Biology and host selection behaviour of the aphid hyperparasitoid Alloxysta victrix in association with the primary parasitoid Aphidius colemani and the host aphid Myzus persicae

The biology of the aphid hyperparasitoid Alloxysta victrix was investigated with respect to its interaction with the primary parasitoid Aphidius colemani and the host aphid Myzus persicae. Laboratory investigations of host selection behaviour indicated that naive A. victrix females could rapidly distinguish between unparasitized aphids and those parasitized by A. colemani, but showed little tendency to discriminate between aphids parasitized by the primary parasitoid three, five, seven, or ten days earlier (even though the latter were already mummified). Host suitability studies, in contrast, indicated that aphids parasitized by A. colemani became progressively less suitable as the primary parasitoid developed. When aphids were exposed to A. victrix more than seven days after being parasitized by A. colemani, survivorship of both primary and secondary parasitoid species decreased, with a significantly higher proportion of mummies producing no viable adult parasitoid of either species. The developmental period (egg to adult eclosion) of surviving hyperparasitoids was also longer in older host stages, and females developed significantly more slowly than males (14.2 days versus 13.6 days, respectively). The sex ratio of hyperparasitoid broods emerging from all host stages was consistently male-biased (31–46% female), but adult longevity was higher for females than for males (37.1 days versus 23.6 days).     

Host handling and specificity of the hyperparasitoid wasp, Dendrocerus carpenteri (Curtis) (Hym., Megaspilidae): importance of host age and species

The oviposition behaviour of Dendrocerus carpenteri (Curtis), an ectophagous hyperparasitoid of aphidiine wasps inside mummified aphids was examined. Hyperparasitoids were provided in the laboratory with pea aphids ( Acyrthosiphon pisum ) which had been parasitized by three different species of aphidiine wasps ( Aphidius ervi, Ephedrus californicus and Praon pequodorum ) ranging in physiological age from the late larval stage to the late pupal stage. Females accepted only the hosts inside mummified aphids; they ignored live aphids, and did not accept dead, but not yet mummified aphids, although the latter were sometimes probed with the ovipositor. Female behaviour in handling A. ervi or E. californicus mummies did not change with experience; handling and oviposition times were stereotypic. However, naive females needed experience to locate the cocoon of P. pequodorum and distinguish it from the empty aphid mummy. Host acceptance and specificity were influenced more by the developmental stage than the species of the primary parasitoid. In dichotomous choice tests, hyperparasitoids 'preferred' prepupae over younger pupae of A. ervi , but they did not distinguish between these stages of E. californicus; older pupae were accepted at a low rate. Host preference was not influenced by conditioning on the rearing host. We consider several constraints on the host range of D. carpenteri , and discuss alternative explanations of differential hyperparasitism in the field.     

Dynamic host-feeding and oviposition behavior of an aphid parasitoid <Emphasis Type="Italic">Aphelinus asychis</Emphasis>

In order to maximize the lifetime reproductive success of parasitoids, they should be induced to dynamically accept individual hosts that have different suitability for oviposition. Parasitoids tend to exhibit higher host-selective behavior when their egg load is limited, and are less selective if they are facing time constraints. Here, we evaluated the effects of parasitoid age on egg load, fecundity and host instar preference of a honey-fed aphid parasitoid, Aphelinus asychis Walker (Hymenoptera: Aphelinidae). Host selective experiment was conducted to measure host-preference of honey-fed A. asychis females at different ages, using the second and fourth instars of the green peach aphid Myzus persicae as their hosts. The results showed that the choice of host-instar for oviposition was significantly influenced by the parasitoid age. Honey-fed parasitoids in the age groups of 1, 5, 10 and 20 days tended to parasitize predominantly second-instar aphids, whereas 15-days old parasitoids showed no significant preference of host instars. On the other hand, host-feeding preference was not affected by parasitoid age. Parasitoid females of all ages preferred younger aphids to older aphids. This result could help evaluate the effectiveness of A. asychis for biological control of M. persicae when they encountered mixed-instar aphids in the field. In addition, the results might be helpful in assessing the host killing effects of other host-feeding parasitoids.     

