Observation on the initial inoculum source and dissemination of Entomophthorales-caused epizootics in populations of cereal aphids

A total number of 1092 migratory alates were trapped from air in wheat grown area of Yuanyang County, Henan Province from early April through May 2002 in order to confirm the source and dissemination of entomophthoralean inocula to cause epizootics of cereal aphids. Those included 415 Sitobion avenae, 642 Rhopalosiphum padi, 22 Metopolophium dirhodum, and 13 Schizaphis graminum. The trapped alates were daily collected and individually reared for 7 days on wheat plants in laboratory. Of those 341 alates died of fungal infection, taking 31.2% in the trapped alates. These included 224 S. avenae, 106 R. padi, 8 M. dirhodum, and 3 S. graminum. Deaths of all infected alates occurred during the first 5 days and 78.9% of the deaths occurred within the first 3 days. Individual examination under microscope proved that all deaths were attributed to entomophthoralean fungi. Of those Pandora neoaphidis accounted for 84.6%, Conidiobolus obscurus for 9.9%, and Entomophthora planchoniana for 5.5%. Four alate deaths die     

Sitobion avenae alatae infected by Pandora neoaphidis: their flight ability, post-flight colonization, and mycosis transmission to progeny colonies

Epizootics caused by the obligate Entomophthorales pathogen Pandora neoaphidis may result from more than one possible means of fungal dissemination among host aphids, but we hypothesize that wide dispersal of the fungus is most likely to be associated with the flight behavior of migratory alates. We tested this hypothesis in a simulation experiment by assessing the flight capability of Sitobion avenae alates infected with P. neoaphidis and the potential of their post-flight survival, colonization, and mycosis transmission to progeny. A total of 281 alates were inoculated with P. neoaphidis, individually flown for up to 5h and 9km in a computer-monitored flight mill system and then reared for 10 days on wheat seedlings. The infected alates were capable of surviving on average for 2.9 days (range 1-7 days) and leaving 4.6 nymphs prior to deaths. Transmission of fungal infection within progeny colonies occurred after the mother alates died from P. neoaphidis mycosis. The level of contagious infection among the nymphs reached up to 16.8% within 7 days but varied with the survival time of the infected mother alates after flight. Based on stepwise polynomial regression analysis, progeny colony size was highly correlated with the interactions of flight time with both post-flight survival time and the number of nymphs left per alate before death (r2 = 0.997). Progeny mortality on day 5 after colonization was inversely correlated with post-flight survival time (r2 = 0.949) whereas infection on day 7 was correlated with flight distance and an interaction of post-flight survival time with fecundity of the infected alates (r2 = 0.970). Progeny mortality observed on day 10 was merely correlated to mortality observed on day 5 (r2 = 0.946). These results indicate a successful transmission of alate-borne P. neoaphidis to progeny colonies and further support our hypothesis on the means of primary dispersal of aphid epizootics by migratory alates in a geographically wide range.     

Wide dispersal of aphid-pathogenic Entomophthorales among aphids relies upon migratory alates

Entomophthoralean mycoses are of general importance in the natural control of aphids, but mechanisms involved in their dissemination are poorly understood. Despite several possible means of fungal survival, the dispersal of the mycoses in aphids has never been related to the flight of their migratory alates that are able to locate suitable host plants. In this study, aphid-pathogenic fungi proved to be widely disseminated among various aphids by their alates through migratory flight based on the following findings. First, up to 36.6% of the 7139 migratory alates (including nine species of vegetable or cereal aphids) trapped from air > 30 m above the ground in three provinces of China were found bearing eight species of fungal pathogens. Of those, six were aphid-specific Entomophthorales dominated in individual cases by Pandora neoaphidis, which occurs globally but has no resting spores discovered to date. Secondly, infected alates were confirmed to be able to fly for hours, to initiate colonies on plants after flight and to transmit fungal infection to their offspring in a laboratory experiment, in which 238 Sitobion avenae alates were individually flown in a computer-monitoring flight mill system after exposure to a spore shower of P. neoaphidis and then allowed to colonize host plants.     

