Ambient temperature affects mechanosensory host location in a parasitic wasp

Certain parasitic wasps (Ichneumonidae, Pimplinae) use self-produced vibrations transmitted on plant substrate to locate their immobile concealed hosts (i.e. lepidopteran pupae). This mechanosensory mechanism, called the vibrational sounding, depends both on physical cues of the environment and physical activity of the parasitoid and is postulated to depend on ambient temperature. We analysed the influences of temperature on vibrational sounding by choice experiments using plant-stem models with hidden host mimics in the temperate species Pimpla turionellae. The results show a significant effect of temperature on host-location activity and on the success of this process. Outside an optimum range, the performance of the wasps decreased both at low and high temperatures. Below 10°C and beyond 24°C, the wasps displayed (1) substantial reduction in responsiveness, i.e. proportion of females showing ovipositor insertions, (2) reduction of quantitative activity with ovipositor insertions in the individuals, and (3) reduced precision of mechanosensory host location. Nevertheless, female wasps were able to locate their host over a surprisingly broad range of ambient temperatures which indicates that the wasps are able to compensate for temperature effects on vibrational sounding.     

Interaction of vibrational and visual cues in parasitoid host location

Female parasitoids are guided by multisensory information during host finding. Individual cues are used in an interactive or a hierarchical manner according to the relative importance on the spatial scale of their effect. Unlike most studies that concentrate on single cues, the present paper investigates the interaction of two physical cues. The interaction of mechanosensory and visual cues was studied in the pupal parasitoid Pimpla turionellae (Hymenoptera: Ichneumonidae). This species uses, amongst other senses, vibrational sounding (echolocation in a solid substrate) to find its mainly endophytic hosts. Location and frequency of ovipositor insertions were scored on cylindrical plant stem models with single or combined cues. Single-cue experiments show that parasitoids use both visual and mechanosensory cues and achieve a similar precision of host location with either cue. The combination of vision and vibrational sounding increased the precision of host location by a factor of approximately two to three. We conclude that the two senses interact, resulting in an additive accuracy. Neither the visual nor the mechanosensory cue was favored when offered adjacent to each other on the same stem model. On the investigated spatial scale, both physical cues are used and seem to be equally important for host location in this species.     

Effect of ambient temperature on mechanosensory host location in two parasitic wasps of different climatic origin

Abstract.  Several parasitic wasps of the Pimplinae (Ichneumonidae) use self-produced vibrations transmitted through plant substrate to locate their concealed immobile hosts (lepidopteran pupae) by reflected signals. This mechanosensory mechanism of host location, called vibrational sounding, depends on the physical characteristics of the plant substrate and the wasp's body and is postulated to depend on ambient temperature. Adaptations of two parasitoid species to thermal conditions of their habitats and the influence of temperature on the trophic interaction during host location are investigated in the tropical Xanthopimpla stemmator (Thunberg) and compared with the temperate Pimpla turionellae (L.). Plant-stem models with hidden host mimics are offered to individual wasps under defined temperature treatments and scored for the number and location of ovipositor insertions. Significant effects of temperature are found on host-location activity and its success. The tropical species possesses an optimum temperature range for vibrational sounding between 26 and 32 °C, whereas the performance decreases both at low and high temperatures. The temperate species reveals substantial differences with respect to performance at the same thermal conditions. With increasing temperature, P. turionellae shows a reduced response to the host mimic, reduced numbers of ovipositor insertions, and decreased precision of mechanosensory host location. In the tropical X. stemmator , the female wasps are able to locate their host with high precision over a broad range of ambient temperatures, which suggests endothermic thermoregulation during vibrational sounding. Environmental physiology may therefore play a key role in adaptation of the host location mechanism to climatic conditions of the species' origin.     

Temperature affects interaction of visual and vibrational cues in parasitoid host location

Parasitoid host location in nature is facilitated by simultaneously using different information sources. How multisensory orientation on the same spatial scale is influenced by environmental conditions is however poorly understood. Here we test whether changes in reliability of cues can cause parasitoids to alter multisensory orientation and to switch to cues that are more reliable under extreme temperatures. In the ichneumonid wasp Pimpla turionellae, multisensory use of thermally insensitive vision and thermally sensitive mechanosensory host location by vibrational sounding (echolocation on solid substrate) was investigated with choice experiments on plant-stem models under optimum temperature (18°C), at high- (28°C) and low-temperature limits (8°C) of vibrational sounding. Temperature affected relative importance of vibrational sounding whereas visual orientation did not vary. At 18°C, parasitoids used visual and vibrational cues with comparable relative importance. At 8 and 28°C, the role of vibrational sounding in multisensory orientation was significantly reduced in line with decreased reliability. Wasps nearly exclusively chose visual cues at 8°C. The parasitoids switch between cues and sensory systems depending on temperature. As overall precision of ovipositor insertions was not affected by temperature, the parasitoids fully compensate the loss of one cue provided another reliable cue is available on the same spatial scale.     

