Plant Resistance Affects the Olfactory Response and Parasitism Success of Cotesia vestalis

Understanding the factors influencing host-selection behavior of parasitoids is essential in studies on host-parasitoid ecology and evolution, and in combining sustainable strategies of pest management, such as host-plant resistance and biological control. The effects of host-plant resistance on the olfactory response and parasitism success by Cotesia vestalis, a parasitoid of diamondback moth (Plutella xylostella) larvae were examined. Here, it was demonstrated that host-plant resistance can strongly influence foraging behavior and parasitism success of the parasitoid. In olfactometer experiments, C. vestalis did not differentiate between crucifer plant types with similar levels of susceptibility or resistance to P. xylostella but showed a strong preference for susceptible compared with partially-resistant host plants. The influence of previous oviposition activity varied with the host-plant type experienced by the parasitoid. In cage experiments, C. vestalis preferred to parasitize P. xylostella larvae on a susceptible plant compared with larvae on a partially resistant host plant when exposed to hosts for 24 h. However, this preference appeared to be transitory, and was not found after 96 h exposure. The present study suggests that combining partial host-plant resistance with biological control by C. vestalis for the control of P. xylostella may in some circumstances be antagonistic and negatively affect parasitism success.     

Relative influences of plant type and parasitoid initial density on host–parasitoid relationships in a tritrophic system

To explore the effects of bottom-up and top-down forces on the relationships between a host, Plutella xylostella (L.) (Lepidoptera, Plutellidae), and its parasitoid, Cotesia vestalis (Haliday) (Hymenoptera, Braconidae), a short-term field experiment was established as a factorial experiment using three different host plants (Brassica pekinensis cv. Yuki F1, Brassica oleracea var. capitata cv. Midorimaru F1 and B. oleracea var. botrytis cv. Snow Crown) in the presence of C. vestalis at two different levels (low and high initial release). The tritrophic interactions were monitored by census counts of live adults 20?days after parasitoid release. The mean numbers of P. xylostella and C. vestalis adults were compared using log-linear analysis of deviance. Also, differences in the levels of parasitism were analysed using logistic analysis of deviance. There was a significant effect of host plant type on the abundance of P. xylostella, the abundance of C. vestalis and the percentage parasitism of P. xylostella by C. vestalis. The mean number of P. xylostella adults per cage on common cabbage or cauliflower was significantly greater than that on Chinese cabbage. The mean number of C. vestalis adults and the proportion of hosts attacked by C. vestalis per cage were significantly greater on Chinese cabbage compared with common cabbage or cauliflower. Indeed, initial parasitoid release did not significantly affect the abundance of P. xylostella but there was a significant influence of initial parasitoid release on the abundance of C. vestalis and the levels of parasitism of P. xylostella by C. vestalis. The mean number of C. vestalis adults and the proportion of P. xylostella parasitised by C. vestalis per cage were greater in high level of parasitoid release compared with low level of parasitoid release. However, there were no significant interacting effect of the factors (plant type?×?parasitoid initial abundance) on the abundance of P. xylostella, the population size of C. vestalis and parasitism of P. xylostella by C. vestalis.     

A food-supply device for maintaining Cotesia vestalis,a larval parasitoid of the diamondback moth Plutella xylostella,in greenhouses

The limited availability of sugar sources (e.g., flowers) in greenhouses may affect biological pest control by parasitoid wasps. However, few studies have focused on feeding devices to provide parasitoids with sugar foods. We investigated the accessibility of a yellow-colored bottle-type feeding device to adult Cotesia vestalis (Halliday), a larval endoparasitoid of the diamondback moth Plutella xylostella (L.). All parasitoids died within four days in a room with no sugar source, whereas 66.7 % of individuals survived if a bottle-type feeding device providing honey solution was installed. We also investigated female longevity in response to different sugar solutions presented in a bottle-type feeding device. Honey and sugar mixtures (glucose and fructose) improved female longevity (38.4 and 39.2 days, respectively) much more than water (3.1 days), indicating these feeding devices containing sugar foods to be potentially useful for maintaining C. vestalis in greenhouses where natural food sources are limited.     

Starvation and herbivore-induced plant volatiles affect the color preferences of parasitic wasps

Using light-emitting diode spotlights, we examined the responses of Cotesia vestalis, a parasitoid of diamondback moth (DBM), Plutella xylostella larvae, with different hunger level to different chromatic cues. Naïve satiated female wasps showed no significant preference for either green, yellow, orange, or red spotlighted areas over a control area with background fluorescent light. When starved for 2 h, female wasps preferred yellow and green light over the control area, but not orange or red light. We also tested the effects of DBM-larvae-induced cabbage-plant volatiles, which attract female wasps, on wasp responses to green versus yellow light. In control experiments with no plant volatiles, starved wasps showed no color preference. However, when synthetic volatiles were present, the wasps preferred green over yellow light. We concluded that both hunger level and herbivore-induced plant volatiles were important factors affecting the response of parasitic wasps to light of different color.     

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.     

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.     

