Showing papers on "Aphididae published in 1991"

Aphid transmission of barley yellow dwarf virus: Acquisition access periods and virus concentration requirements.

Abstract: The duration of access periods and the availability of virus in source plants are two factors that influence the transmission of barley yellow dwarf virus (BYDV) by its aphid vectors. This study was conducted to quantify the relationships among acquisition access period (AAP), virus titer in infected oats, and transmission of three isolates of BYDV from New York by two aphid vector species. Thirteen AAPs, ranging from 15 min to 72 hr, were examined, and virus titer was quantified from each virus source leaf by using enzyme-linked immunosorbent assay (ELISA) (...)

Detection and Mechanisms of Russian Wheat Aphid (Homoptera: Aphididae) Resistance in Barley

Abstract: A collection of 524 barley lines from areas of the world where the Russian wheat aphid, Diuraphts noxia (Mordvilko), and barley, Hordeum vulgare L., have coexisted was evaluated for sympatric resistance to this pest. Mass screening tests were conducted in a growth chamber and greenhouse with seedlingsplanted in flats. Additional teststo determine the mechanisms of resistance were performed with nine of these lines. The lines and their sources are PI 366444, PI 366447, PI 366449, PI 366450, PI 366453 (Afghanistan); CI 1412, PI 430140, PI 430142 (Iran); and PI 447219 (Spain). Various levels of antibiosis and tolerance were exhibited in most of the lines tested in comparison with ‘Wintermalt’ (CI 15767), the susceptible control. For example, in the antibiosis test, an average of 27.3 nymphs per adult were produced on PI 366449 compared with 50.0 on ‘Wintermalt’. In the tolerance test, plant growth and leaf area of some of the resistant entries were not affected by the Russian wheat aphid, whereas growth and leaf area of infested ‘Wintermalt’ plants was only 61% of noninfested ‘Wintermalt’ plants. Plant survivors of these tests have been saved for developing Russian wheat aphid plant-resistant germplasm for the North American barley industry.

Natural Control of Cereal Aphids (Homoptera: Aphididae) by Entomopathogenic Fungi (Zygomycetes: Entomophthorales) and Parasitoids (Hymenoptera: Braconidae and Encyrtidae) on Irrigated Spring Wheat in Southwestern Idaho

Abstract: Data on the natural control of cereal aphids, Diuraphis noxia (Mordvilko), Metopolophium dirhodum (Walker), and Sitobion avenae (F.), by entomophthoralean fungi and hymenopterous parasitoids on furrow-irrigated spring wheat in southwestern Idaho are presented. The important fungi involved were Pandora neoaphidis (Remaudire & Hennebert) Humber and Conidiobolus spp., including C. obscurus (Hall and Dunn) Remaudire, C. thromboides Drechsler, and C. coronatus (Constantin) Batko. During 1986–1989, M. dirhodum and S. avenae populations usually reached or exceeded economic levels. D. noxia did not enter the region until late June 1987. It was the first aphid found infesting the crop in 1988 and reached high densities that summer but was not found in 1989, perhaps because of mortality during an extremely cold winter. Fungal infection occurred ≍2,3, and 6 wk after colonization of the crop by M. dirhodum, S. avenae , and D. noxia, respectively. Epizootics occurred each summer but usually after the crop was damaged by large aphid populations. An exception occurred during 1987 when M. dirhodum and S. avenae populations were effectively suppressed by mycoses, apparently enhanced by unusually frequent rainfall during late May and June. M. dirhodum experienced much higher mortality from fungal infection than did S. avenae and D. noxia. P. neoaphidis was the dominant fungus infecting M. dirhodum and D. noxia, whereas Conidiobolus spp. were most important on S. avenae. Entomophthora chromaphidis Burger & Swain and two Zoophthora species sporadically infected cereal aphids. Parasitoids, mainly Aphidius ervi Haliday for S. avenae and M. dirhodum and Diaeretiella rapae (M'Intosh) for D. noxia, usually attacked cereal aphids earlier than the fungi but were less influential during the decline of host populations. Multiple regression and correlation analysis indicated that mycoses and parasitoids made significant contributions to the reduction of peak populations of each aphid species compared with the influence of crop maturation. Weak correlation between fungal infection and host density and climatic factors ( r2 ranging from 0.35 to 0.53) was found. Development of fungal infection seemed to be better correlated with host density than with climatic factors. Precipitation was more associated with fungal infection of S. avenae, which inhabits upper portions of the host crop, but had little effect on that of M. dirhodum and D. noxia, which inhabit more humid microenvironments. No other climatic factors were significantly correlated with fungal infection. A hypothesis is proposed that the site on a wheat plant typically occupied by an aphid species influences the development of mycoses.