Host Location by Ichneumonid Parasitoids is Associated with Nest Dimensions of the Host Bee Species

Parasitoid fitness depends on the ability of females to locate a host. In some species of Ichneumonoidea, female parasitoids detect potential hosts through vibratory cues emanating from them or through vibrational sounding produced by antennal tapping on the substrate. In this study, we (1) describe host location behaviors in Grotea gayi Spinola (Hymenoptera: Ichneumonidae) and Labena sp. on nests of Manuelia postica Spinola (Hymenoptera: Apidae), (2) compare nest dimensions between parasitized and unparasitized nests, (3) correlate the length of M. postica nests with the number of immature individuals developing, and (4) establish the relative proportion of parasitized nests along the breeding period of M. postica. Based on our results, we propose that these parasitoids use vibrational sounding as a host location mechanism and that they are able to assess host nest dimensions and choose those which may provide them with a higher fitness. Finally, we discuss an ancestral host?Cparasitoid relationship between Manuelia and ichneumonid species.     

Host Location of a Pupal Parasitoid in a Tritrophic System Compared to a Model Offering Mechanosensory Cues Only

Several species of hymenopteran parasitoids are able to locate concealed pupal hosts by vibrational sounding. However, the specific role of this technique of mechanosensory host location has not yet been investigated in a natural, tritrophic system with multiple cues. Here we compared the host location of the pupal parasitoid Xanthopimpla stemmator in a tritrophic system with corn borer pupae in maize stem to the behavior on a paper model offering mechanosensory cues only. In general, the behavioral pattern and the behavioral states exhibited by host-searching female parasitoid were identical in the model and in the tritrophic system, while quantitative aspects differed. Our results demonstrate that vibrational sounding maintains its significance for host location in an environment with multiple cues, and that additional cues may increase responsiveness of females.     

Host preference of Xanthopimpla stemmator (Hymenoptera: Ichneumonidae) and its reproductive performance on selected African lepidopteran stem borers

The Asian pupal parasitoid, Xanthopimpla stemmator Thunberg, was imported into East Africa as a classical biological agent of lepidopteran cereal stem borers. Preference of X. stemmator females for four common borers of maize and sorghum; the crambid Chilo partellus (Swinhoe), the pyralid, Eldana saccharina Walker, and the noctuids, Busseola fusca Fuller and Sesamia calamistis Hampson, was investigated. Pre-adult experience of X. stemmator females did not influence choice of host. In dual choice tests, more B. fusca were attacked than E. saccharina, while E. saccharina were attacked more than Ch. partellus. Life table studies on three of the hosts revealed that the intrinsic rate of increase was highest when X. stemmator was reared on S. calamistis. Net reproductive rates, mean generation times and doubling times were not different between hosts. Results suggest that X. stemmator can be successfully reared on the three stem borer species and released in areas where any combination of the three hosts occurs.     

Host-finding behavior, host acceptance, and host suitability of the parasiteXanthopimapla stemmator

Xanthopimpla stemmator (Thunberg)(Hymenoptera: Ichneumonidae), a solitary endoparasite of pupae of Old World lepidopteran stalkborers, was recently imported into Texas as a candidate for biological control of New World stalkborers. Information on host acceptability, host suitability and cues responsible for host finding were necessary to gain an insight into parasite/host interactions, because of the absence of a coevolutionary history.Xanthopimpla stemmator females were exposed to laboratory-reared one-to six-day-oldDiatraea saccharalis (F.) pupae. An average of 62% of host pupae were accepted and all ages of pupae were equally acceptable. Host suitability decreased with host age. One- to five-day-old host pupae averaged 31–37% suitability, whereas only 19% of 6-day-old pupae were suitable. Successful parasitization, defined as the product of the proportion accepted and the proportion suitable, decreased from 22–23% for 1-, 2- and 3-day-old pupae to 13% for 6-day-old pupae. Sex ratio (female:male) of the parasite progeny increased with host age. Females comprised 47% of total parasite progeny of 1-day-old and 84% of 6-day-old pupae. The increase in percent females was a result of a similar number of females in all age classes, coupled with a decrease in the number of males from older hosts.Xanthopimpla stemmator superparasitized 61% of acceptedD. saccharalis pupae in the laboratory. On dissection, 73% of host pupae with multiple probe wounds were found to contain parasite eggs or larvae; these hosts contained up to 10 eggs or 7 first-instar larvae. Increased numbers of probes by the parasites were associated with an increase in successful parasitization. Host seeking activity inX. stemmator was stimulated by the presence of larval frass, host odor and movement of host pupae. Results suggest thatX. stemmator is a good candidate for biological control ofD. saccharalis and possibly other factitious stalkborer hosts.     

