Preference and performance of a gall-inducing sawfly: plant vigor, sex, gall traits and phenology

Four hypotheses regarding the role of predation in the population dynamics of eruptive small mammal communities were tested using the small mammal assemblage found in mixed forests in New Zealand. Large-scale (750 ha) predator removal was conducted, targeting stoats ( Mustela erminea ). House mouse ( Mus musculus ) and ship rat ( Rattus rattus ) population dynamics during an eruption were compared in areas with and without predator reduction. The success of predator reduction was measured by comparing live-capture rates of predators on treatment and non-treatment areas, and by recruitment rates of the threatened northern brown kiwi ( Apteryx australis mantelli ). Overall, predator reduction was successful, although there was a continual low rate of reinvasion. The predictions and results were that 1) Predators can slow but not prevent a population eruption. Supported: Populations of mice and rats erupted to high densities in areas with and without predator reduction, following synchronous southern beech ( Nothofagus spp.) seeding. 2) Predators cannot truncate peak prey population size. Supported: Peak densities of mice and rats were not significantly different between treatment and non-treatment areas. 3) Predators can hasten the rate of decline in prey populations during the crash phase. Not supported: There was evidence of populations of mice and rats declining slower in areas with predators removed, but none of the trends were significant. 4) Predators can limit low-phase prey populations. Equivocal: Populations of rats in beech forest, and population of mice and rats in coastline habitats were significantly higher in areas with predators removed, but were not significantly different in tawa-podocarp forest. Therefore, the role of food in driving the early stages of the mouse and rat eruption was demonstrated, but the role of predation in the decline and low phases is unclear.     

Hormaphis hamamelidis and gall size: a test of the plant vigor hypothesis

We tested the Plant Vigor Hypothesis by determining the distribution of galls formed on leaves of witch hazel, Hamamelis virginiana , by the aphid Hormaphis hamamelidis , and by determining various factors that affect the fecundity of the gall-forming fundatrices. We also studied the role of the fundatrix in host plant manipulation. While the mean number of galls per leaf was low, galls had an aggregated distribution among leaves. Among trees, the average number of galls per leaf was not related to the mean leaf size, contrary to the preference prediction of the Plant Vigor Hypothesis. While fundatrices preferred the distal leaves of buds, which grew more than the proximal leaves, being on distal leaves conferred no increase in fecundity for fundatrices, contrary to predictions of the Plant Vigor Hypothesis. Gall size was the factor that explained the largest proportion of variation in fundatrix fecundity; fundatrix size explained somewhat less of the variation. Also, gall position on the leaf, number of aphid galls on the leaf, and on which leaf of the bud the gall was located all played small, statistically significant roles in explaining fundatrix fecundity, but their effects were variable between experiments. Removal of fundatrices shortly after galls had enclosed them limited the growth of galls, indicating the role of the fundatrices in gall growth. We compare and contrast this system versus other gall-forming insects, as well as discuss the adaptive significance of the aphid manipulation of the host plant. Much of the data contradict predictions of the Plant Vigor Hypothesis, and we discuss how gall size, as a measure of plant growth caused by insect manipulation, explains the observed patterns.     

Testing a new hypothesis: plant vigor and phylloxera distribution on wild grape in Arizona

Summary Longer, meaning more vigorous, shoots of a wild grape clone (Vitis arizonica) were more susceptible to attack by second and third generations of leaf-galling grape phylloxera,Daktulopsphaira vitifoliae, as the growing season progressed. Although there was no significant difference in mean shoot length between attacked and unattacked shoots within a clone at the beginning of shoot elongation, attacked shoots were significantly longer than unattacked shoots when elongation had ceased (P<0.01). Also, long attacked shoots had a significantly greater population of phylloxera galls than short attacked shoots (P<0.01) as the season progressed. The phylloxera population on long shoots increased rapidly while the population on short shoots remained the same. Longer shoots also produced significantly more axillary shoots than shorter shoots as the season progressed (P<0.001), and the number of axillary shoots accounted for 66 percent of the variance in number of attacked leaves on a shoot. Experimental evidence showed that there was a significantly greater percentage of available leaves attacked on long shoots than on short shoots (P<0.05) and the leaves on long shoots generally had a greater number of galls per leaf. The relationship between shoot length and probability of attack was also tested by comparing shoots lengths of 10 attacked clones and 10 unattacked clones at a second location. Mean shoot lengths of attacked clones were significantly longer than mean shoot lengths of unattacked clones (P<0.05), and mean shoot lengths of attacked shoots within a clone were significantly longer than unattacked shoots (P<0.001). Longer shoot length accounted for 81 percent of the variance in probability of attack. The reason for this pattern of attack was that long shoots produced newly expanding leaves over a longer time during the growing season and multivoltine phylloxera require undifferentiated tissue to initiate gall formation. Patterns of attack within a shoot were characterized by an uneven distribution of galls among leaves. This was due to development time between generations and the current availability of undifferentiated tissue at times of colonization. This study supports the hypothesis that some herbivore species are favored more by vigorous plants than by stressed plants.     

