Lack of evidence for local adaptation to individual plant clones or site by a mobile specialist herbivore

Several studies have documented local adaptation by sedentary insects to individual phenotypes of their host plants. Here, I examined whether a similar phenomenon could be found in a mobile, specialized insect, the sumac flea beetle. Previous work has shown that sumac individuals differ in their suitability as hosts for these beetles and that differences have both an environmental and a genetic basis. Using beetle populations collected as eggs from eight different sumac clones along an east-west transect, a reciprocal transfer experiment was conducted to determine whether there was any evidence for local adaptation by beetles to individual plant clones or to site. Variables examined were larval survivorship past first instar, development time, weight at pupation and patterns of predation by enemies. While no evidence for local adaptation was found, there were significant effects of plant clone on which larvae developed, origin of the larval population and the interaction of these effects on larval performance. For larval weight at pupation, there was also some indication that trade-offs may exist in ability of larvae to use different host plant clones. In addition, there were significant environmental effects on several measures of larval performance. Predation rates differed by plant clone, but not by site or with respect to origin of larvae. While no evidence for local adaptation was found in this study, prerequisites for finding such patterns may exist in this system. Received: 23 May 1996 / Accepted: 26 September 1996     

Costs and consequences of superparasitism in the polyembryonic parasitoid Copidosoma koehleri (Hymenoptera: Encyrtidae)

Abstract.  1. Polyembryonic wasps provide dramatic examples of intra-specific developmental conflict. In these parasitoids, each egg proliferates into a clonal lineage of genetically identical larvae. If more than one egg is laid in a host (superparasitism), individuals of different clones may compete for food resources.
2. In the polyembryonic encyrtid Copidosoma koehleri , one larva per clone can differentiate into a sterile soldier. It is shown that soldiers are always females, and that they attack intra-specific competitors.
3. Research hypotheses were that (a) clones that develop in superparasitised hosts suffer heavier mortality than clones that develop in singly parasitised hosts, and (b) female clones cause higher mortality to their competitors than male clones, hence larval survival is lower in superparasitised hosts that contain females than in male-only broods.
4. The potential frequency of superparasitism in C. koehleri was manipulated by varying parasitoid–host ratios and exposure durations.
5. As parasitoid densities and exposure durations increased, the frequency of superparasitism rose, brood sizes increased, but the number of hosts that completed development was reduced. The number of offspring per parasitoid female decreased with increasing parasitoid–host ratios. Offspring size and longevity were inversely correlated with brood size. As superparasitism rates increased, fewer all-male broods were produced. Male–female broods were female-biased, suggesting selective killing of males by female soldiers. All-female broods were significantly smaller than all-male broods at high parasitoid densities only, possibly reflecting aggression among soldiers of competing clones.
6. The results support the working hypotheses, and suggest that female larvae outcompete males in superparasitised hosts.     

Variation to cause host injury between Russian wheat aphid (Homoptera: Aphididae) clones virulent to Dn4 wheat

Biotypes are infraspecific classifications based on biological rather than morphological characteristics. Cereal aphids are managed primarily by host plant resistance, and they often develop biotypes that injure or kill previously resistant plants. Although molecular genetic variation within aphid biotypes has been well documented, little is known about phenotypic variation, especially virulence or the biotype's ability to cause injury to cultivars with specific resistance genes. Five clones (single maternal lineages) of Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae), determined to be injurious to wheat, Triticum aestivum L., with the Dn4 gene, were evaluated on resistant and susceptible wheat and barley, Hordeum vulgare L., for their ability to cause chlorosis, reduction in plant height, and reduction in shoot dry weight. Variation to cause injury on resistant 'Halt' wheat, susceptible 'Jagger' wheat, and resistant 'STARS-9301B' barley was found among the Dn4 virulent clones. One clone caused up to 30.0 and 59.5% more reduction in plant height and shoot dry weight, respectively, on resistant Halt than other clones. It also caused up to 29.9 and 55.5% more reduction in plant height and shoot dry weight, respectively, on susceptible Jagger wheat. Although STARS-9301B barley exhibited an equal resistant response to feeding by all five clones based on chlorosis, two clones caused approximately 20% more reduction in plant height and shoot dry weight than three other clones. The most injurious clones on wheat were not the most injurious clones on barley. This is the first report of variation to cause varying degrees of plant damage within an aphid biotype virulent to a single host resistance gene. A single aphid clone may not accurately represent the true virulent nature of a biotype population in the field.     

