Effects of host plant drought stress on the performance of the bird cherry-oat aphid, Rhopalosiphum padi (L.): a mechanistic analysis

Abstract.  1. The growth (increase in height and leaf number) of four grass species was reduced by a −0.5 MPa drought stress, but the performance of an associated herbivore, Rhopalosiphum padi (L.), was not affected consistently. The intrinsic rate of increase of R. padi was reduced by drought stress on three grass species, including Dactylis glomerata (L.), but was unaffected on Arrhenatherum elatius (L.). Therefore, there is no general relationship in the effect of plant drought on an insect herbivore, even among closely related host plant species.
2. Drought stress increased the quality of plant phloem sap, as indicated by increased sieve element osmotic pressure and essential amino acid concentrations. Thus, diet quality could not account for the reduced performance of R. padi under drought stress. The concentration of essential amino acids in the phloem of well-watered A. elatius was, however, lower than that of well-watered D. glomerata , correlating with the decreased performance of aphids on well-watered A. elatius .
3. There were no differences in aphid feeding duration between watering treatments or plant species but sap ingestion rates were reduced significantly under drought stress.
4. Using the measure of dietary amino acid concentrations and the estimate of sap ingestion, the essential amino acid flux through aphids was calculated. Compared with the flux through aphids feeding on well-watered D. glomerata , there was a reduction in aphids feeding on drought-stressed D. glomerata and drought-stressed A. elatius due to lower sap ingestion rates. The flux through aphids on well-watered A. elatius was also reduced due to low phloem essential amino acid concentrations. Thus, the performance of an aphid is correlated with the availability and accessibility of essential amino acids.     

Comparisons of biomass,water use efficiency and water use strategies across five genomic groups of Populus and its hybrids

Genetic improvement and hybridization in the Populus genus have led to the development of genotypes exhibiting fast growth, high rooting ability and disease resistance. However, while large biomass production is important for bioenergy crops, efficient use of resources including water is also important in sites lacking irrigation and for maintaining ecosystem water availability. In addition, comparison of water use strategies across a range of growth rates and genetic variability can elucidate whether certain strategies are shared among the fastest growing and/or most water use efficient genotypes. We estimated tree water use throughout the second growing season via sapflow sensors of 48 genotypes from five Populus taxa; P. deltoides W. Bartram ex Marshall × P. deltoides (D × D), P. deltoides × P. maximowiczii A. Henry (D × M), P. deltoides × P. nigra L. (D × N), P. deltoides × P. trichocarpa Torr. & Gray (D × T) and P. trichocarpa × P. deltoides (T × D) and calculated average canopy stomatal conductance (G_S). We regressed G_S and atmospheric vapor pressure deficit (VPD) wherein the slope of the relationship represents stomatal sensitivity to VPD. At the end of the second growing season, trees were harvested, and their dry woody biomass was used to calculate whole tree water use efficiency (WUE_T). We found that D × D and D × M genotypes exhibited differing water use strategies with D × D genotypes exhibiting high stomatal sensitivity while retaining leaves while D × M genotypes lost leaf area throughout the growing season but exhibited low stomatal sensitivity. Across measured taxa, biomass growth was positively correlated with WUE_T, and genotypes representing each measured taxa except D × N and T × D had high 2-year dry biomass of above 6 kg/tree. Overall, these data can be used to select Populus genotypes that combine high biomass growth with stomatal sensitivity and WUE_T to limit the negative impacts of bioenergy plantations on ecosystem water resources.     

Stress‐induced changes in abundance differ among obligate and facultative endosymbionts of the soybean aphid

Bacterial endosymbionts can drive evolutionary novelty by conferring adaptive benefits under adverse environmental conditions. Among aphid species there is growing evidence that symbionts influence tolerance to various forms of stress. However, the extent to which stress inflicted on the aphid host has cascading effects on symbiont community dynamics remains poorly understood. Here we simultaneously quantified the effect of host‐plant induced and xenobiotic stress on soybean aphid (Aphis glycines) fitness and relative abundance of its three bacterial symbionts. Exposure to soybean defensive stress (Rag1 gene) and a neurotoxic insecticide (thiamethoxam) substantially reduced aphid composite fitness (survival × reproduction) by 74 ± 10% and 92 ± 2%, respectively, which in turn induced distinctive changes in the endosymbiont microbiota. When challenged by host‐plant defenses a 1.4‐fold reduction in abundance of the obligate symbiont Buchnera was observed across four aphid clonal lines. Among facultative symbionts of Rag1‐stressed aphids, Wolbachia abundance increased twofold and Arsenophonus decreased 1.5‐fold. A similar pattern was observed under xenobiotic stress, with Buchnera and Arsenophonus titers decreasing (1.3‐fold) and Wolbachia increasing (1.5‐fold). Furthermore, variation in aphid virulence to Rag1 was positively correlated with changes in Arsenophonus titers, but not Wolbachia or Buchnera. A single Arsenophonus multi‐locus genotype was found among aphid clonal lines, indicating strain diversity is not primarily responsible for correlated host‐symbiont stress levels. Overall, our results demonstrate the nature of aphid symbioses can significantly affect the outcome of interactions under stress and suggests general changes in the microbiome can occur across multiple stress types.     

