Plant cues influence searching behaviour and parasitism in the egg parasitoid Trichogramma nr. brassicae

Abstract. 1. The effect of experience of plant cues at emergence on searching behaviour and parasitism was investigated in two strains of Trichogramma nr. brassicae (Hymenoptera: Trichogrammatidae).
2. Wasps reared on Sitotroga cereallela were allowed to emerge on a tomato plant or in a vial, with or without food. For one of the strains, females emerging on tomato searched significantly longer on a tomato seedling than females emerging in a vial.
3. The experience effect lasted between 1 and 2 days. It involves associating plant stimuli with the remains of the host, because females emerging from their host on a tomato plant had an increased searching response to tomato, but females transferred to the plant immediately after emergence did not.
4. The effect of emergence environment on parasitism of the host Helicoverpa punctigera on tomato and lettuce seedlings was tested. Wasps were allowed to emerge on seedlings of tomato or lettuce, or in an empty vial. For one strain, females experienced with tomato at emergence parasitized significantly more hosts on tomato than did females emerging on lettuce or in a vial. Parasitism on lettuce was not influenced by the treatments.     

Life history traits and foraging behaviour of Cotesia nonagriae (Olliff) (Hymenoptera: Braconidae), a newly recognised member of the Cotesia flavipes complex of stemborer parasitoids

Cotesia nonagriae (Olliff) from Australia, a parasitoid of the incidental native pest of sugarcane, Bathytricha truncata (Walker) (Lepidoptera: Noctuidae), was previously thought to be a synonym of Cotesia flavipes Cameron. However, recent studies using DNA sequences, morphology and preliminary biological information show that this parasitoid is clearly a different species than C. flavipes and other members of the species complex. Here we further examine differences in the biology of these species by undertaking a detailed study of the life history traits of C. nonagriae , including adult longevity and the potential and realised fecundity of females. In addition, the influence of learning on microhabitat location and foraging behaviour were investigated. Duration of the larval stages and adult longevity of C. nonagriae were longer than previously recorded for other members of the species complex. The potential fecundity of females was similar to C. flavipes (∼200 eggs); however, C. nonagriae oviposited a average of over 100 eggs into each host, almost three times more than for other species in the C. flavipes complex (30–40). The propensity of C. nonagriae to allocate a large number of eggs to each host may be an evolutionary strategy due to the high mortality rate (50–57%) of ovipositing adult wasps. During microhabitat location, both naïve and experienced females demonstrated a strong response towards the plant host complex, with experienced wasps benefiting by having a more rapid response time to host-induced volatiles and cues.     

Connectivity counts: disentangling effects of vegetation structure elements on the searching movement of a parasitoid

1. A heterogeneous habitat structure can have a profound impact on foraging carnivorous arthropods. In the present study, we examined which elements of complex vegetation structure influence the searching movement of a parasitoid model organism. 2. Previous field work showed that tall and dense vegetation reduces the parasitism success of the eulophid egg parasitoid Oomyzus galerucivorus while the probability of host egg deposition increased close to plant tips. 3. In laboratory bioassays, dried grass stems were arranged according to the natural situation in different setups. The wasps' walking time on stems increased with increasing stem height and density. High stem density decreased the walking time of the parasitoids on the ground and an increased stem height reduced the propensity to fly to the ground. Connectivity had a minor positive effect on the number of stem contacts, but considerably reduced the number of wasps reaching the upper part of grass stems by two‐thirds. 4. Thus, although enhanced vegetation complexity enhances walking activity of the parasitoids in the vegetation, laying eggs at the tip of long grass stems in dense vegetation can be an adaptive strategy for the host, as it maximises the number of connections between plant parts to cross by parasitoids before reaching the host. The connection points disorient the wasps, which lose time, reverse their direction or fly away.     

