Fitness effects of two facultative endosymbiotic bacteria on the pea aphid,Acyrthosiphon pisum,and the blue alfalfa aphid,A. kondoi

The effects of two bacterial endosymbionts, designated PASS and PAR, were evaluated on the pea aphid, Acyrthosiphon pisum (Harris) (Hemiptera:Aphididae), in which they occur facultatively, and on the blue alfalfa aphid, A. kondoi Shinji, in which these bacteria have not been found in natural populations. Subclones of pea aphids and blue alfalfa aphids, derived from parent aphid clones that did not contain PASS or PAR, were infected with one or both bacteria, generating PASS- and/or PAR-positive subclones with minimal genetic differences from the parent clones. Under laboratory conditions at 20 °C, PAR consistently reduced the fecundity (by between 19 and 60%) of subclones derived from three different parent pea aphid clones on bur clover, Medicago hispida Gaertn. PAR had intermediate effects on pea aphids reared on sweet pea, Lathyrus odoratus L., and had no significant effect on pea aphids on alfalfa, Medicago sativa L. The effect of PASS was either neutral or negative, depending on parent clone as well as host plant. Also at 20 °C, PASS reduced fecundity (70–77%) and longevity (40–48%), and increased the age of first reproduction (by up to 1.5 days) of blue alfalfa aphid reared on alfalfa and clover. PAR had a less dramatic effect (e.g., 30–39% reduction in fecundity) on these traits of blue alfalfa aphid. In contrast, PAR and PASS increased the fitness of pea aphid subclones of one parent clone reared for three generations at 25 °C on each of the three test plants. Without facultative bacteria, fecundity of the parent clone was reduced to a mean total of < 6 offspring per adult at this elevated temperature, but with PASS or PAR, mean total fecundity of its subclones was > 35. However, this ameliorative effect of facultative bacteria at 25 °C was not found for two other sets of parent clones and their derived subclones. Alate production in pea aphids was significantly increased in large populations of two PASS- and PAR-positive subclones relative to their parent clones. Attempts to transmit PASS or PAR horizontally, i.e., from aphid to aphid via feeding on host plants (bur clover), were unsuccessful.     

Acceptance of low-saponin lines of alfalfa with varied phenolic concentrations by pea aphid (Homoptera: Aphididae)

This research aims to examine the effect of phenolics on pea aphid (Acyrthosiphon pisum) (Homoptera: Aphididae) development and feeding behaviour, on leaves of selected low-saponin lines of Radius alfalfa (Medicago sativa). There was a slight, negative correlation (Spearman rank correlation r _s = −0.80) between concentrations of saponins and phenols. Lines with higher concentrations of saponins had less phenolics. Levels of phenolics in low-saponin lines of alfalfa cv. Radius were related to their acceptance by the pea aphid. Our data revealed an inverse relationship between level of phenolics and the aphid abundance and its biology on studied alfalfa lines. Larval development of the pea aphid was longer, reproduction period was shorter, and the fecundity was lower on low-saponin lines with higher level of phenolics. There were observed some tendencies in the pea aphid feeding behaviour on these lines: prolonging the probing of the peripheral tissues (epidermis and mesophyll) and shortening the period of phloem sap ingestion. The better hosts for the pea aphid were low-saponin lines with low levels of phenolic compounds.     

Influence of Experience on the Response of Bathyplectes curculionis (Hymenoptera: Ichneumonidae), a Nonaphidophagous Parasitoid, to Aphid Odor