The relationship between guard cell water potential and the aperture of stomata in Populus

Abstract Previous work with clones of Populus trichocarpa demonstrated that the water vapour conductance of leaves from well-watered cuttings of this species does not decline with loss of turgor from the bulk leaf. In the present study, stomatal responses to water potential in Populus were examined with detached epidermal strips. Stomata in epidermal strips from well-watered plants of P. trichocarpa did not close at low water potentials which led to plasmolysis of the guard cells. In contrast, stomata of P. deltoides and a P. trichocarpa×deltoides hybrid closed when the guard cells lost turgor. A period of water stress preconditioning resulted in modified stomatal responses in P. trichocarpa such that stomata of stressed and re-watered plants nearly closed when guard cell turgor was lost.     

The effects of spacing on growth, morphology and biomass production and allocation in two hybrid poplar clones growing in the boreal region of Canada

Intra-clonal competition was studied in young hybrid poplar plantations to assess the effects of spacing on growth, biomass production and allocation, and morphological characteristics of above- and below-ground tree parts. Three spacings were used as whole-plots (1 × 1 m, 3 × 3 m and 5 × 5 m), with two hybrid poplar clones as subplots (BT747, Populus balsamifera L. × P. trichocarpa Torr. & Gray; MB915, P. maximowiczii A. Henry × P. balsamifera L.) in a split-plot design. After six growing seasons, diameter at breast height (dbh) increased by about 120% from the 1 × 1 m to the 5 × 5 m spacing for clone MB915, while there was no significant change in dbh for the other clone. The effect of spacing on height growth was opposite for the clones; it increased by about 175% from the narrowest to the widest spacing for clone MB915, while it decreased by about 27% for clone BT747. Estimates of above-ground biomass production after six growing seasons were significantly reduced with increasing spacing, with 29.6, 4.9 and 3.2 MgDM ha⁻¹ on average from the narrowest to the widest spacing. Branch traits and the vertical distribution of leaf area were the most affected by spacing for both clones, while live crown ratio and percentage of syllepsis did not change. Spacing also affected proportions of biomass allocated to stem, leaves, and branches, but allocation to roots did not change.     

Infection of Arabidopsis by cucumber mosaic virus triggers jasmonate-dependent resistance to aphids that relies partly on the pattern-triggered immunity factor BAK1

Many aphid-vectored viruses are transmitted nonpersistently via transient attachment of virus particles to aphid mouthparts and are most effectively acquired or transmitted during brief stylet punctures of epidermal cells. In Arabidopsis thaliana, the aphid-transmitted virus cucumber mosaic virus (CMV) induces feeding deterrence against the polyphagous aphid Myzus persicae. This form of resistance inhibits prolonged phloem feeding but promotes virus acquisition by aphids because it encourages probing of plant epidermal cells. When aphids are confined on CMV-infected plants, feeding deterrence reduces their growth and reproduction. We found that CMV-induced inhibition of growth as well as CMV-induced inhibition of reproduction of M. persicae are dependent upon jasmonate-mediated signalling. BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 (BAK1) is a co-receptor enabling detection of microbe-associated molecular patterns and induction of pattern-triggered immunity (PTI). In plants carrying the mutant bak1-5 allele, CMV induced inhibition of M. persicae reproduction but not inhibition of aphid growth. We conclude that in wildtype plants CMV induces two mechanisms that diminish performance of M. persicae: a jasmonate-dependent and PTI-dependent mechanism that inhibits aphid growth, and a jasmonate-dependent, PTI-independent mechanism that inhibits reproduction. The growth of two crucifer specialist aphids, Lipaphis erysimi and Brevicoryne brassicae, was not affected when confined on CMV-infected A. thaliana. However, B. brassicae reproduction was inhibited on CMV-infected plants. This suggests that in A. thaliana CMV-induced resistance to aphids, which is thought to incentivize virus vectoring, has greater effects on polyphagous than on crucifer specialist aphids.     