The adaptive significance of host location by vibrational sounding in parasitoid wasps

Vibrational sounding, which is a form of echolocation, is a means of host location by some parasitoid wasps. The wasp taps the substrate (wood, stem or soil) and detects the position of a potential host through the returning 'echoes'. The deployment of vibrational sounding is inferred through the form of the subgenual organ in the female tibia in combination with the presence of modifications to the female antenna used for tapping the substrate. Vibrational sounding and its associated modifications were found in two families. The use of vibrational sounding by parasitoid wasps was positively correlated with the depth of the host in the substrate relative to the size of the parasitoid. There were also significant correlations between the use of vibrational sounding and parasitism of immobile and concealed hosts and between vibrational sounding and idiobiosis. The data suggested that vibrational sounding evolved under a variety of ecological conditions, being employed in the location of wood-boring, stem-boring, soil-dwelling and cocooned hosts and stem-nesting aculeates, often in situations in which the host does not produce vibrations itself.     

Body temperature of the parasitic wasp Pimpla turionellae (Hymenoptera) during host location by vibrational sounding

Abstract.  The pupal parasitoid Pimpla turionellae (L.) uses self-produced vibrations transmitted on the plant substrate, so-called vibrational sounding, to locate immobile concealed pupal hosts. The wasps are able to use vibrational sounding reliably over a broad range of ambient temperatures and even show an increased signal frequency and intensity at low temperatures. The present study investigates how control of body temperature in the wasps by endothermic mechanisms may facilitate host location under changing thermal environments. Insect body temperature is measured with real-time IR thermography on plant-stem models at temperature treatments of 10, 18, 26 and 30 °C, whereas behaviour is recorded with respect to vibrational host location. The results reveal a low-level endothermy that likely interferes with vibrational sound production because it occurs only in nonsearching females. At the lowest temperature of 10 °C, the thoracic temperature is 1.15 °C warmer than the ambient surface temperature whereas, at the high temperatures of 26 and 30 ° C, the wasps cool down their thorax by 0.29 and 0.47 °C, respectively, and their head by 0.45 and 0.61 °C below ambient surface temperature. By contrast, regardless of ambient temperature, searching females always have a slightly elevated body temperature of at most 0.30 °C above the ambient surface temperature. Behavioural observations indicate that searching females interrupt host location more frequently at suboptimal temperatures, presumably due to the requirements of thermoregulation. It is assumed that both mechanisms, producing vibrations for host location and low-level endothermy, are located in the thorax. Endothermy by thoracic muscle work probably disturbs signal structure of vibrational sounding, so the processes cannot be used at the same time.     

Discrimination by Coptera haywardi (Hymenoptera: Diapriidae) of hosts previously attacked by conspecifics or by the larval parasitoid Diachasmimorpha longicaudata (Hymenoptera: Braconidae)

Coptera haywardi (Oglobin) is an endoparasitoid of fruit fly pupae that could find itself in competition with other parasitoids, both con- and heterospecific, already resident inside hosts. In choice bioassays, ovipositing C. haywardi females strongly discriminated against conspecifically parasitised Anastrepha ludens (Loew) pupal hosts. They also avoided pupae previously attacked by Diachasmimorpha longicaudata (Ashmead), a larval–prepupal koinobiont endoparasitoid, and the degree of larval-parasitoid superparasitism had no effect on this avoidance. There was no difference in the number of ovipositor insertions when hosts previously parasitised by a conspecific and D. longicaudata were exposed simultaneously. As females aged the degree of host discrimination declined. An ability to discriminate against pupae previously attacked as larvae suggests low levels of both conspecific and heterospecific competition in the field.     

