Application of synthetic herbivore‐induced plant volatiles causes increased parasitism of herbivores in the field

We previously reported that Cotesia vestalis (Hymenoptera, Braconidae), a parasitoid of diamondback moth (DBM) (Plutella xylostella; Lepidoptera, Plutellidae) larvae, was attracted to volatiles from crucifer plants infested by moth larvae kept in a desktop acrylic box, and that a blend of four DBM‐induced plant volatiles was responsible for this attraction. In this study, using a specially designed dispenser to release the four compounds, we demonstrated that the wasp was attracted to intact komatsuna plants (Brassica rapa var. perviridis). The experiments were performed in a climate‐controlled room, which was approximately 1000 times larger than the acrylic box used previously. Similarly, using the dispenser in the field, C. vestalis females were attracted to intact komatsuna plants with the dispenser from a distance of three metres. We also examined the effect of the volatile blend on the incidence of parasitism of DBM larvae in the field. Three small containers containing DBM‐infested komatsuna plants with dispensers, and three control containers containing only infested plants (control) were arranged in two lines running perpendicular to a komatsuna field in which both DBM larvae and C. vestalis populations were maintained, at distances of 12, 30 and 70 m. The results showed that the incidence of DBM parasitism was significantly higher in containers containing dispensers than in the control containers, suggesting that the blend could potentially be applied to DBM control in agroecosystems.     

Maize plants sprayed with either jasmonic acid or its precursor,methyl linolenate,attract armyworm parasitoids,but the composition of attractants differs

Treatment of both uninfested and armyworm‐infested maize plants with jasmonic acid (JA) is known to attract the parasitic wasp, Cotesia kariyai Watanabe (Hymenoptera: Braconidae). Here, we show that treatment with a methyl ester of a JA precursor, methyl linolenate (MeLin), also causes maize plants to attract this wasp, yet does not cause elevated levels of endogenous JA. The volatile chemicals emitted from either infested or uninfested maize plants treated with MeLin were qualitatively and quantitatively different from those emitted from JA‐treated plants. Among compounds emitted from MeLin‐treated plants, α‐pinene and menthol attracted wasps in pure form in a two‐choice test using a choice chamber. A mixture of methyl salicylate, α‐copaene, and β‐myrcene also attracted wasps. In contrast, (Z)‐3‐hexenyl acetate was among the main attractants for C. kariyai in JA‐treated plants. These data show that in addition to JA, MeLin also has the potential to increase the host‐finding ability of C. kariyai, but that the composition of attractants they induce differs.     

The parasitic fly Nemorilla maculosa exploits host‐plant volatiles to locate the legume pod borer,Maruca vitrata

Nemorilla maculosa Meigen (Diptera: Tachinidae) is a solitary endoparasitoid of the legume pod borer, Maruca vitrata Fabricius (Lepidoptera: Crambidae), a key pest of cowpea, Vigna unguiculata (L.) Walp. (Fabaceae) in Africa. A colony of N. maculosa, introduced for experimental purposes from Taiwan to the laboratories of the International Institute of Tropical Agriculture (IITA) in Benin, was used for our studies. Olfactory reponses of N. maculosa to leaves of infested or uninfested cowpea and yellow peabush, Sesbania cannabina (Retz.) Pers. (Fabaceae), and to M. vitrata larvae were evaluated in a four‐arm olfactometer. For all combinations of odor sources, responses between naïve and oviposition‐experienced female flies did not differ. Nemorilla maculosa females were attracted by odors from uninfested leaves of yellow peabush and flowers of cowpea when compared with clean air, and they were attracted to plants damaged by M. vitrata with larvae removed. However, the female fly did not discriminate between odors from infested and uninfested plants. The parasitic fly N. maculosa proved well able to use volatile compounds from various host plants (peabush and cowpea) to locate its host, with a more pronounced attraction by the combination of host larvae and infested host plant parts. These findings are discussed in light of the prospective use of N. maculosa as a biological control agent against the legume pod borer.     

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.     

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.     

