Role of volatiles emitted by host and non-host plants in the foraging behaviour of Dentichasmias busseolae, a pupal parasitoid of the spotted stemborer Chilo partellus

The role of volatiles from stemborer host and non‐host plants in the host‐finding process of Dentichasmias busseolae Heinrich (Hymenoptera: Ichneumonidae) a pupal parasitoid of Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) was studied. The non‐host plant, molasses grass (Melinis minutiflora Beauv. (Poaceae)), is reported to produce some volatile compounds known to be attractive to some parasitoid species. The studies were conducted to explore the possibility of intercropping stemborer host plants with molasses grass in order to enhance the foraging activity of D. busseolae in such a diversified agro‐ecosystem. Olfactometric bioassays showed that volatiles from the host plants maize, Zea mays L., and sorghum, Sorghum bicolor (L.) (Poaceae), were attractive to the parasitoid. Infested host plants were the most attractive. Volatiles from molasses grass were repellent to the parasitoid. Further tests showed that volatiles from infested and uninfested host plants alone were preferred over those from infested and uninfested host plants combined with the non‐host plant, molasses grass. In dual choice tests, the parasitoid did not discriminate between volatiles from maize infested by either of the two herbivore species, C. partellus or Busseola fusca Fuller (Lepidoptera: Noctuidae). Volatiles from sorghum infested by C. partellus were preferred over those from C. partellus‐infested maize. The study showed that the pupal parasitoid D. busseolae uses plant volatiles during foraging, with those from the plant–herbivore complex being the most attractive. The fact that volatiles from molasses grass were deterrent to the parasitoid suggested that intercropping maize or sorghum with molasses grass was not likely to enhance the foraging behaviour of D. busseolae. Volatiles from the molasses grass may hinder D. busseolae's host location efficiency.     

Associative learning of host presence in non‐host environments influences parasitoid foraging

1. Parasitoids are known to utilise learning of herbivore‐induced plant volatiles (HIPVs) when foraging for their herbivorous host. In natural situations these hosts share food plants with other, non‐suitable herbivores (non‐hosts). Simultaneous infestation of plants by hosts and non‐hosts has been found to result in induction of HIPVs that differ from host‐infested plants. Each non‐host herbivore may have different effects on HIPVs when sharing the food plant with hosts, and thus parasitoids may learn that plants with a specific non‐host herbivore also contain the host. 2. This study investigated the adaptive nature of learning by a foraging parasitoid that had acquired oviposition experience on a plant infested with both hosts and different non‐hosts in the laboratory and in semi‐field experiments. 3. In two‐choice preference tests, the parasitoid Cotesia glomerata shifted its preference towards HIPVs of a plant–host–non‐host complex previously associated with an oviposition experience. It could, indeed, learn that the presence of its host is associated with HIPVs induced by simultaneous feeding of its host Pieris brassicae and either the non‐host caterpillar Mamestra brassicae or the non‐host aphid Myzus persicae. However, the learned preference found in the laboratory did not translate into parasitisation preferences for hosts accompanying non‐host caterpillars or aphids in a semi‐field situation. 4. This paper discusses the importance of learning in parasitoid foraging, and debates why observed learned preferences for HIPVs in the laboratory may cancel out under some field experimental conditions.     

Response of Brassica oleracea to temporal variation in attack by two herbivores affects preference and performance of a third herbivore

1. Plants are frequently under attack by multiple insect herbivores, which may interact indirectly through herbivore‐induced changes in the plant's phenotype. The identity, order, and timing of herbivore arrivals may influence the outcome of interactions between two herbivores. How these aspects affect, in turn, subsequently arriving herbivores that feed on double herbivore‐induced plants has not been widely investigated. 2. This study tested whether the order and timing of arrival of two inducing herbivores from different feeding guilds affected the preference and performance of a subsequently arriving third herbivore, caterpillars of Mamestra brassicae L. (Lepidoptera: Noctuidae). Aphids [Brevicoryne brassicae L. (Hemiptera: Aphididae)] and caterpillars [Plutella xylostella L. (Lepidoptera: Yponomeutidae)] were introduced onto wild Brassica oleracea L. (Brassicaceae) plants in different sequences and with different arrival times. The effects of these plant treatments on M. brassicae caterpillars were assessed in pairwise preference tests and no‐choice performance tests. 3. The caterpillars of M. brassicae preferred to feed from undamaged plants rather than double herbivore‐induced plants. Compared with undamaged plants, they preferred plant material on which aphids had arrived first followed by caterpillars, whereas they avoided plant material with the reverse order of herbivore arrival. Performance of the caterpillars increased with increasing arrival time between herbivore infestations in double herbivore‐induced plants. Although M. brassicae grew faster on plants induced by aphids than on those induced by caterpillars alone, its performance was not affected by the order of previous herbivore arrival. 4. These results imply that the timing of colonisation by multiple herbivores determines the outcome of plant‐mediated herbivore–herbivore interactions.     