Dispersal of apterous aphids (Homoptera: Aphididae) from their host plant and its significance

Abstract: Dispersal of apterae from their natal plant was monitored for three aphid species (Myzus persicae (Sulzer), Brevicoryne brassicae (Linnaeus) and Megoura viciae Buckton), each on a different host plant. Some movement off the plant occurred even in the initial parental and second generations, when the plant appeared to be growing normally and when crowding was unlikely to be the cause. However, in most experiments the most marked increase in apterous dispersal coincided with a reduction or even cessation of plant growth. In nearly all the experiments this dispersal occurred when the population on the natal plant exceeded about 20 adults and their offspring per plant. The main emigrants were fourth instar apterae and prelarviposition adult apterae. The implications of this behaviour are discussed, and it is argued that apterous dispersal might be common in aphids, allowing a fuller utilization of the environment than would be likely by alate dispersal alone.

Identification of Russian Wheat Aphid (Homoptera: Aphididae) Resistance in Wheat

Abstract: Greenhouse and growth chamber evaluations were used to determine sources of resistance in wheat to populations of the Russian wheat aphid, Diuraphis nona (Mordvilko), in Idaho. Sources of resistance to D.noxia previously identified in South Africa were confirmed as resistant to the Idaho population. Low levels of resistance were also identified in three breeding lines from Idaho, one breeding line from Texas, and the variety ‘Treasure.’ Five new sources of resistance from Iran and the Soviet Union (PI 47545, PI 94355, PI 94365, PI 94460, and PI 151918) were also identified.

Field manipulation of populations of individual staphylinid species in cereals and their impact on aphid populations

Abstract: . 1 Four species of staphylinids were caged separately on wheat and their effect on the population development of the grain aphid, Sitobion avenae (F.), was investigated over two seasons. 2 A novel field cage design was used to exclude all but the staphylinid species under investigation. 3 Predator effects were compared with total-exclusion cages in which all predators and parasitoids were kept out. 4 During a high-density phase of aphid population development only Philonthus cognutus (Stephens) significantly influenced aphid population numbers. 5 At low aphid densities, Tachyporus obtusus (L.), T.chrysomelinus (L.) and P.cognatus reduced aphid population levels. 6 The experiments showed that Tachyporus spp. can reduce the numbers of cereal aphids prior to the exponential phase of aphid population increase and that P.cognatus causes some reduction at both the stages of aphid population growth investigated.

Interactions between Solenopsis invicta (Hymenoptera : formicidae), Rhopalosiphum maidis (Homoptera : aphididae), and the parasitoid Lysiphlebus testaceipes cresson (Hymenoptera : aphidiidae)

Abstract: In the presence of the red imported fire ant, Solenopsis invicta Buren, adult emergence of the parasitoid Lysiphlebus testaceipes Cresson from laboratory colonies of the corn leaf aphid, Rhopalosiphum maidis (Fitch), was reduced. Parasitoids abandoned an attack when ants were within a few millimeters and searched elsewhere, increasing search time. However, the increased searching time in the laboratory was compensated by a reduction in parasitoids' resting time resulting in similar levels of parasitism between the controls and the ant-infested containers. A major effect of the red imported fire ant was the removal and destruction of parasitized aphids. Ants began detecting parasitized aphids 6 d after parasitization and removed them to the ant colony. Honeydew production was reduced in parasitized aphids by 6 d and they resembled starved aphids. However, in contrast to parasitized aphids, starved aphids were moved to healthy plants, when available.

