Plant structure and the foraging success of Aphidius rhopalosiphi (Hymenoptera: Aphidiidae)

Abstract. 1. A study of searching behaviour on wheat and host preference of Aphidius rhopalosiphi (DeStefani-Perez) is described.
2. Parasitoids divided their time equally between the leaves but spent very little time on the ear.
3. After contact with honeydew or an aphid host, parasitoids were arrested in a particular area and increased the time spent searching.
4. Aphids feeding on the ear were parasitized less successfully, since their position between the grains protected them from parasitoid attacks.
5. A.rhopalosiphi exhibited no preference for Metopolophium dirhodum (Walker) or Sitobion avenae (Fabricius), although the handling time for the latter was significantly longer; this resulted in fewer S.avenae being parasitized when it was abundant. Parasitoids did not switch between hosts in these experiments.
6. As a result of its searching behaviour, A.rhopalosiphi will encounter and parasitize M.dirhodum feeding on the leaves more frequently than S.avenae, which feeds on the ear. This will limit the parasitoid's ability to regulate populations of the cereal aphid S. avenae.     

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

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

Prey selection and foraging behaviour by Pterostichus cupreus L. (Col., Carabidae) under laboratory conditions

Abstract: Several authors have indicated that carabid beetles offer potential as predators of insect pests on arable crops, but this potential is reduced by their limited ability to climb the crop plants. In the current investigation an initial laboratory experiment indicated that seven species of carabid beetles commonly found in arable fields in the UK would feed on both aphid and collembolan prey. Pterostichus cupreus L. and Bembidion guttula Fabr. were found to accept both live and dead aphids and collembolans. In further laboratory investigations P. cupreus exhibited greater consumption of the cereal aphid Metopolophium dirhodum Walker than the alternative prey species Heteromurus nitidus , Templeton (Collembola, Entomobryidae) and there was some indication of preference for the aphid prey as these were often consumed first. Significantly more handling time was required for the consumption of M. dirhodum compared to H. nitidus. The treatments where P. cupreus was given a diet of aphids or collembolans prior to the prey choice experiment, did not appear to influence the number or type of prey items consumed.
Using a four-arm olfactometer P. cupreus reacted positively to the volatiles from collembolans, but gave a less clear response to those from the cereal aphid M. dirhodum . In an artificial field arena P. cupreus readily climbed barley plants in search of M. dirhodum , perhaps as a response to volatile emissions, but this behaviour changed when H. nitidus were introduced onto the substrate of the arena. Although the current results indicated that P. cupreus has potential for the control of cereal aphids, the presence of alternative prey appeared to reduce this potential and suggest that the carabid is an opportunistic feeder.     

Evidence for the existence of a species closely related to the cereal aphid parasitoid Aphidius rhopalosiphi De Stefani-Perez based on host ranges, morphological characters, isoelectric focusing banding patterns, cross-breeding experiments and sex pheromone specificities (Hymenoptera, Braconidae, Aphidiinae)

Abstract. Host transfer trials with six strains of the cereal aphid parasitoid Aphidius rhopalosiphi De Stefani-Perez revealed the presence of two differing host ranges: three strains parasitized Sitobion uvenae (F.) and S.fragariae (Wlk.) exclusively, whereas the others successfully developed in both Sitobion species and also in Metopolophium dirhodum (Wlk.), M.festucue cerealiurn Stroyan, Rhopalosiphum padi (L.) and Diuraphis muehlei Börner. The two host range groups were poorly distinguishable by means of morphological characters. In con trast, isoelectric focusing proved to be a much better tool for discrimi nation. Reproductive isolation between the two host range groups was clear but not complete, as evidenced by crossing experiments with laboratory-bred and field-collected material. Under field conditions, males showed a specific response to virgin females of the two groups. Hence, the sex pheromones of the females were obviously different. Considering these results, a species ( Aphidius sp. A) closely related to A.rhopalosiphi seems to occur. As a detailed study of morphological characters is still lacking, Aphidius sp. A is not yet formally established here. Due to its dependence upon Sitobion spp., Aphidius sp. A may be an economically important parasitoid of the pest aphid S.avenae .     