Constraints on growth and allocation patterns of Silphium integrifolium (Asteraceae) caused by a cynipid gall wasp

Summary Life-history traits of 101 clones from two populations of Daphnia magna were measured under controlled environmental conditions in the laboratory. Some individuals had four juvenile instars, others had five. This depended on their length at birth and on the population they came from. Females in the group with five juvenile instars were smaller at birth but larger and older at maturity than those with four juvenile instars. Within groups of females with equal numbers of preadult instars (instar groups) age and size at maturity increased with size at birth. This relationship differed significantly among instar groups for both age and size at maturity. Significant differences in age and size at maturity between two populations became non-significant when size at birth was used as a covariable in AN-COVA. Within populations, size at birth depended on the clone and on the parity of the clutch. First-clutch offspring were considerably smaller than those from later clutches. The results suggest that variability in life-history traits is common within and between clones, but that most of this variation can be accounted for by size at birth and the number of pre-adult instars.     

Bottom-up cascading effects in a tritrophic system: interactions between plant quality and host-parasitoid immune responses

Abstract.  1. Little is known about underlying mechanisms by which plants indirectly affect parasitism success in hymenopteran endoparasitoids. The hypothesis that host-plant effects can challenge the innate immune system of an insect host was experimentally tested in this study using a model tritrophic, crucifer – lepidopteran [ Plutella xylostella (L.)] – parasitoid [ Cotesia plutellae (Kurdjumov)], system.
2. The effects of host-plant suitability on herbivore performance and parasitism were examined. The bottom-up effect of plant suitability on host-parasitoid immune responses was then evaluated using measures of cellular and humoral effectors.
3. Host-plant quality showed a significant effect on the encapsulation response of P. xylostella to first instar but not to second instar parasitoid larvae. Encapsulation was never sufficient to prevent parasitoid emergence.
4. Poor host-plant suitability suppressed phenoloxidase activity in the absence of the parasitoid. The suppressive effect of C. plutellae on phenoloxidase activity was much greater and no plant effects were detectable after insects had been parasitized.
5. Despite strong plant effects on parasitism, those on immune effectors of the host were transitory or overwhelmed by the effect of the parasitoid.
6. These results demonstrated that plant-mediated variation in parasitism success by C. plutellae were not as a result of plant nutritional status or other attributes affecting the immune function of P. xylostella , nor to host-plant effects on superparasitism.
7. In these experiments, P. xylostella was a fully permissive host to C. plutellae and host-plant-mediated effects on the innate immune response appeared to play no part in parasitoid survival within hosts.     

Phenotypic plasticity and constancy of life-history traits in laboratory clones of Daphnia magna Straus: effects of neonatal length

1. The phenotypic constancy of four laboratory Daphnia magna clones in fitness-related life-history traits, such as age and clutch size at maturity, was studied among consecutive experimental runs in differing food environments.
2. A significant part of the observed clonal and genetic-by-environmental variation in age and clutch size at maturity was explained by experimentally uncontrollable variations in neonatal body length.
3. Despite food availability, neonatal length determined the number of instars invested to maturity and thus maturation age. Clonal differences in neonatal length and thus in maturation instar occurrence across environments explained most of the clonal variability observed in maturation age.
4. Although interclonal differences in clutch size existed, most of the phenotypic plasticity observed for clutch size was mediated by clonal differences in neonatal length.
5. Clonal differences in neonatal length and in the occurrence of maturation instars across environments dramatically affected the body length of instar IM-2 where provisioning of eggs take place. Since clutch size is determined from clutch mass and clutch mass was strongly related to the body length of instar IM-2, clonal differences across environments in body length of instar IM-2 mirrored clonal differences across environments in clutch size.
6. The results reported in the present study show that maternally mediated traits such as neonatal length affect how genotypes respond to different environmental selection regimes (genetic-by-environmental interaction). Future research needs to focus on the effects of neonatal length on the heritability or genetic variation of the reaction norms, since prediction of the response to selection is a key research objective in quantitative genetic studies.     