Size does matter: variation in herbivory between and within plants and the plant vigor hypothesis

The plant stress and plant vigor hypotheses (PVH) are two of the most widely recognized hypothesis invoked to explain differential distribution of insect herbivores among their host plants. In both cases, the emphasis is on bottom–up processes (i.e. host-plant quality), but a recent meta-analytical review of the literature has shown that the plant stress hypothesis might have limited support among insect herbivores. In this study, we conducted a meta-analysis of the effects of plant vigor on insect herbivore abundance and survivorship by reviewing 71 published articles that explicitly tested the PVH and enabled 161 independent comparisons. Z-transform was used as the metric to standardize the results of all independent comparisons. Our quantitative results have shown that Hymenoptera (sawflies) was the most abundant group in the reviewed studies, representing 28.1% of the independent comparisons, followed by Diptera (25.1%) and Homoptera (22.6%). Amongst all the guilds studied, gall-formers were the most representative group (68.0%), whereas leaf-miners and stem-borers were underrepresented (less than 4.0% of the available comparisons). Insect herbivores were significantly more abundant on more vigorous plants (E++=0.6432, CI=0.7558–0.7280), but no significant effect was detected on herbivore survivorship. When herbivores were categorized into feeding guilds, effects of plant vigor on herbivore abundance were stronger for sap-suckers, leaf-miners and gall-formers. Our results have shown a strong herbivore preference for more vigorous plants, although our results do not support a preference–performance linkage.     

Preference and performance linkage in aPhyllocolpa sawfly on the willow,Salix miyabeana,on Hokkaido

A leaf-folding sawfly in the genusPhyllocolpa (Hymenoptera: Tenthredinidae) attackingSalix miyabeana (Salicaceae) was studied near Sapporo, Hokkaido, along the Ishikari River in 1993. Host plant individuals were young trees 4–7 years old which were growing rapidly, producing some long shoots with large leaves. On a gradient of shoot length classes from 0–5 cm long to over 80 cm long, shoots were much more abundant in the shorter shoot length classes. However, attacks by ovipositing females increased as shoot length increase from 0 attacks on the shortest shoots to 5.17 attacks per shoot on the longest shoots. The frequency of attack per leaf increased from 0 to 0.13 over the same range of shoot lengths. This pattern of attack resulted in a high frequency of larval establishment in feeding sites, between 0.96 and 1.00, in all attacked shoot length categories. However, probability of survival to a late instar larva increased with shoot length and corresponded to the attack pattern, indicating a preference-performance linkage between female ovipositional decisions and larval survival. The patterns found for thisPhyllocolpa species are similar for galling sawflies in North America and Europe, especially in the genusEuura, members of which make stem, bud and leaf midrib galls. Extending the pattern to aPhyllocolpa species broadens identification of pattern and ultimately the generality of the emerging theory on populations of galling sawflies.     