Females of Cotesia vestalis,a parasitoid of diamondback moth larvae,learn to recognise cues from aphid‐infested plants to exploit honeydew

1. To maximise their reproductive success, the females of most parasitoids must not only forage for hosts but must also find suitable food sources. These may be nectar and pollen from plants, heamolymph from hosts and/or honeydew from homopterous insects such as aphids. 2. Under laboratory conditions, females of Cotesia vestalis, a larval parasitoid of the diamondback moth (Plutella xylostella) which does not feed on host blood, survived significantly longer when held with cruciferous plants infested with non‐host green peach aphids (Myzus persicae) than when held with only uninfested plants. 3. Naïve parasitoids exhibited no preference between aphid‐infested and uninfested plants in a dual‐choice test, but those that had been previously fed aphid honeydew significantly preferred aphid‐infested plants to uninfested ones. 4. These results suggest that parasitoids that do not use aphids as hosts have the potential ability to learn cues from aphid‐infested plants when foraging for food. This flexible foraging behaviour could allow them to increase their lifetime reproductive success.     

Density dependence in insect performance within individual plants: induced resistance to Spodoptera exigua in tomato

Net intraspecific density dependence experienced by insect herbivores at the scale of single plants can be a function both of induced resistance in the plant and other interactions among individual herbivores. Theory suggests that non‐linearity in the form of this density dependence can influence the effects of plants on herbivore population dynamics. This study examined both net density dependence at the scale of single plants, and changes in plant quality with herbivore density for Spodoptera exigua caterpillars on tomato plants. One experiment measured the growth of caterpillars moving freely about the plant at different densities, the distribution of damage by these caterpillars, and the quality of the plant as food for caterpillars (growth of caterpillars on undamaged leaf tissue excised from the plant). A second experiment measured plant quality for plants with different amounts of damage by caterpillars confined to particular leaves in mesh bags. Growth of S. exigua caterpillars was found to be negatively density dependent, and this was in part due to decreases in plant quality both as herbivore density increased and as the amount of damage increased. The response of plant quality to herbivores was found to have non‐linear features; there was both a threshold below which no significant decreases in quality (as measured by herbivore growth) occurred, and the decrease in herbivore performance saturated at the highest damage levels. In addition, it was found that caterpillar damage was significantly more aggregated than expected when multiple caterpillars occupy a single plant. This study confirms that host plants have the potential to be a source of density dependence that affects herbivore performance.     

Effects of quantitative and qualitative differences in volatiles from host‐ and non‐host‐infested maize on the attraction of the larval parasitoid Cotesia kariyai

Cotesia kariyai Watanabe (Hymenoptera: Braconidae) is a specialist larval parasitoid of Mythimna separata Walker (Lepidoptera: Noctuidae). Cotesia kariyai wasps use herbivore‐induced plant volatiles (HIPVs) to locate hosts. However, complex natural habitats are full of volatiles released by both herbivorous host‐ and non‐host‐infested plants at various levels of intensity. Therefore, the presence of non‐hosts may affect parasitoid decisions while foraging. Here, the host‐finding efficiency of naive C. kariyai from HIPVs influenced by host‐ and non‐host‐infested maize [Zea mays L. (Poaceae)] plants was investigated with a four‐arm olfactometer. Ostrinia furnacalis Guenée (Lepidoptera: Crambidae) was selected as a non‐host species. One unit (1 U) of host‐ or non‐host‐infested plant was prepared by infesting a potted plant with five host or seven non‐host larvae. In two‐choice bioassays, host‐infested plants fed upon by different numbers of larvae, and various units of host‐ and non‐host‐infested plants (infestation units; 1 U, 2 U, and 3 U) were arranged to examine the effects of differences in volatile quantity and quality on the olfactory responses of C. kariyai with the assumption that volatile quantity and quality changes with differences in numbers of insects and plants. Cotesia kariyai was found to perceive quantitative differences in volatiles from host‐infested plants, preferring larger quantities of volatiles from larger numbers of larvae or plants. Also, the parasitoids discriminated between healthy plants, host‐infested plants, and non‐host‐infested plants by recognising volatiles released from those plants. Cotesia kariyai showed a reduced preference for host‐induced volatiles, when larger numbers of non‐host‐infested plants were present. Therefore, quantitative and qualitative differences in volatiles from host‐ and non‐host‐infested plants appear to affect the decision of C. kariyai during host‐habitat searching in multiple tritrophic systems.     

