Common volatiles are major attractants for neonate larvae of the specialist flea beetle <Emphasis Type="Italic">Altica koreana</Emphasis> (Coleoptera: Chrysomelidae)

Olfactory stimuli play an important role in the host searching of larval phytophagous insects. Previous studies indicate that larvae that have to find feeding sites after hatching are generally attracted to host volatiles. However, there are few studies on the olfactory responses of neonate larvae to host volatiles in cases when those larvae hatched on the host plant. In the present study, we determined the olfactory responses of neonate larvae of the specialist flea beetle, Altica koreana Ogloblin, to host and six non-host plants, using a static-air "arena." Larvae responded significantly to the host plant Potentilla chinensis Ser. and five of six non-host plants, compared to the control. Larvae did not prefer the host plant over the non-host plants (except Artemisia sp.) when offered a choice. Additionally, odours of a non-host plant, which were unattractive to neonate larvae, may have masked the attractive odour of the host plant. These results indicate that common volatiles can play a major role in attracting larvae of this specialist to plants, but attraction to such odours may not be the major mechanism of host choice.     

The larval parasitoid <Emphasis Type="Italic">Microplitis croceipes</Emphasis> oviposits in conspecific adults

Microplitis croceipes (Hymenoptera: Braconidae) is a larval parasitoid of Helicoverpa/Heliothis spp. In the course of mass rearing of M. croceipes, we found that females oviposited in the conspecific adults in rearing cages. When 20 pairs of inexperienced females and males or of experienced females and males were reared in a cage, the males lived for 14–15 days and the females for 18–20 days on average. At their death, 37–42% of the males and 50–57% of the females contained conspecific eggs or first instar larvae in their abdominal cavity. When two of inexperienced females met on a host-infested leaf of soybean, they attempted to sting each other. Of the attacked females, 30% contained a conspecific egg laid in their abdomen. In abdominal cavity of the adults parasitized by a conspecific female, the majority of the parasitoid eggs laid disappeared within 1 day after oviposition. Only 10–30% of the parasitoid eggs laid in conspecific adults hatched 3–4 days after oviposition, but those larvae never molted to second instar. When the adults were stung by one or two conspecific females, their subsequent longevity was significantly shorter than that for the control adults. Oviposition in conspecific adults may be prevalent in other parasitic wasps that quickly oviposit without intensive host examination, and have cuticle and size of abdomen to be stung by conspeicifcs.     

Prenatal cannibalism in an insect

Host selection and infection strategies of parasitoids often correlate with high parental investment and low numbers of progeny. In this study, we investigate how additional internal mechanisms might shape brood size and fitness of the offspring. Emblemasoma auditrix is a parasitoid fly in which about 38 larvae hatch simultaneously in utero. After host location, a single larva is deposited into the host, where it rapidly develops and pupates after about 5 days. The search for hosts can take several weeks, and during that time, the larvae arrest their development and remain in the first larval instar. Nevertheless, the larvae increase in weight within the uterus, and this growth correlates to a decrease in the number of larvae, although no larvae are deposited. Thus, our data indicate a first case of prenatal cannibalism in an invertebrate with larvae feeding on each other within the uterus of the adult.     

The effect of larval and early adult experience on behavioural plasticity of the aphid parasitoid <Emphasis Type="Italic">Aphidius ervi</Emphasis> (Hymenoptera,Braconidae, Aphidiinae)

