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1.

Tree mortality and growth losses following insect defoliation are poorly documented in Scandinavia. In 1990-1991, Diprion pini (L.) caused extensive defoliation to Scots pine in Lauhanvuori national park and surrounding areas in south-western Finland. Most trees lost all their foliage in 1990. In 1991, the outbreak area was sprayed with diflubenzuron (Dimilin®), except in the national park, where trees were severely defoliated again. No further defoliation occurred in 1992. In spring 1993, sprayed trees had ca 30% foliage, whereas unsprayed trees on average carried less than 10% of full foliage. The latter trees were susceptible to attack by Tomicus piniperda (L.), whereas the former largely escaped beetle attack. Beetle attacks peaked in 1993, and depletion of suitable host trees probably terminated the beetle outbreak in the area. Two years of severe defoliation resulted in substantial tree mortality and growth losses. In spring 1997, these unsprayed stands had suffered a ca. 50% loss in basal area which was mainly because of mortality, and about half of the dead trees had been attacked by T. piniperda. Surviving trees had ca 50% of full foliage, and radial growth had still not recovered. Basal area growth was reduced by 40-70%, depending on the amount of foliage left after the second year of defoliation. In contrast, tree mortality and beetle attack in the sprayed stands were negligible, and these trees had recovered full foliage and radial growth by spring 1997. Thus, one year of total defoliation resulted in an estimated loss in basal area growth of approximately 30% during ca. 5 yrs. In conclusion, the spraying operation was economically justified, as it prevented substantial tree mortality and reduced growth losses.  相似文献   

2.
The pine processionary moth (Thaumetopoea pityocampa) is an important defoliating insect of native cedar stands in Northern Africa. In 2002 and 2003, we studied the size of needles of Atlas cedar trees that had been defoliated or not in the previous year, and compared the oviposition preference and larval performance of processionary moth on these two types of trees in cedar stands of central Algeria. Needle length and their number per needle cluster were twice as high on non-defoliated trees than on defoliated trees. There were about ten times fewer egg batches on defoliated than on non-defoliated trees in both study years. Young larvae raised in the laboratory on the foliage of previously defoliated trees were smaller compared to those fed on the foliage from healthy trees. Larval colonies transferred to trees in the field showed two times higher mortality on defoliated trees. It was concluded that the decline in population density in defoliated stands often observed after outbreaks results both from the fact that the female moths avoid laying their eggs on defoliated trees and the lower performance of larvae which later feed on the new-grown needles.  相似文献   

3.
In 1996, the pine looper (Bupalus piniaria) (L.) defoliated 7000 ha of Scots pine (Pinus sylvestris L.) forest at Hökensås, in southern Sweden. To study tree mortality, foliage recovery and top-kill, plots were laid out in stands with varying levels of defoliation in autumn 1997. Tree mortality peaked 2 yrs after defoliation, and amounted to 25% in stands suffering from 90–100% defoliation. Suppressed trees suffered higher mortality than intermediate and dominant trees. In stands suffering <90% defoliation, tree mortality did not exceed 8%. Foliage recovery in moderately and severely defoliated stands was not complete at the end of the 4 yr study period, whereas slightly defoliated stands had regained full foliage in 1998. Top-kill was most frequent in severely defoliated stands, and 50% of all trees in these stands suffered from top-kill at the end of the study period in spring 2001.  相似文献   

4.
I tested the presence of rapid and delayed induced resistance in Pinus sylvestris L. by biotests, rearing three species of diprionid sawflies on defoliated and undefoliated trees. I damaged the pines artificially by cutting mature needles with scissors at the beginning of the growing season. Four levels of defoliation were created on the basis of levels of removed needle biomass. Sawfly larval and cocoon weights were retarded and relative growth rates delayed as a result of an increase in damage level. The larval periods were prolonged at most 2–6 days. The response to the damage was partly species‐specific. The needle water content decreased in the following growing season in connection with low and moderate damage and increased at high damage levels. The nutrient contents of the foliage in the damaged and undamaged trees did not differ significantly. The Scots pine showed damage‐induced changes in food quality after extensive defoliation, which affected, via plant, species using the same host.  相似文献   