Host preference of the facultative hyperparasitoidTetrastichus howardi (Hym.: Eulophidae)

Tetrastichus howardi (Olliff) preferred the lepidopteran hosts,Chilo partellus (Swinhoe) (Pyralidae) andHelicoverpa armigera (Hübner) (Noctuidae) to their parasitoids,Xanthopimpla stemmator (Thunberg) (Hymenoptera: Ichneumonidae) andPalexorista laxa (Curran) (Diptera: Tachinidae). IfT. howardi had previously experienced parasitising a certain host, its preference for that host increased, but not significantly. When reared on a certain host, the preference for that host increased.T. howardi showed no preference to any particular age of its hosts.T. howardi was able to discriminate between parasitised and unparasitised hosts, initially preferring parasitised hosts, but two days later preferring unparasitised hosts.     

Effects of host age on the performance of >Diadromus collaris, a pupal parasitoid of >Plutella xylostella

The ichneumonid >Diadromus collaris(Gravenhorst) (Hymenoptera: Ichneumonidae) is amajor solitary, pupal endoparasitoid of thediamondback moth, >Plutella xylostella(Linnaeus) (Lepidoptera: Plutellidae).Experiments to examine parasitism of the hostpupae of different ages by the parasitoid wereconducted in the laboratory. >Diadromuscollaris preferred host pupae that were in thefirst half of their pupal development. Survivalfrom larva to adult, and size and parasitizingcapacity of the resultant female adultsdecreased dramatically as host pupal ageincreased. When ovipositions were made intohost pupae that were in the last quarter oftheir development, all parasitoids died beforeadult emergence. The performance of >D.collaris, as affected by host pupal age,agrees with the simple diet theory thatpredicts female wasps should select hosts ofhigher nutritional quality for oviposition.     

A comparison of the host-searching efficiency of two larval parasitoids of Plutella xylostella

1. A host specialist parasitoid is thought to have greater efficiency in locating hosts or greater ability to overcome host defence than a generalist species. This leads to the prediction that a specialist should locate and parasitise more hosts than a generalist in a given arena. The work reported here tested these predictions by comparing the host‐searching behaviour of Diadegma semiclausum (a specialist) and Cotesia plutellae (an oligophagous species), two parasitoids of larval Plutella xylostella. 2. Both parasitoids employed antennal search and ovipositor search when seeking hosts but D. semiclausum also seemed to use visual perception in the immediate vicinity of hosts. 3. Larvae of P. xylostella avoided detection by parasitoids by moving away from damaged plant parts after short feeding bouts. When they encountered parasitoids, the larvae wriggled vigorously as they retreated and often hung from silk threads after dropping from a plant. 4. These two parasitoids differed in their responses to host defences. Diadegma semiclausum displayed a wide‐area search around feeding damage and waited near the silk thread for a suspended host to climb up to the leaf, then attacked it again. Cotesia plutellae displayed an area‐restricted search and usually pursued the host down the silk thread onto the ground. 5. Diadegma semiclausum showed a relatively fixed behavioural pattern leading to oviposition but C. plutellae exhibited a more plastic behavioural pattern. 6. The time spent by the two parasitoids on different plants increased with increasing host density, but the time spent either on all plants or a single plant by D. semiclausum was longer than that of C. plutellae. Diadegma semiclausum visited individual plants more frequently than C. plutellae before it left the patch, and stung hosts at more than twice the rate of C. plutellae. 7. The results indicated that the host‐location strategies employed by D. semiclausum were adapted better to the host's defensive behaviour, and thus it was more effective at detecting and parasitising the host than was C. plutellae.     

Acceptance and suitability of four lepidopteran stemborers for the development of the pupal parasitoid Xanthopimpla stemmator (Hymenoptera: Ichneumonidae)

Xanthopimpla stemmator (Thunberg), a solitary endoparasitoid of lepidopteran stemborer pupae, was recently imported into East Africa as a candidate biological control agent of gramineous stemborers. Suitability of Busseola fusca Fuller, Chilo partellus (Swinhoe), Eldana saccharina (Walker) and Sesamia calamistis Hampson, for the development of X. stemmator was studied in the laboratory. One- to 6-day-old laboratory reared pupae of the four stemborer species were exposed to naïve X. stemmator females. All host pupae and ages were acceptable for oviposition. The parasitoids inflicted multiple probe wounds on 67.8% of pupae exposed. B. fusca, C. partellus and S. calamistis were equally suitable with 56.4, 59.4 and 52.3%, respectively, of probed pupae leading to emergence of adult parasitoids. E. saccharina was less suitable with only 22.6% of probed pupae producing parasitoids. Emergence of parasitoids did not differ significantly across the six pupal ages for B. fusca and S. calamistis, but varied for C. partellus and E. saccharina. No parasitoids emerged from 6-day-old E. saccharina pupae. Realized fecundity of females reared on the four stemborer pupae showed that fewer progeny were produced by females emerging from E. saccharina than females reared on the other three stemborer species. Eldana saccharina may be a poor host for X. stemmator in Kenya, but this parasitoid is a potential candidate for biological control of B. fusca, C. partellus and S. calamistis.     