The relevance of the integration of preimaginal and eclosion experiences on the subsequent habitat preferences and mate finding by the adult has been rarely tested in holometabolous insects. In this work, the effect of larval and early adult experiences on the behavioural responses of adult males of the aphid parasitoid, Aphidius ervi, towards volatiles from the host–plant complex (HPC) and from conspecific females were evaluated. Two experience factors were considered: host diet (normal diet=ND; artificial diet=AD), and eclosion, i.e. extraction or non-extraction of the parasitoid larva from the parasitised aphid (extracted=EX; non-extracted=NE). Thus, four treatments were set up: ND/NE, ND/EX, AD/NE and AD/EX. Glass Y-tube olfactometers were used to investigate the responses of adult A. ervi males to the odour sources used. Males from the ND/NE treatment showed a shorter latency to the first choice of olfactometer arms, displayed a marked preference towards the HPC olfactometer arm, and spent more time in the HPC arm than males from the other treatments. Only the interaction of host diet and eclosion experiences proved to be relevant in explaining the differences in latency to first choice, time spent in olfactometers arms, and behaviours displayed in the olfactometer arms. These results show the importance of the integration of larval and eclosion experiences in the development of stereotyped responses of the adults. This process may involve memory retention from the preimaginal and emergence period, but further research is needed to disentangle the contribution of each stage. The response to conspecific females was much less affected by the treatments in relation to first arm choice and times in olfactometer arms, suggesting a pheromone-mediated behaviour, even though a prompter and more intense wing fanning courtship behaviour was registered in the ND/NE males compared to males from the AD/NE treatment. These results show that sexual behaviours are less affected by early experiences than behaviours related with finding a HPC for foraging or oviposition. Taken together, our results demonstrate for first time that larval and eclosion experiences affect in a differential way the early responses of the adult towards environment-derived cues and mate related cues.     

Infanticide by a solitary koinobiont ichneumonid ectoparasitoid of spiders

When encountering an already parasitized host, a parasitoid’s optimal choices (superparasitism, host rejection, host feeding or infanticide) seem to depend on the individual species’ life history, because the same choice may have different fitness consequences. We demonstrate infanticide under laboratory conditions by a polysphinctine, Zatypota albicoxa, which is a solitary koinobiont ectoparasitoid of spiders. The female always removed any previously attached egg or larva from the body of the host spider, Parasteatoda tepidariorum, with a rubbing behaviour. She rubbed her ovipositor back and forth toward the undersurface of the attached egg or of the saddle under the attached larva to pry it off and laid an egg after removal. When removing a larva, the infanticidal female engaged exclusively in unfastening the ‘saddle’ which fastens the larva to the body of the spider. All larvae were removed with the ‘saddle’ attached to the ventral surface of the body. The female invested more time to remove the medium second and the large penultimate instar larvae than to remove eggs and first instar larvae because of the labour involved in unfastening the saddle. Oviposition with infanticide of the medium second and the penultimate instar larvae imposed more time upon the female than that on an unparasitized host. Removal of any previous occupant in spite of the associated labour costs suggests that infanticide will always be adaptive, no matter the time costs to Z. albicoxa, because so much is invested in attacking the host and because the parasitoid cannot detect whether the spider is already parasitized until she achieves subjugation.     

Self-Injection of a Dipteran Parasitoid into a Spider

 A host invasion strategy hitherto unknown from other insect parasitoids was observed in the dipteran Acrocera orbicula (Fabricius) (Diptera: Acroceridae) parasitizing the wolf spider, Pardosa prativaga (L. Koch) (Araneida: Lycosidae). In laboratory experiments the free-living first instar acrocerid larvae attached themselves firmly to the spiders' integument by the mouthparts, cutting a tiny hole through the integument. No first instar larvae invaded the host. A week later the parasitoids molted, and a small, flexible, and glabrous second instar larva left each of the attached first instar exuviae and invaded the host through the attachment hole of the first instar larva. The novel host invasion pattern observed may reduce physical damage to the host in the initial phase of endoparasitism, enhancing parasitoid survival. Received: 14 April 1999 / Accepted in revised form: 2 July 1999     

Olfactory host finding, intermediate memory and its potential ecological adaptation in Nasonia vitripennis

