Responses of holocyclic and anholocyclic Rhopalosiphum padi populations to low‐temperature and short‐photoperiod induction

The different life cycles of aphid species make these organisms good models for studying the short‐term consequences of sex. The bird cherry‐oat aphid Rhopalosiphum padi has a wide geographic distribution and correspondingly different life cycles. In this study, the life cycles of R. padi collected from six different regions in China were characterized experimentally by comparing the responses of holocyclic and anholocyclic populations to low‐temperature and short‐photoperiod induction. Clones collected from Chuzhou, Taian, and Taigu consistently reproduced via obligate parthenogenesis, whereas clones from Hami and Baicheng were holocyclic in their response, and those from Lanzhou were both holocyclic and anholocyclic. Prolonged exposure to low temperature and a short photoperiod (LS) had negative effects on the offspring of anholocyclic aphids with regard to adult lifespan, total longevity, and fecundity compared with aphids maintained at a normal temperature and a long photoperiod (NL). Holocyclic LS R. padi had longer developmental times at all nymph stages, a shorter adult lifespan, shorter total longevity, and a lower fecundity than NL counterparts. The adult prereproduction period of gynoparae was significantly longer than that of virginoparae, and the total longevity of gynoparae was significantly shorter than that of virginoparae. Moreover, the net reproductive and gross reproduction rates, as well as the total fecundity, were roughly fivefold higher in virginoparae than in gynoparae, indicating that there is the short‐term cost of sex. When maintained on their secondary host (Triticum aestivum), gynoparae, males, and oviparae produced by holocyclic populations could survive, and gynoparae produced oviparae. However, under NL conditions, oviparae could not produce overwintering eggs on the secondary host, whereas a few overwintering eggs were generated by oviparae under LS conditions. Taken together, these results illuminate the complexity of insect responses and contribute to a complete understanding of the aphid life cycle and its evolution.     

On the virtue of being the first born: the influence of date of birth on fitness in the mosquitofish, Gambusia affinis

We found in an earlier study that mosquitofish (Gambusia affinis and G. holbrooki) ceased reproduction in the late summer, long before the end of warm weather, stored fat, then utilized reserves to survive the winter and initiate reproduction the following spring. We hypothesized that this pattern of fat utilization was a life history adaptation that enabled the fish to acquire food resources in the autumn then allocate them to reproduction the following spring when the fitness of the young would be greater. Here we evaluate one aspect of this hypothesis by evaluating the probability of survival to maturity and fecundity of young as a function of date of birth. We placed cohorts comprising eight to ten litters of young born early‐, mid‐ or late in the reproductive season in replicate field enclosures. The entire experiment was repeated in two different years. Early‐born young had a significantly higher probability of survival to maturity but did not differ in fecundity relative to the last cohort of the season. Early‐born young also attained maturity early enough to reproduce in their year of birth while late‐born young had to overwinter before reproduction. The fitness consequences to the mother of either producing one more litter of young at the end of the season, versus instead storing fat and reproducing the following spring are not as determinate as are the effects of date of birth on offspring fitness. Females most often gain fitness by not producing one last litter and instead over‐wintering. If, however, the overwinter survival of offspring is not influenced by their size at the end of the season, then a female's fitness could be enhanced by producing one more litter late in the season. If instead the probability of overwinter survival is strongly influenced by the size of offspring at the end of the season, then our results suggest that a female gains more by deferring reproduction and storing for overwinter survival and reproduction the following spring.     