Gamete production and sexual size dimorphism in an insect (Orchesella cincta) with indeterminate growth

Abstract 1. The relationship of growth and body size with reproductive effort in animal species has been studied much less for males than for females. This imbalance applies to Orchesella cincta (L.) (Collembola), an insect with indeterminate growth, in which egg production is related positively to body size and negatively to growth.
2. To allow a comparison of the reproductive effort of male and female O. cincta , development and growth in immature stages of both sexes, and growth and spermatophore production for adult males were studied.
3. Embryonic development time and hatchling size did not differ between prospective males and females, but from hatching on the trajectories diverged, with males growing more slowly and maturing earlier and at a much smaller body size than females.
4. Neither the number of spermatophores deposited in the first adult instar (= inter-moult period) nor the total number of spermatophores deposited during seven instars was related to body size or growth.
5. Differences in growth rate between instars with and without spermatophore deposition indicated that the physiology of spermatophore production inhibits growth, which, however, was compensated for during the next instar.
6. The difference in the relationship of gamete production with body size and growth between males and females explains the divergence of their size at maturity.     

Asymmetric intraspecific competition among larvae of the damselfly Ischnura elegans (Zygoptera: Coenagrionidae)

Abstract. 1. A laboratory competition experiment is described in which the growth and development rates of larvae of the damselfly Ischnura elegans (Lind.) were measured over an entire instar.
2. Two larval instars which commonly occur together in the field were used in the experiment; they were maintained with a superabundance of prey and either larvae from the same or the larger/smaller instar.
3. Small larvae suffered increased development times and decreased size increases at the moult in the presence of large larvae but similar interference effects were not evident when these smaller larvae were in the presence of other small larvae.
4. Development time and size increases of large larvae were not significantly affected by the presence of small larvae.
5. Irrespective of the instar combinations investigated, interference effects were reduced when there were more perches available, although in only a few cases was this reduction significant.
6. The consequences of the asymmetric competition reported in the experiment for the study of lifetime reproductive success in damselflies are discussed. Late emerging adults may incur reduced reproductive success.     

Clonal Analysis of Dominant Female-Sterile, Germline-Dependent Mutations in DROSOPHILA MELANOGASTER

Three allelic, dominant and germline-dependent female-sterile mutations (ovo(D) mutations) can be classified according to the severity of the ovarian abnormalities that they produce. The size and frequency of +/+ germline clones, induced in ovo(D)/+ females, were compared with K10/K10 germline clones induced in K10/+ control females. The frequency of germline clones induced by irradiation of first instar larvae is similar for the three dominant alleles and K10 ; however, the clone size increased with the strength of the allele tested, compared with K10 clones. When clones were induced later in development, the clone frequencies decreased with the strength of the alleles. These results are discussed in the context of the antimorphic nature of these mutations and the characteristics of germline development. The use of these alleles as tools in the genetic analysis of development is discussed.     

Sources of variation in the interaction between three cereal aphids (Hemiptera: Aphididae) and wheat (Poaceae)

The relative contributions of host plant, herbivore species and clone to variation in the interaction between cereal aphids and wheat were investigated using five clones each of three species, Rhopalosiphum padi (Linnaeus), Sitobion avenae (Fabricius) and Schizaphis graminum (Rondani), on seedlings of two cultivars of Triticum aestivum L. and one cultivar of Triticum durum Desf. More individuals and biomass of R. padi than of the other two species were produced on seedlings. The three wheat cultivars lost similar amounts of biomass as a result of infestation by aphids, with the amount lost depending on aphid species: S. avenae caused the lowest loss in biomass. Variation in aphid biomass production was due mostly to differences among aphid species (70%), less to the interaction between wheat type and aphid species (7%), and least to aphid clone (1%). The specific impact of the aphids on the plants ranged from 1.7 to 3.7 mg of plant biomass lost per mg of aphid biomass gained, being lowest for R. padi and highest for S. graminum. Variation in plant biomass lost to herbivory was due mostly to unknown sources of error (95%), probably phenotypic differences among individual seedlings, with 3% due to aphid species and none attributable to aphid clone. For these aphid-plant interactions, differences among aphid clones within species contributed little to variation in aphid and plant productivity; therefore, a small sample of clones was adequate for quantifying the interactions. Furthermore, one clone of each species maintained in the laboratory for about 200 parthenogenetic generations was indistinguishable from clones collected recently from the field.     

