Predator and parasitoid attacking ant-attended aphids: effects of predator presence and attending ant species on emerging parasitoid numbers

Interaction between a predator and a parasitoid attacking ant-attended aphids was examined in a system on photinia plants, consisting of the aphid Aphis spiraecola, the two ants Lasius japonicus and Pristomyrmex pungens, the predatory ladybird beetle Scymnus posticalis, and the parasitoid wasp Lysiphlebus japonicus. The ladybird larvae are densely covered with waxy secretion and are never attacked by attending ants. The parasitoid females are often attacked by ants, but successfully oviposit by avoiding ants. The two ants differ in aggressiveness towards aphid enemies. Impacts of the predator larvae and attending ant species on the number of parasitoid adults emerging from mummies per aphid colony were assessed by manipulating the presence of the predator in introduced aphid colonies attended by either ant. The experiment showed a significant negative impact of the predator on emerging parasitoid numbers. This is due to consumption of healthy aphids by the predator and its predation on parasitized aphids containing the parasitoid larvae (intraguild predation). Additionally, attending ant species significantly affected emerging parasitoid numbers, with more parasitoids in P. pungens-attended colonies. This results from the lower extent of interference with parasitoid oviposition by the less aggressive P. pungens. Furthermore, the predator reduced emerging parasitoid numbers more when P. pungens attended aphids. This may be ascribed to larger numbers of the predator and the resulting higher levels of predation on unparasitized and parasitized aphids in P. pungens-attended colonies. In conclusion, a negative effect of the predator on the parasitoid occurs in ant-attended aphid colonies, and the intensity of the interaction is affected by ant species.     

Effects of leaf epicuticular wax on the movement, foraging behavior, and attack efficacy of Diaeretiella rapae

Plant morphological characters can affect the foraging abilities of natural enemies. Heavy wax blooms have been shown to impede predators searching for herbivores on various species in the genus Brassica (Brassicaceae). This study determined whether epicuticular wax affected the foraging behavior of the braconid wasp Diaeratiella rapae (M’Intosh) (Hymenoptera: Braconidae) as it searched for its aphid host Brevicoryne brassicae L. (Homoptera: Aphididae) on the leaves of cauliflower varieties with heavy or light wax blooms. Wasps on the variety with a heavier wax bloom foraged more slowly, groomed more often and for longer periods of time, fell from the leaves more often, took longer to find colonies of aphids, and attacked them at a lower rate than wasps foraging on the variety with a lighter wax bloom. When epicuticular wax was removed from the leaves, the wasp's foraging efficiency and efficacy improved significantly on the cauliflower variety with a heavy wax bloom. The amount of epicuticular wax present on a leaf was shown to impede the ability of a parasitoid to forage, locate, and attack its host.     

Aphelinus albipodus(Hymenoptera: Aphelinidae) andDiaeretiella rapae(Hymenoptera: Braconidae) Parasitism onDiuraphis noxia(Homoptera: Aphididae) Infesting Barley Plants Differing in Plant Resistance to Aphids

Russian wheat aphid,Diuraphis noxia(Mordvilko), as a pest of small grains, has prompted research into biological control and host plant resistance. In the presence of Russian wheat aphid, leaves of a susceptible barley (Morex) are curled and chlorotic and sustain large densities of this aphid, while leaves of a resistant barley (STARS-9301B) remain flat and green and sustain fewer aphids. Might parasitism of Russian wheat aphid byAphelinus albipodusHayat & Fatima andDiaeretiella rapaeMcIntosh be affected differently by these plant types? When presented the plants separately and based on parasitism rate relative to aphid density, the largerD. rapaewas more effective in parasitizing relatively high densities of aphids within curled leaves of Morex than relatively low densities of aphids on uncurled leaves of STARS-9301B. Parasitism byA. albipodusdid not significantly differ among the plants. When given a choice of plants, approximately equal rates of parasitism occurred on the two plant lines for both parasitoid species, and parasitism byD. rapaewas greater thanA. albipodus.These data indicate that using parasitoid size as an indicator of success in a physically restricted environment may be misleading, when considered in a plant environment responsive in several manners to aphids (chlorosis, curling, and ability to sustain Russian wheat aphid). We expect that use of resistant barley will result in decreased parasitoid abundance as aphid densities decrease. However, parasitism rates are expected to be approximately equal on resistant and susceptible barley. In this system, plant resistance and biocontrol are compatible management strategies.     

