Naive Parasitoids Research Articles

Exposure to sugarcane borer‐induced plant volatile (E)‐caryophyllene enhances parasitoid recruitment

AbstractPredators and parasitoids locate herbivore‐infested plants via detection of herbivore‐induced plant volatiles (HIPVs) that are released in response to pest damage. Synthetic HIPVs have been proposed as tools to enhance the biological control of crop pests. The sugarcane borer, Diatraea saccharalis (Fabricius) (Lepidoptera: Pyralidae), is a key herbivore pest of sugarcane, Saccharum spp. (Poaceae), in the Americas. To manage D. saccharalis in Brazil, more than 3 million ha of sugarcane have been treated with the larval parasitoid Cotesia flavipes (Cameron) (Hymenoptera: Braconidae). In this study, the role of sugarcane HIPVs as cues in the host‐finding process of C. flavipes was investigated using a combination of dynamic headspace collection, chemical analysis, and laboratory behavior experiments. Comparison of volatile organic compounds (VOCs) collected from D. saccharalis‐damaged and healthy sugarcane revealed very similar profiles, apart from significantly higher levels of the sesquiterpene (E)‐caryophyllene released from damaged plants. Naive female C. flavipes spent significantly more time in the olfactometer arm containing VOCs from D. saccharalis‐damaged plants but showed no preference for VOCs from healthy plants. When (E)‐caryophyllene was added to VOCs from healthy plants, parasitoids spent more time in the arm containing the combined treatment. Furthermore, in a dose–response experiment with synthetic (E)‐caryophyllene, naive parasitoids preferred the compound across a dose range of 3–300 ng, and experienced parasitoids – preexposed to (E)‐caryophyllene – responded to doses of (E)‐caryophyllene as low as 0.03 ng. These results suggest that C. flavipes can use (E)‐caryophyllene as a cue to locate D. saccharalis‐infested sugarcane plants. Moreover, experienced females appear to respond to lower doses than naive females. These results potentially pave the way for increasing the efficiency of C. flavipes in biological control of D. saccharalis, the most important pest of sugarcane and maize crops in the Western Hemisphere, and also a major pest of rice and sorghum crops.

Associative learning and memory retention of nectar yeast volatiles in a generalist parasitoid

Understanding how animals learn is crucial to interpreting animal behaviour. Flower-visiting insects, such as bees and parasitoids, are excellent animal models to study visual and olfactory learning, including memory phenomena. The diversity of resources flower-visiting insects exploit predisposes them to learn and remember the colours, shapes and odours associated with rewarding experiences (e.g. flowers), allowing them to focus on the most rewarding resources. Recent research has shown that nectar-living microbes release volatile organic compounds (VOCs) that contribute to overall flower scent. Nevertheless, little is known about the extent to which nectar microbiota mediate insect learning of floral preferences. In this study, we investigated whether VOCs produced by nectar microbes serve as a learning cue to parasitoids and how long any developed preference is maintained. Experiments were performed using the generalist aphid parasitoid Aphidius ervi and three nectar yeasts, including the nectar specialist Metschnikowia reukaufii and the generalist species Hanseniaspora uvarum and Sporobolomyces roseus. Results showed that naive parasitoids had an innate preference for nectar fermented by the nectar specialist M. reukaufii, but not by the other two yeasts which had either a neutral (H. uvarum) or deterrent (S. roseus) effect. When parasitoids were conditioned with yeast-fermented nectar, they were strongly attracted to their odours 2 and 24 h after conditioning, but not after 48 h. Furthermore, when parasitoids were conditioned to one yeast-fermented nectar, they also showed increased attraction to other yeast-fermented nectars. This generalization suggests that their learning ability may have broader ecological consequences. However, this generalized response to other yeast VOCs lasted for only 2 h. We conclude that parasitoids show conditioned responses to the scent of yeast-fermented nectar, and yeasts, therefore, may play an important but understudied role in shaping their foraging behaviour.

The role of herbivore- and plant-related experiences in intraspecific host preference of a relatively specialized parasitoid.