Associative learning of host-associated chemical cues was studied in Nasonia vitripennis, a parasitoid of fly pupae in nests of hole-nesting birds. When females encountered a fly pupa and performed one sequence of host recognition behaviour including drilling the ovipositor into the host in the presence of the artificial odour furfurylheptanoate (FFH), they were afterwards arrested by FFH in olfactometer experiments. The response vanished after 4 days and could be blocked after 3 days by feeding wasps with ethacrynic acid prior and after the training. This indicates the formation of an intermediate form of memory by one host experience in N. vitripennis. Interestingly, the trained wasps avoided odours that were not present during the host encounter, although naive wasps did not react to these odours. This unique behaviour probably causes wasps to focus during host searching on those chemical cues they have experienced in the host environment. Studies in nests of hole-nesting birds revealed that about 30% of all nests contained only one fly pupa, and laboratory studies showed that N. vitripennis females are able to parasitise around 100 fly pupae in their life. It is discussed that under these conditions, the formation of a non-permanent intermediate memory for host-associated odours after one host encounter is adaptive to avoid costs involved with formation and maintenance of memory for misleading cues. The demonstration of associative olfactory learning in N. vitripennis, the first parasitoid species with sequenced genome, opens the gate to study molecular mechanisms of memory formation and its ecological adaptation in parasitoids.     

Effects of fluoride on growth and reproduction of the army worm, Mythimna separata (Walker)

This paper deals with effects of airborne fluorine on the army worm, Mythimna separata (Walker), by rearing the larvae on the wheat foliage exposed to HF or fumigating the larvae on the artificial diet with the pollutant. Larval relative growth rate (GR) and index of population trend of the insect reduced by 5% and 11%, respectively, when the larvae were reared on the foliage taken from the wheat plants exposed to 0.87 g·dm^-2·day^-1 of fluorine compared with those of the control. An extra instar appeared in a majority of the larvae treated. Survival rate and GR of the larvae on the wheat plant being exposure to the same concentration of fluorine in field open-top fumigation device were 40% and 15% lower than that of the control, respectively. Similar experiment with the insect on the artificial diet also showed that direct impact of the pollutant on the army worm was greater than its indirect effect via their host plant.     

Temperature and parasitism by <Emphasis Type="BoldItalic">Asobara tabida</Emphasis> (Hymenoptera: Braconidae) influence larval pupation behaviour in two <Emphasis Type="BoldItalic">Drosophila</Emphasis> species

In holometabolous insects, pupation site selection behaviour has large consequences for survival. Here, we investigated the combined effects of temperature and parasitism by the parasitoid Asobara tabida on larval pupation behaviour in two of its main Drosophila sp. hosts differing in their climate origin. We found that larvae of Drosophila melanogaster—a species with a (sub)tropical origin—placed at 25°C pupated higher in rearing jars than those placed at 15°C. The opposite pattern was observed for Drosophila subobscura larvae—a species from temperate regions—which pupated lower, i.e. on or near the substrate at 25°C, than those placed at 15°C. When placed at 25°C, parasitized larvae of both species pupated closer to the substrate than unparasitized ones. Moreover, the Drosophila larvae that had been exposed and probably stung by A. tabida, but were not parasitized, pupated lower than the control unparasitized larvae. These results provide new insights of host behaviour manipulation by A. tabida larvae.     

Larval host plant origin modifies the adult oviposition preference of the female European grapevine moth <Emphasis Type="Italic">Lobesia botrana</Emphasis>

According to the 'natal habitat preference induction' (NHPI) hypothesis, phytophagous insect females should prefer to lay their eggs on the host species on which they developed as larvae. We tested whether this hypothesis applies to the breeding behaviour of polyphagous European grapevine moth, Lobesia botrana, an important pest in European vineyards. We previously found that different grape cultivars affect several life history traits of the moth. Because the different cultivars of grapes are suspected to provide different plant quality, we tested the NHPI hypothesis by examining oviposition choice of L. botrana among three Vitis vinifera cultivars (Pinot, Chasselas and Chardonnay). In a choice situation, females of L. botrana that had never experienced grapes were able to discriminate between different grape cultivars and preferentially selected Pinot as an oviposition substrate. This 'naive' preference of oviposition could be modified by larval environment: Females raised on grapes as larvae preferred to lay eggs on the cultivar that they had experienced. Furthermore, experience of the host plant during adult emergence could be excluded because when pupae originating from our synthetic diet were exposed to grapes, the emerging adults did not show preference for the cultivar from which they emerged. The NHPI hypothesis that includes the two sub-hypothesis "Hopkins host selection principle" and "chemical legacy" may thus be relevant in this system.     