Bathyplectes curculionis (Thomson) is an introduced natural enemy of the alfalfa weevil in North America. The wasp requires carbohydrate foods as an adult. Adult wasps have increased longevity and fecundity when provided access to pea aphid, Acyrthosiphon pisum (Harris), honeydew in the laboratory, and adults respond positively to the presence of pea aphids in alfalfa fields. However, it is unknown how these wasps find aphid honeydew in the field. In a series of Y-tube olfactometer experiments, we evaluated the response of naïve and experienced adult female B. curculionis to odors from pea aphids, alfalfa, and pea aphids on alfalfa. Naïve adult females did not respond positively to pea aphid odor even when hungry. But adult females were able to learn aphid odor, and the mechanism of learning appears to be associative rather than by sensitization. Naïve females also showed no preference for alfalfa odor but learned alfalfa odor through sensitization. The wasps did not distinguish between alfalfa with aphids and alfalfa without aphids, even after exposure to aphids or alfalfa with aphids. However, they preferred pea aphid odor to alfalfa odor after a feeding experience in the presence of pea aphid odors. But after exposure to mixed odors of aphids and alfalfa while feeding, B. curculionis females preferred the odor of alfalfa to the odor of pea aphids. These results suggest that alfalfa odors mask or override aphid odors when aphids are associated with alfalfa (as happens naturally), thus interfering with the wasp's ability to respond to learned aphid odors. Therefore, although the wasps are capable of learning to find pea aphids and their honeydew in a simplified laboratory setting, it appears unlikely that they do so in the field.     

EVOLUTION OF AN APHID-PARASITOID INTERACTION: VARIATION IN RESISTANCE TO PARASITISM AMONG APHID POPULATIONS SPECIALIZED ON DIFFERENT PLANTS

The evolution of associations between herbivorous insects and their parasitoids is likely to be influenced by the relationship between the herbivore and its host plants. If populations of specialized herbivorous insects are structured by their host plants such that populations on different hosts are genetically differentiated, then the traits affecting insect-parasitoid interactions may exhibit an associated structure. The pea aphid (Acyrthosiphon pisum) is a herbivorous insect species comprised of genetically distinct groups that are specialized on different host plants (Via 1991a, 1994). Here, we examine how the genetic differentiation of pea aphid populations on different host plants affects their interaction with a parasitoid wasp, Aphidius ervi. We performed four experiments. (1) By exposing pea aphids from both alfalfa and clover to parasitoids from both crops, we demonstrate that pea aphid populations that are specialized on alfalfa are successfully parasitized less often than are populations specialized on clover. This difference in parasitism rate does not depend upon whether the wasps were collected from alfalfa or clover fields. (2) When we controlled for potential differences in aphid and parasitoid behavior between the two host plants and ensured that aphids were attacked, we found that pea aphids from alfalfa were still parasitized less often than pea aphids from clover. Thus, the difference in parasitism rates is not due to behavior of either aphids or wasps, but appears to be a physiologically based difference in resistance to parasitism. (3) Replicates of pea aphid clones reared on their own host plant and on a common host plant, fava bean, exhibited the same pattern of resistance as above. Thus, there do not appear to be nutritional or secondary chemical effects on the level of physiological resistance in the aphids due to feeding on clover or alfalfa, and therefore the difference in resistance on the two crops appears to be genetically based. (4) We assayed for genetic variation in resistance among individual pea aphid clones collected from clover fields and found no detectable genetic variation for resistance to parasitism within two populations sampled from clover. This is in contrast to Henter and Via's (1995) report of abundant genetic variation in resistance to this parasitoid within a pea aphid population on alfalfa. Low levels of genetic variation may be one factor that constrains the evolution of resistance to parasitism in the populations of pea aphids from clover, leading them to remain more susceptible than populations of the same species from alfalfa.     

Aphid population dynamics: does resistance to parasitism influence population size?

1. Insect population size is regulated by both intrinsic traits of organisms and extrinsic factors. The impacts of natural enemies are typically considered to be extrinsic factors, however insects have traits that affect their vulnerability to attack by natural enemies, and thus intrinsic and extrinsic factors can interact in their effects on population size. 2. Pea aphids Acyrthosiphon pisum Harris (Hemiptera: Aphididae) in New York and Maryland that are specialised on alfalfa are approximately two times more physiologically resistant to parasitism by Aphidius ervi Haliday (Hymenoptera: Braconidae) than pea aphids specialised on clover. To assess the potential influence of this genetically based difference in resistance to parasitism on pea aphid population dynamics, pea aphids, A. ervi, and other natural enemies of aphids in clover and alfalfa fields were sampled. 3. Rates of successful parasitism by A. ervi were higher and pea aphid population sizes were lower in clover, where the aphids are less resistant to parasitism. In contrast, mortality due to a fungal pathogen of pea aphids was higher in alfalfa. Generalist aphid predators did not differ significantly in density between the crops. 4. To explore whether intrinsic resistance to parasitism influences field dynamics, the relationship between resistance and successful field parasitism in 12 populations was analysed. The average level of resistance of a population strongly predicts rates of successful parasitism in the field. The ability of the parasitoid to regulate the aphid may vary among pea aphid populations of different levels of resistance.     