Russian Wheat Aphid,Diuraphis noxia in the Czech Republic – cause of the significant population decrease

Diuraphis noxia is a newly established alien pest in the Czech Republic. There were two population explosions during the period 1993–2012. A successful overwintering of the anholocyclic populations of D. noxia was the cause of a population explosion in the oncoming growth season. Viviparous parthenogenetic females of the anholocyclic populations and diapausing eggs of holocyclic populations could overwinter at the same time. Analysis of the course of winter temperatures showed that a complete winter mortality of anholocyclic populations depended mainly on the duration of the freezing period, when temperature dropped below ?5°C, and on the intensity of the frost. We proposed the cumulative effective freeze (in minus degree days) as a key parameter for the aphid anholocyclic population's mortality during overwintering. This parameter equalled ?10.1 degree days for relatively warm winter period 2006–2007, whereas in the relatively cold winter period of 2009–2010, this parameter reached ?87.1 DD, which caused complete mortality of anholocyclic populations. A cumulative effective freeze could be used for short‐time prognosis of D. noxia population explosion.     

Influence of temperature on the biological parameters of the anholocyclic species Cinara tujafilina (Hemiptera: Aphidoidea)

Aphids are a good model to study insect reaction to habitat change. Temperature is one of the main factors that influences insects. This paper examines the influence of temperature on developmental stages, fecundity, survival rate and demographic parameters of Cinara tujafilina (Hemiptera: Aphidoidea, Lachnidae), connected with decorative plants of the Cupressaceae family. C. tujafilina was reared in a laboratory on T. orientalis at five constant temperatures of 10, 15, 20, 25 and 28°C, 70% humidity and 14L:10D. The pre-reproduction stage varied from 7 at 25°C to 19 days at 10°C. Developmental threshold was assigned at 3.5°C. The longest reproduction stage for the aphids developing was recorded at 25°C, namely 33 days, while the shortest, at the temperature of 10°C, lasted 8 days. At 25°C this species is characterised by the shortest pre-reproduction stage, the highest fecundity, the highest survival rate and the highest demographic parameters, particularly r_m (0.17). The results suggest that the optimal temperature for the species is 25°C, and indicate that climatic change will favourably influence its development and increase its role as a pest of decorative plants.     

Melatonin in the seasonal response of the aphid Acyrthosiphon pisum

Aphids display life cycles largely determined by the photoperiod.During the warm long-day seasons.most aphid species reproduce by viviparous parthenogenesis.The shortening of the photoperiod in autumn induces a switch to sexual reproduction.Males and sexual females mate to produce overwintering resistant eggs.In addition to this full life cycle(holocycle),there are anholocyelic lineages that do not respond to changes in photoperiod and reproduce continuously by parthenogenesis.The molecular or hormonal events that trigger the scasonal response(i.c,induction of the sexual phenotypes)are still unknown.Although circadian synthesis of melatonin is known to play a key role in vertebrate photoperiodism,the involvement of the circadian clock and/or of the hor-mone melatonin in insect seasonal responses is not so well established.Here we show that melatonin levels in the aphid Acyrthosiphon pisum are significantly higher in holocyclice aphids reared under short days than under long days,while no differences were found between anholoeyelic aphids under the same conditions.We also found that melatonin is localized in the aphid suboesophageal ganglion(SOG)and in the thoracic ganglionic mass(TGM).In analogy to vertcbrates,insect-type arylalkxylamine N-acetyltransferases(i-AANATs)are thought to play a key role in melatonin synthesis.We measured the expression of four I-AANAT genes identified in A.pisum and localized two of them in situ in the insect central nervous systems(CNS).Levels of expression of these genes were compatible with the quantities of melatonin observed.Moreover,like melatonin,expression of these genes was found in the SOG and the TGM.     