Olfactory response of a predatory mirid to herbivore induced plant volatiles: multiple herbivory vs. single herbivory

Plants infested with a single herbivore species can attract natural enemies through the emission of herbivore‐induced plant volatiles (HIPVs). However, under natural conditions plants are often attacked by more than one herbivore species. We investigated the olfactory response of a generalist predators Macrolophus caliginosus to pepper infested with two‐spotted spider mites, Tetranychus urticae, or green peach aphid, Myzus persicae, vs. plants infested with both herbivore species in a Y‐tube olfactometer set up. In addition, the constituents of volatile blends from plants exposed to multiple or single herbivory were identified by gas chromatography‐mass spectrometry (GC‐MS). The mirid bugs showed a stronger response to volatiles emitted from plants simultaneously infested with spider mites and aphids than to those emitted from plants infested by just one herbivore, irrespective of the species. Combined with results from previous studies under similar conditions we infer that this was a reaction to herbivore induced plant volatiles. The GC‐MS analysis showed that single herbivory induced the release of 22 additional compounds as compared with the volatiles emitted from clean plants. Quantitative analyses revealed that the amount of volatile blends emitted from pepper infested by both herbivores was significantly higher than that from pepper infested by a single herbivore. Moreover, two unique substances were tentatively identified (with a probability of 94% and 91%, respectively) in volatiles emitted by multiple herbivory damaged plants: α‐zingiberene and dodecyl acetate.     

The jasmonate pathway alters herbivore feeding behaviour: consequences for plant defences

The jasmonate pathway is a highly conserved defensive cascade in plants that regulates the induction of resistance against herbivores; however, its role in herbivore feeding behaviour remains unknown. We used a mutant tomato plant (def‐1) deficient in the production of jasmonate‐related defensive proteins to test the hypothesis that genotypes with a reduced ability to induce resistance have a higher and more concentrated pattern of herbivore damage. Wild‐type and def‐1 plants received either damage by Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) caterpillars or no damage. After treatment, we tested for systemic responses by allowing a free roaming S. exigua caterpillar to feed on the undamaged portions of plants. Weight‐gain and leaf consumption of S. exigua were highest on def‐1 plants, regardless of prior herbivore damage. Def‐1 plants also had fewer numbers of leaves and leaflets eaten, and fewer feeding holes, which was indicative of a more concentrated distribution of damage on mutant compared to wild‐type plants. Following these results, we mimicked the amount and distribution of feeding damage that wild‐type or jasmonate‐deficient plants would receive on wild‐type plants to test whether changes in feeding behaviour may feedback to influence the expression of induced resistance. We mimicked the distribution of damage in wild‐type and jasmonate‐deficient plants by allowing caterpillars to feed on either one (leaf 1 or 2) or two leaves (leaf 1 and 2). Increased herbivore damage resulted in higher proteinase inhibitor (PI) activity, a jasmonate‐regulated defensive protein, and lower S. exigua performance on wild‐type but not jasmonate‐deficient plants. Compared to undamaged plants, a concentrated pattern of herbivore damage increased systemic resistance; these induced responses were greater on leaflets with stronger vascular connections to the damaged leaf. A more dispersed pattern of caterpillar damage altered the expression of induced responses, but the outcome depended on the specific pattern of damage. When leaf 1 was damaged and then leaf 2, the undamaged (third) leaf (which is more strongly connected to leaf 1 than 2) expressed reduced the PI activity compared to plants receiving concentrated damage to leaf 1; whereas in plants where leaf 2 was first damaged and then leaf 1, there were no differences in PI activity in leaf 3 compared to plants receiving concentrated damage to leaf 2. Thus, induction of the jasmonate pathway may not only determine the amount and distribution of feeding damage by herbivores, but this may feedback to affect the subsequent expression of plant defence.     

