The effect of environmentally induced changes in the bark of young conifers on feeding behaviour and reproductive development of adult Hylobius abietis (Coleoptera: Curculionidae)

Abstract

Young plants of Sitka spruce, Scots and Corsican pine were subject to high and low light, and high and low nitrogen treatments in a polyhouse experiment. The effect of treatments on resin duct size and nitrogen concentration in stem bark was determined together with feeding by Hylobius abietis Linnaeus on the stems of 'intact' plants and on 'detached' stems cut from the plant. Resin duct size was largest on Corsican pine and smallest on Sitka spruce and inherent variation in duct size between the three conifer species appears to determine the pattern of weevil feeding between species. Resin ducts and the flow of resin from them protect the stems of young conifers from weevil feeding not by affecting the total amount of bark eaten but by limiting the depth of feeding and so protecting the inner phloem and cambium. Shallow feeding may increase the likelihood of effective wound repair. Duct size was positively related to plant growth and in particular increased with bark thickness. Overall, ducts were largest in the high light treatment although species differed in their response to the treatment. It is suggested that the effects of plant size, growing conditions and transplantation on susceptibility to attack by H. abietis, reported in various studies, may be due to underlying variation in resin duct size or flow rate. The effect on weevils of superficial feeding on stems is to increase the time for reproductive maturation by reducing consumption of the inner bark which has a higher nitrogen content.

Growth and defence in young pine and spruce and the expression of resistance to a stem-feeding weevil

“…In linear regressions of the relationship between the number of resin ducts and stem diameter, duct area and stem diameter, and between resin mass and duct area, a common slope was Wtted to the data where appropriate. The analysis of bio-assay data was based on an L n transformed ratio of deep/shallow feeding (L n ratio) (Wainhouse et al 2005). Prior to analysis, seven observations of zero shallow feeding were given the value of 0.05 (minimum observed value = 0.06), and one observation with zero deep feeding was given the value of 1 (minimum observed value = 3).…”

“…The amount of bark eaten by each weevil was measured as described by Wainhouse et al (2005). BrieXy, the stem section containing feeding damage was removed from the tree and immersed in pentane for 5 or 10 min to remove exuded resin at the feeding sites.…”

“…Pine and spruce are most vulnerable to attack during the Wrst few years of growth, and the principal defence is provided by resin ducts within the bark on which the weevils feed (Wainhouse et al 2004(Wainhouse et al , 2005. In the study reported here, we examined the physical and physiological constraints on the expression of resin-based defence in two conifer species widely planted in the UK-Sitka spruce [Picea sitchensis (Bong.)…”

Damage by the pine weevil <i>Hylobius abietis</i> to seedlings of two native and five introduced tree species in Sweden

“…Large seedlings are generally more tolerant to pine weevil feeding because of their lower likelihood of being girdled (Selander 1993;Örlander and Nilsson 1999;Thorsén et al 2001). Moreover, the size of resin ducts, which contribute to weevil resistance in young conifers, is correlated to bark thickness and stem base diameter (Wainhouse et al 2005). For high survival rates of seedlings in areas infested with pine weevils, threshold root collar diameters of 8-9 mm for Norway spruce and 12 mm for Scots pine have been suggested (Thorsén et al 2001;Wallertz et al 2005).…”

The influences of forest stand management on biotic and abiotic risks of damage

“…For example, insect defoliators such as the pine sawflies Neodiprion swanei and Neodiprion sertifer (Larsson and Tenow, 1984;Smirnoff and Bernier, 1973), or the poplar chrysomelids Chrysomela tremulae and Phratora vitellinae (Gruppe et al, 1999) are adversely affected by nitrogen fertilization of trees. Similarly, it has been observed that the number and size of resin ducts increases in fertilised conifers (Kytö, 1999;Wainhouse et al, 2005). Because resin compounds are toxic for bark feeders such as bark beetle and weevils or repellent for mammal herbivores such as voles and hares (Harju and Tahvanainen, 1997), it follows that fertilised trees would be less damaged by these agents.…”