Maintenance of soldier-producing aphids on an artificial diet

In our efforts to understand the biology of soldier-producing aphids, we attempted to maintain them in the laboratory using a chemically defined artificial diet. The ability of 16 species from the subfamilies Eriosomatinae and Hormaphidinae, most of which are soldier-producing species, to survive on the artificial diet was examined. Some species neither fed nor grew on the diet, whereas other species accepted the diet, grew to some extent, and managed to produce a small number of short-lived offspring. Although they performed poorly on the diet in general, aphid performance was correlated with the stage in the life cycle and the developmental stadium in that aphids of the gall generation tended to accept the diet and survive on it, whereas aphids of the non-gall generation did not. Also, old insects tended to perform better on the diet than young nymphs. Notably, only one species, Tuberaphis styraci, a gall-forming aphid that produces 2nd instar sterile soldier, showed good performance on the diet. Insects collected from galls (generation G0) survived on the diet, grew well, and produced many progeny. Three successive generations (G1, G2 and G3) were produced on the diet. Developmental period, adult body size, and age of first reproduction were almost constant through G0, G1 and G2 whereas fecundity, adult longevity and daily offspring production declined as the generations proceeded. These results are comparable to previous studies in which pest aphids have been maintained on similar artificial diets for several generations. Therefore, it is suggested that the artificial-diet rearing system will provide a useful tool to investigate various biological aspects of the soldier-producing eusocial aphid, T. styraci.     

Reproductive strategy of winged and wingless morphs of the aphids Sitobion avenae and Metopolophium dirhodum

The reproduction of apterous and alate morphs of the aphids Sitobion avenae and Metopolophium dirhodum is compared on the basis of fecundity in 5- and 10-day periods of adult life. Apterae of both species are consistently more fecund than alatae of comparable weight, producing about three more nymphs on average in any 5-day period. The reproductive differences are related to the number and quality of embryos at eclosion and to ovulation rates, both of which in turn appear to be linked to wing-muscle maintenance. These relationships between weight, embryos and reproduction may be used to predict a newly moulted adult aphid's fecundity, a method which may facilitate the assessment of resistance to aphids in new cereal varieties, by obviating lengthy recording of reproduction. The strategies by which alatae of these and other aphid species minimize the difference between their fecundity and that of apterae are discussed.     

Multiple generation effects of high temperature on the development and fecundity of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) biotype B

Insects are ectotherms and their ability to resist temperature stress is limited. The immediate effects of sub‐lethal heat stress on insects are well documented, but longer‐term effects of such stresses are rarely reported. In this study, survival, development and reproduction of the whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) biotype B, were compared over five consecutive generations at 27, 31 and 35 °C and for one generation at 37 °C. Both temperature and generation significantly affected the fitness of the whitefly. These impacts were more dramatic with increasing generations and temperatures. Among the experimental temperatures, the most favorable for development and reproduction were 27 °C and 31 °C. At 27 °C, survival, development and fecundity were all stable over these five generations. At 31 °C, immature survival rate was the highest in the fifth generation, but female fecundities decreased in the fourth and fifth generations. At 35 °C, egg hatching rate, immature survival rate and female fecundity decreased significantly in the fourth and fifth generations. At 37 °C, survival of B. tabaci was not adversely affected, but female fecundity at 37 °C was less than 10% of that at 27 °C or 31 °C. These results demonstrate that the lethal high temperature for B. tabaci is over 37 °C, and the whitefly population continued expanding in the five generations at 35 °C. The ability of B. tabaci biotype B to survive high temperature stress will play an important role in its population extension under global warming.     

Dissecting insect responses to climate warming: overwintering and post‐diapause performance in the southern green stink bug,Nezara viridula,under simulated climate‐change conditions

The effect of simulated climate change on overwintering and post‐diapause reproductive performance is studied in Nezara viridula (L.) (Heteroptera: Pentatomidae) close to the species' northern range limit in Japan. Insects are reared from October to June under quasi‐natural (i.e. ambient outdoor) conditions and in a transparent incubator, in which climate warming is simulated by adding 2.5°C to the ambient temperatures. Despite the earlier assumption that females of N. viridula overwinter in diapause, whereas males do so in quiescence, regular dissections show that the two sexes overwinter in a state of true diapause. During winter, both sexes are dark‐coloured and have undeveloped reproductive organs. Resumption of development does not start until late March. During winter, the effect of simulated warming on the dynamics and timing of physiological processes appears to be limited. However, the warming significantly enhances winter survival (from 27–31% to 47–70%), which is a key factor in range expansion of N. viridula. In spring, the effect of simulated warming is complex. It advances the post‐diapause colour change and transition from dormancy to reproduction. The earlier resumption of development is more pronounced in females: in April, significantly more females are already in a reproductive state under the simulated warming than under quasi‐natural conditions. In males, the tendency is similar, although the difference is not significant. Warming significantly enhances spring survival and percentage of copulating adults, although not the percentage of ovipositing females and fecundity. The results suggest that, under the expected climate‐warming conditions, N. viridula will likely benefit mostly as a result of increased winter and spring survival and advanced post‐diapause reproduction. Further warming is likely to allow more adults to survive the critical cold season and contribute (both numerically and by increasing heterogeneity) to the post‐overwintering population growth, thus promoting the establishment of this species in newly‐colonized areas.     