The effects of host-plant properties on gall density,gall weight and clone size in the aphidAploneura lentisci (Pass.) (Aphididae: Fordinae) in Israel

Summary Samples of shoots ofPistacia lentiscus carrying galls of the aphid,Aploneura lentisci, were collected at three localities in Israel. Shoots growing near pruning scars carried more galls than elsewhere on the plant, but these galls weighed less and contained fewer aphids (smaller clones). The proportion of empty galls increased with gall density. Crowding of galls at such sites may be due to the early burst of buds at the time of aphid emergence from the overwintering eggs, and not to active search for preferred sites. Shoots bearing larger numbers of leaves carried heavier galls, which contained larger aphid clones. The position of the galled leaf on the shoot had no effect on gall weight nor on clone size. The physiological condition of the plant may be an important environmental (ecological) factor affecting the variation in clone-size and in aphid morphology among galls.     

Phenotypic plasticity of Daphnia life history traits: the roles of predation, food level and toxic cyanobacteria

1. We studied the life history responses of Daphnia pulex under different biotic conditions. In a factorially designed experiment, we tested the impacts of water conditioned by the invertebrate predator Chaoborus, low and high food level (10 000 and 100 000 Scenedesmus cells ml^–1), and exposure to toxic Microcystis (5000 cells ml^–1) on twelve D. pulex clones originating from different habitats. Our aim was to compare the phenotypic plasticity of different clones, and to study the interactions among biotic factors.
2. Individuals cultured in Chaoborus -conditioned water started to reproduce at a larger size than individuals cultured in water not conditioned by the predators. We found interactions between food level and Chaoborus -conditioned water on age at first reproduction and total offspring number. In addition, the impact of the Chaoborus treatment on the size at first reproduction was reduced by Microcystis exposure.
3. Clonal differences were found in the degree of phenotypic plasticity of different life history traits. However, there was no obvious connection between the original habitat of the clone and the degree of plasticity.     

Effects of temperature elevation on the population dynamics of the upland heather psyllid Strophingia ericae (Curtis) (Homoptera: Psylloidea)

1 Cloches permeable to gas and water were used to simulate the effect of climate warming on upland populations of the heather psyllid Strophingia ericae at Moor House National Nature Reserve in the North Pennines, UK.
2 The cloches produced an average warming of about 1 °C in the heather canopy over a period of one year.
3 The density of S. ericae increased markedly in cloches within a few months of erection.
4 Species composition and numbers of potential predators were similar inside cloches and in control plots.
5 In the two year life-cycle of S. ericae , the warming effect advanced the phenology from the second to third instar in the first winter, but in the second winter, fifth instar nymphs did not moult prematurely to adult.
6 The density of S. ericae was higher on Calluna vulgaris at its boundary with Juncus squarrosus than in the pure C. vulgaris sward (in both cloched and control plots).     

Competition and dispersal in the pea aphid: clonal variation and correlations across traits

Abstract.  1. The presence of an across-species trade-off between dispersal ability and competitive ability has been proposed as a mechanism that facilitates coexistence. It is not clear if a similar trade-off exists within species. Such a trade-off would constrain the evolution of either trait and, given appropriate selection pressures, promote local adaptation in these traits.
2. This study found substantial levels of heritable variation in competitive ability of the pea aphid, Acyrthosiphon pisum Harris (Homoptera: Aphididae), measured in terms of relative survival when reared with a single clone of the vetch aphid, Megoura viciae Buckton (Homoptera: Aphididae).
3. Pea aphids can move to new patches by either flying (longer distance dispersal) or walking (local dispersal) from plant to plant. There was considerable clonal variation in dispersal ability, measured in terms of the proportion of winged offspring produced, and ability to survive away from their host plant.
4. Winged individuals showed longer off-plant survival times than wingless forms of the same pea aphid clone.
5. There was no evidence of a relationship between clonal competitive ability and either measure of dispersal ability, although the power of the test is limited by the number of pea aphid clones used in the trial.
6. However, there was a positive correlation between clonal fecundity and the proportion of winged offspring produced. Although speculative, it is suggested that clones that are more likely to either overwhelm their host plant or attract higher numbers of natural enemies as a result of having higher fecundity are more likely to produce winged morphs.     