Larvicidal activity of lectins onLucilia cuprina: mechanism of action

Foraging behaviour and host-instar preference of young and old females of the solitary aphid parasitoid,Lysiphlebus cardui Marshall (Hymenoptera: Aphidiidae), were studied in the laboratory. The analysis of interactions between parasitoids and different stages ofAphis fabae cirsiiacanthoidis Scop. (Homoptera: Aphididae) revealed that encounter rates between aphids and parasitoid females and defence reactions of the aphids influenced the degree to which a particular aphid age class is parasitized. Encounter rates between hosts and parasitoid females depended on the foraging pattern of the parasitoid, which varied with age. In mixed aphid colonies patch residence time increased with parasitoid age. Furthermore, younger parasitoids (≦1 day old) laid more eggs into second and third instars, while older parasitoids (≧4 days old) did not show distinct host instar preferences. It is suggested that the oviposition behaviour ofL. cardui is influenced by the physiological state, i.e. the age of the wasp.     

Host age and fitness-related traits in a koinobiont aphid parasitoid

Abstract.  1. Trade-offs play a key role in species evolution and should be found in host–parasitoid interactions where the host quality may differ between host age categories.
2. The braconid wasp, Aphidius ervi , is a solitary endoparasitoid that allows its aphid hosts to continue to feed and grow after parasitisation. The hypotheses that host age influences their quality and that female parasitoids exploit their hosts based on that quality were tested under laboratory conditions using no-choice tests.
3.  Aphidius ervi females accepted the aphid Myzus persicae for oviposition and their progeny developed successfully in all host ages. The fitness-related traits of parasitoids did not increase linearly with the host age in which they developed. Host quality was found to be optimal at intermediate host ages and the females preferred to parasitise these hosts. The shortest progeny development time and a more female-biased sex ratio were observed in hosts of intermediate age.
4. This study suggests the existence of multiple interactive trade-offs occurring during host–parasitoid interactions according to host age related quality.     

Does mother really know best? Oviposition preference reduces reproductive performance in the generalist parasitoid Aphidius ervi

The reproductive success of female parasitoids is dependent on their ability to accurately assess the suitability of a host for larval development. For generalist parasitoids, which utilize a broad range of species and instars as hosts, a set of assessment criteria determines whether a host is accepted or rejected. The suitability of a host, however, can only be imperfectly assessed by the female parasitoid, which can result in the selection of lesser quality hosts for oviposition. In this study we explored the disparity between host quality and host preference using the generalist koinobiotic parasitoid Aphidius ervi Haliday (Hymenoptera: Aphidiidae) and the host Aulacorthum solani (Harris) (Homoptera: Aphididae), the foxglove aphid. The second instar hosts produced the highest level of reproductive success, while third and fourth instars resulted in a substantially reduced reproductive performance. When given a choice of host instars, parasitoids preferred the older hosts for oviposition disregarding their reduced suitability for larval development. Results are discussed in context of mechanisms involved in A. ervi host selection and biases in the criteria used to assess hosts that may arise when parasitoids transfer host species between generations.     

Foraging behaviour at the fourth trophic level: a comparative study of host location in aphid hyperparasitoids

In studies of foraging behaviour in a multitrophic context, the fourth trophic level has generally been ignored. We used four aphid hyperparasitoid species: Dendrocerus carpenteri (Curtis) (Hymenoptera: Megaspilidae), Asaphes suspensus Walker (Hymenoptera: Pteromalidae), Alloxysta victrix (Westwood) (Hymenoptera: Alloxystidae) and Syrphophagus aphidivorus (Mayr) (Hymenoptera: Encyrtidae), to correlate their response to different cues with their ecological attributes such as host range and host stage. In addition, we compared our results with studies of primary parasitoids on the same plant–herbivore system. First, the olfactory response of females was tested in a Y‐tube olfactometer (single choice: plant, aphid, honeydew, parasitised aphid, aphid mummy, or virgin female parasitoid; dual choice: clean plant, plant with aphids, or plant–host complex). Second, their foraging behaviour was described on plants with different stimuli (honeydew, aphids, parasitised aphids, and aphid mummies). The results indicated that olfactory cues are probably not essential cues for hyperparasitoid females. In foraging behaviour on the plant, all species prolonged their total visit time and search time as compared to the control treatment (clean plant). Only A. victrix did not react to the honeydew. Oviposition in mummies prolonged the total visit time because of the long handling time, but the effect of this behaviour on search time could not be determined. No clear correlation between foraging behaviour and host stage or host range was found. In contrast to specialised primary aphid parasitoids that have strong fixed responses to specific kairomones and herbivore‐induced synomones, more generalist aphid hyperparasitoids seem to depend less on volatile olfactory stimuli, but show similarities with primary parasitoids in their use of contact cues while searching on a plant.     