Host discrimination of a larval parasitoid: the quick movement of Microplitis demolitor

Many parasitoids discriminate previously parasitised hosts from unparasitised ones to avoid mortality of offspring. Parasitoids that parasitise aggressive hosts such as lepidopteran larvae are known to attack hosts very quickly to avoid being attacked. However, little is known about host discrimination of such quick attacking parasitoids. We investigated host discrimination of Microplitis demolitor (Wilkinson) (Hymenoptera: Braconidae) a quick attacking parasitoid of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae). Results showed that ratios of female wasps that rejected the hosts after antennal examination did not differ between parasitised and unparasitised hosts, indicating that M. demolitor did not discriminate hosts by antennal examination. However, 95% of females that inserted ovipositor into unparasitised hosts actually laid eggs, whereas it was only 31% for parasitised hosts, indicating that females discriminated hosts by oviposition insertion. Analyzing video recordings revealed that the ovipositor exploration of the host took 0.3 s. Female wasps that had experienced high-host density of unparasitised hosts readily rejected parasitised hosts, while wasps with experience of low host availability of parasitised hosts tended to accept parasitised hosts. This suggests that host discrimination behaviour of M. demolitor is affected by previous experience of different host availability and host quality.

     

Sensilla on antenna,ovipositor and leg of Eriborus applicitus (Hymenoptera: Ichneumonidae), a parasitoid wasp of Holcocerus insularis staudinger (Lepidoptera: Cossidae)

Eriborus applititus Sheng & Sun (Hymenoptera: Ichneumonidae) is a specialist parasitoid of the small carpenter moth Holcocerus insularis Staudinger (Lepidoptera: Cossidae). Each year damages to trees caused by H. insularis lead to extensive economic and ecological losses. E. applititus is thus a promising candidate for use as a biocontrol agent against H. insularis. To investigate the means by which E. applititus locates and parasitizes H. insularis, we used scanning electron microscopy to determine the morphology and distribution of sensilla on antenna, ovipositor and leg of male and female E. applititus. Eight different sensilla types were found: sensilla chaetica, sensilla trichodea, sensilla placodea, sensilla basiconica, sensilla coeloconica, sensilla pit basiconica, sensilla campaniformia and Böhm's bristles. The sensilla types were differently distributed in the three organs. In addition, differences between sexes were found in the distribution of sensilla trichodea type 2, sensilla placodea and sensilla chaetica type 4. Putative functions of the sensilla are discussed based on the morphological and location data and on previous research. Chemosensitive sensilla are putatively involved in host detection, pheromone detection and host discrimination processes. Mechanoreceptive sensilla likely function as vibrational sensors and are thought to be critical for accurate ovipositor positioning.     

Growth and development of a generalist insect herbivore, Operophtera brumata, on original and alternative host plants

A generalist feeding strategy is common among eruptive insect herbivores but the ultimate reasons for a generalist strategy are not clear. Although generalist insect herbivores are able to complete their life cycle on several species of host plants, there is wide variation in the performance of individuals grown on different hosts. We examined whether different populations of Operophtera brumata are adapted to use the host species which is locally most abundant, and how the host plant affects growth and development of the insect. We reared two allopatric populations (eastern Finland, Prunus padus; south-west Finland, Quercus robur) on four species of host plants (Pr. padus, Populus tremula, Q. robur, Salix phylicifolia) from neonate larvae to the adult stage and measured the growth and development of individuals and the timing of adult hatching. The performance of both populations was best on Pr. padus, and the south-western population, originally on Q. robur, was well adapted to this host. The host affected the growth of females more than that of males. The host plant had an unexpected effect on hatching times of the adults. Individuals grown on the original host hatched in normal synchrony, i.e. males 6–7 days before females; but on alternative hosts this synchrony was disturbed. As is common in eruptive, capital-breeding generalist moths where female fecundity is linked to weight, host quality is critical for the flightless females of O. brumata. We suggest that in a heterogeneous environment the disturbing effect of alternative host plants on adult emergence may decrease the population density and growth rate compared to the potential maximum in a homogeneous environment. Received: 8 July 1999 / Accepted: 29 October 1999     