Host choice by Aphidius colemani: effects of plants, plant–aphid combinations and the presence of intra-guild predators

Abstract.  1. This study first measured the effect of plant (radish, pepper, and wheat), aphid–plant combination (the green peach aphid Myzus persicae on pepper and radish, and the bird cherry-oat aphid Rhopalosiphum padi on wheat) and the host on which Aphidius colemani was reared (the cotton aphid Aphis gossypii on cucumber, M. persicae on radish and pepper, and R. padi on wheat) on host choice behaviour of the parasitoid and the performance of its offspring. Then, the effect of predator presence ( Coccinella undecimpunctata larvae) on host preference of the wasps was tested.
2. When reared on M. persicae on either radish or pepper , wasps preferred the aphid–plant combination from which they had emerged. Wasps reared on A. gossypii (naïve to all hosts offered) and R. padi preferred to parasitise M. persicae on radish and M. persicae on either radish or pepper, respectively. Rhopalosiphum padi on wheat was the least preferred and also the least suitable host, as determined by offspring body size.
3. Contrary to expectations, the presence of predators did not influence the host choice of A. colemani , even when predator-free plants were offered nearby. Results indicate that wasps are more likely to remain in some host systems but not in others, even when facing risk of predation.
4. Results are discussed in respect to field data and the relative importance of risk of predation and host preference to wasp fitness.     

Host specialization in habitat specialists and generalists

Generalists and specialists use different cues to find their habitat and essential resources. While generalists have the advantage of exploiting a wider range of resources, they are predicted to be less efficient in using one particular resource compared to specialists. The level of specialization of parasitoids can be either at the habitat or at the host level; strategies used by either type are expected to differ. We examined interactions between three aphid parasitoid species that are a habitat specialist Aphidius rhopalosiphi, a habitat generalist Aphidius ervi, and a host generalist Praon volucre on three cereal aphids, Sitobion avenae, Metopolophium dirhodum and Rhopalosiphum padi. We compared total parasitism rate across behavioral and physiological variation in a non-choice test. Next, we addressed total parasitism in two phases to examine: (1) the response of parasitoids to different hosts through the behavioral sequence from antennation through oviposition, and (2) the physiological suitability of different hosts for oviposition and larval development. Parasitization typically involved the following behavioral steps: (1) antennal contact, (2) abdominal bending, and (3) ovipositor insertion (acceptance). A. rhopalosiphi had the same number of antennal contacts with the three aphids but showed fewer instances of abdominal bending towards R. padi. Pre-contact host preference was found for A. ervi but it did not correspond to the level of acceptance. The number of antennal contacts by P. volucre corresponded to the parasitization level of the aphid species but more mummies were produced on M. dirhodum than on R. padi. These results suggest that parasitoid species that are habitat specialists react similarly to the different host species present in the same habitat, whereas generalist species exhibit clear preferences during host selection. Preferences were, however, not always related to host suitability.     

Functional responses of aphid parasitoids,Aphidius colemani (Hymenoptera: Braconidae) and Aphelinus asychis (Hymenoptera: Aphelinidae)

We evaluated the functional responses of two aphid parasitoids: Aphidius colemani on the green peach aphid Myzus persicae (Hemiptera: Aphididae), and Aphelinus asychis on M. persicae and the potato aphid, Macrosiphum euphorbiae (Hemiptera: Aphididae). Parasitoid oviposition occurred at host densities of 5, 10, 20, 30, 50, 80 or 100 aphids for A. colemani and 5, 10, 20, 30 or 50 aphids for A. asychis. More M. persicae were parasitized by A. colemani than by A. asychis at an aphid density of 50. Among the three types of functional response, type III best described the parasitoid response to the host densities both in A. colemani and A. asychis. The estimated handling time was shorter for A. colemani than for A. asychis (0.017 and 0.043 d, respectively). The proportion of aphids that were parasitized exhibited the same characteristic curve among the three host-parasitoid combinations: a wave form that appeared to be a composite of a decelerating (as in type II) response at low host density and an accelerating-and-decelerating (as in type III) response at medium to high host density. We hypothesize that the novel host species (and its host plant), density-dependent superparasitism, and/or density-dependent host-killing may have induced the modified type III response.     