Discrimination of parasitized aphids by a hoverfly predator: effects on larval performance, foraging, and oviposition behavior

The choice of oviposition site by female aphidophagous predators is crucial for offspring performance, especially in hoverflies whose newly hatched larvae are unable to move over large distance. Predator and parasitoid interactions within the aphidophagous guild are likely to be very important in influencing the choices made by predatory hoverfly females. In the present study, the foraging and oviposition behavior of the aphidophagous hoverfly Episyrphus balteatus DeGeer (Diptera: Syrphidae) was investigated with respect to the parasitized state of its aphid prey, Acyrthosiphon pisum Harris (Homoptera: Aphididae), that were parasitized by Aphidius ervi Haliday (Hymenoptera: Aphidiidae). We also recorded the number of eggs laid by hoverfly females when subjected to parasitized aphids. Furthermore, we studied the influence of being fed with parasitized aphids on hoverfly larval performance. Hoverfly females did not exhibit any preference for plants infested with unparasitized or aphids parasitized for 7 days. On the other hand, plants infested with mummies or exuvia were less attractive for E. balteatus . These results were correlated with (i) the number of eggs laid by E. balteatus females and (ii) larval performance. Thus, our results demonstrate that E. balteatus behavior is affected by parasitoid presence through their exploitation of aphid colonies. Indeed, hoverfly predators select their prey according to the developmental state of the parasitoid larvae.     

Host range of a newly introduced parasitoid, Binodoxys communis among common aphid species in Hawaii

Binodoxys communis (Gahan) (Hymenoptera:Braconidae), a parasitoid of aphids originally from China, was introduced into Hawaii and evaluated in the laboratory for its ability to detect, accept, oviposit and develop in Aphis gossypii reared on two host plants, plus five other common aphid species. The parasitoid was able to detect all six aphid species and to successfully sting five species, with highest preference for those in the genus Aphis. Aphis species were highly suitable for parasitoid development. Other species were only marginally suitable. Parasitoids spent less time searching on plants of less acceptable aphids. Aphid defensive behaviors did not affect oviposition success, but did lengthen the parasitoid’s handling time of several aphid species. Host acceptance was positively correlated with host suitability, yet one unsuitable host was readily accepted for oviposition.     

Endoparasitism in cereal aphids: molecular analysis of a whole parasitoid community

Insect parasitoids play a major role in terrestrial food webs as they are highly diverse, exploit a wide range of niches and are capable of affecting host population dynamics. Formidable difficulties are encountered when attempting to quantify host–parasitoid and parasitoid–parasitoid trophic links in diverse parasitoid communities. Here we present a DNA-based approach to effectively track trophic interactions within an aphid–parasitoid food web, targeting, for the first time, the whole community of parasitoids and hyperparasitods associated with a single host. Using highly specific and sensitive multiplex and singleplex polymerase chain reaction, endoparasitism in the grain aphid Sitobion avenae (F) by 11 parasitoid species was quantified. Out of 1061 aphids collected during 12 weeks in a wheat field, 18.9% were found to be parasitized. Parasitoids responded to the supply of aphids, with the proportion of aphids parasitized increasing monotonically with date, until the aphid population crashed. In addition to eight species of primary parasitoids, DNA from two hyperparasitoid species was detected within 4.1% of the screened aphids, with significant hyperparasitoid pressure on some parasitoid species. In 68.2% of the hyperparasitized aphids, identification of the primary parasitoid host was also possible, allowing us to track species-specific parasitoid-hyperparasitoid links. Nine combinations of primary parasitoids within a single host were found, but only 1.6% of all screened aphids were multiparasitized. The potential of this approach to parasitoid food web research is discussed.     