Evaluation of the predatory mite <Emphasis Type="Italic">Phytoseiulus macropilis</Emphasis> (Acari: Phytoseiidae) as a biological control agent of the two-spotted spider mite on strawberry plants under greenhouse conditions

The predatory mite Phytoseiulus macropilis is a potential biological control agent of the two-spotted spider mite (TSSM) Tetranychus urticae on strawberry plants. Its ability to control TSSM was recently assessed under laboratory conditions, but its ability to locate and control TSSM under greenhouse conditions has not been tested so far. We evaluated whether P. macropilis is able to control TSSM on strawberry plants and to locate strawberry plants infested with TSSM under greenhouse conditions. Additionally, we tested, in an olfactometer, whether odours play a role in prey-finding by P. macropilis. The predatory mite P. macropilis required about 20 days to achive reduction of the TSSM population on strawberry plants initially infested with 100 TSSM females per plant. TSSM-infested plants attract an average of 27.5 ± 1.0% of the predators recaptured per plant and uninfested plants attracted only 5.8 ± 1.0% per plant. The predatory mites were able to suppress TSSM populations on a single strawberry plant and were able to use odours from TSSM-infested strawberry plants to locate prey in both olfactometer and arena experiments. Hence, it is concluded that P. macropilis can locate and reduce TSSM population on strawberry plants under greenhouse conditions.     

Caterpillar‐induced plant volatiles attract conspecific herbivores and a generalist predator

Plants release volatiles in response to caterpillar feeding that attracts natural enemies of the herbivores, a tritrophic interaction which has been considered to be an indirect plant defence against herbivores. On the other hand, the caterpillar‐induced plant volatiles have been reported to either repel or attract conspecific adult herbivores. This work was undertaken to investigate the response of both herbivores and natural enemies to caterpillar‐induced plant volatiles in apple orchards. We sampled volatile compounds emitted from uninfested apple trees, and apple trees infested with generalist herbivore the pandemis leafroller moth, Pandemis pyrusana (Lepidoptera, Tortricidae) larvae using headspace collection and analysed by gas chromatography/mass spectrometry. Infested apple trees uniquely release six compounds (benzyl alcohol, phenylacetonitrile, phenylacetaldehyde, 2‐phenylethanol, indole and (E)‐nerolidol). These compounds were tested on two species of herbivores and one predator in apple orchards. Binary blends of phenylacetonitrile + acetic acid or 2‐phenylethanol + acetic acid attracted a large number of conspecific male and female adult herbivores. The response of pandemis leafroller to herbivore‐induced plant volatiles (HIPVs) was so pronounced that over one thousand and seven hundred conspecific male and female adult herbivores were caught in traps baited with HIPVs in three‐day trapping period. In addition, significantly higher number of male and female obliquebanded leafroller, Choristoneura rosaceana (Lepidoptera, Tortricidae), was caught in traps baited a binary blend of 2‐phenylethanol + acetic acid, or a ternary blend contains 2‐phenylethanol and phenylacetonitrile + acetic acid. This result challenges the current paradigm hypothesized that HIPVs repel herbivores and question the indirect defensive function proposed for these compounds. On the other hand, a ternary blend of phenylacetonitrile and 2‐phenylethanol + acetic acid attracted the largest numbers of the general predator, the common green lacewing, Chrysoperla plorabunda. To our knowledge, this is the first record of the direct attraction of conspecific adult herbivores as well as a predator to the caterpillar‐induced plant volatiles in the field.     

Birds exploit herbivore‐induced plant volatiles to locate herbivorous prey

Arthropod herbivory induces plant volatiles that can be used by natural enemies of the herbivores to find their prey. This has been studied mainly for arthropods that prey upon or parasitise herbivorous arthropods but rarely for insectivorous birds, one of the main groups of predators of herbivorous insects such as lepidopteran larvae. Here, we show that great tits (Parus major) discriminate between caterpillar‐infested and uninfested trees. Birds were attracted to infested trees, even when they could not see the larvae or their feeding damage. We furthermore show that infested and uninfested trees differ in volatile emissions and visual characteristics. Finally, we show, for the first time, that birds smell which tree is infested with their prey based on differences in volatile profiles emitted by infested and uninfested trees. Volatiles emitted by plants in response to herbivory by lepidopteran larvae thus not only attract predatory insects but also vertebrate predators.     

Different genetic structures revealed resident populations of a specialist parasitoid wasp in contrast to its migratory host

Genetic comparisons of parasitoids and their hosts are expected to reflect ecological and evolutionary processes that influence the interactions between species. The parasitoid wasp, Cotesia vestalis, and its host diamondback moth (DBM), Plutella xylostella, provide opportunities to test whether the specialist natural enemy migrates seasonally with its host or occurs as resident population. We genotyped 17 microsatellite loci and two mitochondrial genes for 158 female adults of C. vestalis collected from 12 geographical populations, as well as nine microsatellite loci for 127 DBM larvae from six separate sites. The samplings covered both the likely source (southern) and immigrant (northern) areas of DBM from China. Populations of C. vestalis fell into three groups, pointing to isolation in northwestern and southwestern China and strong genetic differentiation of these populations from others in central and eastern China. In contrast, DBM showed much weaker genetic differentiation and high rates of gene flow. TESS analysis identified the immigrant populations of DBM as showing admixture in northern China. Genetic disconnect between C. vestalis and its host suggests that the parasitoid did not migrate yearly with its host but likely consisted of resident populations in places where its host could not survive in winter.     