Parasitoids use odor cues from infested plants and herbivore hosts to locate their hosts. Specialist parasitoids of generalist herbivores are predicted to rely more on herbivore-derived cues than plant-derived cues. Microplitis croceipes (Cresson) (Hymenoptera: Braconidae) is a relatively specialized larval endoparasitoid of Heliothis virescens (F.) (Lepidoptera: Noctuidae), which is a generalist herbivore on several crops including cotton and soybean. Using M. croceipes/H. virescens as a model system, we tested the following predictions about specialist parasitoids of generalist herbivores: (i) naive parasitoids will show innate responses to herbivore-emitted kairomones, regardless of host plant identity and (ii) herbivore-related experience will have a greater influence on intraspecific oviposition preference than plant-related experience. Inexperienced (naive) female M. croceipes did not discriminate between cotton-fed and soybean-fed H. virescens in oviposition choice tests, supporting our first prediction. Oviposition experience alone with either host group influenced subsequent oviposition preference while experience with infested plants alone did not elicit preference in M. croceipes, supporting our second prediction. Furthermore, associative learning of oviposition with host-damaged plants facilitated host location. Interestingly, naive parasitoids attacked more soybean-fed than cotton-fed host larvae in two-choice tests when a background of host-infested cotton odor was supplied, and vice versa. This suggests that plant volatiles may have created an olfactory contrast effect. We discussed ecological significance of the results and concluded that both plant- and herbivore-related experiences play important role in parasitoid host foraging.

Herbivore-specific, density-dependent induction of plant volatiles: honest or "cry wolf" signals?

Plants release volatile chemicals upon attack by herbivorous arthropods. They do so commonly in a dose-dependent manner: the more herbivores, the more volatiles released. The volatiles attract predatory arthropods and the amount determines the probability of predator response. We show that seedlings of a cabbage variety (Brassica oleracea var. capitata, cv Shikidori) also show such a response to the density of cabbage white (Pieris rapae) larvae and attract more (naive) parasitoids (Cotesia glomerata) when there are more herbivores on the plant. However, when attacked by diamondback moth (Plutella xylostella) larvae, seedlings of the same variety (cv Shikidori) release volatiles, the total amount of which is high and constant and thus independent of caterpillar density, and naive parasitoids (Cotesia vestalis) of diamondback moth larvae fail to discriminate herbivore-rich from herbivore-poor plants. In contrast, seedlings of another cabbage variety of B. oleracea (var. acephala: kale) respond in a dose-dependent manner to the density of diamondback moth larvae and attract more parasitoids when there are more herbivores. Assuming these responses of the cabbage cultivars reflect behaviour of at least some genotypes of wild plants, we provide arguments why the behaviour of kale (B. oleracea var acephala) is best interpreted as an honest signaling strategy and that of cabbage cv Shikidori (B. oleracea var capitata) as a “cry wolf” signaling strategy, implying a conflict of interest between the plant and the enemies of its herbivores: the plant profits from being visited by the herbivore's enemies, but the latter would be better off by visiting other plants with more herbivores. If so, evolutionary theory on alarm signaling predicts consequences of major interest to students of plant protection, tritrophic systems and communication alike.

Influence of host size on the clutch size and developmental success of the gregarious ectoparasitoid Eulophus pennicornis (Nees)(Hymenoptera: Braconidae) attacking larvae of the tomato moth Lacanobia oleracea (L.) (Lepidoptera: Noctuidae)