Soil calcium and plant disease in serpentine ecosystems: a test of the pathogen refuge hypothesis

Ecologists have long sought mechanistic explanations for the patterns of plant distribution and endemism associated with serpentine soils. We conducted the first empirical test of the serpentine pathogen refuge hypothesis, which posits that the low levels of calcium found in serpentine soils provide associated plants with a refuge from attack by pathogens. We measured the range of soil calcium concentrations experienced by 16 wild population of California dwarf flax (Hesperolinon californicum) and experimentally recreated part of this range in the greenhouse by soaking serpentine soils in calcium chloride solutions of varying molarity. When flax plants grown in these soils were inoculated with spores of the rust fungus Melampsora lini we found a significant negative relationship between infection rates and soil calcium concentrations. This result refutes the pathogen refuge hypothesis and suggests that serpentine plants, by virtue of their association with low calcium soils, may be highly vulnerable to attack by pathogens. This interaction between plant nutrition and disease may in part explain demographic patterns associated with serpentine plant populations and suggests scenarios for the evolution of life history traits and the distribution of genetic resistance to infection in serpentine plant communities.     

Preference and performance of the sawfly Diprion pini on host and non-host plants of the genus Pinus

The sawfly, Diprion pini L., is a pest of Pinus in Europe and is mainly found on P. sylvestris L. and P. nigra laricio Poiret. The relative importance of female oviposition capacity and behaviour, egg development, and larval survival on a new host plant was measured on 11 pine species. Five were natural host plants and six non-host plants, five of which are not indigenous to Europe. Oviposition choice tests showed that females discriminated between the pine species. Egg and larval development also differed between pine species. However, the female choice was not linked with hatching rate and larval development. Results of biological tests clearly indicated that there were different response patterns of D. pini life stages in relation to pine species, and these patterns were the same with insects of four different origins. We discuss the importance of each potential barrier to colonisation of a new host.     

Host stress influences oviposition preference and performance of a pine sawfly

Abstract.

• 1 The effect of ponderosa pine (Pinus ponderosa Doug]. ex Laws) water stress, measured by xylem water potential, on oviposition preference and egg development of Neodiprion fulviceps was determined by two laboratory experiments in 1987 and 1988.
• 2 In the first experiment, adult females were allowed to choose between foliage from water-stressed, watered, and untreated control trees.
• 3 Significant differences were found between treatments in xylem water potential in 1987 but not in 1988.
• 4 Females chose water-stressed trees and a positive correlation was found between female preference and xylem water potential in 1987 but not in 1988.
• 5 In the second experiment, females were placed in globe cages with a single branch from each treatment and allowed to oviposit.
• 6 Significant differences were found among treatments in egg survival in 1987 but not in 1988.
• 7 These results indicate that sawflies can distinguish between stressed and non-stressed trees and that this behaviour is adaptive in that egg survival was greatest on the preferred treatment.

     

The role of female search behaviour in determining host plant range in plant feeding insects: a test of the information processing hypothesis

Recent theoretical studies have suggested that host range in herbivorous insects may be more restricted by constraints on information processing on the ovipositing females than by trade-offs in larval feeding efficiency. We have investigated if females from polyphagous species have to pay for their ability to localize and evaluate plants from different species with a lower ability to discriminate between conspecific host plants with differences in quality. Females of the monophagous butterflies Polygonia satyrus, Vanessa indica and Inachis io and the polyphagous P. c-album and Cynthia cardui (all in Lepidoptera, Nymphalidae) were given a simultaneous choice of stinging nettles (Urtica dioica) of different quality. In addition, the same choice trial was given to females from two populations of P. c-album with different degrees of specificity. As predicted from the information processing hypothesis, all specialists discriminated significantly against the bad quality nettle, whereas the generalists laid an equal amount of eggs on both types of nettle. There were no corresponding differences between specialist and generalist larvae in their ability to utilize poor quality leaves. Our study therefore suggests that female host-searching behaviour plays an important role in determining host plant range.     

The size-grain hypothesis: a phylogenetic and field test

Abstract.  1. The size-grain hypothesis predicts that environmental rugosity results in positive allometric scaling of leg length on body length because of changes in locomotion costs.
2. The scaling of leg length and body length in ants was re-examined using phylogenetic independent contrast methods, and the allometric relationship found by Kaspari and Weiser ( Functional Ecology , 13 , 530–538, 1999) was supported.
3. The size-grain hypothesis was tested further by comparing the body sizes of ants from areas of contrasting habitat complexity in two different savanna habitats. No support for the size-grain hypothesis was found. Small body size classes were no more speciose in the rugose than in the more planar environment, and small ants were more abundant in the planar environment.     