Annual predictions of the peak numbers of Sitobion avenae infesting winter wheat

Sitobion avenae is the most abundant aphid that infests the ears of winter wheat in central Europe. Control of this pest will benefit from prediction of its maximum abundances on ears. We attempted to predict the maximum numbers of this species infesting the ears using earlier counts of this species in ears and on leaves using data collected in the Czech Republic in 2002–2014. The numbers of aphids on particular leaves and the ears were recorded at weekly intervals. The maximum numbers infesting the ears were significantly related to aphid counts on the ears made 1 week, 2 weeks and even 3 weeks before the peak. A regression based only on plots where there were aphids (non‐zero aphid counts) recorded 3 weeks prior to the peak resulted in a critical number of ≥0.44 aphids/ear, which if exceeded resulted in a harmful maximum abundance of ≥5 aphids/ear at the peak. We also regressed maximum numbers infesting ears on numbers of S. avenae on leaves recorded 1, 2 and 3 weeks after earing, which all resulted in reliable predictions. Regression of maximum abundance on non‐zero aphid counts on the leaves in week 3 after earing revealed a critical number of ≥0.59 aphids/tiller. Zero aphid counts resulted in only 3% of cases of a harmful maximum abundance in ears. Using records of the numbers on flag leaves, which make up 58% of the S. avenae population present on the foliage, also provided reliable predictions of maximum abundance in ears. Predicting maximum numbers of aphids from earlier records of the numbers of aphids on leaves or ears is thus possible. The prediction may be further improved by increasing the precision with which the numbers of aphids initially infesting the crop are estimated.     

Effects of pattern and timing of high temperature exposure on reproduction of the rose grain aphid, Metopolophium dirhodum

Metopolophium dirhodum (Walker) (Hemiptera: Aphididae) is one of the three most important cereal aphid species in Germany and central European countries. High temperature is known to play a detrimental role in the reproduction and survival of aphids. Detailed experiments were conducted to assess the effects of high temperatures (27, 29, 31, and 33 °C), exposure pattern (8 h day^?1 for 1, 2, 4, and 6 days), and exposure development stage (2nd, 3rd, 4th instar larvae, and adult) on the reproduction and longevity of M. dirhodum. All three factors have a significant influence on the aphids. Increasing the temperature to over 29 °C significantly decreases their lifetime fecundity and longevity. Mature aphids are more sensitive to high temperatures than younger ones. A high temperature pulse to treat aphids older than 3rd instar larvae greatly shortened their longevity and lessened their lifetime fecundity. Lifetime fecundity and longevity was found to be inversely related to exposure pattern. A high temperature pulse lasting longer than 1 week greatly reduced the lifetime fecundity and longevity. The limitation of the data collected from experiments at constant temperatures for aphid forecasting is discussed.     