Barley Yellow Dwarf Luteovirus (BYDV) Infectivity of Alate Aphid Vectors in Northeast Spain

Rhopalosiphum padi and Sitobion avenae alates were collected from colonised winter cereals and maize in N.E. Spain and fed on young wheat plants for 7–10 days in the glasshouse. Then, aphids were killed and the plants on which aphids reproduced were kept in the glasshouse for 30–40 days. ELISA of infested plants was made using polyclonal and monoclonal antisera against PAV-, RPV- and MAV-like isolates. In autumn and spring, MAV serotypes were transmitted by S. avenae and R. padi , mainly in mixed infections with PAV serotypes. This possibly explains the high frequency of MAV-like isolates and their previously recorded year-to-year stability in maize, grain and forage winter cereals and cereal volunteers. PAV-like isolates were rarely transmitted by S. avenae and its spread thus depends almost exclusively on R. padi. These results confirm the importance of forage cereals and cereal volunteers as virus sources for winter cereal infection in the autumn, and the latter as a source of BYDV for maize in spring.     

Cereal aphids on maize in southern England

In 1978, 1979 and 1980, counts were made of the numbers of cereal aphids on a maize crop near Fordingbridge, Hampshire, England. Maize plants were selected away from the edge of the crop and the number, morph, species and feeding site of the aphids were recorded. Metopolophium dirhodum and Rhopalosiphum padi were found at densities that exceeded those found on small-grain cereals and grasses at the same time of the year. Sitobion avenae and Macrosiphum euphorbiae were found at relatively low densities, and Rhopalosiphum maidis and Metopolophium festucae were observed rarely. Unlike other species, the numbers of R. padi continued to increase throughout the summer and reached a peak in September/October; at this time of the year, R. padi constituted over 98% of the aphids found on maize. M. dirhodum and S. avenae were found mostly on the leaves whereas R. padi appeared to favour the cob until populations became too large to be supported. If the area of maize grown in the UK increases, the crop will probably play an important role in the life cycle of cereal aphids (especially R. padi ). It is suggested that further work should be carried out on the pest status of aphids on maize and on the role that maize may play in the ecology of cereal pests.     

The impact of transgenic wheat expressing GNA (snowdrop lectin) on the aphids Sitobion avenae, Schizaphis graminum, and Rhopalosiphum padi

This study investigated the impact of transgenic wheat expressing Galanthus nivalis agglutinin (GNA), commonly known as snowdrop lectin, on three wheat aphids: Sitobion avenae (F.), Schizaphis graminum (Rondani), and Rhopalosiphum padi (L.). We compared the feeding behavior and the life-table parameters of aphids reared on GNA transgenic wheat (test group) and those aphids reared on untransformed wheat (control group). The results showed that the feeding behaviors of S. avenae and S. graminum on GNA transgenic wheat were affected. Compared with the control group, they had shorter initial probing period, longer total nonprobing period, shorter initial and total phloem sap ingestion phase (waveform E2), shorter duration of sustained ingestion (E (pd) > 10 min), and lower percentage of phloem phase of the total observation time. Moreover, S. graminum made more probes and had a longer total duration of extracellular stylet pathway (waveform C). The fecundity and intrinsic rate of natural increase (r(m)) of S. avenae and S. graminum on the transgenic wheat were lowered in the first and second generations, however, the survival and lifespan were not affected. The effects of the GNA expressing wheat on S. graminum and S. avenae were not significant in the third generation, suggesting rapid adaptation by the two aphid species. Despite the impact we found on S. avenae and S. graminum, transgenic GNA expressing wheat did not have any effects on R. padi.     

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.     

Tillage impacts cereal-aphid (Homoptera: Aphididae) infestations in spring small grains

We compared infestation levels of cereal aphids (Homoptera: Aphididae) in spring-seeded wheat and barley grown with and without preplant tillage for 8 site yr in eastern South Dakota. Crop residue covered approximately 25% of the soil surface with preplant tillage, whereas without preplant tillage 50% or more of surface residue was conserved. Rhopalosiphum padi (L.) comprised nearly 90% of all cereal aphids sampled, and R. maidis (Fitch), Schizaphis graminum (Rondani), and Sitobion avenae (F.) collectively comprised the remainder. R. padi routinely infested lower parts of tillers and were generally concealed by surface residue in plots with no preplant tillage. Across 7 site yr, R. padi were more abundant in plots with no preplant tillage than with preplant tillage (272.6 +/- 54.4 versus 170.1 +/- 37.2 aphid days per 25 tillers). However, in comparisons at individual site years, R. padi were greater in no-preplant tillage plots only once. For all cereal-aphid species combined, infestations were greater in plots with no preplant tillage for 1 of 8 site yr, but did not differ with tillage when compared across all site years. Cereal aphids were never more abundant in plots with preplant tillage. Our results show that conservation tillage leads to greater infestations of R. padi in spring small grains, as increased surface residue provides a favorable microhabitat for this aphid.     