Host volume measurement by the parasitoid wasp Trichogramma minutum: The roles of curvature and surface area

The number of eggs laid into its insect-egg host by Trichogramma minutum Riley (Hymenoptera: Chalcidoidea: Trichogrammatidae) depends on the volume of its lepidopterous host. The wasps allocated fewer progeny into Manduca sexta (L.) (Sphingidae) egg hosts partially embedded in the substrate than into fully exposed hosts, both in daylight and in total darkness. Since the hosts are of identical diameter and surface chemistry, the mechanism of volume determination is neither chemical nor visual, but essentially mechanosensory, based on accessible surface area. Before oviposition, the wasp examines the host surface by walking over it, turning at the edge, and drumming with its antennae. The number of edge turns (N_t) made by wasps when they examined glass beads presented as model hosts was inversely dependent on surface area, but independent of diameter. Conversely, the examination time (T_e) was directly dependent on diameter, but independent of surface area.A mechanism is proposed for host volume measurement. While the wasp examines a potential host, the relative curvature (and hence diameter) is measured, possibly by antennal angle. If this measure is within an acceptable range, the wasp initiates its drumming walk. The measure is also used to set T_c and to establish an estimate of a maximum possible volume (and hence progeny number) to the host object. With T_c fixed, the value for N_t depends on accessible surface area, the wasp turning more often on restricted surfaces. N_t allows the wasp to correct the estimate of host volume such that an appropriate number of eggs is laid in non-spherical hosts. The use of curvature to set T_e increases the accuracy of the wasp's measure of N_t, while also improving its coverage of the surface. The use of curvature and surface area allows Trichogramma to optimize its progeny allocation for a variety of host sizes and shapes.

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Die bestimmung des wirtsvolumen durch die parasitische wespe Trichogramma minutum: Die rolle von krümmung und oberfläche

Zusammenfassung Bei dem Insekten-Eiparasiten Trichogramma minutum (Hymenoptera: Trichogrammatidae) hängt die Anzahl der abgelegten Eier vom Volumen der Wirtseier ab. Es wurden Untersuchungen zur Bestimmung der sensorischen Grundlagen dieses Verhaltens unternommen. Die Wespe weist sowohl im Tageslicht, als auch in totaler Dunkelheit jenen Manduca sexta (L.) (Lepidoptera: Sphingidae) Eiwirten (Diameter c. 1.3 mm) weniger eigene Eier zu, die teils (30%) in Substrat eingebettet sind, als solchen, die ganz herausragen. Da die Wirte einen identischen Radius und gleichen Oberflächengeruch haben, kann der Mechanismus der Volumenbestimmung weder chemisch noch optisch sein, sondern ist dem Wesen nach mechanosensorisch, und von der zugänglichen Oberfläche abhängig. Vor de Eiablage untersucht die Wespe die Wirtsoberfläche, indem sie darüber läuft, an den Ecken umkehrt und mit ihren Antennen trommelt. In Experimenten mit Glaskugelmodellen war die Anzahl der von der Wespe gemachten Wendungen an den Ecken (N_t) umgekehrt proportional zur Oberfläche, aber unabhängig vom Radius. Im Gegensatz dazu, war die Untersuchungszeit (T_e) direkt proportional zum Radius, aber unabhängig von der Oberfläche.Ein Mechanismus für die Volumenmessung wird vorgeschlagen. Gleich beim Besteigen des potentialen Wirtes wird die relative Kurve (und infolgedessen der Radius), wahrscheinlich durch den Antennenwinkel gemessen. Wenn diese Messung innerhalb einer bestimmten Spanne liegt, beginnt die Wespe ihren Trommelgang. Die Messung wird auch dazu benutzt, ein maximales Volumen (und folglich die Anzahl der Nachkommen) des Wirtes und eine maximale Untersuchungzeit (T_e) zu erzielen. Wird T_e konstantgehalten, hängt die Anzahl der Wendungen (N_t) von der zugänglichen Oberfläche ab, wobei die Wespe öfter bei kleineren Oberflächen wendet. Die Anzahl der Wendungen gestattet der Wespe, die ursprüngliche Einschätzung des Wirtsvolumens so zu verändern, daß eine angemessene Anzahl von Eiern in nicht kugelige Wirte gelegt wird. Die Anwendung von kurven-und oberflächenabhängigen Messungen ermöglicht eine optimale Zuweisung von Nachkommen für eine Anzahl von unterschiedlichen Wirtsgrössen und-formen. Die Oberflächenmessung gestattet eine bessere Volumenbestimmung von nicht-kugeligen Objekten. Die gezeigte Abhängigkeit von der Oberfläche ist wahrscheinlich auch eine Anpassung an Wirte, die ihre Eier in Gruppen ablegen. Unter solchen Bedingungen von grosser Wirtsdichte ist es günstiger für die Wespe, weniger Eier in jeden Wirt der Gruppe zu legen, weil diese Verteilung zu grösseren, fruchtbareren Nachkommen führt.