Tolerance of antibiotic and susceptible cereal seedlings to the aphids Metopolophium dirhodum and Rhopalosiphum padi

Some cereal seedlings exhibit antibiotic and antixenotic resistance to the aphids Metopolophium dirhodum (Walker) and Rhopalosiphum padi (L.), because the seedlings contain hydroxamic acids or gramine. The association between tolerance to aphids and aphid antibiosis was investigated for three cereals, Dollarbird wheat Vulcan wheat and Yagan barley. The dry biomass gained by the aphids and the simultaneous reduction in the biomass of the plants (biomass conversion ratio) quantified tolerance. Biomass production and the density dependence of biomass production by the aphids quantified antibiosis more effectively than fecundity. Vulcan wheat, which has more hydroxamic acid than Dollarbird wheat showed the highest level of antibiosis, and the barley was not antibiotic for either aphid. The biomass conversion ratio was a constant; the biomass of an infested plant was reduced by 3 mg for each mg of aphid biomass gained, regardless of aphid species, plant cultivar, or aphid density. The three plants showed no differential tolerance to the aphids, and therefore tolerance is not associated with antibiosis in this case.     

Genotype‐by‐genotype specificity remains robust to average temperature variation in an aphid/endosymbiont/parasitoid system

Genotype‐by‐genotype interactions demonstrate the existence of variation upon which selection acts in host–parasite systems at respective resistance and infection loci. These interactions can potentially be modified by environmental factors, which would entail that different genotypes are selected under different environmental conditions. In the current study, we checked for a G × G × E interaction in the context of average temperature and the genotypes of asexual lines of the endoparasitoid wasp Lysiphlebus fabarum and isolates of Hamiltonella defensa, a protective secondary endosymbiont of the wasp's host, the black bean aphid Aphis fabae. We exposed genetically identical aphids harbouring different isolates of H. defensa to three asexual lines of the parasitoid and measured parasitism success under three different temperatures (15, 22 and 29 °C). Although there was clear evidence for increased susceptibility to parasitoids at the highest average temperature and a strong G × G interaction between the host's symbionts and the parasitoids, no modifying effect of temperature, that is, no significant G × G × E interaction, was detected. This robustness of the observed specificity suggests that the relative fitness of different parasitoid genotypes on hosts protected by particular symbionts remains uncomplicated by spatial or temporal variation in temperature, which should facilitate biological control strategies.     

Food quality of Ephestia eggs,the aphid Rhopalosiphum padi and mixed diet for Orius majusculus

We studied the food quality of the aphid Rhopalosiphum padi to the pirate bug Orius majusculus using Ephestia eggs as high-quality comparison prey. Several performance parameters were tested on individuals that had been reared and maintained on each of the two single-prey diets or on a mixed diet. All fitness parameters were lower in individuals fed aphids only, indicating poor food quality of this prey. Compared with the pure Ephestia egg diet, the mixed diet enhanced teneral mass, while adult survival and female starvation tolerance were negatively affected and all other traits were unaffected. Body protein proportions were constant across diets, whereas lipid proportion was low in the aphid treatment. Preference for aphids was lower following a monotypic aphid diet than when reared on Ephestia eggs or a mixed diet. The results confirm that R. padi is low-quality food for O. majusculus as it is for other generalist predators, even though O. majusculus may contribute significantly to population suppression of the aphid.     

Genetic mapping shows intraspecific variation and transgressive segregation for caterpillar‐induced aphid resistance in maize