5.
The intensity and duration of Neodiprion abietis outbreaks have recently increased in forests of North America that were precommercially thinned more than a decade earlier. We tested the hypotheses that changes in stand structure following thinning increase the fitness (i.e., survival rate × fecundity) of N. abietis by either (a) increasing foliar availability and/or quality (i.e., increased availability of primary metabolites and/or reduced foliar defenses) or (b) by reducing any negative effects on foliar quality and/or availability resulting from herbivory that occurred during the preceding season(s). Effects of thinning and previous herbivory on N. abietis and its host plant (Abies balsamea) were determined through (i) a manipulative field experiment that evaluated the effects of experimental defoliation on N. abietis in a thinned stand, (ii) a manipulative field experiment that examined the effects of thinning on N. abietis in undamaged and naturally defoliated stands, and (iii) a field survey to estimate survival of N. abietis in natural populations.Defoliation caused reductions in the availability of different-aged foliage available to larvae and in the fitness of a subsequent N. abietis generation feeding on defoliated branches, but decreases in fitness were smaller in thinned than unthinned stands. In thinned stands, defoliation was associated with increases in foliage production and foliar contents of monoterpenes and nitrogen, as well as with a decrease in foliar contents of water. Conversely, only small changes in plant growth and foliar contents of nutrients and secondary chemicals were observed in defoliated unthinned stands. This suggests that deleterious effects of defoliation on sawfly fitness were offset by an increase in the foliar content of nitrogen, a primary compound known to improve larval growth in sawflies, which supports the hypothesis that thinning moderates negative effects of previous defoliation on sawfly fitness. The present study demonstrates that forestry practices that alter stand structure by reducing tree density may increase herbivory by affecting the way trees respond to insect attack, even after crown closure, with consequences on the buildup of herbivore populations in attacked trees.  相似文献   

6.
Plantations of Eucalyptus globulus in southwestern Australia are defoliated by Eucalyptus weevil, Gonipterus scutellatus, and a complex of chrysomelid and scarab beetles, yet there is no information on the impact of beetle defoliation to tree growth in southwestern Australia. To address this shortcoming, we used insect exclusion trials, to compare growth of insecticide treated (and thus relatively undamaged) trees with untreated (and thus defoliated) trees to determine whether defoliation by G. scutellatus and other beetles reduced the growth and harvest volume of E. globulus trees. Our results showed some evidence of beetle defoliation reducing growth of E. globulus. Mean defoliation levels of the growing tip of untreated trees ranged from 18% to 33% across the duration of the study and were significantly greater than mean defoliation levels of 5–16% on insecticide treated trees. Seasonal peaks in defoliation of 30–80% to the growing tip of untreated trees were recorded between late spring and early autumn. The greatest impact of defoliation on tree growth was evident during the 2.5 year period of insect exclusion, when higher relative growth rates were recorded for insecticide treated trees, which were significantly different from relative growth rates of untreated trees at two of the four plantations. However, our results showed only a limited impact of beetle defoliation on the total volume at harvest. Initially small trees tended to suffer more severe defoliation than initially large trees. Effects of insect exclusion treatment on harvest volume were modified by the initial tree size and the relationship between the initial tree size and levels of defoliation.  相似文献   

7.
The phloem-feeding aphid Essigella californica represents a potential threat to the productivity of Pinus radiata plantations in south-eastern Australia. Five- and nine-year-old field trials were used to characterize the effects of artificial and natural aphid-induced (E. californica) defoliation, respectively, on shoot photosynthesis and growth. Photosynthetic capacity (A(max)) was significantly greater following a 25% (D25) (13.8 μmol m(-2) s(-1)) and a 50% (D50) (15.9 μmol m(-2) s(-1)) single-event upper-crown artificial defoliation, 3 weeks after defoliation than in undefoliated control trees (12.9 μmol m(-2) s(-1)). This response was consistently observed for up to 11 weeks after the defoliation event; by Week 16, there was no difference in A(max) between control and defoliated trees. In the D50 treatment, this increased A(max) was not sufficient to fully compensate for the foliage loss as evidenced by the reduced diameter increment (by 15%) in defoliated trees 36 weeks after defoliation. In contrast, diameter increment of trees in the D25 treatment was unaffected by defoliation. The A(max) of trees experiencing upper-crown defoliation by natural and repeated E. californica infestations varied, depending on host genotype. Despite clear differences in defoliation levels between resistant and susceptible genotypes (17 vs. 35% of tree crown defoliated, respectively), growth of susceptible genotypes was not significantly different from that of resistant genotypes. The observed increases in A(max) in the lower crown of the canopy following attack suggested that susceptible genotypes were able to partly compensate for the loss of foliage by compensatory photosynthesis. The capacity of P. radiata to regulate photosynthesis in response to natural aphid-induced defoliation provides evidence that the impact of E. californica attack on stem growth will be less than expected, at least for up to 35% defoliation.  相似文献   