     

Effect of host availability on reproduction and survival of the parasitoid wasp Trichogramma minutum

Abstract.

• 1 We tested the hypothesis that females of the egg parasitoid, Trichogramma minutum Riley (Hymenoptera: Trichogrammatidae), could adjust their fecundity schedule according to host availability and that there was a negative correlation between reproduction and survival in these wasps.
• 2 Newly-emerged females were provided with an unlimited or limited number of hosts in the first trial and with either unlimited, limited or zero hosts in the second trial.
• 3 When hosts were unlimited, wasps had the highest rate of reproduction in the first day, which decreased dramatically thereafter. When hosts were limited, wasps from the two trials differed in their response. In Trial I, females with limited hosts had lower first-day fecundity than, and the same subsequent-day fecundity as, those with unlimited hosts. However, in Trial II, females with limited host had a lower first-day but a higher subsequent-day fecundity than those with unlimited hosts. This indicates variation in Trichogramma's ability to shift its fecundity schedule in response to host availability.
• 4 There was a positive (rather than a negative) correlation between reproduction and survival. Wasps that oviposited (in host-unlimited treatment) had greater longevity than those that could not (in host-unavailable treatment).
• 5 The sex ratio of the progeny produced by wasps in both host-unlimited and limited treatments shifted gradually from a female to a male bias as the wasps aged.
• 6 We consider the ability of parasitoids to adjust their fecundity schedule as an adaptation to changing host resources and discuss our findings with regard to theories of life history evolution.

     

Influence of environmental conditions and parasitoid quality on field performance of Trichogramma minutum

The effects of weather conditions and two parasitoid quality attributes, realized fecundity and host acceptance, were assessed on the field efficacy of mass-released Trichogramma minutum. Temperature was the most important single variable, explaining up to 75% of the variation in field parasitism. There were significant positive relationships between both the sum of the maximum temperatures and the number of degree-hours above a 15 °C threshold, accumulated in the three days following the release, and parasitism in the field. There was a significant negative relationship between the mean relative humidity and the odds of parasitism in the field. Quality parameters based on parasitoid biology were not effective for predicting field efficacy if poor weather conditions persisted after a release. If weather conditions were good (i.e. accumulated maximum temperatures above 62 °C, in the 3 days following the release), then parameters such as release rate and fecundity in the lab were useful predictors of field performance. There was no relationship between host acceptance measured in the lab and field parasitism. Given the importance of field temperatures for field performance, selection for cold tolerance of T. minutum would be desirable.     

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.     

Variation in plant stem density and its effects on foraging success of juvenile bluegill sunfish

Synopsis Juvenile bluegill sunfish, Lepomis macrochirus, are known to use beds of aquatic vegetation as a refuge from predators. This study examines the effects of increasing plant stem density on juvenile bluegill foraging. Three stem densities (100, 250 and 500 stems m⁻²), varying in their refuge potential for bluegills from predators, were tested. Results demonstrate that stem densities chosen as a refuge from predation (i.e. 500 stems m⁻²) significantly reduced bluegill foraging success and increased time required to capture prey. Therefore, juvenile bluegills seeking safety in vegetation may be faced with a trade-off between foraging success and effective refuge from predation when choosing among plant stem densities.     

Natural and artificial envenomation of Ceratitis capitata by Eupelmus urozonus and the search for new bioinsecticides

This study was designed to indentify novel bioactive molecules in the venom of the parasitoid Eupelmus urozonus Dalman (Hymenoptera: Eupelmidae). Parasitism by E. urozonus induces apparent paralysis in the larvae of the host Ceratitis capitata (Wiedemann) (Diptera: Tephritidae) and it arrests the development of host pupae. Parasitoid eggs were transferred from stung to unparasitized host pupae to determine whether adult female stings or bites from the first instars were responsible for the above effects. The two treatments gave the same results, indicating that both parasitoid adults and larvae produced venoms capable to compromise host development. A protocol was developed to artificially microinject E. urozonus venom into healthy host pupae and adults at known concentrations to study the effects. The microinjection of venom was found to produce the same macroscopic result as natural parasitization, indicating that host developmental arrest was caused by molecules produced by the parasitoid venom glands. One‐tenth, one‐twentieth, and one‐hundredth of the contents of a female venom reservoir was sufficient to compromise the development of 100, 90, and 50% of the microinjected host pupae, respectively. The microinjection of 0.1 female venom equivalents into host adults always caused death within 24 h. Extraction and freezing did not affect the activity of the E. urozonus venom, which facilitates its storage, whereas denaturation treatments demonstrated that the bioactive molecules were proteins. The venom was also found to prevent the hosts from decaying for over 2 weeks and it promoted the accumulation of unknown subspherical granules in the host haemocoel. These results suggest the potential identification of novel molecules with interesting biological activity with various possible applications.     