Morph identification in natural populations of alate female bird cherry aphids, Rhopalosiphum padi L., by multivariate methods

The effects of six burning treatments combined with two insecticide treatments of alfalfa (Medicago sativa L.) stubble on alfalfa plant bug, Adelphocoris lineolatus (Goeze) and Lygus spp. populations were evaluated over an 8-yr period (1982–1989). The burn treatments were: burning every spring and autumn, burning in alternate springs, burning at 50–100 mm and 150–200 mm of spring growth, and an unburned control. Alfalfa plant bug, which overwinters in the egg stage in alfalfa stems, were reduced by the spring burns. First generation populations of Lygus spp., were enhanced by the burn treatments before spring growth appeared, but were reduced when alfalfa was burned over 50 mm of spring growth. Counts of the minute pirate bug, Onus tristicolor White, were enhanced in 1983 by all burn treatments, and in 1985 by some of the burn treatments. Of all the predators, only spiders were significantly reduced and only in 1986 by the burned every autumn and spring treatments.     

The use of visual cues in host evaluation by aphidiid wasps I. Comparison between threeAphidius parasitoids of the pea aphid

Host evaluation behaviour was examined in three species of aphid parasitoids,Aphidius ervi haliday,A. pisivorus Smith, andA. smithi Sharma & Subba Rao (Hymenoptera: Aphidiidae). Parasitoids were provided under laboratory conditions with three kinds of hosts representing two aphid species: (green) pea aphid,Acyrthosiphon pisum (Harris), and green and pink colour morphs of the alfalfa aphid,Macrosiphum creelii Davis. Females of all threeAphidius species distinguished between aphids on the basis of colour, movement, and host species. Patterns of host acceptance by parasitoids were species-specific. InA. ervi, host preference was the same in light and dark conditions: pea aphid>green alfalfa aphid≫pink alfalfa aphid. In contrast,A. pisivorus attacked and accepted pea aphid and green alfalfa aphid equally in the light and preferred both of these over pink alfalfa aphid; however, it made no distinction between pea aphid and pink alfalfa aphid in the dark. Females ofA. smithi attacked all three kinds of hosts (pea aphid>green alfalfa aphid≫pink alfalfa aphid) but apparently laid eggs only in pea aphid. The frequencies of attack and oviposition by all wasps were higher on ‘normal’ pea aphids than on those anaesthetized with CO₂. Host recognition is confirmed by chemical cues in the aphid cuticle that are detected during antennation, and host acceptance is dependent on an assessment of host quality during ovipositor probing.     

Aggregation of polyphagous predators in response to multiple prey: ladybirds (Coleoptera: Coccinellidae) foraging in alfalfa

The spatial distribution of polyphagous predators may often reflect the integration of aggregative responses to local densities of multiple species of prey, and as such may have consequences for the indirect linkages among the prey sharing these predators. In a factorial field experiment in which we manipulated local prey densities within a field of alfalfa in Utah (USA), we tested whether aphidophagous ladybirds would aggregate not only in response to their primary aphid prey, but also in response to an abundant alternative prey, the alfalfa weevil (Hypera postica [Gyllenhal]). Native North American ladybirds (primarily Hippodamia convergens Guerin and H. quinquesignata quinquesignata [Kirby]) responded only to spatial variation in aphid density. In contrast, the introduced ladybird, Coccinella septempunctata L., aggregated also at local concentrations of the weevil late in the experiment when weevil density was high and aphid density was relatively low throughout all experimental plots. The results support the hypothesis that C. septempunctata is more responsive than are native ladybirds to the availability of alternative prey in alfalfa, which may account in part for the displacement of native ladybirds from alfalfa by the introduced species as aphid numbers have declined. The differing responses of the native and introduced ladybirds to spatial patterns of the alternative prey underscore the importance of extending the study of predator aggregation to understand better how polyphagous predators distribute themselves in response to spatial patterns of multiple species of potential prey.     