8.
Developing integrated pest management practices against the defoliating beetle Mesoplatys ochroptera is an important aspect of the adoption of Sesbania sesban as an improved fallow species in southern Africa. The effect of defoliation by M.ochroptera on the growth of S. sesban(provenance Kakamega) was studied during 1998–2000 at Msekera Research Station in eastern Zambia. To determine the relationship between M. ochroptera densities and degree of defoliation, potted seedlings were infested manually with different densities of larvae and adults. Infestation of two to three month old seedlings with 5–30 larvae or adults resulted in less than 20% defoliation. Infestation of seedlings with 90–150 larvae(>3 masses of eggs) led to 80–100% and 50–80% defoliation in two and three months old seedlings, respectively. The time of infestation and degree of defoliation that lead to reduction in growth and biomass were determined using simulated (manual) defoliation of one to three months old S. sesban seedlings. Manual removal of 50–100% of the foliage atone and two months after transplanting (MAP) appeared to reduce plant height, basal diameter, primary branches, leaf and wood biomass compared to that done atthree MAP. Removal of 25–50% of the sesbania foliage three months after transplanting apparently leads to overcompensation. In sesbania, compensatory growth occurred when 25–50% of the leaves were defoliated three months after transplanting. Therefore, farmers need to protect sesbania seedlings from defoliation against insects such as M. ochroptera only during the first two months after transplanting.This revised version was published online in November 2005 with corrections to the Cover Date.  相似文献   

9.
In 1989, the first recorded outbreak of hemlock looper (Lambdina fiscellaria fiscellaria (Guen.)) occurred in New Brunswick, Canada. Data were collected from ten plots established in an area infested from 1992–1994, to assess impacts of hemlock looper. Ocular and branch sample assessments of current defoliation and ocular assessments of total defoliation (all age classes of foliage) were conducted for balsam fir (Abies balsamea [L.] Mill.), white spruce (Picea glauca [Moench] Voss), and black spruce (Picea mariana (Mill.) B.S.P.). Stand response was assessed and related to cumulative defoliation. Ocular assessments were found to accurately estimate defoliation, which was significantly related to tree mortality. Ninety-two percent of balsam fir trees that had cumulative defoliation >90% died. Mortality of balsam fir was significantly (p < 0.05) related to tree size, in both lightly and severely defoliated plots; trees with DBH <11 cm sustained 22–48% higher mortality than larger trees. Mortality of balsam fir, in terms of both percent stems/ha and m3/ha merchantable volume, increased exponentially in relation to three estimates of cumulative (summed) plot mean defoliation. The strongest relationships (r2 = 0.75–0.79) were between mortality and the ocular defoliation assessment for 1990–1993 foliage. Tree mortality caused by the looper outbreak ranged from 4–14% stems/ha in lightly defoliated and from 32–100% in severely defoliated plots; merchantable volume killed was 3–14 m3/ha and 51–119 m3/ha, respectively. Relationships between mortality and defoliation were similar when defoliation was assessed for 1987–1993 and 1990–1993 foliage age classes.  相似文献   

10.
Insect damage to production forests has the potential to reduce financial returns by retarding tree growth and causing mortality, however, long-term realised quantification of these losses is rare. In order to help elucidate economic damage thresholds for making spray decisions we capitalised on a natural outbreak of autumn gum moth, Mnesampela privata, in a 2-year-old Eucalyptus nitens plantation. Following the partial chemical control of this insect outbreak we measured the tree growth variables diameter at breast height over bark and height of five differing tree defoliation classes for 75 months following tree damage. At the end of this period a threshold model was fitted to describe the relationship between tree defoliation and realised tree wood volumes. The model revealed that realised stand wood volume was not significantly affected up until defoliation exceeded 60% and then declined sharply after this defoliation level was reached. Further support for this defoliation threshold was evident from multiple comparisons among defoliation classes that showed 50% defoliated trees did not have significantly different wood volume compared to more lightly defoliated trees, but did have significantly greater wood volume compared to trees that were 72% or more defoliated. To determine if the realised differences in wood volume resulted in differences in yield over a plantation rotation the E. nitens growth model NITGRO was used to on-grow trees to age 15 years for a ‘best case’ (type 1 growth response, constant growth rates from last inventory until harvest) and ‘worst case’ (type 2 growth response, divergent growth rates from last inventory until harvest) scenario. The threshold model was then fitted to the outcomes of both scenarios and the economic consequences of defoliation were clearly dependent on the growth function assumed.  相似文献   