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

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

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

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

Influence of the aphid pathogen Pandora neoaphidis on the foraging behaviour of the aphid parasitoid Aphidius ervi

Abstract.  1. The parasitoid Aphidius ervi and the entomopathogenic fungus Pandora neoaphidis both require successful invasion of an aphid host to complete their life cycle. A shorter developmental period allows P. neoaphidis to out-compete A. ervi. Aphidius ervi may reduce this fitness cost by avoiding aphid colonies containing P. neoaphidis . Here the response of A. ervi towards P. neoaphidis was assessed using sequential experiments designed to replicate different stages of parasitoid foraging behaviour.
2. Entry rate experiments showed that A. ervi entered aphid colonies containing P. neoaphidis -sporulating cadavers and that there was no significant difference in the attraction of A. ervi to aphid-damaged Vicia faba plants containing either healthy Acyrthosiphon pisum or P. neoaphidis -sporulating cadavers.
3. Observational behavioural experiments indicated that the presence of P. neoaphidis did not affect the search time or total foraging time of A. ervi on V. faba plants infested with either healthy A. pisum or P. neoaphidis -sporulating cadavers.
4. In Petri dish bioassays using aphids infected with P. neoaphidis over a period of 120 h, A. ervi showed no difference in attack rate against uninfected aphids or living aphids infected with P. neoaphidis for 1, 24, 48, 72, or 96 h. However, sporulating cadavers (120 h infection) were not attacked.
5.  Aphidius ervi appears only able to detect the presence of P. neoaphidis once the host is dead and sporulation has started. The fitness of A. ervi may therefore be severely reduced when foraging in P. neoaphidis -infected aphid colonies.     

Variability in characters commonly used to distinguish Aphidius species (Hymenoptera: Aphidiidae)

Abstract Characters traditionally used to separate some Aphidius species commonly parasitizing pea and cereal aphids are shown to be more variable than previously supposed. Aphidius ervi, picipes and rhopalosiphi (Hymenoptera: Aphidiidae) were cultured and the variability of the numbers of antennal segments, sculpture on the anterolateral area of the petiole and the tentorial index were recorded. The number of antennal segments in each of the three species varied by four in males and by three in females. The numbers of ridges on the anterolateral area of the petiole of A.rhopalosiphi varied widely from six to eighteen. The tentorial index range from 0.35–0.62, 0.34–0.54 and 0.35–0.60 for A.ervi, picipes and rhopalosiphi respectively, compared with previously published ranges of 0.30–0.40, 0.28–0.37 and 0.30–0.35. A.rhopalosiphi and uzbekistanicus cannot be distinguished reliably by the wing-vein length ratios previously used.     

The effect of parasitoid host-size preference on host population growth rates: an example of Aphidius colemani and Aphis glycines

Abstract.  1. Direct observations of Aphidius colemani foraging for Aphis glycines demonstrated that A. colemani attacks large aphid size classes selectively, in contrast to other Aphidiinae (Hymenoptera: Braconidae) species that generally prefer small or medium-sized nymphs of different host species.
2. To determine the effect of this size preference on the potential ability of A. colemani to control A. glycines outbreaks, the stage-dependent survival and fecundity of A. glycines were measured to parameterise a stage-structured demographic model for the density-independent population growth rate of the aphid.
3. Compared with hypothetical parasitoids that show either no size preference or preference for medium-sized host nymphs, the preference of A. colemani for large hosts caused a greater reduction in the population growth rate of A. glycines . This occurs in the model because, by attacking reproductive adults, A. colemani kills those aphids that have the greatest immediate effect on the population growth rate.
4. The strong effect of size preference by A. colemani is diminished somewhat by the continued reproduction of A. glycines adults, which can reproduce for up to 3 days following parasitism. Nonetheless, reproduction by parasitised aphids is not enough to compensate for the stronger, negative effect of the preference of A. colemani for large aphids, which removes individuals with the greatest reproductive value from the A. glycines population.     