Folivory versus florivory—adaptiveness of flower feeding

The distribution of resources and defence is heterogeneous within plants. Specialist insects may prefer tissue with high concentrations of the plant’s characteristic defence compounds. Most herbivorous butterfly or sawfly larvae are considered to be folivores, so also the turnip sawfly Athalia rosae (Hymenoptera: Tenthredinidae), a specialist herbivore on Brassicaceae. We investigated which tissue larvae choose to feed upon and how they perform on flowers, young or old leaves of Sinapis alba. Furthermore, constitutive and inducible levels of glucosinolates and myrosinases were investigated and nutrients analysed. Larvae moved from leaves to flowers for feeding from the third larval instar on. Flowers were not actively chosen, but larvae moved upwards on the plant, regardless of how plants were orientated (upright or inverted). Flower-feeding larvae were heavier and developed faster than larvae feeding on young leaves, and adults laid more eggs. Old leaves as food source resulted in the lowest growth rates. Flowers contained three and ten times higher myrosinase activities than young and old leaves, respectively, whereas glucosinolate concentrations and nitrogen levels of flowers and young leaves were comparable. Glucosinolate concentrations of old leaves were very low. Changes in tissue chemistry caused by larval feeding were tissue specific. Defence levels did not change in flowers and old leaves after A. rosae feeding in contrast to young leaves. The high insect performance on flowers cannot be explained by differences in chemical defence. Instead, the lack of mechanical defence (trichomes) is probably responsible. Movement to the flowers and folivory is overall highly adaptive for this sawfly species.     

Transgenic insect-resistant corn affects the fourth trophic level: effects of<Emphasis Type="Italic"> Bacillus thuringiensis</Emphasis>-corn on the facultative hyperparasitoid<Emphasis Type="Italic"> Tetrastichus howardi</Emphasis>

As hyperparasitoids may have a considerable influence on the control of herbivorous arthropods, analyzing the host-mediated impact of Bacillus thuringiensis-plants (Bt-plants) on hyperparasitoids is of interest. Laboratory-scale experiments were conducted in order to assess the potential effect of Bt-corn leaf material on the facultative hyperparasitoid Tetrastichus howardi (Olliff) (Hymenoptera: Eulophidae), mediated through the herbivore Chilo partellus Swinhoe (Lepidoptera: Crambidae) and its primary parasitoid Cotesia flavipes (Cameron) (Hymenoptera: Braconidae). In the Bt-group, significantly less C. flavipes larvae per host spun cocoons and pupated than in the control, and the mean fresh weight of a single C. flavipes cocoon was significantly reduced compared to the control. All C. flavipes cocoons of one host formed cocoon clusters. T. howardi females of the Bt-group parasitized significantly less cocoon clusters than in the control. Moreover, significantly fewer C. flavipes cocoons per cocoon cluster were successfully parasitized as compared to the control. As a consequence, T. howardi females of the control had more offspring than in the Bt-group. Adult female T. howardi offspring of the Bt-group weighed significantly less than in the control, but there was no significant weight difference between males of both groups. Our results suggest that transgenic insect-resistant plants could affect hyperparasitoids indirectly. However, it remains to be determined whether facultative hyperparasitoids prefer to develop as primary or secondary parasitoids under field conditions.     

Short- and long-range cues used by ground-dwelling parasitoids to find their host

Parasitoids of phytophagous insects face a detectability–reliability dilemma when foraging for hosts. Plant-related cues are easily detectable, but do not guarantee the presence of the host. Host-related cues are very reliable, but much harder to detect from a distance. Little is known in particular about the way coleopteran parasitoid females use these cues when foraging for a suitable place to lay their eggs. The question is of interest because, unlike hymenopteran larvae, coleopteran parasitoid larvae are highly mobile and able to forage for hosts on their own. We assessed whether females of the parasitoid rove beetle Aleochara bipustulata (L.) (Coleoptera: Staphylinidae) are attracted to plant (Swede roots, Brassica napus) and host-related cues [pupae of the cabbage root fly Delia radicum (L.) (Diptera: Anthomyiidae)]. In the field, A. bipustulata adult females were captured in selective pitfall traps containing pieces of roots damaged by D. radicum larvae, but not in traps containing pieces of healthy roots or D. radicum pupae. However, in the laboratory, the odour of D. radicum pupae attracted A. bipustulata females to mini-pitfalls. Video monitoring in the laboratory showed that foraging A. bipustulata females preferred a zone containing D. radicum pupae and larval tracks rather than one containing an extract of D. radicum-infested roots. Our results suggest a behavioural sequence where A. bipustulata females use plant-related cues at a distance, but then switch their preference to host-related cues at a close range. This would be the first observation of this behaviour in coleopteran parasitoids.     