The relationship between clutch size, host size and progeny survival in the gregarious ectoparasitoid Eulophus pennicornis was investigated in a number of scenarios. When naive parasitoids were exposed singly to Lacanobia oleracea hosts, clutch size was strongly correlated with the size of the host. However, survival of parasitoid offspring was negatively affected by the size of the host such that, in larger hosts, greater wasp larval and pupal mortality was recorded. As a result, no gain in realised fecundity was achieved through parasitizing L. oleracea larvae of mass >0.4 g over hosts of mass between 0.2-0.3 g. When exposed to populations of mixed stadium hosts (larvae in the fourth, fifth and sixth instars) during the entire lifespan of the wasp, host size and clutch size were correlated in early ovipositions (first three ovipositions). However, as the wasps aged, the relationship was much less apparent. When the parasitoid was restricted to foraging upon populations of sixth instar hosts only, no relationship between host size and clutch size was apparent. Exposure of the parasitoid to mixed and fixed stadium host populations showed that final (sixth) stadium hosts were the most frequently parasitized (ca. 96% of parasitized hosts) and that the average numbers of eggs laid per wasp, and the number of hosts parasitized, was significantly lower when the parasitoid was provided with fourth or fifth instar hosts only. The results indicate that the reproductive success of E. pennicornis does not increase with increasing host size or greater resource availability above a certain threshold, and that the physiological status of the host at the time of parasitism is the governing factor determining oviposition decisions and parasitoid survival. We conclude that E. pennicornis has been selected to preferentially utilize those hosts that maximize progeny survival and to adapt clutch size to the size of such hosts. We hypothesize that the major driver leading to the evolution of this strategy is the ability of the parasitoid to physiologically regulate the host.

Responses of Metaphycussp. nr. flavus to semiochemicals released from a scale host, Coccus hesperidum

Metaphycus sp. nr. flavus (Encyrtidae: Hymenoptera) is a parasitoid species collected from the Mediterranean region which lays its eggs in the immature stages of several economically important soft scale insects (Hemiptera: Coccidae), including brown soft scale, Coccus hesperidum L. (= host insect). Preliminary tests suggested that the parasitoid is most successful in producing offspring when it oviposits in the younger stages of brown soft scale. In Y-olfactometer bioassays measuring wasp choices and residence times, naive parasitoids were significantly more attracted to yucca leaves infested with 26, 27, or 28 d-old scale than to uninfested leaves, whereas leaves with older (29-30 d-old) scale were no more attractive than uninfested leaves. Parasitoids also spent significantly more time in the arm with yucca leaves infested with 26 d-old scale than in the arm with uninfested leaves. These results are consistent with observations of the parasitoid’s reproductive success on scale of different ages, whereby older scale are more likely to encapsulate the developing eggs of M. sp. nr. flavusfemales than are younger scale. Further bioassays determined that yucca leaves that had been infested with 26 d-old scale but from which the scale had been removed were as attractive as infested leaves. In contrast, infested yucca leaves from which scale had been removed and the leaves subsequently washed with distilled water were less attractive than infested leaves. Furthermore, the wash water containing scale residues was attractive to female wasps. In total, these results suggest that Metaphycussp. nr. flavus females utilize volatile, water soluble compounds produced by brown soft scale as cues to locate suitable hosts.

Responses to aphid sex pheromones by the pea aphid parasitoids Aphidius ervi and Aphidius eadyi

Abstract Aphidius ervi and Aphidius eadyi, two parasitoids of the pea aphid Acyrthosiphon pisum, were attracted to components of the aphid sex pheromone in laboratory bioassays. Pre‐test experience with host aphids in the presence of aphid sex pheromone did not affect the response of A. ervi to pheromone in a 4‐way olfactometer, compared with that of naive parasitoids. Aphidius ervi females exposed only to the pheromone prior to testing did not respond in the olfactometer, suggesting habituation to the foraging cue by the parasitoid. In a wind tunnel, aphid sex pheromone increased the attraction of A. ervi to the plant‐host complex (Vicia faba/A. pisum), suggesting an additive effect when two different foraging cues are present simultaneously.

The attractiveness of different odour sources from the fruit–host complex on Leptopilina boulardi, a larval parasitoid of frugivorous Drosophila spp.

SummaryIn parasitoid insects, successful offspring development depends on the female’s ability to find a suitable host. Specific recognition is often based on responses to olfactory cues, but their source and nature have rarely been determined.–This paper deals with the recognition of odours involved in host location by Leptopilina boulardi[Barbotin, Carton & Kelner‐Pillault] (Hymenoptera: Eucoilidae), a larval parasitoid of Drosophila species that develops in mature fruits. The nature and origin of volatile stimuli recognized among odours of the host–fruit complex, and the effect of learning on this recognition, were investigated. Oriented responses to these odours were observed in a four‐armed olfactometer and were analysed with the observer software (Noldus Information Technology). Fruit odours alone (banana and pear) were not spontaneously attractive to naive parasitoids, whereas naturally‐infested bananas were highly attractive. The attraction was related to the odour that adult Drosophila left on the substrate but not to Drosophila oviposition activity or larval development. A synergism between some fruit odours (banana and pear) and the odour left by adult Drosophila on damp filter paper was observed. However, when testing a non‐fruit substrate (mushroom), no synergism was observed. Thus, female L. boulardi may innately recognize host–food substrate odours associated with odours from the adult stage of their host. In addition, an oviposition experience on an infested banana allows L. boulardi females to memorise the fruit odour itself through associative learning. The adaptive significance of this process is discussed.