SEASONAL DECLINES IN OFFSPRING FITNESS AND SELECTION FOR EARLY REPRODUCTION IN NYMPH-OVERWINTERING GRASSHOPPERS

In this study, I examine the effects of natural and experimentally induced variation in life cycle timing on offspring fitness in Arphia sulphurea and Chortophaga viridifasciata, to understand the selective pressures shaping phenology in these two species of nymph-overwintering grasshoppers. Because these species lack embryonic diapause, hatching varies over a two month range under natural conditions. I used a cold treatment to delay hatching of some egg pods and extend the natural range of hatching dates. Due to the shorter time for growth and poorer growing conditions late in the fall, late-hatching nymphs of both species grew to a smaller size before winter and suffered higher overwinter mortality, compared to early nymphs. In addition, late nymphs that did survive the winter became reproductive later in the following year's breeding season. Size- dependent mortality of offspring during the winter is a strong selective pressure favoring early reproduction in these species. Female adult life history traits appear responsive to the seasonal declines in offspring fitness, in that late-maturing females began reproducing sooner after adult maturation and reproduced at a more rapid rate, even at the expense of having shorter adult longevity and producing fewer total egg pods. Experimental manipulations were crucial in understanding the fitness consequences of intrapopulation variation in the timing of specific life-cycle events for these species.     

Relationships within aphids Cinara (Cupressobium) (Hemiptera) based on mitochondrial and nuclear DNA sequences

The relationships between Cinara (Cupressobium) aphids inhabiting woody parts and leaves of conifers belonging to Cupressaceae have been studied using a mitochondrial gene (COI) and a nuclear gene (EF1-α). Based on the COI sequences, genetic distances between species ranged from 5.6 % between Cinara (C.) tujafilina (del Guercio) and Cinara (C.) juniperi (De Geer) to 10.5 % between C. (C.) tujafilina and Cinara (C.) mordvilkoi (Pa?ek). Genetic distances among EF1-α sequences were lower and showed from 0.1 % between C. cupressi and C. juniperi to 2.3 % between C. tujafilina and C. mordvilkoi. Molecular phylogenetic trees were constructed using the Bayesian inference (BI) phylogenetic analysis and maximum parsimony (MP) criterion. Phylogenetic trees obtained based on COI and EF1-α marker genes created two sister clades. Our results indicate that Cinara (Cupressobium) are a monophyletic group of aphids. Phylogenetic relationships amongst Cupressobium aphids do not result from the association with the host plant, but from the feeding site on the host plant or an ability to change the microhabitat on the plant. As closely related species inhabit similar microhabitats on different host plants, it suggests that the host switching is the main mode of speciation in this subgenus.     

Life‐history trade‐offs vary with resource availability across the geographic range of a widespread plant

• Trade‐offs between reproduction, growth and survival arise from limited resource availability in plants. Environmental stress is expected to exacerbate these negative correlations, but no studies have evaluated variation in life‐history trade‐offs throughout species geographic ranges. Here we analyse the costs of growth and reproduction across the latitudinal range of the widespread herb Plantago coronopus in Europe.
• We monitored the performance of thousands of individuals in 11 populations of P. coronopus, and tested whether the effects of growth and reproduction on a set of vital rates (growth, probability of survival, probability of reproduction and fecundity) varied with local precipitation and soil fertility. To account for variation in internal resources among individuals, we analysed trade‐offs correcting for differences in size.
• Growth was negatively affected by previous growth and reproduction. We also found costs of growth and reproduction on survival, reproduction probability and fecundity, but only in populations with low soil fertility. Costs also increased with precipitation, possibly due to flooding‐related stress. In contrast, growth was positively correlated with subsequent survival, and there was a positive covariation in reproduction between consecutive years under certain environments, a potential strategy to exploit temporary benign conditions.
• Overall, we found both negative and positive correlations among vital rates across P. coronopus geographic range. Trade‐offs predominated under stressful conditions, and positive correlations arose particularly between related traits like reproduction investment across years. By analysing multiple and diverse fitness components along stress gradients, we can better understand life‐history evolution across species’ ranges, and their responses to environmental change.