Aphid dispersal flight disseminates fungal pathogens and parasitoids as natural control agents of aphids

Abstract.  1. Dispersal flight, a well-known strategy for aphids to locate suitable plants, was studied for its possible role in disseminating fungal pathogens and parasitoids as natural control agents of aphids by air captures in Hangzhou, China during 2001–2005. Up to 3183 migratory alates of green peach aphid Myzus persicae were captured from air using a yellow-plus-plant trap on the top platform of a six-storey building in an urbanised area, and individually reared in a laboratory for ≥ 7 days.
2. Among the captured alates, 28.9% survived on average for 2.5 days and then died from mycoses attributed to 10 species of obligate or non-obligate aphid pathogens. These were predominated by Pandora neoaphidis , which was causative of 80% of the mycosed alates. Another 4.4% survived for an average of 3.7 days, followed by mummification of Aphidius gifuensis (52.9%) and Diaeretella rapae (47.1%).
3. Numerous progeny colonies individually initiated by infected, parasitised, or healthy alates were monitored daily for 12 days, and fitted well to a logistic equation depicting the potential of their post-flight colonisation and fecundity. Both infected and parasitised alates from air were highly capable of initiating progeny colonies independently, although their potential fecundity was greatly reduced compared with that of healthy counterparts.
4. Our results confirmed that both obligate and non-obligate pathogens can be widespread with aphid dispersal flight, and demonstrated that parasitoids also took advantage of the host flight for their dispersal. This study provides new insights into aphid dispersal biology.     

Influence of aphid species and barley yellow dwarf virus on soft red winter wheat yield

Yield loss in soft red winter wheat, Triticum aestivum L., caused by aphid-transmitted barley yellow dwarf virus (family Luteoviridae, genus Luteovirus, BYDV) was measured over a 2-yr period in central Missouri. Rhopalosiphum padi (L.) was the most common and economically important species, accounting for > 90% of the total aphids. Schizaphis graminum (Rondani), Rhopalosiphum maidis (Fitch), and Sitobion avenae (F.) made up the remainder of the aphids. Aphid numbers peaked at wheat stem elongation in 2003 with 771 R. padi per meter-row. In the 2003-2004 growing season, aphid numbers averaged seven aphids per meter-row in the fall and peaked at 18 aphids per meter-row at jointing. Wheat grain yield was reduced 17 and 13% in 2003 and 2004, respectively. Thousand kernel weights were reduced 10 and 5% in the untreated plots compared with the treated control in 2003 and 2004, respectively. Padi avenae virus was the predominate strain, accounting for 81 and 84% of the symptomatic plots that tested positive for BYDV in 2003 and 2004. Our results indicate that economic thresholds for R. padi are 16 aphids per meter-row in the fall and 164 aphids per meter-row at jointing.     

Patterns of resistance to three cereal aphids among wheats in the genus Triticum (Poaceae)

Forty-one accessions of wild and cultivated wheats belonging to 19 Triticum species were tested in the field for resistance to three species of aphids, Rhopalosiphum padi Linnaeus, Sitobion avenae Fabricius and Schizaphis graminum Rondani. Antibiotic resistance was estimated by the increase in biomass of aphids over 21 days on adult plants. Overall resistance was estimated by the plant biomass lost due to aphid infestation. All three species of aphids survived and reproduced on all wheats, and reduced spike biomass compared to uninfested controls. The level of antibiosis varied among wheat species and among accessions, with accessions from three, five and one species showing antibiosis to R. padi, S. avenae and S. graminum, respectively. Overall resistance to the three aphid species was observed in five to seven accessions per aphid species. Resistance was usually specific to one aphid species. The frequency of accessions with antibiosis or overall resistance was associated with the ploidy level of the plant species. Except for overall resistance to R. padi, resistance was highest for diploid species and lowest for hexaploid species. No consistent relationship between resistance and level of domestication was detected. Accessions of the wild wheats, Triticum boeoticum Bois, Triticum tauschii (Coss.) Schmal. and Triticum araraticum Jakubz. exhibited high levels of resistance to aphids, as did Triticum monococcum L. which is derived from T. boeoticum. Nevertheless, individual susceptible or resistant accessions occurred at all levels within the evolutionary tree of wheat.     