The Role of Chemical Cues in Host and Mate Location in the Pear Psylla Cacopsylla bidens (Homoptera: Psyllidae)

Chemical communication was shown to play a role in the pear psylla, Cacopsylla bidens. Electrophysiological (EAG) and behavioral responses were investigated in males and females pear psylla . Males were found to be attracted to females, and especially to those on host plants, but not to males, uninfested host plants, or plants infested with conspecific larvae. On the other hand, females were not attracted to males or females but displayed some attraction to host plants. Furthermore, females showed a preference for uninfested pear versus plants infested with conspecific larvae. The antennae of males gave highest electroantenographic response to volatiles from pears infested with females but not males, while females, responded also toward the volatiles of pear alone. These results indicate that females of C. bidens emit sex pheromones that are attractive to the males and suggest that, host volatiles may play a role in host selection by pear psylla females.     

Interactions between white mealybugs and red spider mites sequentially colonizing coffee plants

Plants under herbivore attack often respond defensively by mounting chemical and physical defences. However, some herbivores can manipulate plant defences to their own benefit by suppressing the expression of induced defences. These herbivore‐induced changes specific to the attacking herbivore can either facilitate or impede the colonization and establishment of a second herbivore. Although recent studies have focused on the effect of multiple herbivory on plant induced response and the third trophic level, few have examined the ecological relevance of multiple herbivores sharing the host. Here, we investigated whether herbivory by the white mealybug Planococcus minor (Maskell) (Hemiptera: Pseudococcidae) or the red spider mite Olygonychus ilicis (McGregor) (Acari: Tetranychidae), two herbivores that peak in coffee plantations during the dry season, may facilitate the colonization and establishment of the other species in coffee plants. Dual‐choice arena tests showed that white mealybugs preferred mite‐infested over uninfested coffee plants as hosts. Fifteen days after the release of 50 first‐instar P. minor nymphs, greater numbers of nymphs and adults were found on mite‐infested than uninfested plants, indicating superior performance on mite‐infested plants. On the other hand, female red spider mites did not show clear preference between uninfested and mealybug‐infested plants and deposited similar numbers of eggs on both treatments. In a no‐choice test, red spider mites performed poorly on mealybug‐infested plants with a smaller number of eggs, nymphs, females and males found in mealybug‐infested plants relative to uninfested plants. Thus, our results indicate that coffee plants are more likely to be infested by the red spider mite before white mealybug, rather than the inverse sequence (i.e. mealybug infestation followed by red spider mites). Our findings are discussed in the context of plant manipulation reported for pseudococcid mealybugs and spider mites.     

Host‐searching responses to herbivory‐associated chemical information and patch use depend on mating status of female solitary parasitoid wasps

1. In a tritrophic interaction system consisting of plants, herbivores, and their parasitoids, chemicals released from plants after herbivory are known to play important roles for many female parasitoids to find their hosts efficiently. On the plant side, chemical information associated with herbivory can act as an indirect defence by attracting the natural enemies of the host herbivores. 2. However, mated and virgin females of haplodiploid parasitoids might not necessarily respond to such chemical cues in the same way. Since virgin females can produce only sons, they might refrain from searching for hosts to invest eggs until copulation, in order to produce both sexes. 3. Here, we investigated differential host‐searching behaviours shown by mated and virgin females in the solitary parasitoid wasp, Cotesia vestalis, in response to herbivory‐associated chemical information from cruciferous plants infested by their host larvae, Plutella xylostella. 4. Mated females showed a significantly higher flight preference for host‐infested plants over intact plants, while no preference was observed with virgin females. Mated females also showed more intensive antennal searching and ovipositor probing behaviours to leaf squares with wounds caused by hosts than did virgin females. Furthermore, mated females stayed longer in host patches with higher parasitism rates than virgin females. 5. These results indicate that mating status of C. vestalis females clearly influences their host‐searching behaviour in response to herbivory‐associated chemical information and patch exploitation. Female parasitoids seem to forage for hosts depending on their own physiological condition in a tritrophic system.     