Flight initiation in the egg parasitoid Trichogramma minutum: Effects of ambient temperature, mates, food, and host eggs

Emergence, preening, and flight initiation were studied in laboratory-reared Trichogramma minutum Riley (Hymenoptera: Trichogrammatidae). Male parasitoids emerged first and flew before females. When both sexes were present in flight cylinders, female parasitoids flew before males. Flight propensity in males was negatively related to the number of emerging females, while flight propensity in females was independent of the number of males present. Ambient temperature significantly affected the propensity and timing of flight; between 70–80% of the parasitoids flew at 25 and 30 °C while less than 4%, mostly males, flew at 20 °C. No flights were observed at 15 °C. The presence of fresh host eggs caused a reduction in the proportion of female parasitoids that flew and a delay in the time to flight for the females that did fly. The presence of food increased the flight propensity of female parasitoids, but did not affect the timing of flight. The relationship between flight behaviour and the efficiency of mass-reared Trichogramma is discussed in terms of its importance for inundative release programmes.     

Effects of plant diversity and structural complexity on parasitoid behaviour in a field experiment

1. In natural ecosystems, plants containing hosts for parasitoids are often embedded within heterogeneous plant communities. These plant communities surrounding host‐infested plants may influence the host‐finding ability of parasitoids. 2. A release‐recapture‐approach was used to examine whether the diversity and structural complexity of the community surrounding a host‐infested plant influences the aggregation behaviour of the leaf‐miner parasitoid Dacnusa sibirica Telenga and naturally occurring local leaf‐miner parasitoids. Released and locally present parasitoids were collected on potted Jacobaea vulgaris Gaertn.plants infested with the generalist leaf‐miner Chromatomyia syngenesiae Hardy. The plants were placed in experimentally established plant communities differing in plant diversity (1–9 species) and habitat complexity (bare ground, mown vegetation, and tall vegetation). Additionally, parasitoids were reared out from host mines on the trap plants. 3. Plant diversity did not influence the mean number of recaptured D. sibirica or captures of other locally present parasitoids but the number of recaptured parasitoids was influenced by habitat complexity. No D. sibirica parasitoids were recaptured in the bare ground plots or plots with mown vegetation. The mean number of recaptured D. sibirica generally increased with increasing complexity of the plant community, whereas locally present parasitoids were captured more frequently in communities with more bare ground. There was a unimodal relationship between the number of reared out parasitoids and diversity of the surrounding vegetation with the highest density of emerged parasitoids at intermediate diversity levels. 4. The present study adds to the thus far limited body of literature examining the aggregation behaviour of parasitoids in the field and suggests that the preference of parasitoids to aggregate in complex versus simple vegetation is association specific and thus depends on the parasitoid species as well as the identity of the plant community.     

The influence of light and moisture gradients on the attack rate of parasitoids foraging for hosts in a laboratory arena (Hymenoptera: Pteromalidae)

Microhabitat preferences of four parasitoids (Hymenoptera: Pteromalidae) of house fly pupae were measured in a two-dimensional arena containing perpendicular light and moisture gradients. Parasitism by Muscidifurax raptorand Nasonia vitripenniswas greatest in dry substrates, with the latter preferring bright illumination and the former tending to prefer dark. Urolepis rufipesselected bright illumination and moist substrates, attacking the most hosts at bright-moist and dim-moist microhabitats. Spalangia cameroniexhibited no main-effect preference for light intensity or moisture, although parasitism was highest at dim-moist, dark-wet, and dark-moist microhabitats. These results demonstrate that simple abiotic attributes, such as light intensity and substrate moisture concentration, are important in defining some dimensions of the niches of these parasitoids.     