Plants in nature have inducible defences that sometimes lead to targeted resistance against particular herbivores, but susceptibility to others. The metabolic diversity and genetic resources available for maize (Zea mays) make this a suitable system for a mechanistic study of within‐species variation in such plant‐mediated interactions between herbivores. Beet armyworms (Spodoptera exigua) and corn leaf aphids (Rhopalosiphum maidis) are two naturally occurring maize herbivores with different feeding habits. Whereas chewing herbivore‐induced methylation of 2,4‐dihydroxy‐7‐methoxy‐1,4‐benzoxazin‐3‐one glucoside (DIMBOA‐Glc) to form 2‐hydroxy‐4,7‐dimethoxy‐1,4‐benzoxazin‐3‐one glucoside (HDMBOA‐Glc) promotes caterpillar resistance, lower DIMBOA‐Glc levels favour aphid reproduction. Thus, caterpillar‐induced DIMBOA‐Glc methyltransferase activity in maize is predicted to promote aphid growth. To test this hypothesis, the impact of S. exigua feeding on R. maidis progeny production was assessed using seventeen genetically diverse maize inbred lines. Whereas aphid progeny production was increased by prior caterpillar feeding on lines B73, Ki11, Ki3 and Tx303, it decreased on lines Ky21, CML103, Mo18W and W22. Genetic mapping of this trait in a population of B73 × Ky21 recombinant inbred lines identified significant quantitative trait loci on maize chromosomes 1, 7 and 10. There is a transgressive segregation for aphid resistance, with the Ky21 alleles on chromosomes 1 and 7 and the B73 allele on chromosome 10 increasing aphid progeny production. The chromosome 1 QTL coincides with a cluster of three maize genes encoding benzoxazinoid O‐methyltransferases that convert DIMBOA‐Glc to HDMBOA‐Glc. Gene expression studies and benzoxazinoid measurements indicate that S. exigua ‐induced responses in this pathway differentially affect R. maidis resistance in B73 and Ky21.     

Feeding location affects demographic performance of cabbage aphids on winter canola

The cabbage aphid, Brevicoryne brassicae L. (Hemiptera: Aphididae), is a perennial pest that specializes on plants of the Brassicaceae family, attacking winter canola (Brassica napus L.) mainly during and after flowering. Under field conditions, cabbage aphid colonizes the upper flowering canopy. Population dynamics of aphids in the flowering canopy could be regulated by differences in either plant quality (bottom‐up) or predatory (top‐down) forces. The goal of our study was to determine the effect of feeding location on cabbage aphid demography. A stage‐structured matrix population model was constructed for aphids restricted to reproductive or vegetative plant tissues of canola. We found that feeding location had a large impact on demography of cabbage aphid; the finite rate of increase (λ ± SEM) was higher when aphids were restricted to reproductive tissues, compared to aphids feeding on vegetative tissues: 1.25 ± 0.01 vs. 1.17 ± 0.01 (leaves). Aphids confined to reproductive tissues with higher λ exhibited shorter generation times (T = 14.2 ± 0.2 days) and 53–75% higher net reproductive rates (R₀ = 23.3 ± 1.7) than aphids feeding on vegetative tissues. Prospective analyses showed that there was a nymph‐skewed stable stage distribution, and elasticity values revealed that λ is most sensitive to changes in stasis of adults staying in the adult stage and to adult survival. Retrospective analyses indicated that variation in adult fecundity (value of 0.05) had the largest effect on population dynamics but collectively, growth of nymphal stage 2–3, 3–4, and 4 to adult accounted for most of the difference in λ between the treatments. Monitoring programs should target adults and penultimate instars colonizing reproductive tissues of canola plants in the field as aphids on these plant structures contribute most to population growth.     

Seasonal changes in the density of the symbionts Buchnera and Wolbachia of the aphid Tuberculatus macrotuberculatus on Quercus dentata

Seasonal changes in leaf traits and tannins, including hydrolysable tannins (HTs) and condensed tannins (CTs), affect aphid body size and demographic patterns. The aphid Tuberculatus macrotuberculatus Essig & Kuwana (Hemiptera: Aphididae) feeds on the leaves of the daimyo oak, Quercus dentata Thunberg (Fagaceae), does not alternate hosts, and is attended continuously by ants. Buchnera aphidicola Munson et al. (a γ‐proteobacterium, hereafter Buchnera) is the primary symbiont of most aphid species. It provides essential amino acids to host aphids. Wolbachia sp. (an α‐proteobacterium) is present in some aphid species. This study investigated the effects of seasonal tannin and leaf trait fluctuations in Q. dentata on aphid performance and Buchnera and Wolbachia densities in T. macrotuberculatus. As the season progressed, the water content and HT concentration in Q. dentata leaves decreased, CT concentration in Q. dentata leaves increased, and T. macrotuberculatus performance decreased. Buchnera density varied in accordance with host aphid performance, whereas Wolbachia density did not vary throughout the season, suggesting that although Buchnera depends on the host plant and host aphid performance for growth, Wolbachia may have a high tolerance for oligotrophic environments and may exist independent of the nutritional metabolism of the host aphid. Although the role of Wolbachia in T. macrotuberculatus remains unclear, it may be associated with resistance to parasitoid wasps and plant secondary metabolites.     