Similar attractiveness of maize volatiles induced by Helicoverpa armigera and Pseudaletia separata to the generalist parasitoid Campoletis chlorideae

Campoletis chlorideae Uchida (Hymenoptera: Ichneumonidae), a major larval endoparasitoid of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), also attacks many other noctuid caterpillars. We investigated the attractiveness of H. armigera‐ and Pseudaletia separata (Lepidoptera: Noctuidae)‐infested maize [Zea mays L. (Poaceae)] plants to C. chlorideae, and analyzed the volatiles emitted from infested plants and undamaged plants. Considering the reported specific induction of plant volatiles by elicitors in the caterpillar regurgitant, we also tested the response of the parasitoid to mechanically damaged plants treated with caterpillar regurgitant or water and measured the volatiles released by these plants. In wind‐tunnel bioassays, C. chlorideae was strongly attracted to herbivore‐induced maize volatiles. Mechanically damaged plants, whether they were treated with caterpillar regurgitant or water, were more attractive to the parasitoid than undamaged plants. The parasitoid did not distinguish between maize seedlings infested by the two noctuid insects, nor did they show a difference in attraction to mechanically damaged plants treated with caterpillar regurgitant or water. Coupled gas chromatography–mass spectrometer (GC‐MS) analysis revealed that 15 compounds were commonly emitted by herbivore‐infested and mechanically damaged maize plants, whereas only two compounds were released in minor amounts from undamaged plants. Infestation by H. armigera specifically induced four terpenoids, β‐pinene, β‐myrcene, D‐limonene, and (E)‐nerolidol, which were not induced by infestation of P. separata and mechanical damage, plus caterpillar regurgitant or water. Two compounds, geranyl acetate and β‐sesquiphellandrene, were also induced by the infestation of H. armigera, but not by the infestation of P. separata. All treated maize plants released volatiles in significantly larger total amounts than did undamaged plants. Maize plants infested by H. armigera emitted greater amounts of volatiles than plants infested by P. separata. The treatment with caterpillar regurgitant resulted in larger amounts of volatile emission than the treatment with water did in mechanically damaged plants. The amounts of emissions of individual compounds were also different between differently treated plants.     

Herbivore species identity rather than diversity of the non‐host community determines foraging behaviour of the parasitoid wasp Cotesia glomerata

Extensive research has been conducted to reveal how species diversity affects ecosystem functions and services. Yet, consequences of diversity loss for ecosystems as a whole as well as for single community members are still difficult to predict. Arthropod communities typically are species‐rich, and their species interactions, such as those between herbivores and their predators or parasitoids, may be particularly sensitive to changes in community composition. Parasitoids forage for herbivorous hosts by using herbivore‐induced plant volatiles (indirect cues) and cues produced by their host (direct cues). However, in addition to hosts, non‐suitable herbivores are present in a parasitoid's environment which may complicate the foraging process for the parasitoid. Therefore, ecosystem changes in the diversity of herbivores may affect the foraging efficiency of parasitoids. The effect of herbivore diversity may be mediated by either species numbers per se, by specific species traits, or by both. To investigate how diversity and identity of non‐host herbivores influence the behaviour of parasitoids, we created environments with different levels of non‐host diversity. On individual plants in these environments, we complemented host herbivores with 1–4 non‐host herbivore species. We subsequently studied the behaviour of the gregarious endoparasitoid Cotesia glomerata L. (Hymenoptera: Braconidae) while foraging for its gregarious host Pieris brassicae L. (Lepidoptera: Pieridae). Neither non‐host species diversity nor non‐host identity influenced the preference of the parasitoid for herbivore‐infested plants. However, after landing on the plant, non‐host species identity did affect parasitoid behaviour, whereas non‐host diversity did not. One of the non‐host species, Trichoplusia ni Hübner (Lepidoptera: Noctuidae), reduced the time the parasitoid spent on the plant as well as the number of hosts it parasitized. We conclude that non‐host herbivore species identity has a larger influence on C. glomerata foraging behaviour than non‐host species diversity. Our study shows the importance of species identity over species diversity in a multitrophic interaction of plants, herbivores, and parasitoids.     