Influence of elevated CO2 on interspecific interactions at higher trophic levels

Abstract We report the results of a study investigating the influence of elevated CO₂ on species interactions across three trophic levels: a plant (Brassica oleracea), two aphid herbivores (the generalist Myzus persicae and the specialist Brevicoryne brassicae), and two natural enemies (the coccinellid Hippodamia convergens (ladybird) and the parasitoid wasp Diaeretiella rapae). Brassica oleracea plants reared under elevated CO₂ conditions (650 ppmv vs. 350 ppmv) were larger and had decreased water and nitrogen content. Brevicoryne brassicae reared on plants grown in elevated CO₂ were larger and accumulated more fat, while there was no change in M. persicae traits. Fecundity of individual aphids appeared to be increased when reared on plants grown in elevated CO₂. However, these differences were generally lost when aphids were reared in colonies, suggesting that such changes in plant quality will have subtle effects on aphid intraspecific interactions. Nevertheless, CO₂ treatment did influence aphid distribution on plants, with significantly fewer M. persicae found on the shoots, and B. brassicae was only found on senescing leaves, when colonies were reared on plants grown in elevated CO₂. We reared B. brassicae and M. persicae in competition on plants grown at both the CO₂ concentration treatments. We found a significantly lower ratio of M. persicae: B. brassicae on plants grown under elevated CO₂ conditions, strongly suggesting that increasing CO₂ concentrations can alter the outcome of competition among insect herbivores. This was also reflected in the distribution of the aphids on the plants. While the CO₂ treatment did not influence where B. brassicae were found, fewer M. persicae were present on senescing leaves under elevated CO₂ conditions. Changes in plant quality resulting from the CO₂ treatments did not appear to alter aphid quality as prey species, as the number consumed by the ladybird H. convergens, and the number parasitised by the parasitoid wasp D. rapae, did not change. To our knowledge, this study provides the first empirical evidence that changes in host plant quality mediated by increasing levels of CO₂ can alter the outcome of interspecific competition among insect herbivores.     

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.     

<Emphasis Type="Italic">Aphidius ervi</Emphasis> Preferentially Attacks the Green Morph of the Pea Aphid, <Emphasis Type="Italic">Acyrthosiphon pisum</Emphasis>

The pea aphid, Acyrthosiphon pisum Harris (Hemiptera: Aphididae) is found in red and green color morphs. Previous work has suggested that the aphidiine parasitoid Aphidius ervi Haliday preferentially attacks green pea aphids in the field. It is not clear whether these results reflect a real preference, or some unknown clonal difference, such as in immunity, between the aphids used in the previous studies. We used three susceptibility-matched pairs of red and green morph pea aphid clones to test for preferences. In a no-choice situation, the parasitoids attacked equal proportions of each color morph. When provided with a choice, A. ervi was significantly more likely to oviposit into colonies formed from green morphs when the neighboring colony was formed from red morph aphids. In contrast, red morphs were less likely to be attacked when their neighboring colony was of the green morph. By preferentially attacking green colonies, A. ervi may reduce the likelihood of intraguild predation, as it is suggested that visually foraging predators preferentially attack red aphid colonies. Furthermore, if this host choice behavior is replicated in the field, we speculate that color morphs of the pea aphid may interact indirectly through their shared natural enemies, leading to intraspecific apparent competition.     

The Foraging Behavior of <Emphasis Type="Italic">Lysiphlebus fabarum</Emphasis> (Marshall), a Thelytokous Parasitoid of the Black Bean Aphid in Iran

A series of laboratory experiments examined the foraging behavior of a thelytokous strain of Lysiphlebus fabarum (Marshall), a strongly proovigenic parasitoid of Aphis fabae Scopoli, in Iran. Females use chemical camoflage to forage undisturbed in ant-tended aphid colonies and solicit honeydew from aphids in the manner of ants. Rates of oviposition are very low (∼ 1.2 eggs / h) despite many aphid encounters and persistent ovipositor probing which appears to prime aphids for subsequent honeydew solicitation. Starved females spent 3.6 times longer in host patches (leaf disks with 15 2nd–3rd instar A. fabae) than did females sated on honey, and 40% of this time was spent soliciting honeydew. Five d-old females spent longer in patches than did one d-old females, and parasitized three times as many aphids. A 24 h pre-trial foraging experience did not reduce mean egg load significantly compared to a one h experience, but was sufficient to reduce patch residence time and number of aphids parasitized. Wasps reared under short day conditions (L:D = 10:14) were more pessimistic foragers (remained longer in patches and parasitized more aphids) than females raised under long days (L:D = 16:8). Wasps that encountered aphids previously parasitized by conspecifics began to superparasitize and remained longer in patches than females that encountered only unparasitized aphids. Encounters with other females had no effect on foraging behavior, possibly because cuticular camoflage interferes with conspecific recognition. The exceptionally low oviposition rate of this wasp may reflect a life history in which individual fitness has evolved to be strongly dependent on continued ant attendance throughout the period of progeny development.     