Predator foraging success and habitat complexity: quantitative test of the threshold hypothesis

Summary Numerous studies have demonstrated a negative relationship between increasing habitat complexity and predator foraging success. Results from many of these studies suggest a non-linear relationship, and it has been hypothesised that some threshold level of complexity is required before foraging success is reduced significantly. We examined this hypothesis using largemouth bass (Micropterus salmoides) foraging on juvenile bluegill sunfish (Lepomis macrochirus) in various densities of artificial vegetation. Largemouth foraging success differed significantly among the densities of vegetation tested. Regression analysis revealed a non-linear relationship between increasing plant stem density and predator foraging success. Logistic analysis demonstrated a significant fit of our data to a logistic model, from which was calculated the threshold level of plant stem desity necessary to reduce predator foraging success. Studies with various prey species have shown selection by prey for more complex habitats as a refuge from predation. In this stydy, we also examined the effects of increasing habitat complexity (i.e. plant stem density) on choice of habitat by juvenile bluegills while avoiding predation. Plant stem density significantly effected choice of habitat as a refuge. The relationship between increasing habitat complexity and prey choice of habitat was found to be positive and non-linear. As with predator foraging success, logistic analysis demonstrated a significant fit of our data to a logistic model. Using this model we calculated the threshold level of habitat complexity required before prey select a habitat as a refuge. This density of vegetation proved to be considerably higher than that necessary to significantly reduce predator foraging success, indicating that bluegill select habitats safe from predation.Implications of these results and various factors which may affect the relationships described are discussed.     

Effect of host age on life cycle and morphological characteristics of Glyptapanteles liparidis (Hymenoptera: Braconidae), a parasitoid of Acronicta rumicis (Lepidoptera: Noctuidae)

The life cycle of Glyptapanteles liparidis was 23.75 ± 1.26, 21.95 ± 2.44 and 20.83 ± 0.78 days when fed on the first, second and third instar larvae of Acronicta rumicis, respectively. Although insufficient numbers hindered statistical analysis, the life cycle of G. liparidis appeared to be shortest, 19 days, when fed on fourth instar larvae. The life cycle of G. liparidis tends to shorten as the larvae of A. rumicis fed upon are more advanced. The body length, forewing length and head capsule width of female G. liparidis fed on first instar larvae of A. rumicis were greater than those of males, while the antennae of males were longer than those of females. When fed on second instar larvae, there was no difference in body length and head capsule width between males and females, but the male antennae were longer than the female, and the female forewings were longer than the male. When fed on third instar larvae, there was no significant difference in head capsule width between the sexes, but female body length and forewing length were greater than the male, and the male antennae were longer than the female. On the whole, females were bigger than males in terms of body length and forewing length, while antennae of the males were longer than those of the females. There was no difference in head capsule width between males and females. Body length, antenna length, forewing length and head capsule width of male and female G. liparidis were relatively larger when fed on first instar larvae of A. rumicis than when fed on second and third instar larvae.     

The influence of host's age on the selectivity of parasitism and fecundity ofTrichogramma

Laboratory studies showed that ♀♀ ofTrichogramma embryophagum (Hartig) andT. semblidis (Aurivillius) when offered an alternative of young and old eggs of the grain moth,Sitotroga cerealella Oliv., prefer ovipositing into young eggs. The fecundity of ♀♀ that are simultaneously offered both young and old eggs was lower in comparison to ♀♀ offered only young eggs (inspite of the fact that in both variants young eggs were in excess). The results suggest that old eggs can inhibit the oviposition of some species ofTrichogramma.      

Patch choice from a distance and use of habitat information during foraging by the parasitoid Ibalia leucospoides

1. In environments in which resources are distributed heterogeneously, patch choice and the length of time spent on a patch by foragers are subject to strong selective pressures. This is particularly true for parasitoids because their host foraging success translates directly into individual fitness. 2. The aim of this study was to test whether: (i) females of the parasitoid Ibalia leucospoides (Hymenoptera: Ibaliidae) can discriminate among patches according to host numbers; (ii) the surrounding context affects the initial choice of patch, as well as time spent on patch; and (iii) the perceived quality of a given patch is affected by the quality of the surrounding patches. 3. Each female was randomly exposed to one of three different three‐patch environments which differed in host number per patch, mean environment host number and host distribution among patches. For each treatment level, the first patch chosen and the time allocated to each patch visited by the female were recorded. 4. Females of I. leucospoides were able to discriminate different levels of host numbers among patches from a distance. The patch bearing the highest number of hosts was, predominantly, the first choice. Patch host number in association with mean habitat profitability influenced the length of time spent on the first patch visited. By contrast, variance in habitat profitability did not influence time allocation decisions. Contrary to the study prediction, there were no significant habitat‐dependent time allocation differences among patches holding the same number of hosts. 5. The results indicate that, for I. leucospoides, patch exploitation decisions are partially influenced by information obtained from the habitat as a whole, a behaviour that may prove to indicate adaptive ability in highly patchy environments, as well as suggesting the presence of good cognitive abilities in this parasitoid species.     

