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1.
  • 1 Invasive species pose significant threats to native and managed ecosystems. However, it may not always be possible to perform rigorous, long‐term studies on invaders to determine the factors that influence their population dynamics, particularly when time and resources are limited. We applied a novel approach to determine factors associated with mortality in larvae of the sawfly Profenusa thomsoni Konow, a leafminer of birch, and a relatively recent invader of urban and rural birch forests in Alaska. Classification tree analysis was applied to reveal relationships between qualitative and quantitative predictor variables and categorical response variables in a large data set of larval mortality observations.
  • 2 We determined the state (living or dead) of sawfly larvae in samples of individual leaves. Each leaf was scored for variables reflecting the intensity of intra‐specific competition and leaf quality for leafminers, year of collection and degree‐days accumulated were recorded for each sample. We explored the association of these variables with larval state using classification tree analysis.
  • 3 Leafminer mortality was best explained by a combination of competition and resource exhaustion and our analysis revealed a possible advantage to group feeding in young larvae that may explain previously observed patterns of resource overexploitation in this species. Dead larvae were disproportionately found in smaller leaves, which highlights the potential effect of competition on mortality and suggests that smaller‐leaved species of birch will better able to resist leafminer damage.
  • 4 We show that classification tree analysis may be useful in situations where urgency and/or limited resources prohibit traditional life‐table studies.
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2.
Summary The effect of leaf damage simulating the feeding of early season insect herbivore species, e.g. Epirrita autumnata, to mountain birch, Betula pubescens ssp. tortuosa, on the performance of insect larvae was studied with eleven leaf-chewing sawfly species. I found variation in the results that was due to short- and long-term inducible responses and to the phenology of herbivore species. In general, early and mid-season species were more strongly affected by induced reactions than late-season species. This finding is in accordance with earlier results but I could show that the persistance of induced reactions rather than the influence of timing of damage is responsible for the result. The growth of the larvae of mid-season sawfly species was affected by both short- and long-term induced reactions. This result shows that early season species may escape short-term induced reactions of mountain birch in current year but may not avoid long-term effects. It is supposed that seasonal deterioration of leaf quality either masks the effects of induced defences or late-season species are better adapted to low-quality leaves. Some species show variation in their response to induced defence in different years. This may be due to yearly differences in induced reactions as well as to species-specific responses. Induced defence reactions may play a role in competitive interactions between herbivore species in leaf-chewing guild of mountain birch.  相似文献   

3.
Diversity of birch sawfly responses to seasonally atypical diets   总被引:4,自引:0,他引:4  
Most insect herbivores are specialised on a particular plant taxon. To have a better understanding of host shift functions and consequences for insect herbivores, it is essential to gather more information on the effects of variation in host quality on specialists across species and environments. We examined the effects of seasonally atypical food on mortality, developmental time, and final body mass of six sawfly species (Hymenoptera: Symphyta) feeding on the foliage of mountain birch (Betula pubescens ssp. czerepanovii), whose pooled larval feeding periods form a gradient and cover the growing season. Insect phenology was manipulated so that the larvae of early-season species would feed on atypically mature leaves and mid- or late-season species would feed on atypically young leaves of their major host plant. Mortality increased dramatically for all species when the larval feeding schedule was advanced or delayed. This indicates a high degree of specialisation not only on a particular host but also to its phenological phases. The main cause of mortality on novel food was a rejection of the diet by the young larvae and their subsequent starvation. An interesting observation was that late-season species showed this response on nutritious young foliage. The effects of seasonally atypical diets on larval development and growth were species-specific and milder than the effects on mortality. Interestingly, for those individuals that accepted it, atypical food seemed to be most beneficial for species appearing at both ends of the seasonal gradient, which might be related to a wider exposure to variable food quality in natural conditions compared with other species. The diversity of responses to atypical food among closely related herbivore species with overlapping feeding periods on the same host plant is the most crucial finding of this study.  相似文献   