The estimation of crop evacuation rates in Chaoborus larvae (Diptera: Chaoboridae) using natural prey

SUMMARY. 1. Crop evacuation rates were estimated for the first time in Chaoborus larvae, using natural prey.
2. Fourth instar C. americanus Matheson digested copepods (Diaptomus leptopus S.A. Forbes) 48.7% faster than daphnids (Daphnia rosea Leydig) of similar size; meal size did not significantly affect the instantaneous rate of digestion (IRD) within each prey type. Prey specific IRD has not been reported before for zooplankton.
3. Prey specific differences in IRD require the use of natural prey when digestion experiments are to be used to estimate natural rates of food consumption of animals.     

Understanding gregariousness in a larval Lepidopteran: the roles of host plant, predation, and microclimate

Abstract.  1. Many moth and butterfly larvae are gregarious early in development, but become solitary in late instars. This ontogenetic variation in behaviour is probably the result of temporal changes in the costs and benefits associated with gregariousness. This study provides observational and experimental evidence that, in one particular moth species, a series of different ecological factors influence larval behaviour at different times during development.
2. Field observations show that young caterpillars of the limocodid Doratifera casta form large aggregations while foraging, but that mature larvae are largely solitary.
3. A field experiment revealed that individual first to third instar larvae in larger groups develop more rapidly, but that group size had no detectable influence on survival. The developmental advantage associated with gregariousness is affected by host plant species, but not by predator exclusion, suggesting that group living in these cryptic early instar larvae promotes feeding facilitation, but does not provide individuals with protection from natural enemies.
4. Laboratory experiments revealed that aposematic fourth instar caterpillars in large groups were less likely to be attacked by a generalist insect predator than those in small groups.
5. Field observations provided no evidence that group living affects body temperature, suggesting that microclimatic factors do not favour gregariousness in this species.
6. It is concluded that gregariousness in D. casta confers at least two different advantages on larvae at different stages early in development, but that these advantages disappear, or are outweighed by costs associated with intraspecific competition, in final instars.     

外寄生性花绒寄甲的寄生选择及其发育表现

"选择-表现"假说认为,成虫应该选择有利于子代发育的高品质寄主,但在寄主选择中,除了寄主品质外,其他因素也可能影响寄主选择决策。寄主选择研究通常以成虫为对象,而对那些初龄幼虫选择寄主的寄生性昆虫很少关注。以1龄幼虫积极搜寻寄主的寄生性花绒寄甲为模式生物,采用双选试验设计,观察了花绒寄甲初孵幼虫在不同体重青杨天牛幼虫之间、在已被寄生与健康的黄粉虫蛹之间的寄生选择性;然后采用回归设计,观察了花绒寄甲寄生若干不同体重的青杨天牛幼虫后的发育表现。研究结果表明,花绒寄甲1龄幼虫对体型较大的青杨天牛幼虫的选择偏好显著大于对体型较小的寄主幼虫的选择,选择大体型幼虫的比值比是选择小体型幼虫的4.55倍;对已被寄生的寄主黄粉虫蛹的选择偏好显著大于对健康寄主蛹的选择,选择已被寄生寄主的比值比是选择健康寄主的12.57倍。寄生青杨天牛幼虫的花绒寄甲幼虫发育历期平均为11.49 d、蛹历期为26.67 d、幼虫发育至成虫的羽化率50%,这些发育表现与寄生时青杨天牛幼虫的体重没有显著关系。但刚羽化寄甲成虫体重与寄生时寄主的体重存在显著的正直线关系:寄生时的寄主体重每增大0.01 g,羽化出的寄甲成虫体重增大近0.08%;方差分析寄甲成虫体重在不同寄主体重水平之间的差异表明,从体型较大寄主中羽化的寄甲成虫体重显著大于从体型较小寄主中羽化的成虫。研究结果说明,花绒寄甲初孵幼虫在寄主选择决策时,在寄主体型大小与被寄生状态之间可能采取折衷对策,而且对体型大小不同的寄主选择与子代发育适合度表现存在一致性,从而支持"选择-表现"假说。     