Morphometric and electrophoretic study of Aphidius species (Hymenoptera: Aphidiidae) reared from a variety of aphid hosts

Abstract. Morphological and electrophoretic variation is evaluated in 272 female and 424 male specimens of Aphidius reared from known hosts representing seventeen aphid-host species. Much of this material was reared under experimental conditions in order to examine the effects of environmental conditions such as temperature and different host species, and to induce as much variation as is possible in known single populations. Multivariate analysis and electrophoresis are used to demonstrate the degree to which entities defined by morphological characters and electrophoretic bands correspond to collection from a common host.
Many characters commonly used in the identification of this group, e.g. sculpturing on the propodeum, are shown to be very variable. Despite this, several morphologically and electrophoretically definable groups of Aphidius are demonstrated, many of them corresponding to individual aphid-hosts, while others, especially those parasitizing pea and cereal aphids were found to occur on more than one host. The possible nominal species associated with each defined group are discussed and morphological keys given to these complexes, and to the species of of Aphidius known to parasitize pea and cereal aphids.     

Responses to aphid sex pheromones by the pea aphid parasitoids Aphidius ervi and Aphidius eadyi

Aphidius ervi and Aphidius eadyi, two parasitoids of the pea aphid Acyrthosiphon pisum, were attracted to components of the aphid sex pheromone in laboratory bioassays. Pre-test experience with host aphids in the presence of aphid sex pheromone did not affect the response of A. ervi to pheromone in a 4-way olfactometer, compared with that of naive parasitoids. Aphidius ervi females exposed only to the pheromone prior to testing did not respond in the olfactometer, suggesting habituation to the foraging cue by the parasitoid. In a wind tunnel, aphid sex pheromone increased the attraction of A. ervi to the plant-host complex (Vicia faba/A. pisum), suggesting an additive effect when two different foraging cues are present simultaneously.     

Host instar preference and functional response of Aphidius transcaspicus, a parasitoid of mealy aphids (Hyalopterus species)

Aphidius transcaspicus Telenga (Hymenoptera: Braconidae: Aphidiinae) has been selected for use as a biological control agent against Hyalopterus pruni Geoffroy (Hemiptera: Aphididae) in prune orchards in California. In this study we focus on the host instar preference and functional response of A. transcaspicus. Host instar preference was determined using laboratory arenas by allowing female parasitoids to attack aphids on field-collected leaves of Prunus domestica L. supporting a natural range of H. pruni instars. A relative preference index indicated that A. transcaspicus was 1.4 times more likely to attack intermediate instars (2–4) than 1st or adult instars. The functional response of A. transcaspicus to H. pruni density was determined on enclosed P. domestica shoots in a prune orchard. The functional response to host density was linear (Type I) at the lowest aphid densities, and the asymptotic number of aphids attacked at higher aphid densities was 64.3?±?6.7 aphids day^?1.     

Prey preferences of six species of Carabidae (Coleoptera) and one Lycosidae (Araneae) commonly found in UK arable crop fields

Abstract: Laboratory experiments were used to determine the feeding preferences of six carabid beetles and one lycosid spider on aphid and collembolan prey. The first investigation used only five species of carabid Pterostichus melanarius Illiger, Pterostichus madidus F., Harpalus rufipes DeGeer, Nebria brevicollis F. and Carabus violaceus L., which had been caught most commonly in pitfall traps in the headland region of arable crop fields. When offered Brevicoryne brassicae L., Sitobion avenae F., Metopolophium dirhodum Walker and Rhopalosiphum padi L. as prey items, the species consistently consumed in high numbers was M. dirhodum . In subsequent experiments four carabids Pterostichus cupreus L., P. melanarius, P. madidus. H. rufipes and a lycosid spider Trochosa ruricola DeGeer whose distribution was shown by pitfall trapping to extend throughout the arable crop, were the chosen predators. These predators were offered a choice between M. dirhodum and entomobryid collembolans (a recognised alternative prey item) to gauge their preference between the two prey types. Both male and female P. cupreus and P. melanarius showed a significant preference for the aphid prey, while there was no significant preference displayed by the other species. The effect of temperature on the voracity of these five predators fed on M. dirhodum was investigated. There were significant differences in the number of aphids consumed by the species and sexes at the different temperatures. Regression analysis on the mean numbers of aphids eaten by each sex of the five predators, showed that in the majority of cases there was a significant increase in predation with increasing temperature. In considering the dietary preferences illustrated by these experiments, it appeared that P. cupreus and P. melanarius offered the greatest potential in controlling aphids on arable crops.     