Effect of SO2 on performance of the turnip moth,Agrotis segetum Schiff

Effect of SO₂-enriched air on the turnip moth, Agrotis segetum Schiff.was investigated by rearing the larvae on rape leaves that had been exposed to 40 or 80 ppb of the air pollutant in field fumigation chambers. An examination on the 11th day showed that the larvae in both treatments survived more, developed markedly faster, their fresh v/eig'ht and mean relative growth rate were significantly greater than those of control insect. Improvement of their growth and development resulted in decrease of total larval duration by 0.5 - 1.0 day. Pupal and adult performances were little affected by SO₂ level to which larval food plant was exposed. Possible reason responsible for enhanced growth and development of the insect species was discussed.     

Larval competition in weevils Revena rubiginosa (Coleoptera: Curculionidae) preying on seeds of the palm Syagrus romanzoffiana (Arecaceae)

Inter- and intraspecific local resource competition may lead to the selection of specific adaptive individual characteristics to overcome interference competition. A highly selective scenario is predictable for interference competition among seed preying weevil larvae that live in and feed upon a single host seed. This scenario is found in Syagrus romanzoffiana palm seeds which are predated by Revena rubiginosa (Curculionidae) larvae. Although multiple infestation of one seed by weevil larvae can occur, invariably only one individual survives and develops in each host seed. A strong competition between the first instar larvae in a restricted window of host fruit development stages leads to physical interactions of conspecifics by ovicide or direct fighting using falcate mandibles. The occurrence of this type of mandible is synchronized with fruit development and restricted to instars with probable competition, as infestation occurs only while the endocarp is soft. Only after lignification of the endocarp the larva changes into the next instar. Mandibles of subsequent instars differ markedly from those of the first instar. The new mandibles can scrape the solid endosperm but are unable to perforate and kill conspecifics. These findings give strong evidence for the selective pressure of intraspecific competition, where special behaviour, mandible morphology and synchronization of its changes with the seed development contribute to individual benefit that involves the killing of conspecifics, since one host seed can only maintain a single larva throughout its complete development.     

The effect of a green leaf volatile on host plant finding by larvae of a herbivorous insect

 The role of a general green leaf volatile (glv) in host finding by larvae of the oligophagous chrysomelid Cassida denticollis was investigated using a new bioassay which takes into account the need for neonate larvae of this species to climb fresh host plants from the ground. A "stem arena" was designed in which plant stems of the host, tansy (Tanacetum vulgare), and stem dummies (tooth picks), both wrapped in perforated filter paper, were offered to neonate larvae. The wrapping allowed olfactory responses to be tested by preventing access to contact stimuli of stems and dummies. Larvae significantly preferred to climb the wrapped tansy stems over dummies after a period of 15 min. The test glv, (Z)-3-hexen-1-ol, was not attractive when applied to dummies. However, when the glv was applied to the bottom of the arena, the ability of larvae to discriminate between host stems and untreated dummies was significantly enhanced. More larvae climbed wrapped host stems than dummies even within 5 min. While numerous other herbivorous insects are known to be directly attracted by glv, this study shows that a singly offered glv on its own is unattractive to an herbivore but enhances the herbivore's ability to differentiate between host and nonhost plants. Received: 19 October 1999 / Accepted in revised form: 28 February 2000     

No evidence for visual context-dependency of olfactory learning in <Emphasis Type="Italic">Drosophila</Emphasis>