11.
Abstract

Two phenological forms of the pedunculate oak co-occur in the same habitats throughout the species range: the early trees (Quercus robur var. praecox) develop leaves up to 5 weeks before the late ones (Quercus robur var. tardiflora). This study tests the idea that late leaf flushing serves as an antiherbivore defence, i.e. late trees, which develop leaves asynchronously with eclosion of folivorous caterpillars, avoid the costs of defoliation, which could offset the costs of a later onset of the growing season. Effects of folivorous caterpillars foraging on oaks were observed in 1998–2006 in remnants of primeval temperate lowland forest preserved in the Bia?owie?a National Park (eastern Poland). Observations covered trough and outbreak years of the major defoliator, the winter moth [Operophtera brumata L. (Geometridae)]. In seven out of nine seasons, including all peak caterpillar years, the amount of frass produced by folivorous caterpillars on late trees (n=8–18) was significantly (up to 7.1 times) lower than on the early ones (n=12–32). Assessment of the degree of defoliation in 2002–2006 showed that the late oaks were visibly defoliated only during a caterpillar peak (2003), while the early trees were affected in all years and in 2003 almost completely lost their leaves. The results confirm the effect of late budburst on lowering herbivore damage, and give support to the idea that late leaf flushing acts as an antiherbivore defence.  相似文献   

12.
Summary Effects of the gypsy moth defoliation on growth and mortality of oak stands Investigations were carried out in Rumania in three oak stands (Quercus pedunculata) situated in two different phytoclimatic regions where the defoliation was observed one year or more consecutive years. In four other forests were used artificial defoliations in young (15–20 years) stands.The growth (ig) was estimated in permanent trial plots (with minimum 150 trees each plot) in defoliated stands and in control stands where gypsy moth were controlled. p ]Radial growth (ir) was measured by microauxometer and by increment cores sampled with Pressler borer.The results show that one year defoliation influenced the stands growth in that year and in the next two years. In this case the whole growth difference — as compared with control stands — was almost similar to the equivalent of one year growth. If the defoliation occurred two consecutive years, the whole growth difference was similar to 1.3–2.2 years growths, in old stands and in young stands respectively.Oak stands defoliation, unassociated with other limitative factors, does not lead to oak mortality but only to increase of natural elimination process.  相似文献   

13.
Regeneration of beech (Fagus crenata) forests depends on the formation of canopy gaps. However, in Japan Sea-type beech forests, a dwarf bamboo (Sasa kurilensis) conspicuously occupies sunny gaps. Therefore,F. crenata seedlings must escape the severe interference ofS. kurilensis in the gaps and persist beneath a closed canopy of the beech forest. We hypothesized that the growth ofF. crenata seedlings in the understory would be favored by their being more plastic thanS. kurilensis in photosynthetic and morphological traits, which would support the matter production ofF. crenata seedlings in a wide range of light availabilities. To examine this hypothesis, the photosynthetic-light response of individual leaves and the biomass allocation in aboveground parts (i.e., the culm/foliage ratio) were surveyed at sites with contrasting light availabilities in a Japan Sea-type beech forest in central Japan. InF. crenata, photosynthetic light utilization efficiency at relatively low light was greater, and the dark respiration rate was smaller in the leaves of seedlings (10 cm in height) beneath the closed canopy than in the leaves of saplings at the sunny forest edge. The culm/foliage (C/F) ratio of theF. crenata seedlings at the shady site was small, suggesting effective matter-production beneath the beech canopy. On the other hand,S. kurilensis both in the gap and beneath the beech canopy showed low plasticity in photosynthesis and the culm/foliage ratio. Because the shoot density ofS. kurilensis was smaller beneath the beech canopy than in the gap, the light availability at the bottom of theS. kurilensis layer was greater beneath the beech canopy. These results suggest that the photosynthetic productivity of theF. crenata seedlings would be enough for the seedlings to survive in the understory with a low density ofS. kurilensis shoots beneath the closed beech canopy.  相似文献   