Plant water stress intensity mediates aphid host choice and feeding behaviour

1. The effects of drought-induced changes in plant quality on aphid performance and population growth is well-studied. The response of aphid behaviour to plant water limitation has received less attention. Water limitation may affect host-plant colonization by altering the attractiveness of plants. Additionally, plant water limitation may inhibit feeding site establishment and phloem ingestion.
2. Our goal was to examine bird cherry-oat aphid (Rhopalosiphum padi L.) host selection and feeding behaviour under water limitation. We assessed aphid response to well-watered, mildly-stressed, and highly-stressed wheat (Triticum aestivum L.) by evaluating (i) host-plant selection through two-choice assays, (ii) feeding behaviour using the electrical penetration graph technique, and (iii) phloem ingestion by quantifying honeydew production.
3. Aphids were less likely to select highly stressed plants than a mildly stressed or well-watered alternative. Aphids did not distinguish between mildly stressed and well-watered plants. Aphid feeding behaviours, including duration of phloem ingestion, were not affected by water availability. However, honeydew production was reduced under both levels of water limitation. These results suggest that the volume of phloem ingested by aphids per unit time declined on stressed plants. The combination of lower colonization and diminished access to food on stressed plants may lead to a reduction in aphid abundance, independent of the direct effects of nutrition on individual aphid performance.
4. This study highlights the potential contribution of herbivore behaviour to documented changes in aphid abundance on stressed plants and underscores the important role of plant water stress intensity in mediating plant-herbivore interactions.

     

Drought‐stress and plant resistance affect herbivore performance and proteome: the case of the green peach aphid Myzus persicae (Hemiptera: Aphididae)

Little is known about the simultaneous effects of drought stress and plant resistance on herbivorous insects. By subjecting the green peach aphid Myzus persicae Sulzer to well‐watered and drought‐stressed plants of both susceptible and resistant peach (Prunus persica), the effects of both stressors on aphid performance and proteomics are tested. Overall, the influence of the water treatment on aphid performance is less pronounced than the effect of host plant genetic resistance. On the susceptible cultivar, aphid survival, host acceptance and ability to colonize the plant do not depend on water treatment. On the resistant cultivar, aphid survival and ability to colonize are higher on drought‐stressed than on well‐watered plants. A study examining the pattern of protein expression aiming to explain the variation in aphid performance finds higher protein expression in aphids on the drought‐stressed susceptible cultivars compared with the well‐watered ones. In the susceptible cultivar, the regulated proteins are related to energy metabolism and exoskeleton functionality, whereas, in the resistant cultivar, the proteins are involved with the cytoskeleton. Comparison of the protein expression ratios for resistant versus susceptible plants reveals that four proteins are down‐regulated in well‐watered plants and 15 proteins are down‐regulated in drought‐stressed plants. Drought stress applied to the susceptible cultivar induces the regulation of proteins in M. persicae that enable physiological adaptation to maintain an almost unaltered aphid performance. By contrast, for aphids on the resistant cultivar subjected to drought stress, the down‐regulation of proteins responds to an induced host susceptibility effect.     

Thermal biology and establishment potential in temperate climates of the aphid parasitoid, <Emphasis Type="Italic">Lysiphlebus testaceipes</Emphasis>

Lysiphlebus testaceipes (Cresson) (Hymenoptera: Braconidae, Aphidiinae) is a parasitic wasp which plays an important role in the biological control of a number of aphid species. Through assessment of its thermal biology and low temperature tolerance, this study ascertains the establishment potential of L. testaceipes in cool temperate climates typical of northern Europe. The developmental threshold of L. testaceipes was 5.8°C. Rearing of parasitoids at shorter day lengths and lower temperatures indicated no ability to enter a diapause state. The supercooling points (SCP) of non-acclimated and acclimated parasitoid life stages were between −24.6°C and −17.7°C, with LTemp₅₀ temperatures approaching these values, indicating a high level of cold tolerance in short exposures. At 5°C the LTime₅₀ of acclimated larvae within parasitized aphids was 42.8 days. Acclimated pupae continued to develop with 54% adult emergence from mummies within 60 days. Acclimated parasitoid larvae and pupae, within living and mummified aphids, continued to develop during 70 days of winter field exposure and emerging adult parasitoids were reproductively viable under field conditions. These data indicate that where suitable host species are available throughout the year, L. testaceipes would be able to establish in northern Europe.     