How is behaviour organised across sensory modalities? Specifically, we ask concerning the fruit fly Drosophila melanogaster how visual context affects olfactory learning and recall and whether information about visual context is getting integrated into olfactory memory. We find that changing visual context between training and test does not deteriorate olfactory memory scores, suggesting that these olfactory memories can drive behaviour despite a mismatch of visual context between training and test. Rather, both the establishment and the recall of olfactory memory are generally facilitated by light. In a follow-up experiment, we find no evidence for learning about combinations of odours and visual context as predictors for reinforcement even after explicit training in a so-called biconditional discrimination task. Thus, a ‘true’ interaction between visual and olfactory modalities is not evident; instead, light seems to influence olfactory learning and recall unspecifically, for example by altering motor activity, alertness or olfactory acuity.     

Behavioral evidence for host fidelity among populations of the parasitic wasp, Diachasma alloeum (Muesebeck)

The concept of host fidelity, where host-specific mating occurs in close proximity to the oviposition site and location of larval development, is thought to impart a pre-mating isolation mechanism for sympatric speciation (sensu members of the genus Rhagoletis). The apple maggot fly, Rhagoletis pomonella, and the blueberry maggot fly, R. mendax, are morphologically similar sibling species thought to have speciated in sympatry by divergence of host plant association. Both of these fly species are attacked by the specialist braconid parasitoid, Diachasma alloeum. The current study demonstrates that both male and female D. alloeum exhibit a behavioral preference for the odor of the fruit of their larval Rhagoletis host species. Specifically, those D. alloeum emerging from puparia of R. pomonella are preferentially attracted to hawthorn fruit and those emerging from puparia of R. mendax are preferentially attracted to blueberry fruit. However, male D. alloeum reared from either R. pomonella or R. mendax were equally attracted to females originating from both Rhagoletis species. We suggest that the data herein present evidence for host fidelity, where populations of D. alloeum exhibit a greater tendency to mate and reproduce among the host plants of their preferred Rhagoletis hosts. Furthermore, host fidelity may have resulted in the evolution of distinct host races of D. alloeum tracking the speciation of their larval Rhagoletis prey.     

Integument and defence in larva and prepupa of a sawfly living on a semi-aquatic plant

The larvae of the sawfly Rhadinoceraea micans live and feed on a semi-aquatic plant, Iris pseudacorus, and their integument is strongly hydrophobic. The hydrophobicity is part of a chemical defence strategy, easy bleeding, also known from congeners. The prepupae burrow into the soil where they form a cocoon in which they pupate, thus implying different micro-environmental conditions. The cuticle structure and wetting defensive effectiveness of R. micans were compared between larvae and prepupae. The two stages were similarly well defended against attacking ants by the bleeding of a deterrent hemolymph, whereas they were dissimilar in the cuticle surface that presented sculptures and wax crystals at the larval stage only. The integument of prepupae was less structured, and hydrophilic. Larvae of R. micans exhibit, among sawflies, an exceptional cuticle structuring and we assume that they occupy this particular niche of a semi-aquatic environment to avoid encounters with ground-dwelling predators whereas prepupae may benefit from the chemical defence acquired at larval stage.     

Olfactive detection of fig wasps as prey by the ant<Emphasis Type="Italic"> Crematogaster scutellaris</Emphasis> (Formicidae; Myrmicinae)

In the species-specific and obligate mutualism between the fig (Ficus carica) and its pollinator (the fig wasps Blastophaga psenes), a third participant, the ant Crematogaster scutellaris, is a predator of the wasps. Here, we ask how ant workers can rapidly localise such prey, whose availability is limited in time and space. Using a Y-tube olfactometer, we tested ant response to odours emitted by different types of figs (receptive female, ripe female or male figs) and by fig wasps (pollinators or non-pollinators). We demonstrate that ants were significantly attracted only to odours emitted by pollinators, either alone or associated with odours of male figs (releasing wasps). Detection of prey odour by ants is an important trait that can explain their observed high rate of predation on pollinators, and could have important implications on the stability of the fig/fig wasp mutualism.