4.
Diet and growth of leaf-shredding caddisfly larvae, Pycnopsyche spp.,were examined in streams draining a reference catchment and a 16-year-oldclear-cut (disturbed) catchment at Coweeta Hydrologic Laboratory insouthwestern North Carolina, USA. The objective was to explain why shredderproduction is higher in the disturbed streams despite the larvae having lessfood (i.e., leaves) available. We predicted larvae would grow faster onfast-decaying leaf material representative of the disturbed streams. Larvaeconsumed mostly leaf detritus in three streams draining each catchment overthree seasons (fall, winter, and spring), which showed larvae did notconsume higher quality foods (e.g., algae and animal material) in disturbedstreams. When fed 2-month-old conditioned black birch (Betula lenta L.) (afast-decaying leaf species) and white oak (Quercus alba L.) (a slow-decayingleaf species) leaves in the laboratory, larvae grew significantly faster onthe birch leaves. However, when larvae were fed the same leaf types after3-months conditioning, larvae grew significantly faster on oak leaves. Afield growth experiment conducted for 42 d using mixed-species leaf dietsrepresentative of each catchment and initially conditioned for 2 monthsfound that Pycnopsyche grew significantly better on the diet representativeof the reference catchment. The reference diet contained more oak leaveswhich apparently became a more acceptable food as the experiment proceeded.High shredder production in the disturbed streams could not be explained byhigh Pycnopsyche growth rates on fast-decaying leaves. Instead, larvae grewbetter on leaves that were apparently conditioned optimally regardless ofconditioning rate.  相似文献   

5.
Abstract Genetic variance‐covariance structures (G), describing genetic constraints on microevolutionary changes of populations, have a central role in the current theories of life‐history evolution. However, the evolution of Gs in natural environments has been poorly documented. Resource quality and quantity for many animals and plants vary seasonally, which may shape genetic architectures of their life histories. In the mountain birch‐insect herbivore community, leaf quality of birch for insect herbivores declines profoundly during both leaf growth and senescence, but remains stable during midsummer. Using six sawfly species specialized on the mountain birch foliage, we tested the ways in which the seasonal variation in foliage quality of birch is related to the genetic architectures of larval development time and body size. In the species consuming mature birch leaves of stable quality, that is, without diet‐imposed time constraints for development time, long development led to high body mass. This was revealed by the strongly positive phenotypic and genetic correlations between the traits. In the species consuming growing or senescing leaves, on the other hand, the rapidly deteriorating leaf quality prevented the larvae from gaining high body mass after long development. In these species, the phenotypic and genetic correlations between development time and final mass were negative or zero. In the early‐summer species with strong selection for rapid development, genetic variation in development time was low. These results show that the intuitively obvious positive genetic relationship between development time and final body mass is a probable outcome only when the constraints for long development are relaxed. Our study provides the first example of a modification in guild‐wide patterns in the genetic architectures brought about by seasonal variation in resource quality.  相似文献   

6.
A non-native invasive sawfly, the amber-marked birch leaf miner Profenusa thomsoni (Konow), was first detected in south-central Alaska in 1996 and is now widely distributed throughout urban and wild birch trees in Alaska. Impacts have been considered primarily aesthetic because leaf miners cause leaves of birch trees (Betula spp.) to senesce prematurely, but the leaf miners likely also reduce birch vigour and thereby increase susceptibility to diseases and other insects. We tested the ability of commercially available biological control agents to control P. thomsoni. The entomopathogenic fungus Beauveria bassiana (Bals.-Criv.) Vuillemin GHA strain and the entomopathogenic nematode Steinernema carpocapsae (Weiser) were applied in aqueous suspension to the soil/litter surface beneath infested birch trees in Alaska at one site in 2007 and 2008 and two sites in 2010. There was no evidence the fungus or nematode controlled P. thomsoni. Instead, there was evidence the fungus increased the density of this pest insect at two sites, likely by reducing its predators. As tested, B. bassiana and S. carpocapsae do not appear effective as biological controls of P. thomsoni.  相似文献   