Density dependence in cereal aphid populations

Long sequences of data on the incidence of cereal aphids from five European countries were analysed for evidence of density dependent processes occurring between years. Using a randomisation test (Pollard, Lakhani & Rotheray, 1987), density dependence was revealed in all (16) population censuses of Metopolo-phium dirhodum , 60% of Rhopalosiphum padi censuses (10 of 17) but only 25% of Sitobion avenae population censuses (4 of 16). Correcting for density independent effects of weather revealed the existence of significant direct density dependence in some populations censuses where it was previously undetected. The implications of density dependence in cereal aphid populations are considered.     

Comparing growth patterns among field populations of cereal aphids reveals factors limiting their maximum abundance

Cereal stands in central Europe are commonly infested with three species of aphids that may become serious pests. With increasing abundance, the proportion of a particular species in the total aphid population may remain constant, suggesting a density-independent exponential growth, or the proportion can change, suggesting density-dependent constraints on growth. The constraints that affect particular species, and thus their relative abundance, were studied. The proportionality between maximum abundances of the cereal aphids was studied using a 10-year census of the numbers of aphids infesting 268 winter wheat plots. For two species their abundance on leaves and ears was compared. With increasing aphid density the maximum abundance of Rhopalosiphum padi (Linnaeus) remained proportional, but not that of Sitobion avenae (Fabricius), which was constrained by the smaller surface area of ears compared to leaves. There was no evidence of inter-specific competition. Maximum abundance of R. padi and Metopolophium dirhodum (Walker) on leaves did not change proportionally as the proportion of M. dirhodum decreased with increasing overall aphid density. This decrease was probably caused by the restricted distribution of M. dirhodum, which is confined to leaves, where space is limiting. No change in proportion between populations was detected when the average densities were below 0.54 aphids per leaf or ear. Non-proportional relationships between aphid populations appeared to be due to spatial constraints, acting upon the more abundant population. Detecting the limitation of population growth can help with the assessment of when density-independent exponential growth is limited by density-dependent factors. This information may help in the development of models of cereal aphid population dynamics.     

Targeted Dispersal of the Aphid Pathogenic Fungus Erynia neoaphidis by the Aphid Predator Coccinella septempunctata

The potential of adult and larval C. septempunctata to vector the aphid-specific entomopathogenic fungus E. neoaphidis was assessed through a series of laboratory and field experiments. The ability of coccinellids to vector conidia from a colony of E. neoaphidis -infected pea aphids, Acyrthosiphon pisum, to a colony of uninfected A. pisum was demonstrated in a laboratory study. Adult coccinellids which had previously foraged on plants infested with different densities of sporulating cadavers (1, 5, 15, 30 cadavers per plant) initiated infection in a proportion of uninfected pea aphids (4, 0, 2 and 8%, respectively) when subsequently allowed to forage on A. pisum infested bean plants. Further laboratory studies demonstrated that fourth instar larvae and adult coccinellids artificially inoculated with conidia initiated infection in 11 and 13% of an A. pisum population in which they foraged, respectively. Furthermore, a proportion of A. pisum placed on bean plants which had previously been foraged on by inoculated larval and adult coccinellids also died from infection (3 and 10% of A. pisum, respectively). However, although coccinellid adults inoculated with conidia initiated infection in 19% of A. pisum, cereal aphids, S. avenae , exposed to the inoculated coccinellids did not become infected. A further laboratory study demonstrated that infection of A. pisum only occurred if inoculated coccinellids were transferred to A. pisum populations immediately post inoculation. However, a proportion of A. pisum placed on bean plants which had been foraged on by inoculated coccinellids transferred 0, 4 and 24 h post inoculation died from infection (9, 3 and 7%, respectively). A field study further demonstrated the potential of coccinellids to vector E. neoaphidis. Single spring sown field bean plants (Long Hoos Experimental Plots, IACRRothamsted Farm) were enclosed within nylon mesh bags and 25 adult A. pisum were added to each bag with one of the following treatments: no further addition (control), coccinellid adult (control), inoculated coccinellid adult, inoculated A. pisum or sporulating A. pisum cadavers. No aphids died of E. neoaphidis in the control treatments; 5, 16 and 33% of aphids were infected with E. neoaphidis on the other treatments, respectively.     