Pine sawfly defence and variation in host plant resin acids: a trade-off with growth

Abstract.

• 1 The defence capability of pine sawfly larvae reared on Scots pines with low or high concentrations of resin acids was determined in a field experiment, where larvae were exposed to predatory ants, and in a laboratory experiment in which the rate of defence droplet replenishment was measured.
• 2 Larvae on low resin-acid shoots disappeared faster than larvae on high resin-acid shoots when exposed to predatory ants.
• 3 Larvae fed high resin-acid needles produced 50% larger defence droplets than larvae fed low resin-acid needles.
• 4 In an additional experiment high predation pressure was simulated by repeatedly removing defence droplets. Larvae responded by feeding more on tissue rich in resin acids, but at the cost of a reduction in growth rate.
• 5 Based on these data and earlier findings that survival and development of sawfly larvae are negatively affected by high resin-acid concentrations, we conclude that pine sawflies face a dilemma of conflicting demands, i.e. although high needle resin-acid concentrations enhance defence capability, they also reduce growth and survival. Our data also suggest that it is not necessarily the ovipositing female that has to balance this trade-off: plasticity in larval feeding behaviour provides larvae with a means to at least partly solve the dilemma.

     

The phylogeographical history of the Iberian steppe plant Ferula loscosii (Apiaceae): a test of the abundant-centre hypothesis

The geology and climate of the western Mediterranean area were strongly modified during the Late Tertiary and the Quaternary. These geological and climatic events are thought to have induced changes in the population histories of plants in the Iberian Peninsula. However, fine-scale genetic spatial architecture across western Mediterranean steppe plant refugia has rarely been investigated. A population genetic analysis of amplified fragment length polymorphism variation was conducted on present-day, relict populations of Ferula loscosii (Apiaceae). This species exhibits high individual/population numbers in the middle Ebro river valley and, according to the hypothesis of an abundant-centre distribution, these northern populations might represent a long-standing/ancestral distribution centre. However, our results suggest that the decimated southern and central Iberian populations are more variable and structured than the northeastern ones, representing the likely vestiges of an ancestral distribution centre of the species. Phylogeographical analysis suggests that F. loscosii likely originated in southern Spain and then migrated towards the central and northeastern ranges, further supporting a Late Miocene southern-bound Mediterranean migratory way for its oriental steppe ancestors. In addition, different glacial-induced conditions affected the southern and northern steppe Iberian refugia during the Quaternary. The contrasting genetic homogeneity of the Ebro valley range populations compared to the southern Iberian ones possibly reflects more severe bottlenecks and subsequent genetic drift experienced by populations of the northern Iberia refugium during the Pleistocene, followed by successful postglacial expansion from only a few founder plants.     

Effect of forest stand type on host plant quality and direct and indirect effects on pine sawfly performance

1. Observed lower levels of herbivory in mixed compared with monoculture stands have been hypothesized to depend on top-down forces, through higher predation pressure by natural enemies or through bottom-up mechanisms through plant quality effects on herbivore performance.
2. In this study, we compared the performance measured as host plant induced mortality, cocoon weight, and predation mortality of the European pine sawfly Neodiprion sertifer (Geoffroy) (Hymenoptera, Diprionidae) in mixed and monoculture forest stands.
3. We did not observe a difference in host plant induced mortality, cocoon weight, or predation mortality between mixed and monoculture forest stands. We did find an effect of local conditions around each experimental tree on pine sawfly performance. For example, the nitrogen content of pine needles is negatively affected by the proportion of pine around the experimental tree, which in turn increases the survival of sawfly larvae.
4. The results suggest that local conditions around individual trees are more important for the performance of the European pine sawfly than stand type, i.e. mixed or monoculture plant stands.
5. We conclude that the ongoing trend for diversification within commercial forestry calls for more research where the effects of both bottom-up and top-down effects are studied at several spatial scales.