The influence of host food plants on host recognition by four aphidiine parasitoids (Hymenoptera: Braconidae)

Abstract: The attack rate of four parasitoids against different aphid species was investigated in the absence and presence of leaves from aphid food plants. The parasitoids used, Aphidius ervi Haliday, A. rhopalosiphi De Stefani, A. picipes (Nees) and Ephedrus plagiator Nees, varied in the extent of their host ranges. The presence of plant leaves significantly influenced the attack rate of A. ervi and A. rhopalosiphi , the two parasitoids with the most restricted host ranges, but did not appear to affect attack rates by the more polyphagous A. picipes and E. plagiator. Consideration of these results together with those from previous studies on the host preferences of laboratory populations of A. ervi and A. rhopalosiphi leads to the conclusion that genotype is probably determining the responses of individual parasitoids to semiochemicals involved in host recognition. It is proposed that field populations of oligophagous aphid parasitoids consist of a mixture of specialist and generalist individuals.

Aphid transmission of barley yellow dwarf virus : inoculation access periods and epidemiological implications

Abstract: In this study, we examined the efficiency of transmission of three isolates of barley yellow dwarf virus (BYDV) from New York by two aphid species for the purpose of assessing the relative importance of these species as vectors. The influence of the duration of the aphid inoculation access period (IAP) on transmission was investigated for RPV and PAV isolates of BYDV transmitted by Rhopalosiphum padi and for MAV and PAV isolates transmitted by Sitobion avenae. For each aphid-isolate combination, 15 IAPs, ranging from 30 min to 72 hr, were tested (...)

Mycophagy as a factor limiting predation of aphids (Hemiptera: Aphididae) by staphylinid beetles (Coleoptera: Staphylinidae) in cereals

Abstract: Previous work demonstrated the potential of the staphylinids Tachyporus hypnorum (Fabricius), T. chrysomelinus (Linnaeus), T. obtusus (Linnaeus) and Philonthus cognatus (Stephens) to feed on the grain aphid, Sitobion avenae (Fabricius). However, gut dissection and observations of foraging behaviour showed that mycophagy could be a factor influencing the potential of Tachyporus spp. to control aphids. The main fungal material detected in the guts of Tachyporus spp. was spores (conidia) of Erysiphe spp. (powdery mildews). In the present study, numbers of conidia (pustules of mildew on leaf-sections) were offered with aphids in food-choice experiments to test how fungal food affected feeding on aphids by these staphylinids. An index of food preference was calculated for each species, sex and for two age classes of larvae, based on the number of conidia and aphids remaining after feeding. Results showed a preference for mildew conidia by T. hypnorum, no preference by T. chrysomelinus and Tachyporus spp. larvae and a preference for aphids by T. obtusus and P. cognatus (adults and larvae). Food-preferences compared well with dietary composition in recent studies of field collected staphylinids. Futhermore, a functional response was detected to increasing numbers of mildew conidia presented (in mildew only consumption-rate experiments) and to the increase in total food density in food-choice experiments. Mycophagy limited aphid predation and explained the numerical response of staphylinid beetles to areas of cereals with high aphid densities, whilst, in other studies, the proportion of aphids eaten by those beetles did not increase.