First report of Pandora neoaphidis resting spore formation in vivo in aphid hosts

The entomopathogenic fungus Pandora neoaphidis is a recognized pathogen of aphids, causes natural epizootics in aphid populations, and interacts and competes with aphid predators and parasitoids. Survival of entomophthoralean fungi in periods of unsuitable weather conditions or lack of appropriate host insects is accomplished mainly by thick-walled resting spores (zygospores or azygospores). However, resting spores are not known for some entomophthoralean species such as P. neoaphidis. Several hypotheses of P. neoaphidis winter survival can be found in the literature but so far these hypotheses do not include the presence of resting spores. Resting spores were found in an aphid population where P. neoaphidis was the only entomophthoralean fungus observed during surveys conducted in organic horticultural crops in greenhouses and open fields in Buenos Aires province, Argentina. This study sought to use molecular methods to confirm that these resting spores were, in fact, those of P. neoaphidis while further documenting and characterizing these resting spores that were produced in vivo in aphid hosts. The double-walled resting spores were characterized using light and transmission electron microscopy. The Argentinean resting spores clustered together with P. neoaphidis isolates with bootstrap values above 98 % in the small subunit ribosomal RNA (SSU rRNA) sequence analysis and with bootstrap values above 99 % the Internal Transcribed Spacer (ITS) II region sequence analysis. This study is the first gene-based confirmation from either infected hosts or cultures that P. neoaphidis is able to produce resting spores.     

Natural enemy ravine revisited: the importance of sample size for determining population growth

Abstract. 1. The population growth of three aphid species, Metopolophium dirhodum (Walker), Rhopalosiphum padi (L.), and Sitobion avenae (F.), on winter wheat, was analysed by regression. The calculations were based on censuses of aphids made in 268 plots at 3- or 7-day intervals for 10 years on leaves and 6 years on ears. The calculations were made separately for each plot each year, then repeated on the pooled data from all plots monitored in a year.
2. At the level of individual plots, no population growth was detected at very low densities. At high densities, the populations grew exponentially and the growth rates did not decrease with increasing aphid density.
3. Significant growth was always detected in the pooled data. These growth rates decreased significantly at the highest densities. Field estimates of the intrinsic rate of increase derived from these data ranged from 0.010 to 0.026 for M. dirhodum , 0.0071–0.011 for R. padi , and 0.00078–0.0061 and 0.0015–0.13 for S. avenae , on leaves and ears respectively .
4. The apparent lack of growth in the individual plots at low densities is attributable to small sample size. It is concluded that the natural enemy ravine in the population dynamics of cereal aphids, identified by Southwood and Comins (1976), is a consequence of low population densities at which population increase is undetectable unless very large samples are taken.     

取食感染大麦黄矮病毒的小麦后介体麦长管蚜和非介体禾谷缢管蚜体内抗氧化酶和解毒酶活性的变化

【目的】小麦黄矮病［病原为大麦黄矮病毒(barley yellow dwarf virus, BYDV)］是危害小麦生产的主要病害之一,GAV是BYDV在我国的主流株系,且其在田间与介体麦长管蚜 Sitobion avenae 和非介体禾谷缢管蚜Rhopalosiphum padi同时发生,对小麦产量造成严重影响。本研究旨在探究大麦黄矮病毒胁迫下介体和非介体蚜虫体内重要保护酶和解毒酶活性变化规律,为揭示病毒 蚜虫互作生理生化机理提供参考。【方法】利用生化方法测定取食健康小麦(空白对照组),取食前期经无毒麦二叉蚜Schizaphis graminum 3龄若蚜为害72 h的小麦(条件对照组)及取食前期经携带BYDV-GAV麦二叉蚜3龄若蚜为害72 h的小麦(处理组)7 d后,介体麦长管蚜与非介体禾谷缢管蚜成蚜体内过氧化物酶(POD)、超氧化物歧化酶(SOD)和过氧化氢酶(CAT)等重要保护酶及乙酰胆碱酯酶(AchE)、酸性磷酸酶(ACP)和碱性磷酸酶(AKP)等重要解毒酶活性变化。【结果】取食前期经BYDV-GAV感染的麦二叉蚜3龄若蚜为害的小麦7 d后,介体麦长管蚜成蚜体内POD, SOD和AKP活性比空白对照组显著上升,但与条件对照组相比无显著差异;且条件对照组比空白对照组显著上升。取食经BYDV-GAV感染的麦二叉蚜3龄若蚜为害的小麦7 d后,非介体禾谷缢管蚜成蚜体内SOD,AKP和AchE活性比空白对照组显著下降,ACP活性显著上升,但与条件对照组相比仅ACP活性显著上升;且条件对照组禾谷缢管蚜体内AKP和AchE比空白对照组显著下降,ACP活性显著上升。【结论】取食感染BYDV-GAV小麦后介体麦长管蚜成蚜体内POD, SOD和AKP酶活性升高是前期蚜虫的为害造成,与BYDV-GAV无关;非介体禾谷缢管蚜成蚜体内AKP和AchE酶活性下降是前期蚜虫为害引起,SOD酶活性下降是前期蚜虫为害和BYDV-GAV的综合作用,而BYDV-GAV和前期蚜虫为害均能使禾谷缢管蚜体内ACP酶活性上升。     