Leaf surface wax and plant morphology of peas influence insect density

Insect predators and parasitoids adhere better, forage more effectively, and take more aphid prey on pea plants (Pisum sativum L.) (Leguminosae) with mutations that reduce the crystalline wax bloom on the plant surface. To assess the agronomic potential of this trait for pest management, abundance of pea aphids (Acyrthosiphon pisum L.) (Homoptera: Aphididae) and coccinellid predators, and percent parasitism of the aphids were evaluated on pea lines differing in wax bloom and plant architecture over two field seasons. Three pairs of pea lines were evaluated, each pair with a different architecture and differing within the pair in the amount of surface wax bloom (reduced or normal). The trials included plots treated with a narrow spectrum insecticide (pymetrozine) to reduce aphid populations and untreated controls. Reduced wax peas had significantly fewer aphids per plant in 2002 but not in 2003. Total natural enemy abundance was greater on reduced wax than on normal wax pea lines in both years of the study. Pymetrozine reduced aphid densities significantly in both years. Among the four pea lines evaluated for yield, seed yield per plant was affected by plant morphology and insecticide treatment. Yield was greatest on semileafless plants and on pymetrozine sprayed plots in both years. Yield of the reduced wax line in the semileafless background was similar to or exceeded yield in its normal wax sister line, suggesting that this morphological type was best for an agronomically viable reduced wax phenotype. Pea weevil (Bruchus pisorum L.) (Coleoptera: Bruchidae) damage to seed was overall more frequent on seeds from reduced wax varieties than from normal wax varieties. The results illustrate the trade‐offs associated with a reduced wax trait in peas but also show that certain combinations of reduced wax and gross morphology lead to reduced pea aphid populations and yields similar to those of normal wax peas.     

Host-dependent association and its implications on the relative abundance of blue alfalfa aphid and pea aphid (Homoptera: Aphididae) on alfalfa in Oklahoma

Field populations of blue alfalfa aphid and pea aphid on alfalfa were sampled during 1985 and 1986 to determine the association of co-occurrence, interspecific interactions and comparative temporal variations in the spatial dispersion patterns of these species in Oklahoma. Relative abundance of these species is discussed in the light of above analyses.Cole' s coefficient revealed a high degree of association between these species in terms of their occurrence on the same alfalfa stems in the field. Regression analyses indicated that the species populations tended to increase in concert on the same stems without evidence of competitive displacement. Spatial dispersion patterns of both species were highly aggregated at low population densities early in the season. Over time, both species tended to disperse and became less aggregated as numbers increased. It was concluded that magnitude of interspecific interactions between the blue alfalfa aphid and the pea aphid were not of a nature that they could be termed as competing species. On the contrary, a concept of an “ecospecies” is proposed for practical applications such as sampling plans and economic threshold determinations.     

Host effects of glandular-haired alfalfa on alfalfa weevil (Coleoptera: Curculionidae) and potato leafhopper (Homoptera: Cicadellidae) populations in Virginia