Patterns of host selection by four species of aphidiid (Hymenoptera) parasitoids: influence of host switching

Abstract: . 1 We tested switching behaviour in four species of aphidiid parasitoids, using a two-aphid experimental system consisting of second-instar nymphs of pea aphid (Acyrthosiphon pisum (Harris)) and alfalfa aphid (Macrosiphum creelii Davis) feeding on broad beans in the laboratory. 2 Aphidius ervi Haliday, A.pisivorus Smith, A.smithi Sharma & Subba Rao, and Pram pequodorum Viereck showed an innate preference for pea aphid when both host species were provided in equal numbers. 3 Wasps encountered both aphid species equally but differed in their acceptance of alfalfa aphid. Females of A.pisivorus and P.pequodorum accepted alfalfa aphids when few pea aphids were available, but A. smithi always concentrated attacks on pea aphid. Aphidius ervi super-parasitized an increasing proportion of pea aphids as their availability declined. 4 Switching to the alfalfa aphid occurred in A.ervi and P.pequodorum (but not in A.pisivorus and A.smithi) under the condition of a 1:3 ratio of pea aphids:alfalfa aphids. Wasps did not switch when more pea aphids than alfalfa aphids were provided (3:1 ratio). 5 Alfalfa aphids were more likely than pea aphids to escape from parasitoid attack. 6 Switching to the most abundant host may not be adaptive in these four species of aphid parasitoids. A foraging wasp incurs a potentially higher cost in lost opportunity time when attacking (and failing to oviposit in) alfalfa aphids. In addition, alfalfa aphids may have lower host quality than pea aphids, a difference that could influence offspring fitness.

Are ant-aphid associations a tritrophic interaction? Oleander aphids and Argentine ants.

Abstract: Oleander aphids, (Aphis nerii), which are sporadically tended by ants, were used as a moded system to examine whether host plant factors associated with feeding site influenced the formation of ant-aphid associations. Seasonal patterns of host plant utilization and association with attendant ants were examined through bi-weekly censuses of the aphid population feeding on thirty ornamental oleander plands (Nerium oleander) in northern California in 1985 and 1986. Colonies occurred on both developing and senescing plant terminals, including leaf tips, floral structures, and pods. Aphids preferentially colonized leaf terminals early in the season, but showed no preference for feeding site during later periods. Argentine ants (Iridomyrmex humilis) occasionally tended aphid colonies. Colonies on floral tips were three to four times more likely to attract ants than colonies on leaf tips, even though the latter frequently contained more aphids. Ants showed a positive recruitment response to colonies on floral tips, with a significant correlation between colony size and number of ants. There was no recruitment response to colonies on leaf tips. These patterns were reproducible over two years despite large fluctuations in both aphid population density and ant activity. In a laboratory bioassay of aphid palatability, the generalist predator,Hippodamia convergens, took significantly more aphids reared on floral tips compared to those reared on leaf tips. The patterns reported here support the hypothesis that tritrophic factors may be important in modifying higher level arthropod mutualisms.

Potato leafroll virus spread in relation to densities of green peach aphid (Homoptera : Aphididae) : implications for management thresholds for Minnesota seed potatoes

Abstract: Tolerances for potato leafroll virus in potato, Solanum tuberosum L., seed are low in Minnesota, necessitating intensive insecticide use to control the vector, green peach aphid, Myzus persicae (Sulzer). We sought to determine the relation between low green peach aphid densities and potato leafroll spread, and to learn if insecticide use in seed potatoes could be reduced using green peach aphid apterae action thresholds. Plots containing leafroll infected plants were sprayed with methamidophos on thresholds of 0, 10, 30, and 100 apterae per 100 leaves in 1987 and 0, 3, 10, and 30 per 100 leaves in 1988. Percent potato leafroll infection was related to cumulative green peach aphid apterae-days (number of aphids per 100 leaves × number days present) and time of leafroll source introduction (R2 = 0.79). Leafroll infected sample units were more often adjacent than would be expected if occurrence were random in 17 of 18 apterae action threshold plots. The pattern of leafroll spread and the relation with green peach aphid apterae-days suggested that apterae were primarily responsible for spread of the virus. Percent potato leafroll infection in potato, cv. Russet Burbank, was related to the green peach aphid apterae thresholds at which insecticidal sprays were applied, although variation was large within a threshold. Data from these studies suggest that the threshold concept might be employed, but that the threshold for treatment would be very low, perhaps in the range of 3-10 green peach aphid apterae per 100 leaves. Experiments in 1988 showed little spread early in the season, even though aphids and leafroll infected plants were present.