The influence of host plant species on parasitism of pollen beetles (Meligethes spp.) by Phradis morionellus

We investigated the response of the specialist insect predator Oligota kashmirica benefica (Coleoptera: Staphylinidae) to volatiles from lima bean leaves infested with the spider mite Tetranychus urticae (Acari: Tetranychidae), both in a Y-tube olfactometer and in a field in Kyoto, Japan. Adult male and female predators were significantly more attracted to T. urticae-infested leaves than to clean air. Adult male and female predators were not more attracted to uninfested leaves, artificially damaged leaves, or the spider mites and their visible products when compared to clean air. In a field trap experiment, 12 adult predators were caught in three traps containing T. urticae-infested lima bean plants over 13 days, whereas no adult predators were trapped in three traps containing uninfested lima bean plants during the same period. These results showed that O. kashmirica benefica adults responded to herbivore-induced plant volatiles from T. urticae-infested lima bean leaves under both laboratory and field conditions.     

The distribution of herbivores between leaves matches their performance only in the absence of competitors

Few studies have tested how plant quality and the presence of competitors interact in determining how herbivores choose between different leaves within a plant. We investigated this in two herbivorous spider mites sharing tomato plants: Tetranychus urticae, which generally induces plant defenses, and Tetranychus evansi, which suppresses them, creating asymmetrical effects on coinfesting competitors. On uninfested plants, both herbivore species preferred young leaves, coinciding with increased mite performance. On plants with heterospecifics, the mites did not prefer leaves on which they had a better performance. In particular, T. urticae avoided leaves infested with T. evansi, which is in agreement with T. urticae being outcompeted by T. evansi. In contrast, T. evansi did not avoid leaves with the other species, but distributed itself evenly over plants infested with heterospecifics. We hypothesize that this behavior of T. evansi may prevent further spread of T. urticae over the shared plant. Our results indicate that leaf age determines within‐plant distribution of herbivores only in absence of competitors. Moreover, they show that this distribution depends on the order of arrival of competitors and on their effects on each other, with herbivores showing differences in behavior within the plant as a possible response to the outcome of those interactions.     

Yeheb (<Emphasis Type="Italic">Cordeauxia</Emphasis><Emphasis Type="Italic">edulis</Emphasis>) extract deters feeding and oviposition of <Emphasis Type="Italic">Plutella xylostella</Emphasis> and attracts its natural enemy

The effects of yeheb (Cordeauxia edulis Hemsl.) leaf extract on feeding and oviposition by diamondback moth (DBM) (Plutella xylostella L.) and the behavior of DBM parasitoid, Cotesia vestalis (Haliday), were studied. Volatile organic compounds (VOCs) from the headspace of intact and DBM-damaged broccoli plants sprayed with yeheb extracts (YE) were also analyzed. Larval feeding and growth, and oviposition by adult DBM were strongly inhibited by the extract. Cotesia vestalis were attracted to volatile blends from intact or DBM-damaged broccoli plants sprayed with YE over intact plants sprayed with water or methanol. Analyses of VOCs in the headspace of broccoli plants revealed that both intact and DBM-damaged plants sprayed with YE showed remarkable differences in sesquiterpene compounds compared to intact control treatments. These combined negative effects of YE on DBM fitness together with positive effects on the parasitoid show that yeheb is a potential source of compounds for use in integrated pest management to control damage caused by DBM.     

An ecological cost of plant defence: attractiveness of bitter cucumber plants to natural enemies of herbivores

Plants produce defences that act directly on herbivores and indirectly via the attraction of natural enemies of herbivores. We examined the pleiotropic effects of direct chemical defence production on indirect defence employing near‐isogenic varieties of cucumber plants (Cucumis sativus) that differ qualitatively in the production of terpenoid cucurbitacins, the most bitter compounds known. In release–recapture experiments conducted in greenhouse common gardens, blind predatory mites were attracted to plants infested by herbivorous mites. Infested sweet plants (lacking cucurbitacins), however, attracted 37% more predatory mites than infested bitter plants (that produce constitutive and inducible cucurbitacins). Analysis of the headspace of plants revealed that production of cucurbitacins was genetically correlated with large increases in the qualitative and quantitative spectrum of volatile compounds produced by plants, including induced production of (E )‐β‐ocimene (3E )‐4,8‐dimethyl‐1,3,7‐nonatriene, (E,E)‐α‐farnesene, and methyl salicylate, all known to be attractants of predators. Nevertheless, plants that produced cucurbitacins were less attractive to predatory mites than plants that lacked cucurbitacins and predators were also half as fecund on these bitter plants. Thus, we provide novel evidence for an ecological trade‐off between direct and indirect plant defence. This cost of defence is mediated by the effects of cucurbitacins on predator fecundity and potentially by the production of volatile compounds that may be repellent to predators.     