Non‐pathogenic rhizobacteria interfere with the attraction of parasitoids to aphid‐induced plant volatiles via jasmonic acid signalling

Beneficial soil‐borne microbes, such as mycorrhizal fungi or rhizobacteria, can affect the interactions of plants with aboveground insects at several trophic levels. While the mechanisms of interactions with herbivorous insects, that is, the second trophic level, are starting to be understood, it remains unknown how plants mediate the interactions between soil microbes and carnivorous insects, that is, the third trophic level. Using Arabidopsis thaliana Col‐0 and the aphid Myzus persicae, we evaluate here the underlying mechanisms involved in the plant‐mediated interaction between the non‐pathogenic rhizobacterium Pseudomonas fluorescens and the parasitoid Diaeretiella rapae, by combining ecological, chemical and molecular approaches. Rhizobacterial colonization modifies the composition of the blend of herbivore‐induced plant volatiles. The volatile blend from rhizobacteria‐treated aphid‐infested plants is less attractive to an aphid parasitoid, in terms of both olfactory preference behaviour and oviposition, than the volatile blend from aphid‐infested plants without rhizobacteria. Importantly, the effect of rhizobacteria on both the emission of herbivore‐induced volatiles and parasitoid response to aphid‐infested plants is lost in an Arabidopsis mutant (aos/dde2‐2) that is impaired in jasmonic acid production. By modifying the blend of herbivore‐induced plant volatiles that depend on the jasmonic acid‐signalling pathway, root‐colonizing microbes interfere with the attraction of parasitoids of leaf herbivores.     

Effects of single and multiple herbivory by host and non‐host caterpillars on the attractiveness of herbivore‐induced volatiles of sugarcane to the generalist parasitoid Cotesia flavipes

It is well known that parasitoids are attracted to volatiles emitted by host‐damaged plants; however, this tritrophic interaction may change if plants are attacked by more than one herbivore species. The larval parasitoid Cotesia flavipesCameron (Hymenoptera: Braconidae) has been used intensively in Brazil to control the sugarcane borer, Diatraea saccharalisFabricius (Lepidoptera: Pyralidae) in sugarcane crops, where Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae), a non‐stemborer lepidopteran, is also a pest. Here, we investigated the ability of C. flavipes to discriminate between an unsuitable host (S. frugiperda) and a suitable host (D. saccharalis) based on herbivore‐induced plant volatiles (HIPVs) emitted by sugarcane, and whether multiple herbivory (D. saccharalis feeding on stalk + S. frugiperda feeding on leaves) in sugarcane affected the attractiveness of HIPVs to C. flavipes. Olfactometer assays indicated that volatiles of host and non‐host‐damaged plants were attractive to C. flavipes. Even though host‐ and non‐host‐damaged plants emitted considerably different volatile blends, neither naïve nor experienced wasps discriminated suitable and unsuitable hosts by means of HIPVs emitted by sugarcane. With regard to multiple herbivory, wasps innately preferred the odor blend emitted by sugarcane upon non‐host + host herbivory over host‐only damaged plants. Multiple herbivory caused a suppression of some volatiles relative to non‐host‐damaged sugarcane that may have resulted from the unaltered levels of jasmonic acid in host‐damaged plants, or from reduced palatability of host‐damaged plants to S. frugiperda. In conclusion, our study showed that C. flavipes responds to a wide range of plant volatile blends, and does not discriminate host from non‐host and non‐stemborer caterpillars based on HIPVs emitted from sugarcane. Moreover, we showed that multiple herbivory by the sugarcane borer and fall armyworm increases the attractiveness of sugarcane plants to the parasitoids.     

Infochemically mediated tritrophic interaction webs on cabbage plants

In response to damage by herbivores, plants are known to emit infochemicals that enhance the effectiveness of insect parasitoids. Studies on plant–parasitoid interactions mediated by such infochemicals have focused on the tritrophic systems in which plants are infested by a single herbivore species. In natural ecosystems, however, plants are often simultaneously infested by several herbivorous species. The present study focuses on two herbivorous species that simultaneously attack crucifer plants and their respective parasitic wasps. We first show the specific responses of the two specialist parasitic wasps [Cotesia plutellae and C. glomerata (Hymenoptera: Braconidae)] to infochemicals originating from cabbage plants (Brassica oleracea cv. Sikidori) infested by each of their respective host larvae [Plutella xylostella (Lepidoptera: Yponomeutidae) and Pieris rapae (Lepidoptera: Pieridae)]. We then coupled the two tritrophic systems on the same cabbage plants. These experiments demonstrated the presence of indirect interactions between the two species of herbivores. Overall, the results indicate the presence of infochemically mediated tritrophic interaction webs on a single plant. Received: September 1, 2000 / Accepted: February 8, 2001     