The role of experience in plant foraging by the aphid parasitoidDiaeretiella rapae (Hymenoptera: Aphidiidae)

Experiments were conducted to test the hypothesis that plant learning by a relative plant-specialist parasitoid wasp should influence the probability of orienting to plant odors (plant finding) and the duration of searching on a plant after landing (plant examining). The insect tested was Diaeretiella rapaeM'Intosh (Hymenoptera: Aphidiidae), a parasitoid wasp that usually attacks aphids on cruciferous plants, but occasionally on other plants. Laboratory experiments using collard as the cruciferous plant and potato as the novel plant demonstrated that postemergence (adult) plant experience affected plant examining only on the less preferred plant, potato, and was reversible and relatively long-term (that is, lasted >2 days). Postemergence experience with potato did not increase orientation to potato odor in a wind tunnel, but postemergence experience with collard resulted in a trend of increased likelihood of flying to collard odor. Preemergence treatments affected plant finding but not plant examining.     

The use of honeydew in foraging for aphids by larvae of the ladybird beetle, Coccinella septempunctata L. (Coleoptera: Coccinellidae)

Abstract.  1. To clarify the use of honeydew in foraging for aphids by larvae of the ladybird beetle, Coccinella septempunctata L., searching behaviour of ladybird larvae for Aphis craccivora Koch and Acyrthosiphon pisum Harris and the abundance of honeydew under aphid colonies were examined in laboratory experiments.
2. More larvae responded by climbing the plants with aphids than responded to plants without aphids. When the plants were replaced with sticks, in order to exclude visual and olfactory cues from plants and aphids, more larvae of C. septempunctata climbed sticks above the area that contained honeydew than climbed sticks above the area that did not contain honeydew. Then, ladybird larvae use honeydew as a contact kairomone when foraging for aphids.
3.  Aphis craccivora deposited a larger number of honeydew droplets beneath the plants than did similar numbers of A. pisum. Thus, C. septempunctata larvae licked more frequently the honeydew of A. craccivora than that of A. pisum and spent longer searching on the area containing honeydew of A. craccivora than that of A. pisum . Consequently, a larger number of larvae climbed a stick above honeydew of A. craccivora than that of A. pisum.
4. It may be also considered that C. septempunctata larvae can distinguish honeydew of the two aphid species and respond more strongly to A. craccivora than A. pisum.     

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.     

Behavioral responses of the parasitoid Psyllaephagus pistaciae (Hymenoptera: Encyrtidae) to host plant volatiles and honeydew

The behavioral responses of the parasitoid Psyllaephagus pistaciae, the major biocontrol agent of the common pistachio psylla, Agonoscena pistaciae, to volatiles emanating from its host plant and host honeydew, were examined using a four‐arm airflow olfactometer. In addition, the arrestment behavior of this parasitoid on clean and honeydew‐treated leaves of the pistachio, Pistacia vera, was monitored. The infested pistachio leaves were the most favored source of the volatile attracting the parasitoids. The parasitoid clearly distinguished and responded to infochemicals emitted by psyllid honeydew but at a lower level than to the volatiles from infested host plants. However, the searching time, locomotory behavior, antennal drumming and ovipositor probing were all affected when they encountered honeydew‐contaminated zones on pistachio leaves. These findings suggest that the psyllid honeydew releases kairomones that stimulate the parasitoids to greater searching activity, as well as providing a directional cue. The intensive searching activities in the presence of the volatiles tested were very similar to responses by the parasitoid females when encountering patches treated with psyllid honeydew. Such behavior could retain the parasitoid in a favorable area, thereby increasing the probability of additional host encounters.     