7.
We examined whether larvae of the gall midge Rabdophaga rigidae (Diptera: Cecidomyiidae) can modify the seasonal dynamics of the density of a leaf beetle, Plagiodera versicolora (Coleoptera: Chrysomelidae), by modifying the leaf flushing phenology of its host willow species, Salix serissaefolia and Salix eriocarpa (Salicaceae). To test this, we conducted field observations and a laboratory experiment. The field observations demonstrated that the leaf flushing phenology of the willows and the seasonal dynamics of the beetle density differed between shoots with stem galls and shoots without them. On galled shoots of both willow species, secondary shoot growth and secondary leaf production were promoted; consequently, leaf production showed a bimodal pattern and leaf production periods were 1 to 2 months longer than on non‐galled shoots. The adult beetle density on galled shoots was thus enhanced late in the season, and was found to change seasonally, synchronizing with the production of new leaves on the host willow species. From the results of our laboratory experiment, we attributed this synchrony between adult beetle density and willow leaf flush to beetles’ preference to eat new leaves rather than old. Indeed, beetles consumed five times more of the young leaves when they were fed both young and old leaves. These results indicate that stem galls indirectly enhance the adult beetle density by enhancing food quality and quantity late in the beetle‐feeding season. We therefore conclude that midge galls widen the phenological window for leaf beetles by extending the willows’ leaf flush periods.  相似文献   

8.
This study used experiments at several spatial scales to determine whether (1) intraspecific competition occurs among larvae of the leafmining sawfly Profenusa thomsoni (Konow) (Hymenoptera: Tenthredinidae) on birch (Betula spp.), (2) oviposition site preferences of P. thomsoni maximize offspring performance, and (3) early‐season damage by external folivores or the leafminer Fenusa pumila Leach (Hymenoptera: Tenthredinidae) affects oviposition preferences or larval performance of P. thomsoni. Larval P. thomsoni competed at natural densities; survival and weight of larvae were reduced under crowded conditions. Despite this, females of P. thomsoni tended to lay eggs on leaves already bearing eggs from other females and discriminated only weakly among leaves of different sizes on a branch. Both damage by F. pumila and artificial damage to leaves early in the season decreased survival of P. thomsoni larvae on the same branch, and ovipositing P. thomsoni females avoided damaged leaves but not other leaves on the same branch. In general, oviposition choices by P. thomsoni reduced larval survival. Possible reasons for the lack of a strong preference–performance relationship in P. thomsoni are discussed.  相似文献   

9.
The sawfly Rhadinoceraea nodicornis Konow (Hymenoptera: Tenthredinidae) is a member of a closely related group of species, the tribe Phymatocerini, which feed on the Liliales and Ranunculales. It is known to sequester steroid alkaloids from its host plants, species in the genus Veratrum (Liliales: Melanthiaceae), and to use them as a defence against predators. There are known chemical relationships between the hosts of R. nodicornis and hosts of related sawfly species. We tested whether the R. nodicornis larvae would accept hosts of closely- and more distantly-related sawflies, but found that they accepted only plant species in the genus Veratrum. This specificity was apparently innate, as it was independent of early larval experience. A feeding bioassay showed that the steroid alkaloids from Veratrum nigrum were phagostimulatory for R. nodicornis larvae, suggesting that they may be involved in host recognition. We discuss the possibility that the evolution of recognition of specific compounds may represent the mechanism of host radiation within the Phymatocerini.  相似文献   

10.
Abstract.  1. Population density of Epirrita autumnata (Lepidoptera: Geometridae) reaches outbreak densities regularly in northernmost Scandinavia. During these outbreak years, the most abundant host species, the mountain birch ( Betula pubescens ssp. czerepanovii ), is regularly exhausted, although larvae may rescue themselves from starvation by using alternative host species.
2. In this paper, the effects of the shift of host species on the immune defence and other life-history traits of E. autumnata were investigated, and possible consequences for population dynamics were briefly discussed. Moth larvae were reared on the leaves of the main host, mountain birch, until larvae reached their third instar. After this, larvae were allocated randomly to five treatments: larvae were either allowed to finish larval stage on the mountain birch or were shifted onto four alternative host species that are typical species for the area.
3. As expected, the host species had a major effect on fitness traits: body weight, development, and survival rate of the moths. The pupal weight was lower and development rates slower on the three alternative host species, Salix myrsinifolia Salisb., Vaccinium uliginosum L., and Betula nana L., than on the main host, mountain birch.
4. The immunity was, however, the same or better on the alternative hosts than on the main host. The immunity and pupal weights were negatively related, suggesting a trade-off between body size and immunocompetence.
5. The decreased body size and fecundity of E. autumnata during outbreak years may be partly due to the shift to alternative host species whereas the host-plant species probably does not affect markedly the rate of parasitism.  相似文献   

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