     

A facultative endosymbiont in aphids can provide diverse ecological benefits

Ecologically important traits of insects are often affected by facultative bacterial endosymbionts. This is best studied in the pea aphid Acyrthosiphon pisum, which is frequently infected by one or more of eight facultative symbiont species. Many of these symbiont species have been shown to provide one ecological benefit, but we have little understanding of the range of effects that a single strain can have. Here, we describe the phenotypes conferred by three strains of the recently discovered bacterium known as X‐type (Enterobacteriaceae), each in their original aphid genotype which also carries a Spiroplasma symbiont. All comparisons are made between aphids that are coinfected with Spiroplasma and X‐type and aphids of the same genotype that harbour only Spiroplasma. We show that in all cases, infection with X‐type protects aphids from the lethal fungal pathogen Pandora neoaphidis, and in two cases, resistance to the parasitoid Aphidius ervi also increases. X‐type can additionally affect aphid stress responses – the presence of X‐type increased reproduction after the aphids were heat‐stressed. Two of the three strains of X‐type are able to provide all of these benefits. Under benign conditions, the aphids tended to suffer from reduced fecundity when harbouring X‐type, a mechanism that might maintain intermediate frequencies in field populations. These findings highlight that a single strain of a facultative endosymbiont has the potential to provide diverse benefits to its aphid host.     

Prolonged diapause and seed predation by the acorn weevil,Curculio robustus,in relation to masting of the deciduous oak Quercus acutissima

Many researchers have studied the relationship between masting by trees and seed predation by insects. Most of these studies have been plant centered, with little focus on the insect perspective. To estimate the effect of mast seeding on insect seed predators, the life‐history traits of these insects must also be considered because some seed insects can survive lean years by prolonged diapause. In this study, I examined larval infestation of acorns and life‐history traits of the acorn weevil, Curculio robustus (Roelofs) (Coleoptera: Curculionidae), in relation to acorn production of the deciduous oak Quercus acutissima Carruthers (Fagaceae) in a coppice stand in central Japan in 2004–2009. Curculio robustus females oviposit into Q. acutissima acorns, inside which the larvae develop. Mature larvae leave acorns and burrow into the soil, where they overwinter. Although germination did occur in acorns infested by weevil larvae, the percentage of germination was lower in acorns damaged by many larvae. Acorn production in Q. acutissima varied considerably among years. Both the number of C. robustus larvae infesting acorns and the percentage of acorns infested were affected by the amount of acorns produced by Q. acutissima, and two successive lean years appeared to have a considerable impact on C. robustus population size. Consequently, only a small fraction of the acorns produced were lost to predation in a mast year after two successive lean years. However, C. robustus could survive the two successive lean years because of prolonged larval diapause, probably leading to a marked decrease in population size. These findings suggest that masting in Q. acutissima succeeds as a predator satiation strategy in response to acorn damage by C. robustus, and that C. robustus has developed prolonged diapause as a counter‐adaptation.     

Genetic diversity of the green spruce aphid (Elatobium abietinum Walker) in north-west Europe

Abstract 1 The Random Amplified Polymorphic DNA (RAPD) method was used to investigate genetic diversity of anholocyclic populations of the green spruce aphid, Elatobium abietinum Walker, in north‐west Europe. 2 The results showed that the aphid in this region was divided genetically into three major groups. Aphids from the British Isles and north‐west France comprised the first group, the second group consisted of aphids from Denmark and Iceland, and the third group consisted of aphids from Norway. 3 The results indicated a significant level of gene flow within and between sites and geographical regions, especially in the British Isles and north‐west France. Lateral migration of the aphid and/or sexual reproduction is likely to have facilitated the gene flow. 4 The implications of these findings on management of the green spruce aphid are discussed.     