Evolutionary patterns of host utilization by ichneumonoid parasitoids (Hymenoptera: Ichneumonidae and Braconidae)*

The four major biological strategies of ichneumonoid parasitoids, koinobiont and idiobiont, ecto-and endoparasitism, are discussed and the evolutionary radiations of the two families Ichneumonidae and Braconidae compared in an attempt to relate differences in patterns of host utilization to differences in evolutionary history. The most primitive members of both families are idiobiont ectoparasitoids of hosts concealed in plant tissue. Idiobiont ectoparasitic braconids are all still primarily associated with such hosts, but idiobiont ectoparasitic ichneumonids have radiated to attack hosts in other situations, such as in aculeate nests or in cocoons. A shift in emphasis between the behavioural steps, host habitat location and host location, is envisaged as being important in such evolutionary change. Idiobiont endoparasitism is postulated as having arisen amongst ectoparasitoids attacking cocooned hosts, as an adaptation that allows them to exploit pupae and puparia in relatively exposed positions; it is a fairly common strategy in the Ichneumonidae, but virtually unknown in the Braconidae. Koinobiosis is perceived as having evolved in association with hosts which feed in a relatively weakly concealed position, but pupate in a more secluded and safe location. The strategy is advantageous as it allows a parasitoid to oviposit on an easily discoverable host, but to use the host's pupation concealment to complete its own development. The evolution of koinobiosis has allowed parasitoids to exploit hosts that feed in exposed positions, and to attack hosts at a younger and numerically more common stage in the host's life cycle. Koinobiont ectoparasitism is envisaged, in some braconid and ichneumonid groups, to occupy an evolutionary transitional position between idiobiosis and endoparasitic koinobiosis; only in the Ichneumonidae have large radiations of koinobiont ectoparasitoids occurred. Endoparasitic koinobiosis is hypothesized as having arisen in the Braconidae in association with lepidopterous/coleopterous hosts, whilst in the major lineage of endoparasitic koinobiont ichneumonids, this habit is hypothesized as having arisen in association with symphytan hosts. The great majority of braconids are koinobiont endoparasitoids, but only about 50% of the Ichneumonidae have this habit. Very few koinobiont braconids develop as endoparasitoids of hymenopterous hosts, although many endoparasitic ichneumonids attack Hymenoptera. However, lineages of the Braconidae have radiated to exploit adult insects and exopterygote nymphs; ichneumonids do not utilize such hosts.     

Evaluation of Diachasmimorpha longicaudata (Hymenoptera: Braconidae) as a mortality factor of Ceratitis capitata (Diptera: Tephritidae) infesting Citrus species under laboratory and field-cage conditions

Ceratitis capitata (or medfly) is one of the major pests currently affecting fruit crops in northwestern Argentinian Citrus-producing areas. Medfly populations are sustained in large exotic fruits, such as Citrus paradisi, Citrus aurantium and Citrus sinensis, which are known to hinder the activity of almost all native parasitoid species. Therefore, a feasible approach to controlling medfly involves the use of exotic parasitoids such as Diachasmimorpha longicaudata. In this study, the prediction that parasitoid females would be proficient at finding medfly larvae infesting the Citrus species mentioned earlier was tested. Particularly, the variation in fruit species preference by parasitoid females, the efficacy of the parasitoid to kill medfly and the effect of host density on parasitoid performance were determined. Parasitoids were allowed to forage for 8 h on grapefruits and oranges artificially infested with medfly larvae under controlled (laboratory) and uncontrolled (field cage) environmental conditions. Fruit choice and no-choice tests were performed. Results were assessed by comparing the number of female visits to and ovipositor insertions into the fruit, and parasitoid emergence, parasitism and additional host mortality percentages. Parasitoid preference for visiting larger fruits (sour orange and grapefruit) may be related to the increased fruit surface area. Ovipositional activity on fruit was influenced by the variation of the larval host density per unit of fruit surface. The higher parasitism rates recorded from sweet orange would be mainly a result of both increased host density and fruit physical features. Nevertheless, D. longicaudata showed the capacity to parasitise hosts in all Citrus species tested.