Collective Defense of Aphis nerii and Uroleucon hypochoeridis (Homoptera,Aphididae) against Natural Enemies

The prevalent way aphids accomplish colony defense against natural enemies is a mutualistic relationship with ants or the occurrence of a specialised soldier caste typcial for eusocial aphids, or even both. Despite a group-living life style of those aphid species lacking these defense lines, communal defense against natural predators has not yet been observed there. Individuals of Aphis nerii (Oleander aphid) and Uroleucon hypochoeridis, an aphid species feeding on Hypochoeris radicata (hairy cat''s ear), show a behavioral response to visual stimulation in the form of spinning or twitching, which is often accompanied by coordinated kicks executed with hind legs. Interestingly, this behaviour is highly synchronized among members of a colony and repetitive visual stimulation caused strong habituation. Observations of natural aphid colonies revealed that a collective twitching and kicking response (CTKR) was frequently evoked during oviposition attempts of the parasitoid wasp Aphidius colemani and during attacks of aphidophagous larvae. CTKR effectively interrupted oviposition attempts of this parasitoid wasp and even repelled this parasitoid from colonies after evoking consecutive CTKRs. In contrast, solitary feeding A. nerii individuals were not able to successfully repel this parasitoid wasp. In addition, CTKR was also evoked through gentle substrate vibrations. Laser vibrometry of the substrate revealed twitching-associated vibrations that form a train of sharp acceleration peaks in the course of a CTKR. This suggests that visual signals in combination with twitching-related substrate vibrations may play an important role in synchronising defense among members of a colony. In both aphid species collective defense in encounters with different natural enemies was executed in a stereotypical way and was similar to CTKR evoked through visual stimulation. This cooperative defense behavior provides an example of a surprising sociality that can be found in some aphid species that are not expected to be social at all.     

Microsatellite isolation and characterization in the beneficial parasitoid wasp Diaeretiella rapae (M’Intosh) (Hymenoptera: Braconidae: Aphidiinae)

The primary parasitoid wasp, Diaeretiella rapae (M’Intosh), is used as a biological control agent in insect pest management strategies to combat aphid pests, mainly on cruciferous plants. To investigate the spatial scales over which this beneficial parasitoid wasp moves within an agricultural landscape, we developed five polymorphic microsatellite markers. The number of alleles per locus ranged from 11 to 21. Cross‐species amplification indicated that these loci may be useful in some other members of the Aphidiinae.     

Cascading effects of soil organic and inorganic fertilizers on tri-trophic interactions: Plant–aphid–parasitoid wasp as a study system

Understanding the mechanisms underlying tri-trophic interactions between insect herbivores, their host plants and natural enemies is an important aim in ecology. In the present study, the effect of urea fertilizer and vermicompost on a tri-trophic level cascade, comprising safflower, Carthamus tinctorius, safflower aphid, Uroleucon carthami and its primary parasitoid wasp, Praon yomenae, was investigated. Vermicompost increased the number of leaves, leaf area, fresh weight, dry weight, the total number of aphids and reduced the number of winged aphids and aphid load (Aphid load = number of aphids / plant fresh weight). Only two variables, plant phenol content and relative water content, were not significantly affected by vermicompost. Urea fertilizer had no impact on all variables except a significant effect on plant height. In another experiment, the effect of urea fertilizer and vermicompost on the wasp parasitoid was studied. The number of parasitoid mummies, mummification time, developmental time, the number of emerged adults, sex ratio, percentage of parasitism and hind tibia length was measured. Vermicompost had no significant effect on any of the measured parameters, but urea fertilizer increased the hind tibia length of the parasitoid. Vermicompost increased plant growth parameters and had an indirect and inhibiting effect on the safflower aphid itself. There was evidence of a bottom-up cascade to the third trophic level by adding fertilizers in this system: Urea fertilizer enhanced plant height but seemingly had no impact on the attacking herbivore. It is interesting that the effect of urea can be transferred to the third trophic level, that is parasitoid. This suggests that vermicompost could be used simultaneously with urea fertilizer, because urea fertilizer had a positive impact on the parasitoid and vermicompost had a positive impact on plant growth as well as the ability to reduce aphid load.     