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.     

Host plant selection behaviour of Chilo partellus and its implication for effectiveness of a trap crop

Female lepidopterans can display a hierarchy of preference among potential host species, a trait thought to arise from the balance between attractants and deterrents to which the insects respond. Host plant ranking by moths and larvae of Chilo partellus Swinhoe (Lepidoptera: Crambidae), an important pest of cereals in Africa, was investigated, and whether eggs deposited on specific host plants yield larvae of particular host preferences. Trap plants are used in management of this pest. However, any ‘disagreement’ in host ranking between moths and larvae could potentially reduce effectiveness of trap crops as larvae emigrate to the main crop from the parent’s preferred trap plant. We also investigated whether host plant preference is influenced by the diet upon which larvae fed as part of an integrated assessment of the relationship between host plant selection and learning in C. partellus. Five host plants (all Poaceae) were used: maize (Zea mays L.), sorghum (Sorghum bicolor Moench), Napier grass (Pennisetum purpureum Schumach), and two varieties of signal grass [Brachiaria brizantha (A. Rich.) Stapf], viz., local (henceforth signal grass) and improved (‘Mulato’). In multiple choice tests, C. partellus female moths preferentially oviposited on Napier grass, followed by sorghum, maize, and signal grass, and least preferred ‘Mulato’. Larvae however equally orientated and settled on leaf cuts of maize, sorghum, signal grass, and Napier grass, but least preferred ‘Mulato’. Moreover, eggs from specific host plants did not yield larvae of particular host preferences. Furthermore, oviposition preference was not altered by the larval food. These results imply only a slight ‘disagreement’ in host ranking behaviour between moths and larvae, which is beneficial for trap cropping as larvae would not ‘reject’ the trap plant and appreciably disperse to the neighboring plants. Moreover, absence of larval learning behaviour indicates that regardless of the larval food C. partellus moths would still be attracted to the selected trap plant.     

Parasitism rates and sex ratios of a parasitoid wasp: effects of herbivore and plant quality

Summary We studied interactions among collards, Brassica oleracea var. acephala, the diamondback moth (DBM), Plutella xylostella (Lepidoptera: Yponomeutidae) and its parasitoid Diadegma insulare (Hymenoptera: Ichneumonidae) by manipulating plant nitrogen (N) concentrations in field and laboratory experiments. Parasitoid abundance strongly reflected DBM abundance and was related to total leaf N. Parasitism rates were high (70.7%) and density-independent. Wasp sex ratios varied markedly (3–93% female) in response to the herbivores, the plants, or both. Higher proportions of female wasps emerged from DBM larvae on plants with high leaf N than on unfertilized plants. More female wasps also emerged from larvae parasitized as larger instars. We suggest that wasps have the potential to control DBM populations through long-term numerical responses mediated by variable sex ratios.     

Olfactory responses of Cotesia flavipes (Hymenoptera: Braconidae) to target and non-target Lepidoptera and their host plants

The attraction of Cotesia flavipes Cameron to volatiles from a range of non-target lepidopteran larvae and their host plants (grasses and trees) or food substrate (honeycomb) was evaluated using a Y-tube olfactometer. The non-target host larvae used in the study included Galleria mellonella (L.), Charaxes cithaeron Felder, Bombyx mori L., and Eldana saccharina Walker. The target insects, Chilo partellus (Swinhoe) and Chilo orichalcociliellus (Strand), were used as controls. Host plants included Afzelia quanzensis Welw., Morus alba L., Cyperus papyrus L., Pennisetum purpureum Schumach, and Zea mays L. The response of C. flavipes to volatiles from the non-target larvae and their food was variable. Attraction to uninfested maize was not significantly different from uninfested plants of non-target hosts or honeycomb. Only maize and honeycomb were preferred over clean air. C. partellus infested maize plants were significantly more attractive than M. alba, A. quanzensis, and honeycomb infested with their herbivores. Infested maize and C. papyrus were more attractive than uninfested ones. When odors from naked larvae were tested, C. flavipes preferred odors from C. partellus larvae over those of E. saccharina and C. cithaeron and larvae of C. partellus and G. mellonella were preferred to clean air. The implications of these findings for biological control and its effect on non-target organisms are discussed.