Seasonal dynamics of cereal aphids on Russian wheat aphid (Homoptera: Aphididae) susceptible and resistant wheats

Field experiments were conducted in 1997 and 1998 to evaluate the impact of resistance to Russian wheat aphid, Diuraphis noxia (Mordvilko), on the cereal aphid complex in wheat. Two spring wheats were planted: the variety "Centennial" (Russian wheat aphid susceptible) and the advanced line IDO488 (Russian wheat aphid resistant). IDO488 incorporates the resistance found in PI 294994 into a Centennial background. Field plots were artificially infested with adult D. noxia and sampled weekly. The most abundant aphid species in 1997 were Metopolophium dirhodum (Walker), Sitobion avenae (F.), D. noxia, and Rhopalosiphum padi (L.). In 1998, the order of abundance was M. dirhodum, R. padi, S. avenae, and D. noxia. The resistant genotype had significantly fewer D. noxia than the susceptible one during both years. However, plant genotype had no significant effect on the other aphid species in either year. Both the initial density of D. noxia and plant growth stage, had a significant effect on D. noxia population development, but had no effect on the other aphid species. There was no interaction between D. noxia resistance and the population density of the other aphid species observed.     

Population dynamics of cereal aphids: influence of a shared predator and weather

1 Aphid populations may show strong year-to-year fluctuations, but questions remain regarding the dominance of factors that cause this variation, especially the role of natural enemies. To better understand the dynamics of aphid species that occur as pests in cereals, we investigated the relative influence of top-down control by a predator and weather (temperature and precipitation) on population fluctuations of three cereal aphid species.
2 From 1987 to 2005, populations of Metopolophium dirhodum , Sitobion avenae and Rhopalosiphum padi in insecticide-free stands of winter wheat were monitored in the Praha-Ruzyné region of the Czech Republic. Densities of an aphidophagous predator, the ladybeetle Coccinella septempunctata , were recorded from an overwintering site in the landscape. Weather was quantified using historical records.
3 A significant bottom-up effect of densities of aphids on those of C.   septempunctata was found, but evidence of direct top-down regulation of aphids by C.   septempunctata was only significant in the case of R.   padi . There was no significant periodicity in the dynamics of the aphid or C.   septempunctata , suggesting that there was no clear predator-prey cycle. Combinations of C.   septempunctata and weather variables could be used to explain M.   dirhodum and R.   padi per capita rate of change. There were also indications that weather directly affected peak density of M.   dirhodum .
4 We conclude that regional estimates of C.   septempunctata densities are not sufficient to determine whether aphid population dynamics are driven by predator–prey interactions. Feasibility of time series analysis as an investigative tool in aphid population dynamics studies is discussed.     

The reactions of two cereal aphid parasitoids, Aphidius uzbekistanicus and A. ervi to host aphids and their food-plants

ABSTRACT. A Y-tube olfactometer was used to test the reactions of the hymenopteran cereal aphid parasitoids Aphidius uzbekistanicus Luzhetski and A. ervi Haliday to odours from aphids and their host plants. Only females responded to aphids but both sexes responded to plant odours. A. uzbekistanicus responded to the cereal aphids Sitobion avenae (F.) and Metopolophium dirhodum (Walker) whilst A. ervi , which has a broad host range, responded to M. dirhodum and the pea aphid, Acyrthosiphon pisum. Female A. uzbekistanicus responded to wheat leaves only but males responded to a range of plant material. Both male and female A. ervi responded to wheat and bean leaves. The failure of A. ervi to respond to either nettle aphids, Microlophium carnosum (Bukt.), or nettle leaves, despite its frequent parasitization of this aphid in the field, suggests the existence of more than one race of the parasitoid and casts doubts on the usefulness of alternative hosts as reservoirs for A. ervi in integrated control programmes. Males of both species responded to their respective females suggesting the presence of a sex specific attractant.