Oviposition avoidance of parasitized aphid colonies by the syrphid predator Episyrphus balteatus mediated by different cues

Oviposition decisions made by members of a guild of natural enemies can have evolved to avoid intraguild predation, potentially avoiding the disruption of the extraguild prey control. We have studied the oviposition preference of the aphidophagous predator Episyrphus balteatus De Geer (Diptera: Syrphidae) within colonies of Myzus persicae Sulzer (Hemiptera: Aphididae) in the presence of two developmental stages of the aphid parasitoid Aphidius colemani Viereck (Hymenoptera: Aphidiidae). Results from a greenhouse choice experiment showed that E. balteatus females lay significantly fewer eggs in colonies with mummified aphids than in unparasitized colonies. Colonies of parasitized, but not yet mummified did not contain significantly fewer eggs than colonies with unparasitized aphids. In three no-choice experiments, we assessed stimuli coming from aphid honeydew, from the aphids themselves and also from extracts of the aphid bodies, and all of these stimuli mediate the discrimination of mummified aphids from healthy aphids. To a lesser extent these stimuli also contribute to the discrimination against aphids that are parasitized but not yet mummified. These results suggest that the effects of these two species could be complementary for the control of M. persicae, since the species that acts as an intraguild predator, E. balteatus, avoids ovipositing on aphid colonies parasitized by the intraguild prey, A. colemani.     

Effects of prey aphid species on the abundance of a parasitoid and two predator species in aphid colonies attended by ants on citrus

This study focused on three species of enemies, the parasitoid wasp Lysiphlebus japonicus Ashmead (Hymenoptera: Aphidiidae), the ladybird Scymnus posticalis Sicard (Coleoptera: Coccinellidae) and the predatory gall midge Aphidoletes aphidimyza (Rondani) (Diptera: Cecidomyiidae), all of which are able to exploit aphids attended by ants. I experimentally evaluated the effects of prey aphid species on the abundance of each of the three enemy species in ant‐attended aphid colonies on citrus. The aphids compared were Aphis gossypii Glover versus Aphis spiraecola Patch in late spring, and Toxoptera citricidus (Kirkaldy) versus A. spiraecola in late summer (all, Hemiptera: Aphididae). Colonies of the three aphid species were attended by the ant Pristomyrmex punctatus Smith (Hymenoptera: Formicidae). The initial number of attending ants per individual aphid did not differ significantly between the colonies of the two aphid species compared in each season. Between A. gossypii and A. spiraecola, there was no significant difference in the number of mummies formed by the parasitoid or foraging larvae of each of the two predators per aphid colony. A significant difference was detected between T. citricidus and A. spiraecola for each of the three enemy species, with a far greater number of L. japonicus mummies in T. citricidus colonies and distinctly more larvae of each of the two predators in A. spiraecola colonies. Thus, the abundance of each of the three enemy species in ant‐attended aphid colonies was significantly influenced by the species of the prey aphids, with the three enemies showing different responses to the three aphid species.     

Behavioural responses of the aphid parasitoid Diaeretiella rapae to volatiles from Arabidopsis thaliana induced by Myzus persicae

In response to herbivory by insects, several plant species have been shown to produce volatiles that attract the natural enemies of those herbivores. Using a Y‐tube olfactometer, we investigated responses of the aphid parasitoid Diaeretiella rapae MacIntosh (Hymenoptera: Aphidiidae) to volatiles from Arabidopsis thaliana Columbia (Brassicaceae) plants that were either undamaged, infested by the peach‐potato aphid, Myzus persicae Sulzer (Homoptera: Aphididae), or mechanically damaged, as well as to volatiles from just the aphid or its honeydew. In dual‐choice experiments, female D. rapae given oviposition experience on A. thaliana infested with M. persicae were significantly attracted to volatiles from A. thaliana infested with M. persicae over volatiles from undamaged A. thaliana and similarly were significantly attracted to plants that had been previously infested by M. persicae, but from which the aphids were removed, over undamaged plants. Diaeretiella rapae did not respond to volatiles from M. persicae alone, their honeydew, or plants mechanically damaged with either a pin or scissors. We conclude that an interaction between the plant and the aphid induces A. thaliana to produce volatiles, which D. rapae can learn and respond to. Poor responses of D. rapae to volatiles from an A. thaliana plant that had two leaves infested with M. persicae, with the two infested leaves being removed before testing, suggested the possibility that, at this stage of infestation, the majority of volatile production induced by M. persicae may be localized to the infested tissues of the plant. We conclude that this tritrophic interaction is a suitable model system for future investigations of the biochemical pathways involved in the production of aphid‐induced volatiles attractive to natural enemies.     