Feeding behaviour and performance of different populations of the black currant‐lettuce aphid,Nasonovia ribisnigri,on resistant and susceptible lettuce

When crops are bred for resistance to herbivores, these herbivores are under strong selection pressure to overcome this resistance, which may result in the emergence of virulent biotypes. This is a growing problem for crop species attacked by aphids. The Nr‐gene in lettuce confers near‐complete resistance against the black currant‐lettuce aphid, Nasonovia ribisnigri (Mosely) (Hemiptera: Aphididae). Since 2007, populations of N. ribisnigri have been reported in several locations in Europe to infest resistant lettuce varieties that possess the Nr‐gene. The objective of this study was to analyse the behaviour and level of virulence of several N. ribisnigri populations observed to have colonized Nr‐locus‐containing lettuce lines. We analysed the stylet penetration and feeding behaviour, and the performance of these N. ribisnigri populations on resistant and susceptible lettuce lines. Large variation in the degree of virulence to the Nr‐locus‐containing lettuce lines was found among populations of the Nr:1 biotype. The German population was highly virulent on the Nr‐containing resistant lettuce lines, and showed similar feeding behaviour and performance on both the susceptible and resistant lettuces. The French population from Paris was the second most virulent, though reproduction on the resistant lines was reduced. The French population from Perpignan and a population from Belgium, however, showed reduced performance and feeding rate on the resistant compared to the susceptible lettuces. The lettuce background in which the Nr‐gene is expressed influences the level of resistance to the various Nr:1 aphid populations, because the performance and feeding behaviour differed between the aphids on the cultivars (romaine lettuce) compared to the near‐isogenic lines (butterhead/iceberg lettuce). This study also shows that being able to feed on a plant not automatically implies that a population can successfully develop on that plant, because aphids showed phloem ingestion during the 8‐h recording period on resistant lettuce, but were not able to survive and reproduce on the same lettuce line.     

Positive effects of an arbuscular mycorrhizal fungus on aphid life history traits

Two generations of two aphid species (Myzus ascalonicus and M. persicae) were reared on Plantago lanceolata plants, with and without root colonization by the arbuscular mycorrhizal fungus, Glomus intraradices. Life history traits of the aphids measured were nymphal development time, teneral adult weight, growth rate, total fecundity, adult longevity and duration of post-reproductive life. For both aphids in both generations, mycorrhizal colonization increased aphid weight and fecundity, while other traits were unaffected. The increases were consistent between generations. In a second experiment, M. persicae was reared on plants with and without the fungus, under varying N and P regimes. The results of N addition were inconclusive because there was high aphid mortality. However, under P supplementation, positive effects of the mycorrhiza on aphid growth were seen at low and medium P levels, while at high P levels these effects disappeared. The positive effects of mycorrhizal colonization reported here are contrary to the majority of previous studies with chewing insects, which have reported negative effects. A number of possible mechanisms for this apparent discrepancy are discussed. Received: 1 February 1999 / Accepted: 22 March 1999     

The cost of being able to fly in the milkweed-oleander aphid,Aphis nerii (Homoptera: Aphididae)

Summary In the wing dimorphic milkweed-oleander aphid,Aphis nerii, winged aphids begin reproducing about 1.5 days after wingless aphids. The longer maturation period is primarily due to slower development since even adult eclosion by winged aphids takes place after wingless aphids begin reproducing. The delay is not due to a post-eclosion, pre-reproductive flight since, beginning with the fourth instar, larval winged aphids were reared at a density of one per plant and the vast majority were not stimulated to fly under such low-density conditions. Thus, the ability to fly incurs a fitness cost in terms of delayed reproduction, irrespective of whether flight actually occurs. We did not observe a difference between morphs for lifetime fecundity, even though wingless aphids have larger abdomens than winged aphids and for both morphs there is a significant correlation between abdomen width and fecundity. Offspring produced by wingless aphids over the first four days of reproduction are larger than those produced by winged aphids, and the size difference at birth is maintained into adulthood. However, there are no differences in life history traits between these offspring, including maturation period and lifetime fecundity. Thus, reduced body size does not increase the cost of being able to fly, at least under the conditions of these experiments. The cost of being able to fly in this species should favor reduced production of winged individuals in populations that exploit more permanent host plants.