Innate responses of the parasitoidsCotesia glomerata andC. rubecula (Hymenoptera: Braconidae) to volatiles from different plant-herbivore complexes

To determine and compare innate preferences of the parasitoid speciesCotesia glomerata andC. rubecula for different plant-herbivore complexes, long-range (1-m) foraging behavior was studied in dual-choice experiments in a wind tunnel. In this study we tested the hypothesis that naive females of the specialistC. rubecula should show more pronounced preferences for different plant-herbivore complexes than females of the generalistC. glomerata. The herbivore species used were the pieridsPieris brassicae, P. rapae, P. Napi, andAporia crataegi and the nonhostsPlutella xylostella andMamestra brassicae. All herbivore species feed mainly on cabbage and wild crucifers, exceptAporia crataegi, which feeds on species of Rosaceae. Both parasitoid species preferred herbivore-damaged plants over nondamaged plants. NeitherC. rubecula norC. glomerata discriminated between plants infested by different caterpillar species, not even between plants infested by host-and nonhost species. Both parasitoid species showed preferences for certain cabbage cultivars and plant species. No differences were found in innate host-searching behavior betweenC. glomerata andC. rubecula. The tritrophic system cabbage-caterpillars-Cotesia sp. seems to lack specificity on the herbivore level, whereas on the plant level differences in attractiveness to parasitoids were found.     

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 herbivorederived 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. I terestingly, naive parasitoids attacked more soybeathan 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.     

Local and systemic responses induced by aphids in Solanum tuberosum plants

The aphids Macrosiphum euphorbiae (Thomas) and Myzus persicae (Sulzer) (Homoptera: Aphididae) are serious pests of potato (Solanum tuberosum L.) (Solanaceae), notably in transmitting several plant viruses. Heterospecific interactions may occur between these two species as they are often seen at the same time on the same potato plant in the field. As aphid infestation is known to induce both local and systemic changes, we conducted experiments to determine the effect of previous infestation on probing behaviour and feeding‐related parameters. We used the DC electrical penetration graph technique to characterize the influence of previous infestation by conspecific M. persicae or by heterospecific Ma. euphorbiae on M. persicae feeding behaviour at both local and systemic levels, i.e., on previously infested leaves and on non‐previously infested leaves of infested plants, respectively. Conspecific and heterospecific infestation led to similar modification of M. persicae feeding activities. However, the effects of previous infestation occurring at the local level were opposite to those observed at the systemic level. Myzus persicae food acceptance was slightly enhanced on previously infested leaves, whereas it was inhibited on non‐infested leaves of infested plants, which indicated an induced resistance mechanism. Our results advance the understanding of the mechanisms involved in aphid–host plant acceptance and colonization processes on potato plants in conspecific and heterospecific situations.     

Asymmetric effects of a leaf‐chewing herbivore on aphid population growth

1. Plant responses to herbivory are often specific to the feeding guild of the attacking herbivore. These phytochemical responses to herbivore damage can affect herbivore performance and activity. Comprehensive studies on the ecological consequences of multi‐herbivore plant interactions are key to understanding plant–herbivore community dynamics. 2. This study examined how feeding damage by co‐occurring herbivores from separate feeding guilds, Myzus persicae (Sulzer), a sucking herbivore, and Leptinotarsa decemlineata (Say), a chewing herbivore, alter plant chemistry and indirectly affect herbivore performance. Performance was measured when each insect fed on plants individually, sequentially, or simultaneously in laboratory and field experiments. Phytohormone and glycoalkaloid content were measured for each feeding sequence to evaluate plant responses to herbivory by each guild. Mid‐season and end‐of‐season tuber yield were evaluated in the field study. 3. Damage by L. decemlineata negatively impacted M. persicae performance in both laboratory and field settings. Damage by M. persicae did not affect L. decemlineata performance in laboratory assays. However, L. decemlineata performance was positively affected by M. persicae herbivory in the field, but this effect was temporary. Although phytohormones and plant defences varied across treatments, they provide little resolution on interaction outcomes. 4. These results confirm that the presence of multiple feeding guilds on a single plant can affect these chewing and sucking herbivores differentially, but given the variability in our phytochemical analyses compared with other studies, the mechanism remains unclear. The study's findings show that aphids are negatively affected by chewing herbivores across systems, while aphids temporarily affected beetles positively.     