Bioassay of Four Entomophthoralean Fungi (Entomophthorales) Against Diuraphis noxia and Metopolophium dirhodum (Homoptera: Aphididae)

Abstract: A spore shower technique was used to test four species of aphid-derived entomophthoralean fungi, Pandora neoaphidis (Remaudiere & Hennebert) Humber, Zoophthora radicans (Brefeld) Batko, Conidiobolus thromboides Drechsler, and Conidiobolus coronatus (Constantin) Batko, against the cereal aphids, Diuraphis noxia (Mordvilko) and Metopolophium dirhodum (Walker). Six endemic and five foreign isolates of the four fungal species were tested. There was variability in virulence among different isolates and species of the fungi and in susceptibility between the two aphid species. Low LC50's (i.e., high virulence) were produced with two C. coronatus isolates (2.2–4.0 spores/mm2), three C. thromboides isolates (2.3–13.2 spores/mm2), and two P. neoaphidis isolates (1.4–8.1 spores/mm2). Two Z. radicans isolates exhibited moderate virulence with LC50's ranging from 25.1 to 46.8 spores/mm2. Two isolates of C. thromboides that had been in long-term storage displayed relatively low virulence. Aphid colonies on leaves were more suitable for bioassays than aphids confined in wire mesh inoculation chambers. The latter were disadvantageous. because the aphid movement within the chamber adversely affected the determination of fungal inoculum exposure rates. These conditions also often induced the production of resting spores instead of primary conidia.

The longest 18S ribosomal RNA ever known : nucleotide sequence and presumed secondary structure of the 18S rRNA of the pea aphid, Acyrthosiphon pisum

Abstract: An EMBL4 recombinant phage which encodes one of the full length of the aphid ribosomal DNA has been isolated from the aphid genomic library. Determination of the complete nucleotide sequence of the aphid 18S rRNA gene revealed that it is 2469 bp with a G + C content of 59%. The aphid 18S rRNA gene studied here is the longest and has the highest G + C content among the 18S rRNA genes examined so far. Evidence provided by the S1 nuclease assay suggests that the aphid 18S rRNA gene examined in this study is not a pseudogene containing an insertion sequence. Based on the nucleotide sequence of the 18S rRNA gene, we constructed a presumed secondary-structure model of the aphid 18S rRNA. In the aphid 18S rRNA, the eucaryote-specific E21 and 41 region are supposed to be longer and more complex than the counterparts of other 18S rRNA.

Geographical and temporal variation in response to insecticides in various life stages of Aphis gossypii (Homoptera: Aphididae) infesting cotton in California.

Abstract: Five California populations of cotton aphid, Aphis gossypii Glover, ere tested for variability in their responses to the organophosphorus insecticides oxydemeton-methyl, chlorpyrifos, and dicrotophos; the pyrethroid biphenate; and the chlorinated hydrocarbon endosulfan. In the dipped-leaf contact bioassay, alatiform nymphs were significantly more tolerant to all five insecticides compared with apterous adults. A subsequent geographical survey for resistance with single discriminating concentrations revealed that 6 of 13 earlyseason A. gossypii populations had significantly greater survival when exposed to one or more of the organophosphates or endosulfan than the other populations. These populations were collected primarily from the eastern range of the major cotton-growing region of California. A survey of aphid response to pesticides over time showed that survival declined in some of the populations as the season progressed. Populations that had significant survival at the discriminating concentrations as adults also showed improved survival of offspring exposed to residues.