Plant quality vs. risk of parasitism: within-plant distribution and performance of the corn leaf aphid, Rhopalosiphum maidis

1 Colonies of the aphid Rhopalosiphum maidis on Johnsongrass, Sorghum halepense, usually occur inside the whorl. The present work assessed the role of two biotic factors, plant quality and parasitism by Lysiphlebus testaceipes, in determining within‐plant distribution and performance of R. maidis. The mean relative growth rate of aphids inside the whorl and on a mature leaf was compared, and the concentration of hydroxamic acids in those tissues determined as an indicator of plant quality. Parasitism effectiveness and parasitoid behaviour were evaluated using three treatments: aphid colonies placed (1) wholly inside the whorl, (2) on the inner and outer surfaces of the whorl, and (3) on a mature leaf. 2 The mean relative growth rate of aphids was lower on the whorl than on the mature leaf, and hydroxamic acid concentration in the whorl was higher than in the mature leaf. 3 The number of parasitized dead aphids (mummies) inside the whorl was considerably lower than in the other two treatments. Mummies were present in 80% of the replicates in the whorl, whereas mummies were present in all replicates of the other treatments. 4 Host location time of the parasitoid was increased on the mature leaf compared with the other treatments. No significant differences in the residence time of the parasitoid in the aphid colony occurred between treatments. 5 Host location time showed a negative correlation, and residence time of the parasitoid a positive correlation with the number of aphids on the outer surface of the whorl.     

Nectar feeding increases exploratory behaviour in the aphid parasitoid Diaeretiella rapae (McIntosh)

Feeding on floral nectar has multiple positive effects on parasitic wasps, including increased longevity and fecundity, and in addition, nectar feeding can also alter parasitoid behaviour. To advance understanding of the effects of nectar feeding on Diaeretiella rapae (McIntosh) [Hymenoptera: Braconidae], the activities of 1‐day‐old female D. rapae with or without a prior buckwheat (Fagopyrum esculentum) nectar meal were quantified. Nectar increased searching time of D. rapae by a factor of 40 compared with individuals provided with water only and reduced the time spent stationary. The number of attacks to aphids by nectar‐fed parasitoids was not significantly (P = 0.06) higher than that of unfed D. rapae, suggesting that the conditions of the experiment facilitated host finding by ‘quiet’ parasitoids. Nevertheless, nectar feeding modified the behaviour of D. rapae in a way that parasitoids were more explorative and less inactive. This represents one additional mechanism through which nectar feeding impacts parasitoid biology and suggests that a synergy between increased host searching, increased longevity and increased fecundity should lead to an enhancement of biocontrol when D. rapae females have access to nectar in the field.     

Host location in Oomyzus gallerucae (Hymenoptera: Eulophidae), an egg parasitoid of the elm leaf beetle Xanthogaleruca luteola (Coleoptera: Chrysomelidae)

Eggs of the elm leaf beetle Xanthogaleruca luteola are often heavily attacked by the chalcidoid wasp Oomyzus gallerucae. We studied the chemical signals mediating interactions between the egg parasitoid, its host, and the plant Ulmus campestris. Olfactometer bioassays with O. gallerucae showed that volatiles of the host-plant complex attract the parasitoid. In order to determine the source of attractive volatiles within this host-plant-complex, we tested separately the effect of odours of eggs, gravid elm leaf beetle females, faeces of the beetles and elm twigs (with undamaged leaves and leaves damaged either mechanically or by feeding of the beetles). Odours of faeces of the elm leaf beetle were attractive, whereas neither volatiles from eggs nor from gravid females acted as attractants. Volatiles from undamaged or damaged plants did not elicit a positive reaction in O. gallerucae, whereas volatiles from feeding-damaged plants onto which host eggs had been deposited were attractive. This latter result suggests that it is not feeding but deposition of host eggs onto elm leaves that induces the production of plant volatiles attractive to the egg parasitoid. Investigations of the search patterns of O. gallerucae within the habitat by laboratory bioassays revealed that the egg parasitoid encounters host eggs by chance. Contact kairomones from faeces were demonstrated to be important in microhabitat acceptance, while contact kairomones isolated from the host eggs are relevant for host recognition. Received: 12 February 1997 / Accepted: 29 April 1997