Mating or ovipositing? A crucial decision in the life history of the cabbage aphid parasitoid Diaeretiella rapae (M’Intosh)

1. The reproductive fitness of a parasitoid depends on its mating and ovipositing success. Virgin haplodiploid females can reproduce, but produce only males, and may diminish fitness by producing more male offspring than required. Therefore, females must decide on whether to mate or oviposit first. 2. This study was conducted to assess the mating versus ovipositing decision and its impact on the reproductive fitness of Diaeretiella rapae (Hymenoptera: Aphididae), an endoparasitoid of the cabbage aphid Brevicoryne brassicae (Hemiptera: Aphididae). 3. When newly emerged females were given a choice between mating and ovipositing, about 62% of D. rapae females preferred to mate before ovipositing. Those females who oviposited before mating parasitised only 10% of the available aphids. After mating, females superparasitised their hosts with fertilised eggs, which resulted in a highly female‐biased sex ratio in the offspring. 4. Mating success was very high (91%) in the presence of hosts (cabbage aphid nymphs) compared with that in the absence of aphids. However, mating success was not influenced by the quality (size) of the hosts present in the mating arena, despite a parasitoid preference for larger hosts during oviposition. The time between pairing and mating was also shorter in the presence of host aphids. The mean number of aphids parasitised and the parasitism rate were significantly greater after mating.     

Influence of aphid size,age and behaviour on host choice by the parasitoid wasp Ephedrus californicus: a test of host-size models

Summary When host quality varies, parasitoid wasps are expected to oviposit selectively in high-quality hosts. We tested the assumption underlying host-size models that, for solitary species of wasps, quality is based on host size. Using Ephedrus californicus, a solitary endoparasitoid of the pea aphid, we evaluated the influence of aphid size (= mass), age and defensive behaviours on host selection. Experienced parasitoid females were given a choice among three classes of 5-day-old apterous nymphs: small aphids that had been starved daily for 4 h (S4) and 6 h (S6) respectively, and large aphids permitted to feed (F) normally. Wasps attacked more, and laid more eggs in, small than large aphids (S6>S4>F). This rank-order for attack did not change when females could choose among aphids of the same size that differed in age; however, wasps oviposited in all attacked aphids with equal probability. Host size did not influence parasitoid attack rates when aphids were anaesthetized so that they could not escape or defend themselves. As predicted by host-size models, wasp size increased with host size (F>S4; S6), but large wasps required longer to complete development than their smaller counterparts (S4E. californicus reflects a trade-off between maximization of fitness gains per egg and the economics of search-time allocation. Because large aphids are more likely to escape parasitization, a wasp must balance her potential gain in fitness by ovipositinng in a high-quality (large) aphid against her potential cost in terms of lost opportunity time if the attack fails.     

Host-plant use in the range expansion of the pine processionary moth, Thaumetopoea pityocampa

Abstract.  1. During range expansions of phytophagous insects, secondary or novel hosts may allow colonisation of areas without primary hosts. Because plant species often differ in their relative attractiveness and suitability for insects, insect preference for, and performance on, these hosts can determine recruitment potential in the current and future expansion areas.
2. This study explores the relative roles of female preference and larval performance in an important pine defoliator, Thaumetopoea pityocampa (Denis & Schiffermüller) (Lepidoptera, Notodontidae), which colonises three Pinus species at its current range margin in the Italian Alps: P. nigra (primary host), P. sylvestris (secondary host), and P. mugo (novel host).
3. Host use patterns in multiple insect populations were studied through choice and no-choice oviposition experiments in cages, field surveys of mixed stands, and laboratory and field monitoring of larval growth and mortality. It was predicted that a specific life-history trait – time limitation of short-lived females to lay a single batch of eggs – would act as a component of female performance, and lead to similar rates of host acceptance in no-choice settings.
4. In the choice experiment, P. nigra was accepted the most frequently while P. sylvestris was accepted the least frequently, confirming nest density patterns in the field. Contrary to prediction, females remained discriminating in no-choice settings in spite of time limitation. In contrast, relative growth rate (RGR) and mortality of larvae did not differ significantly among the three hosts, highlighting a discrepancy between female preference and larval performance.
5. Recruitment potential of T. pityocampa in future expansion into stands of P. sylvestris and P. mugo is evaluated by combining host quality, conservatism in oviposition behaviour, habitat suitability, and the opportunity for local adaptation.     