     

Resource polyphenism increases species richness: a test of the hypothesis

A major goal of evolutionary biology is to identify the causes of diversification and to ascertain why some evolutionary lineages are especially diverse. Evolutionary biologists have long speculated that polyphenism—where a single genome produces alternative phenotypes in response to different environmental stimuli—facilitates speciation, especially when these alternative phenotypes differ in resource or habitat use, i.e. resource polyphenism. Here, we present a series of replicated sister-group comparisons showing that fishes and amphibian clades in which resource polyphenism has evolved are more species rich, and have broader geographical ranges, than closely related clades lacking resource polyphenism. Resource polyphenism may promote diversification by facilitating each of the different stages of the speciation process (isolation, divergence, reproductive isolation) and/or by reducing a lineage''s risk of extinction. Generally, resource polyphenism may play a key role in fostering diversity, and species in which resource polyphenism has evolved may be predisposed to diversify.     

Localized outbreak of a willow leaf beetle: plant vigor or natural enemies?

The possible roles of plant quality (vigor) and natural enemies in the development of a localized out-break of the leaf beetle Galerucella lineola (Coleoptera: Chrysomelidae) in a stand of Salix cinerea were investigated. Caged and uncaged larvae on six bushes in the outbreak area were compared with caged and uncaged larvae on six bushes in an adjacent nonoutbreak area in terms of performance. In 1997, when the studies were performed, the natural density of the insect (beetles plus eggs) was six times higher in the outbreak area compared with the nonoutbreak area. Even though the vigor (measured as shoot length) of bushes in the outbreak was 72% higher than that of bushes in the nonoutbreak area, we found no difference between areas in the performance (survival, developmental time, pupal weight) of caged larvae or in the willingness of caged females to lay eggs. Among larvae exposed to natural enemies, the disappearance rate was significantly higher in the nonoutbreak area. The density of generalist predators was significantly higher in the nonoutbreak than in the outbreak area. We conclude that differences in plant quality, despite the observed difference in plant vigor, could not explain the observed difference in beetle density between areas. Lower predation pressure in the outbreak area could, however, not be excluded as a possible reason for the higher density of leaf beetles in this area. Received: October 18, 1999 / Accepted: February 4, 2000     

Defection by plants in the yucca–yucca moth association: a test of the cheater plant hypothesis for Yucca treculeana

In mutualisms, an underlying conflict of interests may select for defection from providing benefits. In the obligate mutualism between yuccas and yucca moths, where pollination service and seeds for pollinator larvae are traded, it has been suggested that some individuals in a population of Y. baccata may defect by preventing pollinator egg or larvae from development. We tested this hypothesis in Y. treculeana , another species suggested to contain cheater plants. Five specific predictions were tested during two years of study. A prediction that a surplus of plants without pollinator larvae should be present was met. Predicted existence of two distinct fruit morphs was rejected, and none of several highly variable morphological traits were linked to presence/absence of larvae. Predicted excess of intact seeds in the fruits of plants without larvae was not found; in fact, such plants produced fewer seeds, contrary to the hypothesis. A suggestion that inverse frequency-dependent fitness could explain the pattern was rejected. Contrary to prediction, distribution of larvae of a closely related cheater yucca moth was positively associated with pollinator larvae, even though it would not be affected by the proposed killing mechanism. The results together provide strong support against the existence of cheater plants in Y. treculeana .     

Post-dispersal seed anchorage to soil in semiarid plant communities, a test of the hypothesis of Ellner and Shmida

In this paper we test the Ellner and Shmida’s hypothesis that in semiarid environments aridity may select for the lack of seed dispersal mechanisms (atelechory) of many plants, whereas post-dispersal selective forces such as soil erosion, seed predation or limitations to water uptake by seeds may select for structures facilitating seed anchorage to the ground (antitelechory). We analyzed the proportions of species with seed anchorage mechanisms and that of atelechoric species in shrublands colonizing flat areas and hillslopes in two sites differing in climate dryness. Their relation with several soil properties involved in runoff generation, seed–soil contact and water uptake by seeds and with nest density of granivorous ants was also explored. Our results support the hypothesis that in semiarid shrublands the proportion of species with seed anchorage mechanisms increases because of soil erosion but not because of climate dryness. This is the first time that a direct relation between the proportion of species with seed anchorage mechanisms and soil erosion is shown in plant communities; supporting the view that soil erosion shapes species composition in communities.