Can plants mediate the humoral immunity and biochemical metabolism of entomopathogen‐infected caterpillars?

Plant‐insect herbivore‐entomopathogen interactions are one of the hot topics in biological control and humoral immunity, and biochemical metabolism are important responses of herbivores to pathogen infection. Entomopathogens are key biocontrol agents of caterpillars, but how plants affect the responses of caterpillars to these organisms is not well understood. We studied hormonal immunity (lysozyme and phenoloxidase activities) and biochemical metabolism (total protein and lipid contents) of Beauveria bassiana‐infected beet armyworm (Spodoptera exigua) larvae that feed on five different host plants (soya bean, Chinese cabbage, edible amaranth, water convolvulus and pepper). Results indicated that plant species differentially affected lysozyme and phenoloxidase activity and lipid content, but had no effect on protein content of pathogen‐infected caterpillars. Both lysozyme and phenoloxidase activities were generally higher in entomopathogen‐infected larvae that feed on edible amaranth or water convolvulus compared with the other three plants from days 1 to 5 after treatment. Plant species did not affect in regular changes during the 5 days in the lipid content of infected or non‐infected caterpillars. Our study reveals that plants fail to affect the biochemical metabolism but plants can mediate the humoral immunity of caterpillars to defend against pathogens. This study provides insight into plant‐mediated effects on the response of herbivores to pathogens.     

Diamondback moth females oviposit more on plants infested by non‐parasitised than by parasitised conspecifics

Abstract 1. When offered a choice, female diamondback moths (Plutella xylostella) oviposited more eggs on plants with non‐parasitised conspecific larvae than on plants with parasitised larvae. 2. The leaf area consumed by parasitised larvae was significantly lower than that by non‐parasitised larvae. However, this quantitative difference in larval damage did not explain the female’s ability to discriminate between plants with parasitised and non‐parasitised larvae, as females showed an equal oviposition preference for plants infested by higher or lower densities of non‐parasitised larvae. 3. Pupal weight and duration of the larval stage of P. xylostella were independent of whether larvae were reared on plants that were previously infested by either non‐parasitised or parasitised larvae. 4. The larval parasitoid Cotesia vestalis did not distinguish between plants infested by non‐parasitised larvae and plants infested by larvae that had already been parasitised by conspecific wasps. 5. Based on these data, it can be concluded that the moth oviposition preference for plants infested by non‐parasitised conspecifics relative to plants infested by parasitised conspecifics was not explained by plant quality or by the attractiveness of plants towards wasps. It is hypothesised that one of the reasons for this preference is avoidance of plants where a relatively high risk of parasitism is expected due to the emergence of parasitoids from the parasitised host larvae.     

Inhibitory effects of permethrin on flight responses,host‐searching,and foraging behaviour of Cotesia vestalis (Hymenoptera: Braconidae), a larval parasitoid of Plutella xylostella (Lepidoptera: Plutellidae)

Although it is well known that the application of broad‐spectrum synthetic insecticides reduces the effectiveness of natural enemies, the details of the actual mechanisms, including the lethal and sublethal effects of this reduction, are not fully understood. The inhibitory effects of a pyrethroid insecticide (permethrin), Adion 20% EC on the flight responses, host‐searching behaviour and foraging behaviour of Cotesia vestalis (Hymenoptera: Braconidae), a larval parasitoid of the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae), were investigated under laboratory conditions. In choice trials, the wasps showed significant preference for P. xylostella‐infested Komatsuna plants over insecticide‐treated plants, suggesting an inhibitory effect of the insecticide on the flight response of C. vestalis. When offered a pair of plants, the wasps showed a significant preference for P. xylostella‐infested plants compared to uninfested plants. However, significantly more wasps were attracted to infested permethrin‐treated plants than to uninfested plants, suggesting that the wasps are attracted to the volatile infochemicals from the infested plants, even if treated with permethrin. The searching time was significantly shorter and the mortality of C. vestalis adults on the insecticide‐treated plants significantly higher than in the control plants treated with distilled water. These results suggest that the application of the insecticide had an inhibitory effect on the wasps’‐searching behaviour and consequently reduced the effectiveness of C. vestalis as a biological control agent against P. xylostella. In addition, the strength of the inhibitory effect of permethrin on the attraction of the wasps to the plants is critical to the survival of C. vestalis. Our results suggest that the attraction of the wasps to the permethrin‐treated infested plants increases the risk of their exposure to this insecticide.