Estimating costs of aphid resistance to parasitoids conferred by a protective strain of the bacterial endosymbiont Regiella insecticola

Heritable bacterial endosymbionts are common in aphids (Hemiptera: Aphididae), and they can influence ecologically important traits of their hosts. It is generally assumed that their persistence in a population is dependent on a balance between the costs and benefits they confer. A good example is Hamiltonella defensa Moran et al., a facultative symbiont that provides a benefit by strongly increasing aphid resistance to parasitoid wasps, but becomes costly to the host in the absence of parasitoids. Regiella insecticola Moran et al. is another common symbiont of aphids and generally does not influence resistance to parasitoids. In the green peach aphid, Myzus persicae (Sulzer), however, one strain (R5.15) was discovered that behaves like H. defensa in that it provides strong protection against parasitoid wasps. Here we compare R5.15‐infected and uninfected lines of three M. persicae clones to test whether this protective symbiont is costly as well, i.e., whether it has any negative effects on aphid life‐history traits. Furthermore, we transferred R5.15 to two other aphid species, the pea aphid, Acyrthosiphon pisum (Harris), and the black bean aphid, Aphis fabae Scopoli, where this strain is also protective against parasitoids and where we could compare its effects with those of additional, non‐protective strains of R. insecticola. Negative effects of R5.15 on host survival and lifetime reproduction were limited and frequently non‐significant, and these effects were comparable or in one case weaker than those of R. insecticola strains that are not protective against parasitoid wasps. Unless the benefit of protection is counteracted by detrimental effects on traits that were not considered in this study, R. insecticola strain R5.15 should have a high potential to spread in aphid populations.     

QTL analysis for aphid resistance and growth traits in apple

The rosy apple aphid (Dysaphis plantaginea), the leaf-curling aphid (Dysaphis cf. devecta) and the green apple aphid (Aphis pomi) are widespread pest insects that reduce growth of leaves, fruits and shoots in apple (Malus × domestica). Aphid control in apple orchards is generally achieved by insecticides, but alternative management options like growing resistant cultivars are needed for a more sustainable integrated pest management (IPM). A linkage map available for a segregating F₁-cross of the apple cultivars ‘Fiesta’ and ‘Discovery’ was used to investigate the genetic basis of resistance to aphids. Aphid infestation and plant growth characteristics were repeatedly assessed for the same 160 apple genotypes in three different environments and 2 consecutive years. We identified amplified fragment length polymorphism (AFLP) markers linked to quantitative trait loci (QTLs) for resistance to D. plantaginea (‘Fiesta’ linkage group 17, locus 57.7, marker E33M35–0269; heritability: 28.3%), and to D. cf. devecta (‘Fiesta’ linkage group 7, locus 4.5, marker E32M39–0195; heritability: 50.2%). Interactions between aphid species, differences in climatic conditions and the spatial distribution of aphid infestation were identified as possible factors impeding the detection of QTLs. A pedigree analysis of simple sequence repeat (SSR) marker alleles closely associated with the QTL markers revealed the presence of the alleles in other apple cultivars with reported aphid resistance (‘Wagener’, ‘Cox’s Orange Pippin’), highlighting the genetic basis and also the potential for gene pyramiding of aphid resistance in apple. Finally, significant QTLs for shoot length and stem diameter were identified, while there was no relationship between aphid resistance and plant trait QTLs. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

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.     

Occurrence of cotton aphids (Aphis gossypii) and lady beetles (Hamonia axyridis) on Hibiscus syriacus Linne: Are the aphids a pest of cucurbits?

Populations of cotton aphid on Hibiscus syriacus increased rapidly from 17 to 24 May 2007, and then decreased as its predator, the lady beetle Hamonia axyridis, increased in number. There was a 10 day time lag between peak populations of aphids and lady beetles. The infestation of aphids on H. syriacus produced some damage, but H. syriacus recovered soon after the lady beetles arrived. Cotton aphid clones from H. syriacus were transferred to other summer host plants: to five different vegetables on two dates, and to cucumber on three dates. Apart from one case where reproduction occurred on eggplant, most H. syriacus aphid clones did not survive on the vegetables. The cotton aphid on H. syriancus was prey and a food source for H. axyridis and acted to conserve natural enemies.