The influence of temperature on size as an indicator of host quality for the development of a solitary koinobiont parasitoid

The influence of aphid size on the host quality assessment and progeny performance of aphidiine parasitoids was examined using the mealy plum aphid parasitoid, Aphidius transcaspicus Telenga (Hymenoptera: Braconidae) and the black bean aphid, Aphis fabae Scopoli (Homoptera: Aphididae), as a readily acceptable alternate host. Aphid size in relation to stage of development was manipulated by rearing synchronous aphid cohorts at either 15 or 30 °C. At 15 °C, 2nd instar aphids were approximately the same size as 4th instar aphids reared at 30 °C. Cohorts of 30 aphids from each instar, reared at each temperature, were exposed to parasitism by a single parasitoid female for a period of 5 h. Overall susceptibility to parasitism did not vary between aphid cohorts, but the parasitoid response to aphid size differed significantly between rearing temperatures for both progeny sex ratio (parent female assessment of host quality) and larval growth and development (host suitability for parasitoid development). For aphids reared at 15 °C, the proportion of female progeny and emerging adult size for the parasitoid increased linearly with aphid size at the time of attack, while development time remained constant. In contrast, for aphids reared at 30 °C, the proportion of female progeny, emerging adult size, and the development time of the parasitoid all declined with aphid size at the time of attack. The contrasting responses of the parasitoid to host size for aphids reared at the two temperatures suggest that host quality is only indirectly related to aphid size among aphidiine parasitoids. The possible effects of higher temperatures on nutritional stress, obligate endosymbionts, and future growth potential of the aphids are discussed as explanations for the variation in host quality for parasitoid development.     

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.     

Foraging behaviour and sequential multisensory orientation in the aphid parasitoid, Pauesia picta (Hym., Aphidiidae) at different spatial scales

Abstract: Foraging females of Pauesia picta, a parasitoid of the conifer aphid Cinara pinea on pine, used a variety of host plant or host-borne cues to find and recognize its aphid hosts. For long-range detection, females rely mainly on secondary plant compounds to locate potential host plants. In the medium-range (i.e. on a given host plant), females detected the presence of hosts by encounters with honeydew-collecting workers of the ant Formica polyctena and by perceiving host-borne cues. For short-range detection, after the discovery of an aphid colony, P. picta females recognize hosts visually. Subsequent antennal tapping, a tactile stimulus, prevents dispersal of aphids and increases the oviposition success of the parasitoid. The final host acceptance is obviously triggered by signals located in the epicuticle of the host. The results demonstrate the importance of sequential multisensory foraging that correspond to a diminishing distance between the parasitoid and the host.     

Host specificity and seasonal distribution of Alloxysta pleuralis, a cynipoid hyperparasitoid of aphids in India

Abstract. 1. Alloxysta pleuralis (Cameron) is a cynipoid (Hymenoptera) hyperparasitoid of aphids (Homoptera) through their aphidiid (Hymenoptera) parasitoids.
2. It was recorded attacking five aphidiids, Lysiphlebus delhiensis (Subba Rao & Sharma), Trioxys indicus Subba Rao & Sharma, Aphidius uzbekistanicus Luzhetzki, Lipolexis scutellaris Mackauer and Lysiphlebia mirzai Shuja-Uddin (in order of preference) out of eight species of aphidiid parasitoids tested.
3. Percentage of hyperparasitism of aphids infesting cucurbits, cereals and pulses is higher than in solanaceous crops. Maximum hyperparasitism was recorded on the T. indicus/Aphis gossypii Glover/cucurbit complex.
4. The aphid host as well as the food plants significantly influence its efficiency.
5. A.pleuralis is abundant in northeastern Uttar Pradesh in winter (December–March) when large areas of pulses, cereals and cucurbits are infested with aphids serving as hosts for the primary parasitoids.     