Trade-offs in the life history and energy budget of the parthenogenetic collembolan Folsomia candida (Willem)

Three clones of Folsomia candida from different locations in Europe were compared in four experiments investigating genetic and phenotypic correlations between life-history traits. The first three experiments focused on the effects of food type, clone and temperature on traits associated with the first clutch. Differences in clutch size between clones and treatments were almost completely attributable to body size. Clones differed in length of the juvenile period, but the difference decreased at low temperatures. Age and weight at first reproduction were negatively correlated in the food type experiment and positively correlated in the temperature experiment, an often-encountered result for which no general explanation is as yet available. In the temperature experiment egg size variation was considerable, and was highest at low temperatures. The fourth experiment, with two clones at two feeding levels, aimed at finding trade-offs, in particular between reproduction and survival. It was hypothesized that higher fecundity led to increased scenescence through a higher metabolic rate. The trade-off was clearly present among the clones: one combined fast growth, late reproduction and high lifetime fecundity with lower survival, while in the other the relation between these traits was opposite. The proposed mechanism, however, was not confirmed, as no difference in metabolic rate was found. The effect of food level was too small to result in significant differences in the life-history traits in either of the clones.     

Oviposition choice and larval survival of Dasineura marginemtorquens (Diptera: Cecidomyiidae) on resistant and susceptible Salix viminalis

Abstract.

• 1 Substantial intraspecific variation exists in Salix viminalis resistance to the gall midge Dasineura marginemtorquens. Earlier work has found this variation to have a large genetic component. Willow clones are stable in their resistances between midge generations and different nutrient levels in both field and laboratory culture.
• 2 This study reports the results of laboratory experiments on female oviposition choice and larval survival on potted plants from clones that are very different in resistance as determined in field studies.
• 3 In choice experiments using pairs of plants, the average female midge did not prefer susceptible willow clones over resistant ones for oviposition. In about one third of the replicates, midges actually laid more eggs on the resistant clone. Further work is necessary to examine the nature of variation among midges in discrimination of these plant types.
• 4 Resistance is manifested as great differences in larval survival. Six days after oviposition survival was 92% on susceptible plants but only 6% on resistant ones. Galls developed on all of the susceptible plants, while in 73% of the resistant plants galls were not even initiated.
• 5 The plant traits causing resistance are enigmatic. Larval behaviour suggests that resistant plants interfere with feeding behaviour. On resistant plants, most larvae wander for more than 24 h without initiating any galls before dying. On susceptible plants many first instar larvae begin feeding and initiate galls within this period.

     

Instar duration, adult consumption, oviposition and nitrogen utilization efficiencies of leafhoppers feeding on different quality food (Auchenorrhyncha: Homoptera)

Abstract. 1. Plant material of Holcus lanatus L. removed from an artificially fertilized trial was used to rear leafhoppers of three feeding types: Dicrano-tropis hamata Boheman and Ebmana sulphurella Zetterstedt, (phloem feeders), Eucelis incisus Kirschbaum (xylem feeder) and Zyginidia scutellaris Herrich-Schaeffer (mesophyll feeder).
2. In the laboratory, leafhopper instar durations, adult consumption rate, oviposition rate, and nitrogen utilization efficiency were influenced by the quality of the food consumed. Adult longevity was unaffected by the level of food quality normally found in plants.
3. Z.scutellanis consumed mesophyll contents equivalent to 72% of its body weight each day but only digested about 30% of its intake. Eincicus had a low dry weight consumption rate (12% body weight/day) but digested its food more efficiently (60%) than the other feeding types. The two phloem feeders consumed mounts equivalent to 25% of their body weight each day and digested 40–50% of their intake.
4. Each species reached maximum nitrogen utilization efficiencies at different plant nitrogen levels. This suggests that leafhopper species may be associated with a particular nitrogen concentration in the grasses.