Cultivars of glandular-haired alfalfa, Medicago sativa L., such as '54H69', are currently available and marketed as being resistant to potato leafhopper, Empoasca fabae (Harris). Between 2000 and 2002, studies were conducted to compare the effects of '54H69' and a standard, nonglandular-haired alfalfa cultivar, 'Choice', on alfalfa weevil, Hypera postica (Gyllenhal), and potato leafhopper populations at Campbell and Montgomery counties, Virginia. '54H69' had no effect on alfalfa weevil populations. At each location, densities of alfalfa weevil in '54H69' and 'Choice' were similar, but pest pressure was higher at Campbell Co. than at Montgomery Co. and always exceeded the economic threshold before insecticide was applied. Densities of potato leafhopper also did not differ between '54H69' and 'Choice' in any year at the two locations. Insecticide treatment effectively reduced potato leafhopper densities in the two cultivars, although populations were below the economic threshold at both locations when the insecticides were applied. Overall, postinsecticide treatment comparisons showed that the densities of alfalfa weevil and potato leafhoppers were similar or higher in untreated '54H69' compared with insecticide-treated 'Choice'. In addition, there were no differences in seasonal dry yields between '54H69' and 'Choice' in any year at either location. Our results indicate that the glandular-haired alfalfa '54H69' does not provide acceptable resistance to potato leafhopper and also does not offer a yield advantage to growers in Virginia.     

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.

     

Hybridization and cytoplasmic incompatibility among alfalfa weevil strains

Cross-matings were conducted among eastern, western, and Egyptian alfalfa weevil strains. Fully viable progenies were produced in reciprocal crosses between eastern and Egyptian weevils. The cross between western males and females of eastern or Egyptian strains was incompatible, producing infertile eggs, while the reciprocal cross yielded viable progeny but with a distorted sex ratio, predominantly female. The cause of incompatibility is due to the presence in the western weevil of a rickettsia, which is transmitted through the female parent. Of eight weevil populations surveyed, only the western weevil and a weevil population from the Netherlands harbored the rickettsiae. We conclude that all alfalfa weevil populations in the United States belong to the same species, Hypera postica (Gyllenhal), and that they are potentially interbreeding populations.

---

Résumé Des croisements ont été effectués entre 3 lignées d'H. postica et H. brunneipennis. Des descendants totalement viables ont été obtenus par croisements réciproques entre H. postica de Beltsville et H. brunneipennis. Les croisements entre mâles de H. postica de Logan et femelles de H. postica de Beltsville ou de H. brunneipennis ont été incompatibles, produisant des oeufs stériles, tandis que les croisements réciproques donnaient une descendance viable, mais avec un taux sexuel modifié avec prédominance de femelles. La cause de l'incompatibilité est due à la présence d'une rickettsia dans les H. postica de Logan, transmise par la femelle. Des 8 populations examinées, seule celle de Logan et 1 des pays Bas hébergeaient des rickettsia. Nous en concluons que toutes les populations des USA appartiennent à H. postica Gyllenhal, et qu'elles sont toutes potentiellement interfécondes.

     

The use of visual cues in host evaluation by aphidiid wasps

We examined host evaluation behaviour in three species of aphid parasitoids, Ephedrus californicus Baker, Monoctonus paulensis (Ashmead), and Praon pequodorum Viereck (Hymenoptera: Aphidiidae). Mated females were provided with pairwise choices among three kinds of hosts in the laboratory: (green) pea aphid, Acyrthosiphon pisum (Harris), and a green and a pink colour morph of alfalfa aphid, Macrosiphum creelii Davis. Patterns of attack and host acceptance were species-specific. Females of E. californicus did not respond to the presence of aphids prior to making antennal contact. Variations in rates of parasitization (pea aphid>green alfalfa aphid>pink alfalfa aphid) were consistent with differences in aphid defensive behaviours; no ‘preference’ for any host type was evident when aphids were anaesthetized with carbon dioxide. In M. paulensis, the order of preference (pea aphid>green alfalfa aphid>pink alfalfa aphid) did not vary when aphids were immobilized, or presented in the dark, or both. Host movement did not influence the rate of attack by M. paulensis. In contrast, the ranked order of preference in P. pequodorum varied with circumstance. In the light, females attacked pea aphid and green alfalfa aphid with equal frequency, but parasitized significantly more of the former; both kinds of aphids were attacked and parasitized at higher rates than pink alfalfa aphid. In the dark, P. pequodorum females parasitized green and pink alfalfa aphids equally and at higher rates than pea aphids. Whereas E. californicus was more successful ovipositing in immobilized hosts, P. pequodorum females attacked and laid more eggs in normal than anaesthetized aphids. Patterns of host recognition and evaluation are compared across six species representing four genera in the family Aphidiidae.     