Within-patch foraging behaviour of the aphid parasitoidAphidius funebris: plant architecture, host behaviour, and individual variation

The influence of plant architecture, host colony size, and host colony structure on the foraging behaviour of the aphid parasitoidAphidius funebris Mackauer (Hymenoptera: Aphidiidae) was investigated using a factorial experimental design. The factorial design involved releasing individual parasitoid females in aphid colonies consisting of either 10 or 20 individuals ofUroleucon jaceae L. (Homoptera: Aphididae) of either only larval instar L3 or a mixture of host instars, both on unmanipulated plants and on plants that had all leaves adjacent to the colony removed. Interactions between the parasitoid and its host were recorded until the parasitoid had left the plant. The time females spent on the host plant and the number of eggs laid varied greatly among females. Host colony size significantly affected patch residence time and the number of contacts between parasitoids and aphids. Plant architecture influenced the time-budget of the parasitoids which used leaves adjacent to the aphid colony for attacking aphids. Female oviposition rate was higher on unmanipulated plants than on manipulated plants. No further significant treatment effects on patch residence time, the number of contacts, attacks or ovipositions were found. Oviposition success ofA. funebris was influenced by instar-specific host behaviour. Several rules-of-thumb proposed by foraging theory did not account for parasitoid patch-leaving behaviour.     

The developmental strategy of an idiobiont ectoparasitoid, Dendrocerus carpenteri : influence of variations in host quality on offspring growth and fitness

Dendrocerus carpenteri (Curtis) (Hymenoptera: Megaspilidae) is a solitary hyperparasitoid, which attacks prepupal and pupal stages of hymenopteran parasitoids inside mummified aphids. The larva feeds externally on the host, which is envenomed by the female at oviposition. To evaluate the influence of variations in host quality on the growth, development and fitness of D. carpenteri, we varied the size and developmental stage of the primary parasitoid host (Aphidius ervi Haliday), which was reared on different instars of pea aphid [Acyrthosiphon pisum (Harris)] and English grain aphid [Sitobion avenae (F.)]. Within each kind of host, females eclosed from the relatively larger mummies, while males eclosed from the smaller mummies. Host size and hyperparasitoid size were correlated, and females were larger than males. In hyperparasitoids developing on prepupal and pupal hosts, development time from oviposition to adult eclosion was proportional to size; females required more time for development than males. The mean relative growth rate was the same in males and females and increased with host quality, as predicted by the growth model of Mackauer and Sequeira (1993) for idiobiont parasitoids. Larvae developing on late-pupal stages and pharate adults of A. ervi were unable to consume sclerotized host tissues; they were smaller and needed more time for development. The average number of mature eggs at eclosion was six, except in females developing on suboptimal hosts, which contained only one egg or none. Egg volume was correlated with female size, possibly reflecting differences in larval ontogeny. We provide equations describing the relationship between host quality as indexed by hind-tibia length of the mummified aphid and adult body size in terms of dry mass, development time and mean relative growth rate of D. carpenteri. We discuss the usefulness of host size as a proxy of host quality for idiobiont parasitoids, and provide examples of exceptions. Received: 14 December 1997 / Accepted: 23 July 1998     

Host-related fitness trade-offs in a presumed generalist parasitoid, Diaeretiella rapae (Hymenoptera: Aphidiidae)

Abstract.  1. Host ranges of parasitoid wasps are mediated by behavioural responses to hosts and their environment (infectivity), and development in hosts (virulence). Determinants of host range were measured in Diaeretiella rapae (Hymenoptera: Aphidiidae), which has been described as a generalist that attacks more than 60 species.
2. In northern Colorado, this wasp mainly attacks two hosts: cabbage aphid ( Brevicoryne brassicae ) and Russian wheat aphid ( Diuraphis noxia ). Here, laboratory experiments are described in which D. rapae originating from these two hosts were offered several hosts for oviposition. Both infectivity and virulence were measured.
3. Infectivity included host acceptance and handling time, while virulence was measured as productivity (number of progeny), survival of immatures within hosts, development time, and sex ratio.
4. Wasps had higher productivity and survival when attacking 'home' hosts than 'alternate' hosts, and trade-offs were found by quantitative genetic analyses to be genetically determined. Sex ratio and development times also showed trade-offs, but mainly related to the host environment in which females were reared.
5. In previous genetic studies in northern Colorado, populations were genetically subdivided on the scale of 1 km. The fitness differences described here could be strong enough to create populations adapted to different hosts, but it appears that gene flow is sufficient to prevent formation of separate lineages on the two hosts.
6. Rather than being a generalist with a broad host range, D. rapae is a serial specialist, attacking particular hosts according to availability in different seasons or in different geographical areas.