14.
Summary The aim of the present study was to investigate whether the resistance of beech foliage(Fagus sylvatica) against oxidative stress was affected by soil quality, nitrogen or CO2 fertilisation, or lachnid infestation(Phyllaphis fagi). For this purpose young beech trees were grown for four years in reconstructed calcareous or acidic forest soils in open top chambers under ambient or elevated CO2 concentrations with two levels of nitrogen fertilisation. At harvest lachnid colonisation was observed, preferentially on leaves from trees in calcareous soil and on leaves from trees fertilised with the high nitrogen level. General leaf characteristics such as pigment concentrations, dry mass, and leaf mass ber area were not affected by the soil type, nitrogen fertilisation or CO2 regime. Leaves colonised with lachnids displayed slightly increased leaf mass per area. When the stress resistance was challenged by exposure to paraquat — a herbicide inducing oxidative stress — leaves from trees grown on calcareous soil maintained significantly longer membrane integrity and, thus, were better protected against stress than leaves from trees on acidic soil. Other experimental variables had negligible or no effects on the resistance against oxidative stress.   相似文献   

15.
The impacts of elevated temperature and CO2 on young silver birch (Betula pendula Roth) saplings after 0, 25, 50 or 75% artificial defoliation were assessed by measuring plant height and dry mass of aboveground compartments and roots and various morphological and physiological variables. Defoliation either increased or decreased plant growth depending on the severity of damage and the climatic treatment. At 21 °C and 400 mg L?1 CO2, defoliated plants were not able to compensate for the lost foliage, but growth compensation and adaptation to the changed conditions were greater; growth of young defoliated silver birch saplings increased, which led to increased height and a tendency to enhance final aboveground and root biomass and leaf nitrogen and carbon content compared to the nondefoliated controls. Nevertheless, the short-term effect of the different climatic conditions did not result in a significant overgrowth of defoliated plants. A slight increase in temperature and CO2 were the most acceptable conditions for defoliated plants; however, a 4 °C increase with correspondingly higher CO2 was more stressful as shown by less growth in height and biomass allocation to leaves, stems and roots. The findings from the pilot experiment are more applicable to young birch trees, but stress on young trees may be reflected in future tree growth.  相似文献   

16.

Assessing defoliation and measuring litterfall are two different ways of estimating the shedding of needles and leaves from the forest canopy. Both variables can be said to reflect the crown condition, but the two methods have rarely been compared. In this study the visual observations of defoliation of individual trees were compared with sampled litterfall data for the two main tree species in Denmark, Fagus sylvatica and Picea abies. Defoliation assessments and litterfall measurements were performed in seven level II plots. Six of the stands were 40 yrs old and the remaining stand was 81 yrs old. Both a positive and a negative correlation, which were not significant, were observed between defoliation and total leaf litterfall on the beech sites. Similarly, no significant correlation was observed between defoliation and the yearly needle litterfall on the younger Norway spruce plots. However, the defoliation and the yearly needle litterfall at the old stand at Klosterhede were positively correlated on a 10% significance level. A positive correlation was also apparent at the younger stands between the defoliation and the needle litterfall from the period April-July of the same year in which defoliation was assessed. The absence of a clear connection between the two assessments is discussed. The two assessments are apparently of widely different origin, which makes comparison difficult.  相似文献   

17.
We examined the effects of fertilization and gypsy moth defoliation on condensed tannin concentration (%CT) of hybrid poplar (Populus x canadensis cv 'Eugeneii') fine roots in the summers of 1997 and 1998. This factorial experiment included two defoliation treatments (defoliated and a foliated control) and fertilization treatments (100 kg nitrogen (N) ha(-1) and an unfertilized control). Gypsy moth (Lymantria dispar L.) populations were experimentally increased to obtain defoliation in the summers of 1996, 1997 and 1998; fertilization subplots were supplemented with NH4NO3 (100 kg N ha(-1)) in the spring of each year. Despite the severity of defoliation, the effects were small, and significant on only two sampling dates: in May 1997, when fine root %CT was 23% lower in the defoliated trees, and in November 1997, when trees in the defoliated unfertilized plots had 35% higher root %CT than trees in all other plots. Defoliation effects on root %CT did not follow the same seasonal pattern as defoliation effects on root starch content, N uptake capacity or leaf %CT. Regulation of root condensed tannin concentration appeared to be partially uncoupled from these traits. The small transient effects on root defense reflect the resilience of this early successional tree to severe early season defoliation.  相似文献   