Occurrence and Transmission of Facultative Endosymbionts in Aphids

The occurrence of a secondary bacterial symbiont (PASS) of pea aphid, Acyrthosiphon pisum (Harris), was detected by polymerase chain reaction (PCR) with specific nucleotide primers based on PASS 16S rDNA nucleotide sequences from over 80% (50/57) of clones of pea aphid collected from widely separated locations in California. PASS was also detected by PCR in both red and green phenotypes of rose aphid, Macrosiphum rosae (L.), but not in six other species of aphids examined, including blue alfalfa aphid (A. kondoi Shinji). The nucleotide sequences of the PCR-amplified, partial 16S rDNAs (1060 bp) from pea aphid and rose aphid were identical and 99.9% similar to the published 16S rDNA of PASS. PASS and a recently described new rickettsia of pea aphid (PAR) were transmitted by needle injection of hemolymph from positive pea aphid clones into negative clones and into blue alfalfa aphids. Both PASS and PAR were maintained in the offspring of some of the injected mother aphids via high rate of maternal transmission. Received: 18 September 1996 / Accepted: 30 September 1996     

Quantifying the relationship between disease severity and the amount of Aphanomyces euteiches detected in roots of alfalfa and pea with a real-time PCR assay

A real-time PCR assay was used to quantify the relationship in alfalfa and pea between disease severity and the amount of Aphanomyces euteiches detected in roots. The study included isolates of race 1 and race 2 of the alfalfa pathovar of A. euteiches and an isolate obtained from diseased pea. Spearman rank correlations between pathogen DNA content and disease severity index (DSI) ratings were positive ( ? 0.57) and significant (P  0.0007) for individual alfalfa plants, bulked alfalfa plant samples, and individual pea plants. In all experiments, significantly more pathogen was detected in susceptible populations than in resistant populations. The results clearly demonstrate that resistance to A. euteiches in both alfalfa and pea is characterized by a reduction in pathogen colonization relative to levels observed for susceptible reactions. The assay was very specific for A. euteiches, producing very linear assays with DNA extracted from pathogen isolates obtained from alfalfa, pea, and bean. Possible applications of the assay in conjunction with other real-time PCR assays specific to other legume pathogens are discussed in relation to simultaneous disease screening for multiple plant pathogens and the study of microbial population dynamics in mixed plant infections.     

Increased nitrogen fertilization inhibits the biocontrol activity promoted by the intercropping partner plant

The examination of the compatibility between agricultural practices and biocontrol activities is crucial for establishing an efficient, eco-friendly, and sustainable pest management program. In this study, we examined the population dynamics of two specialist aphids, the English grain aphid (Sitobion avenae) on potted wheat and the pea aphid (Acyrthosiphon pisum) on potted alfalfa, as well as the biocontrol activity of a generalist predator, the harlequin ladybird beetle (Harmonia axyridis). We investigated their responses to the presence of the intercropping partner plant species (alfalfa and wheat, respectively) through plant volatiles or visual cues at three nitrogen fertilizer levels in a greenhouse. In the absence of the predator, the English grain aphid population growth rate increased significantly with increasing nitrogen levels, whereas the pea aphid population increased significantly more slowly in response to high nitrogen levels. The English grain aphid and pea aphid population dynamics were unaffected by the presence of the intercropping partner. However, the presence of the intercropping partner enhanced the control of both aphid populations by the harlequin ladybird beetle. Increasing nitrogen fertilizer levels decreased the predation rates, which were otherwise increased by the intercropping partner. The beneficial effects of the intercropping partner were eventually non-existent at the highest nitrogen level tested. These results imply that the interaction between the presence of intercropping partner and the nitrogen fertilizer application affects the biocontrol activity of the natural enemies of insect pests. Thus, the compatibility between agricultural intensification and biocontrol strategies in integrated pest management programs need to be investigated.     