Epicuticular Lipids of Alfalfa Relative to Its Susceptibility to Spotted Alfalfa Aphids (Homoptera: Aphididae)

Abstract: The epicuticular lipids of alfalfa ( Medicago sativa L.) were analysed by gas chromatography to determine if differences exist between plants with varying degrees of resistance to spotted alfalfa aphids, Therioaphis maculata (Buckton). Leaf surfaces of some resistant plants were found to have as much as 50% more wax esters than did those from plants that were susceptible to spotted alfalfa aphids. This difference was found in one set of sibling plants, which had been selected for spotted alfalfa aphid resistance, but not in a resistant plant population with a wider genetic base. Amounts of epicuticular lipids were found to be similar for leaves of the same age from stems of the same plant and from plants with similar genetic backgrounds which were grown under the same conditions.

Plant biochemistry and aphid populations: Studies on the spotted alfalfa aphid, Therioaphis maculata

Abstract: Feeding by the spotted alfalfa aphid, Therioaphis maculata (Buckton), on susceptible alfalfa, Medicago sativa L., results in dramatic changes in plant biochemistry that in turn have profound effects on aphid physiology. These aphids select older leaves on the plant as feeding sites. One component of this selection process may be the amount and composition of plant epicuticular lipids, which vary with leaf age. Feeding aphids induce a senescence-like state in the leaf that is characterized by loss of chlorophyll, decreased levels of soluble protein and fatty acids, and increased production of ethylene. This process involves lipid peroxidation and, like senescence, is probably free-radical-mediated. Leaves of alfalfa having resistance to spotted alfalfa aphid contain higher activities of catalase than do susceptible leaves. This enzyme may function in concert with other antioxidant enzymes to quench aphid-induced free radical damage and thus impart resistance. Aphid fatty acid metabolism is altered by changes in plant metabolism and thus reflects the close relationship between aphid and plant biochemistry.

Antibiosis and Antixenosis to Pea Aphid (Homoptera: Aphididae) in Cultivars of Field Peas

Abstract: Life tables and host preference tests were used to assess the resistance of six cultivars of field peas, Pisum sativum L., to pea aphid, Acyrthosiphon pisum (Harris), in the laboratory. Antibiosisresistance in ‘Tipu’ and ‘Century’ was expressed as decreased aphid fecundity and longevity. Although differences in the reproductive rate were considerable, because prereproductive periods were similar among cultivars, there were only slight differences in the aphids’ intrinsic rate of increase (r m) among cultivars. At 10 d after infestation, ‘Tipu’ exhibited the most and ‘Triumph’ the least antixenotic resistance. At 20 d, ‘Triumph’ was still the preferred cultivar. ‘Trapper’, on which aphids had the greatest r m value, appeared to have some tolerance to aphid feeding.

Alternate hosts of the Russian wheat aphid (Homoptera : Aphididae) in Northeastern Colorado

Abstract: A 2-yr survey of 25 grass species in northeastern Colorado showed that crested wheatgrass, Agropyron cristatum (L.) Gaertn., and Canada wildrye, Elymus canadensis (L.), were dominant hosts of the Russian wheat aphid, Diuraphis noxia (Mordvilko). These two grasses served as hosts between the time winter wheat was harvested and planted. Russian wheat aphids showed greater life stage diversity when collected from Canada wildrye compared with any other grass species. Several additional perennial and annual grasses were able to support Russian wheat aphids but less consistently and in lower numbers. Russian wheat aphids were found on a wide range of grass species early in the summer. Thereafter, numbers of Russian wheat aphid hosts decreased as the summer progressed. Total numbers of Russian wheat aphids collected from all grass hosts for 1989 were seven times greater than for 1988. The 1989 survey indicated a more suitable year for Russian wheat aphid migratory flight, host seeking, and survival. Russian wheat aphid suction trap captures corresponded with grass survey data and further verified the differences between the two survey years.