18.
Both hemlock looper (Lambdina fiscellaria fiscellaria (Guen.)) and balsam fir sawfly (Neodiprion abietis (Harris)) undergo periodic outbreaks in eastern Canada and cause significant growth and mortality losses to forests. Tree growth and mortality are closely related to cumulative defoliation estimates, which integrate annual defoliation over multiple years. Our objective was to determine a method to estimate cumulative defoliation of balsam fir (Abies balsamea [L.] Mill) due to these insects in western Newfoundland, using aerial defoliation survey data, as an essential input to modeling impacts for Decision Support Systems. Interpretation of aerial defoliation survey data for hemlock looper and balsam fir sawfly is problematic because both insects feed upon multiple age classes of foliage. Current-year (2008) aerial defoliation survey data were compared with ground estimates of defoliation by age class from 45 plots (450 trees and 395 mid-crown branch samples), representing a range of defoliation severity classes for each insect. Cumulative defoliation was calculated using defoliation per foliage age class, weighted by relative foliage mass for a given age of foliage. Three significantly different severity classes were defined based on cumulative defoliation values derived from aerial defoliation survey: (i) 1-year moderate (30–70%) defoliation, (ii) 1-year severe (71–100%) defoliation with calculated cumulative defoliation values of 19 and 39%, respectively, for balsam fir sawfly, 21 and 34% respectively for hemlock looper; and (iii) 2–3 years of moderate–severe defoliation, with cumulative defoliation ranging between 59 and 64% for balsam fir sawfly and 49% for hemlock looper. Defoliation severity from aerial defoliation survey alone hence can be misleading if defoliation measurements are not converted to cumulative defoliation values.  相似文献   

19.
Infestations of Essigella californica following the installation of post-thinning fertilizer trials in Pinus radiata plantations provided an opportunity to examine the impact of repeated defoliation over a period of 8 years (1997–2005). Replicated treatments (n = 4) of nil fertilizer (control), N (300 kg ha−1) as urea, P (80 kg ha−1) and S (45 kg ha−1) as superphosphates were applied immediately after thinning at three sites and this was followed by a second application of NPS fertilizers 6 years later with N applied at 300 kg ha−1 as urea and ammonium sulphate and P at 80 or 120 kg ha−1. Defoliation of untreated P. radiata gradually increased to 50% over a period of 8 years. Basal area growth was negatively correlated with average defoliation for two consecutive post-fertilizer periods of 6 and 2 years. Growth responses to fertilizer varied considerably between sites but the largest improvement in growth was due to NPS fertilizer, this increased basal area by 30–80%. Application of N fertilizer raised total N levels in foliage and increased defoliation with a commensurate loss in growth under conditions of deficiencies of S or P. Repeated infestations gradually increased the percentage of trees with severe defoliation (>80% loss of foliage) indicating that nutrient-deficient trees have a reduced capacity for foliage recovery between episodes of peak infestation. In contrast, treatment with N fertilizer in combination with S- and P-corrected deficiencies of these nutrients, raised levels of total N in foliage and reduced defoliation to approximately 20%. Basal area growth responses to NPS fertilizers reflected improved nutrition as well as reduced insect damage. The reduction in defoliation under conditions of balanced tree nutrition was most likely due to enhanced needle retention following correction of P deficiency as well as greater availability of nutrients enabling a more vigorous recovery of P. radiata after an episode of E. californica activity. Treatment with fertilizer therefore reduced the long-term impact of aphid damage and improved growth of P. radiata.  相似文献   

20.
Many experiments have tested the hypothesis that stress‐induced changes in plant quality enhance insect performance. Data available for insects feeding on woody plants indicate that phloem‐feeding insects’ response is stronger (i.e. enhanced performance) than that of chewing leaf‐feeding insects. However, with the exception of one study, performance data for these two insect groups emanate from separate experiments. Here we report on the performance of a phloem‐feeding aphid (Cinara costata) and larvae of a folivorous sawfly (Gilpinia hercyniae) feeding on drought‐stressed mature Norway spruce (Picea abies). Sawfly larvae did not survive or grow any better on drought stressed trees than on control trees. Surprisingly, there was no significant difference in the build‐up of aphid densities between trees of the two treatments, in contrast to most other stress experiments. Our experiment involved mature trees, while most other studies have used seedlings. It is possible that there are fundamental differences between seedlings and mature trees in their response to environmental stress. Possible mechanisms for such differences are discussed.  相似文献   

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