Quality of different aphids species as hosts for the parasitoid Aphidius ervi Haliday (Hymenoptera: Braconidae: Aphidiinae)

The suitability of Macrosiphum euphorbiae (Thomas), Aulacorthum solani (Kaltenbach) and Acyrthosiphon kondoi Shinji (Hemiptera: Aphididae) as hosts for the aphid parasitoid Aphidius ervi Haliday was evaluated by assessing host size, host preference, and host quality. Tests were carried out in an environmental chamber at 22±1oC, 70±10% RH and 12h photophase. Replicates (11) consisted of one 24h-old mated female of A. ervi without a previous oviposition experience. Female was released into a Petri dish (5 cm) with 20 2nd and 3rd instars of one of each aphid species tested on a leaf disc of the host plant onto a 1% water-agar layer. Parasitoid emergency was lower in A. kondoi (78.7%) compared to M. euphorbiae (92.2%) and A. solani (91.7%). Acyrthosiphon kondoi (0.36 mm) was the smallest host. The parasitoid showed preference (74.0% parasitism) for M. euphorbiae, the largest host (hind tibia length=0.73 mm), which in turn yielded larger A. ervi females (0.75 mm).     

Reactions of adult female parasitoids, particularly Aphidius rhopalosiphi, to volatile chemical cues from the host plants of their aphid prey

Abstract. In Y-tube olfactometer tests, Aphidius ervi Hal., Trioxys sp., Praon sp., Aphelinus flavus (Nees), Lysiphlebus fabarum (Marsh.) and Aphidius rophalosiphi De Stef. responded positively to the odour of the plant on which aphid mummies containing them had been collected. The response to host plant odour was greater than the response to the odour of host aphids, their honeydew or a combination of the two. The strongest response was to a combination of plant and host aphids. A. rhopalosiphi showed a strong positive response to three wheat volatiles (cis-3-hexenyl acetate, cis-3-hexen-1-ol and trans-2-hexenal) as well as to indole-3-acetaldehyde (a breakdown product of tryptophan in aphid honeydew). In both olfactometer tests with odours and choice trials with whole plants, newly emerged A. rhopalosiphi distinguished and preferred the variety of wheat on which their development had occurred to other wheat varieties.     

Limitation of superparasitism by Aphidius rhopalosiphi: a consequence of aphid defensive behaviour

Abstract. 1. The behavioural mechanisms limiting superparasitism by Aphidius rhopalosiphi (DeStefani-Perez) are described.
2. A.rhopalosiphi did not discriminate between parasitized and unparasitized aphids, or avoid attacking and ovipositing in previously parasitized aphids.
3. Aphid defensive behaviour was increased for a short period after a parasite stabbing attack. In response to this behaviour the number of parasite stabbing attacks subsequently undertaken declined.
4. Once a stabbing attack had been initiated, parasite oviposition success was unaffected by aphid defensive behaviour.
5. It is concluded that applied defensive behaviour, by limiting the number of stabbing attacks undertaken by A.rhopalusiphi , effectively limits the number of parasite eggs laid and the extent to which super parasitism occurs.     

Factors determining the pest status of the rose-grain aphid, Metopolophium dirhodum (Walker), on winter barley in the United Kingdom

To test the hypothesis that early maturation of barley affords it some resistance to cereal aphids, the colonisation, survival, growth, reproduction and emigration of M. dirhodum on barley was measured and compared with previously published results for this aphid on wheat.
Barley plants were colonised as readily as wheat plants. Survival to maturity was similar on both hosts. Relative growth rate was higher on wheat than on barley for most of the season while reproduction was similar on both. Emigration, measured alate production, was significantly higher on mature wheat than on mature barley.
It is proposed that the increase of M. dirhodum populations on barley is normally terminated early in the year directly plant maturation and consequent drying out of the leaves, resulting in the death of aphids. This is different to the situation on wheat, which matures later, and where emigration following plant induced alate induction has been shown to be the major factor affecting the decline in population growth.
These results support the hypothesis that early maturation of barley provides some resistance to M. dirhodum.