Effects of Preconditioning Host Plants on Population Development of Russian Wheat Aphids (Homoptera: Aphididae)

Abstract: The effect of preconditioning hosts of Russian wheat aphid, Diuraphis noxia (Mordvilko), was evaluated. Three consecutive generations produced about twice as many progeny on ‘Stephens’ wheat as on ‘Border’ oats with preconditioning on ‘Stephens’ wheat. In the second and third generations, if the preceding generation had developed on ‘Border’ oats, there were significant reductions in the number of aphids produced. Smallest numbers of Russian wheat aphid progeny were produced in the third generation when two preceding generations were reared on ‘Border’ oats. Preconditioning of aphids is crucial if detailed responses of aphids or plants are to be noted, as in the evaluation of host plant resistance mechanisms.

Resistance to Russian Wheat Aphid (Homoptera: Aphididae) in Barley Genotypes

Abstract: Experiments were carried out to determine the relative levels of antibiosis, tolerance, and antixenosisin four barley ( Hordeum vulgare L.) genotypes, S12. (ASE/2CM//B.7.6.B.B.), S13 (Gloria/Come), S16 (‘Esperanza’) and S17 (‘Shyri’) to the Russian wheat aphid, Diuraphis noxia (Mordvilko). Oats (Avena sativa L. var. ‘Paramo’) were used as a resistant check. From field screenings, S12 and S13 were selected as resistant to D. noxia and S16 and S17 susceptible. S12 and S13, in comparison with S16 and S17, showed high levels of antibiosis, demonstrated by low rates of nymph production on whole plants and on excised leaves. S13 showed higher tolerance than the other genotypes; mature plant height was reduced slightly by D. noxia feeding, and dry mass foliage loss per unit of aphid mass produced was relatively low. No antixenosis was demonstrated for the barley genotypes. Overall, S12 and S13 were more resistant to D. noxia in terms of antibiosis and tolerance than S16 and S17. S13 is a widely tested, high-yielding barley genotype with multiple disease resistance and represents for breeders a useful source of resistance to D. noxia in a superior genetic background.

Effects of Aphis fabae and Uromyces viciae-favae on the growth of a susceptible and an aphid resistant cultivar of Vicia faba.

Abstract: Summary Damaging effects of either black bean aphid (Aphis fabae), broad bean rust (Uromyces viciae-fabae), or the combination of both were investigated on a susceptible (cv. Diana) and an aphid resistant (cv. Bolero) cultivar of Vicia faba. When compared with rust, aphids caused greater reductions of root dry weight, shoot dry weight, leaf area, and mean relative growth rate. The mean unit leaf rate was also reduced whereas the leaf area ratio was not affected. The damage caused per aphid was highest on the susceptible cultivar. Rust induced damage did not differ between the cultivars. Concomitant infestation with both pests only resulted in additive damage. The population development of aphids was delayed on partially resistant plants. High temperature and rust infection reduced the total number of aphids the plants were able to support but not the level of resistance. Thus the specific damaging effect per aphid was increased.

Biotypes and Clonal Variation in Greenbug (Homoptera: Aphididae) Populations from a Locality in California

Abstract: Considerable genetic variation in virulence, fecundity, and adult weight was found between and within clones of greenbug, Schizaphis graminum (Rondani), populations near Davis, Calif. Eight clones were established from individual apterous aphids collected from four fields within 50 km of each other. Aphids from these clones differed in their ability to damage wheat variety ‘TAM 107’ and in their weight and fecundity on several sorghum varieties. Based on these differences, the clones could be separated into four distinct groups. Lineages of four clones, maintained for 14–16 successive generations by selecting either large or small aphids, differed in their fecundities. These differences could not be explained by maternal effects. If biotypes of the greenbug are determined by the damage they cause to particular plant varieties, as recently proposed, then clones that differ in fitness traits such as fecundity and weight cannot be considered different biotypes. However, to develop strategies for reducing the rate of aphid adaptation to resistant varieties, we must understand such variation in fitness on a local scale, information that is not obtained in general biotype surveys.