotion and 6% on feeding. Females spend more time feeding than males (7.1% versus 4.2%), possibly because they have higher food requirements (e.g. for egg production). Females also spend more time in total on the seedlings than males (26.3% versus 7.0%). The present study reveals, in high temporal resolution, the diel feeding and locomotion behaviour and time budget of male and female pine weevils.

Ants protect conifer seedlings from feeding damage by the pine weevil Hylobius abietis

1 Ants that protect food resources on plants may prey on (or deter) herbivores and thereby reduce damage. Red wood ants (of the Formica rufa group) are dominant ants in boreal forests of Eurasia and affect the local abundance of several herbivorous species. 2 The pine weevil Hylobius abietis (L.) is a herbivore that causes severe damage by feeding on the bark of coniferous seedlings within areas of forest regeneration. 3 We investigated whether ants can protect conifer seedlings from pine weevil feeding. In a manipulative experiment, ants were attracted to sugar baits attached to spruce seedlings and the damage caused by pine weevils was compared with control seedlings without ant‐baits. 4 The feeding‐scar area was approximately one‐third lower on the seedlings with ant‐baits compared with the controls. Besides red wood ants, Myrmica ants were also attracted in high numbers to the ant baits and the relative effects of these species are discussed. 5 The results obtained in the present study support the trophic cascade hypothesis (i.e. damage to herbivores is suppressed in the presence of predators). The decreased pine weevil feeding on the baited seedlings was probably a result of nonconsumptive interactions [i.e. the presence of (or harassment by) ants distracting pine weevils from feeding]. 6 Understanding the role of ants may have important implications for future strategies aiming to control pine weevil damage. For example, maintaining suitable conditions for ants after harvesting stands may be an environmentally friendly but currently unexploited method of for decreasing weevil damage.

Seeing the forest through the trees: Differential dispersal of hylobius warreni within modified forest habitats

ABSTRACT Hylobius warreni Wood, also known as the Warren root collar weevil, is a flightless insect that feeds on conifers throughout the boreal forests of Canada. Mature trees typically can withstand feeding, but larval feeding around the root collar may cause mortality to young trees. Recently, a large outbreak of mountain pine beetle (Dendroctonus ponderosae Hopkins) has killed a high proportion of mature lodgepole pine (Pinus contorta Douglas variety latifolia) across British Columbia, Canada. This raises concerns that adult weevils may migrate from mature forests with reduced host pools into adjacent young forests that had been salvaged and replanted. To study movement of these walking weevils in different habitat types, we constructed three research plots consisting of various combinations of live-, dead-, and mixed (i.e., live and dead)-tree habitats. We observed dispersal patterns of individually labeled insects using a novel insect trap attached to the base of trees. Approximately 35% of insects were recaptured over 1 mo. Weevils were least likely to be recaptured proximate to the release location when released in a habitat with dead trees. Movement rates therein were almost double the rates of insects moving through live- or mixed-tree habitats. Our findings support the hypothesis that H. warreni may disperse out of habitats with dead trees into areas with higher proportions of green trees. Our findings are discussed in the context of habitat discrimination and potential increases in herbivory by H. warreni in western Canada given salvage harvesting activities after outbreaks of mountain pine beetle.