Learning pays off: influence of experience on host finding and parasitism in Lariophagus distinguendus

Abstract.1. The influence of experience on egg maturation, parasitism rate, and behaviour during host searching was investigated for Lariophagus distinguendus (Först.) (Hymenoptera: Pteromalidae) parasitizing larvae of the granary weevil Sitophilus granarius (L.) in grains of wheat Triticum aestivum L.2. Dissection of female parasitoids and parasitism bioassays at high host density revealed that experience with hosts (e.g. by oviposition or by host feeding) is not required either for triggering oogenesis or for oviposition.3. In parasitism experiments at low host density, when single host‐infested grains were offered within a bulk of healthy grains, host finding and parasitism rate were increased by experience.4. Behavioural observations revealed that searching time required for finding an infested grain was shorter for experienced parasitoids than for naive parasitoids, because travel time from grain to grain is shorter for experienced parasitoids, and because experienced parasitoids spend less time than naive parasitoids on non‐infested grains.5. In conclusion, experience due to host exposure increases parasitism and thereby the fitness of the parasitoids. It is discussed that this increase is more likely due to learning than to different egg load dynamics of experienced parasitoids.

Clavicipitaceous Endophytic Infection in Ryegrass Influences Attack Rate of the Parasitoid Microctonus hyperodae (Hymenoptera: Braconidae, Euphorinae) in Listronotus bonariensis (Coleoptera: Curculionidae)

Laboratory experiments were done to examine the effect of ryegrass infection by the endophytic fungus Acremonium lolii Latch, Christensen & Samuels (Ascomycetes: Clavicipitaceae) on Microctonus hyperodae Loan, a parasitoid of Listronotus bonariensis (Kuschel). Infection of ryegrass did not influence the rate of parasitism in weevils confined with naive parasitoids to individual plants and to ryegrass swards in small plots. In contrast, M. hyperodae with prior experience of hosts feeding on endophyte infected grass subsequently were more efficient in parasitizing hosts on this diet. Feeding on individual plants and small plot swards by weevil for 3–4 d increased subsequent parasitoid attack rate.

Influence of physiological state and experience on the responsiveness ofLysiphlebus testaceipes (Cresson) (hymenoptera: aphidiidae) to aphid honeydew and to host plants

The effect of physiological state (hunger) and experience on the responsiveness of the aphid parasitoid, Lysiphlebus testaceipes(Cresson), to clean and honeydew-contaminated host plants was investigated in laboratory bioassays. Both fed and unfed parasitoids spent significantly longer examining honeydew-contaminated plants compared to uncontaminated controls, but the presence of honeydew did not influence attack latency (i.e., the speed with which naive parasitoids found and attacked hosts). Hunger, however, had a significant negative effect on attack latency, presumably as a result of a physiologically based shift from host- to food-location behavior in starved parasitoids. The parasitoid 's response to clean plants was significantly increased as a result of classical conditioning procedures, whereas the response to honeydew-contaminated plants was not.

Larval parasitoid uses aggregation pheromone of adult hosts in foraging behaviour: a solution to the reliability-detectability problem.