Odor specificity testing in the assessment of efficacy and non-target risk for Trichogramma ostriniae (Hymenoptera: Trichogrammatidae)

Trichogramma ostriniae Pang and Chen (Hymenoptera: Trichogrammatidae) is an indigenous egg parasitoid of Ostrinia furnacalis Guenée (Lepidoptera: Crambidae) in China. We evaluated T. ostriniae's responses in olfactometer and wind tunnel assays to various host and plant odors that are likely to impact the efficacy and non-target risk of utilizing T. ostriniae as an augmentative biocontrol agent against Ostrinia nubilalis Hübner in the USA. In a Y-olfactometer, female T. ostriniae exhibited innate positive responses to the egg mass volatiles, scale volatiles, and synthetic sex pheromones of O. nubilalis. When exposed to O. nubilalis pheromone while walking on a platform in a wind tunnel, the wasps manifested significant changes in patch exploration behavior, including delayed dispersal from the platform and slowed walking. The wasps did not respond innately to the synthetic pheromone of a non-target species, Spodoptera frugiperda J. E. Smith (Lepidoptera: Noctuidae), however. Exposing wasps to S. frugiperda pheromone together with eggs of a factitious rearing host prior to testing also did not alter the wasps’ lack of response to the pheromone, indicating that associative learning of the novel odor did not occur. Lastly, wasps showed no innate responses to leaf volatiles from corn (Zea mays L.) and pepper (Capsicum annuum L.), two crops attacked by O. nubilalis. We conclude that T. ostriniae is likely to be highly efficient at finding O. nubilalis eggs in the field through the exploitation of host chemical cues. Further, T. ostriniae's response to moth pheromone appears to be relatively host-specific, since the wasps responded to the pheromone of a congener to their natural host, but not to the pheromone of a more distantly related non-target species. This type of odor-specificity could be an important mechanism for reducing the risk of T. ostriniae attack on non-target species.     

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.     

Influence of host distribution on foraging behaviour in the hyperparasitoid wasp, Dendrocerus carpenteri

The foraging behaviour of Dendrocerus carpenteriCurtis (Hymenoptera: Megaspilidae), an ectophagous hyperparasitoid of aphidiine wasps inside mummified aphids, was examined in the laboratory with an experimental system consisting of broad bean, Vicia fabaL, the pea aphid, Acyrthosiphon pisumHarris, and a primary parasitoid, Ephedrus californicusBaker. Pea aphids parasitised by E. californicusoften disperse from their feeding sites (or off host plants) before dying and mummifying. Response of female hyperparasitoids to host distribution was evaluated at two spatial scales. At the first scale, behaviour of hyperparasitoids was examined on individual plants with different densities of hosts. At the second scale, habitat complexity and host location were manipulated in large foraging cages containing several plants. I show that patterns of density-dependent hyperparasitism can result from the foraging behaviour of D. carpenteri. However, dispersal of parasitised aphids may not reduce the incidence of hyperparasitism if hyperparasitoids systematically search the habitat.     

Influence of foliar and systemically applied azadirachtin on host‐plant evaluation behaviour of the sweetpotato whitefly,Bemisia tabaci

Abstract The behaviour of female adult Bemisia tabaci is observed for a period of 20 min after initial contact with untreated cucumber leaves, or leaves either foliar or systemically treated with azadirachtin, to determine whether application of azadirachtin affects the host‐evaluation behaviour and whether the behaviour on treated leaves differs between application methods. Application of azadirachtin deters settling of the whiteflies on host plants. The whiteflies probe for shorter duration and less frequently but spend longer and engage more frequently in labial grooming on treated leaves than on untreated leaves. Behavioural transition between probing and other behavioural elements is less common, and that between labial grooming and other behavioural elements more common, on both treated leaves than on untreated leaves. No difference is detected in the host‐evaluation behaviour of B. tabaci between leaves foliar treated and systemically treated with azadirachtin.