Mobility of salivary components as a possible reason for differences in the responses of alfalfa to the spotted alfalfa aphid and pea aphid

The spotted alfalfa aphid (SAA), Therioaphis trifolii maculata (Buckton), causes a characteristic veinal chlorosis and necrosis in the growing tips of susceptible cultivars of alfalfa. The pea aphid, Acyrthosiphon pisum (Harris), causes general degenerative changes in alfalfa but no specific, local symptoms. Biochemical and electrophoretic analyses detected similar enzymes in the ejected saliva of either species: pectin methylesterase, endopolygalacturonase and at least three isozymes of a copper dependent oxido-reductase that showed both catechol oxidase and peroxidase activity. Pectinase and catechol oxidase activities per unit of soluble protein were much greater in the saliva of pea aphid compared with that of SAA. The isozymes of the oxidase from SAA were roughly half the molecular weights of the corresponding isozymes from pea aphid, however, and radiotracer studies showed that soluble secretions injected into alfalfa by SAA travelled to growing tips considerably faster than the secretions of pea aphid. It is suggested that differences in the lesions caused by these aphids may be due to reaction kinetics rather than specific salivary toxins; that the rate of arrival of salivary components, possibly the oxidases, at phloem unloading sites may determine whether the plant's local defensive system is able to repress the immediate challenge or undergoes a run-away reaction leading to necrosis.     

Overwintering conditions affect cold hardiness,survival, and post-overwintering fitness of the pea leaf weevil

Overwintering conditions affect the physiological state of ectotherms, and therefore, their cold hardiness and survival. A measure of the lethal and sublethal impacts of overwintering conditions on pest populations is crucial to predict population dynamics and to manage pests the following spring. The impact of winter conditions can be most intense for invasive insects undergoing range expansion. Insect herbivores can display plastic host use behaviours that depend on their body condition following winter. The pea leaf weevil, Sitona lineatus L. (Coleoptera: Curculionidae), is an invasive pest of field peas, Pisum sativum L., and faba bean, Vicia faba L. (Fabaceae). Pea leaf weevil has expanded its range in North America to include the Prairie Provinces of Canada. This study investigated the effects of temperature and microhabitat on overwintering survival and cold hardiness of pea leaf weevil in its expanded range. Further, we investigated the sublethal effect of overwintering temperature and duration on post-overwintering survival, feeding, and oviposition of pea leaf weevil. We also investigated the role of juvenile hormone in modulating body condition of overwintering weevils. The overwintering survival of pea leaf weevil adults increased with soil temperature and varied with region and microhabitat. More weevils survived winters when positioned near tree shelterbelts compared to open alfalfa fields. The supercooling point of pea leaf weevil varied throughout its expanding range but did not differ for weevils held in the two microhabitats. The average threshold lethal temperature of pea leaf weevil at all three sites was −9.4 °C. Weevils that overwintered for a longer duration and at a higher temperature subsequently fed more on faba bean foliage and laid more eggs compared to those which overwintered for a shorter duration at a lower temperature. Our findings highlight that warm winters would increase overwintering survival and post-overwintering fitness, facilitating further pea leaf weevil invasion northward in the Prairie Provinces of Canada.     

Time-specific life tables for the pea aphid,Acyrthosiphon pisum (Harris), on alfalfa

Time-specific life tables were constructed for three pea aphid, Acyrthosiphon pisum (Harris ) (Homoptera: Aphididae), populations using a modification ofHughes' analytical procedure. All populations were studied on second-growth alfalfa (mid-June to mid-July) in south central Wisconsin; data for two populations were collected during 1980, and data for the third population were collected during 1982. The intrinsic rate of increase (r_m) estimated on a physiological time (day-degree) scale under field conditions but in the absence of natural enemies, provided a reliable estimate of potential population growth rate and was used in preference toHughes' approach of estimating potential population growth rates directly from stage structure data. Emigration by adult alatae and fungal disease were the major sources of A. pisum mortality in each of the three populations studied. These factors were most important because of their impact on reducing birth rates within the local population. Parasitism was never greater than 9 percent. Mortality attributable to predation ranged from 0.0 to about 30.0%; however, even at the highest predator densities A. pisum populations increased exponentially.