Parasitoids that forage for herbivorous hosts by using infochemicals may have a problem concerning the reliability and detectability of these stimuli: host stimuli are highly reliable but not very detectable at a distance, while stimuli from the host's food are very detectable but generally not very reliable in indicating host presence. One solution to this problem is to learn to link highly detectable stimuli to reliable but not very detectable stimuli. Ample knowledge is available on how associative learning aids foraging parasitoids in the location of suitable microhabitats. However, in this paper we report on another solution to the reliability-detectability problem and present evidence for an essential, but as yet overlooked, aspect of Drosophila parasitoid ecology. For the first time it is shown that a parasitoid of Drosophila larvae spies on the communication system of adult Drosophila flies to locate potential host sites: naive parasitoids strongly respond to a volatile aggregation pheromone that is deposited in the oviposition site by recently mated female flies. Thus, the parasitoids resort to using highly detectable information from a host stage different from the one under attack (i.e. infochemical detour). The function and ecological implications of these findings are discussed.

Influence of adult experience on host microhabitat location by the generalist parasitoid,Campoletis sonorensis (Hymenoptera: Ichneumonidae)

The effect of adult experience on microhabitat location behavior of the generalist parasitoid Campoletis sonorensis (Cameron)was examined in a wind tunnel bioassay. Responses were tested to the odors of two host plants (cotton and sesame) of Heliothis virescens (F.) or a nonhost plant (potato), either damaged and infested with host larvae and host products (host/plant complex) or intact, clean and uninfested. Parasitoid females remained naive or were allowed one oviposition experience on either of the plants, 1 min, 2 h, or 24 h prior their introduction into the wind tunnel. In a no-choice test, parasitoids experienced 1 min prior to bioassay completed significantly more flights to sesame and potato host/plant complexes than did naive parasitoids. However, 24 h after experience, only females experienced on potato completed more flights to the host/ plant complex than did naive females. Parasitoids experienced 1 min prior to flight to undamaged plants showed a slight increase in flight response (significant only for potato) but, after 24 h, completed only as many flights as naive parasitoids. In a dual-choice situation, parasitoids did not show a preference for either of the two host plants but did prefer a host to a nonhost plant. This innate plant preference was not changed by a single oviposition experience. The potential significance of these results to the microhabitat location behavior of C. sonorensisin the field is discussed.

Effect of Host Diet on Flight Behavior of the Parasitoid Campoletis sonorensis (Hymenoptera: Ichneumonidae)

The effect of larval host diet on the flight behavior and microhabitat preference of the parasitoid, Campoletis sonorensis (Cameron), was studied in a wind tunnel bioassay. Campoletis sonorensis was reared from Heliothis virescens F. larvae fed either artificial diet, cotton (Gossypium hirsutum L.) foliage or sesame (Sesamum indicum L.) foliage. In a no-choice situation, naive female parasitoids were allowed to fly to undamaged host plants or to naturally damaged host plants containing host larvae and host products (host/plant complex). Parasitoids completed significantly more flights to the host/plant complex (73% of flights) than to undamaged plants (8% of flights). However, the percentage of females completing flights did not differ according to the diet of the host on which the parasitoid was reared. In a choice test, parasitoids allowed to fly in a wind tunnel containing an artificially-damaged cotton leaf and an artificially-damaged sesame leaf exhibited similar landing preferences regardless of whether their hosts were reared on artifical diet, cotton or sesame. The implications of these results on the host microhabitat location behavior of a generalist parasitoid are discussed.

Honeydew as a source of host-searching kairomones for the aphid parasitoid Aphidius nigripes (Hymenoptera: Aphidiidae)

The parasitoid Aphidius nigripes spent more time searching on potato plants previously infested with aphids than on fresh plants, suggesting the presence of searching stimulants on infested plants. Water extracts of aphid honeydew applied to the surface of foliage or to filter paper discs increased searching time and affected the locomotory behavior of parasitoids. When crossing a honeydew-treated area on filter paper, female parasitoids exhibited arrestment, antennal examination, abdominal protraction, reduced walking speed, and increased turning. Upon losing contact with honeydew-contaminated substrate, parasitoids often exhibited a klinotactic response which drove them back to the treated area. The time spent by individual parasitoids within a honeydew-treated area on filter paper decreased with successive visits, but the initial response level was restored when the insect was prevented from contacting honeydew for 1 h. The retention response exhibited by parasitoids contacting a honeydew-treated area increased with honeydew concentration, but levelled off at high concentrations. Naive parasitoids were more responsive to the retention effect of honeydew than experienced individuals.