菜蛾盘绒茧蜂卵携带的免疫抑制因子

抑制寄主昆虫的免疫反应是内寄生蜂存活的关键。菜蛾盘绒茧蜂Cotesia vestalis（Haliday）寄生小菜蛾Plutella xylostella （L.）幼虫后,蜂卵如何逃避和抑制寄主的免疫攻击,尚未得到全面揭示。本文采用电镜技术系统观察了菜蛾盘绒茧蜂卵表面的超微结构。结果显示：蜂卵表面覆盖有纤维层和絮状的类病毒样纤丝（VLFs）,同时携带了含多分DNA病毒粒子（PDV）的萼液。在寄生初期,包裹在蜂卵表面的纤维层和VLFs首先起到保护蜂卵不被小菜蛾血细胞包囊的被动防御作用。随后,PDV发挥主动的免疫抑制作用。通过假寄生手段,证明了菜蛾盘绒茧蜂PDV (CvBV) 具有较持久的克服寄主免疫攻击的能力,是主要的免疫抑制因子。在假寄生后连续8 d的观察时间内,菜蛾盘绒茧蜂的蜂卵均未被包囊。结果提示,在菜蛾盘绒茧蜂-小菜蛾寄生体系中,菜蛾盘绒茧蜂采取被动防御和主动攻击两种方式应对寄主小菜蛾的免疫攻击。     

Movement of transgenic plant-expressed Bt Cry1Ac proteins through high trophic levels

The movement of Bacillus thuringiensis (Berliner) (Bt) Cry1Ac endotoxin through high trophic levels was assessed to help elucidate the effects of Bt toxin on non‐target insects. The diamondback moth (Plutella xylostella L., Lepidoptera: Plutellidae), the parasitic wasp (Cotesia vestalis Haliday, Hymenoptera: Braconidae) and the predatory green lacewing Chrysoperla carnea (Stephen) (Neuroptera: Chrysopidae) were used as a model system in this laboratory study. Bt‐resistant P. xylostella larvae fed Cry1Ac‐expressing transgenic oilseed rape (OSR, Brassica napus L., Cruciferae), before and after parasitization by C. vestalis, consumed Cry1Ac with the ingested plant material but only a proportion of Cry1Ac consumed was recovered from the bodies and faeces of P. xylostella larvae. Cry1Ac was not detected in newly emerged parasitoid larvae. In contrast, Cry1Ac was detected in C. carnea larvae fed on resistant P. xylostella larvae reared on Bt OSR. However, no Cry1Ac could be detected in C. carnea larvae when the lacewings were transferred to P. xylostella larvae reared on conventional OSR and tested 24–48 h. The metabolizing ability of Cry1Ac is discussed for the larvae of P. xylostella and C. carnea.     

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.     

Gamma irradiation on canola seeds affects herbivore-plant and host-parasitoid interactions

As an agricultural modernization, gamma irradiation is an important method for enhancing crop yield and quality. Nevertheless, its use can alter other plant traits such as nutrition and resistance to different biotic/abiotic stresses that consequently affect plant–insect interactions. A tritrophic system was utilized based on two canola mutant lines produced through gamma irradiation (RGS 8–1 and Talaye 8–3). Plutella xylostella (L.), as a worldwide pest of Brassicaceae and Cotesia vestalis (Holiday) as a key biocontrol agent of P. xylostella were examined for the potential indirect effects of canola seed irradiation on the experimental insects’ performance when acting on the respective mutant lines. This study showed that physical mutation did not affect plant nitrogen and herbivore-damaged total phenolics; however, phenolic compounds showed greater concentration in damaged leaves than undamaged leaves of both mutant and control plants. The relative growth rate and pupal weight of P. xylostella reared on RGS 8–1 were significantly higher than those reared on the control RGS. There was no significant difference by performance parameters of the parasitoid, C. vestalis, including total pre-oviposition period, adult longevity, adult fresh body weight of males and females, pupal weight, forewing area, and total longevity of both sexes on tested canola cultivars in comparison with their mutant lines. Life table parameters of C. vestalis on mutant lines of both cultivars, RGS and Talaye, were not significantly different from their control treatments. Comprehensive studies should be conducted to find out the mechanisms under which gamma rays affect plant–insect interactions.     

Fitness parameters of Plutella xylostella (L.) (Lepidoptera; Plutellidae) at four constant temperatures by using age-stage,two-sex life tables

Different temperature zones have significant impact on the population dynamics of Plutella xylostella. Effective management of P. xylostella requires the knowledge of temperature tolerance by different life stages. In the current study, fitness parameters of diamondback moth were reported by using age-stage, two-sex life table traits at four constant temperatures (15, 20, 25 and 30 °C). The life cycle of P. xylostella was significantly longer at 15 °C. The 20 °C level of temperature was found optimal for fecundity, gross reproductive rate (51.74 offspring) and net reproductive rate (44.35 offspring per individual). The adult pre-oviposition period was statistically at par at all four level of temperatures. However, the survival was maximum at 20 °C as compared to other three temperature ranges. Based on the current study, it was concluded that temperature has a great role in population build-up of P. xylostella and effective management tactics should be applied to prevent significant damage to cabbage and other cruciferous crops when the temperature in the field is near 20 °C.     

Phenology of Parasetigena silvestris (Diptera: Tachinidae), gypsy moth (Lymantria dispar) (Lepidoptera: Lymantriidae) larval parasitoid and its efficiency for parasitisation

Parasetigena silvestris is a univoltine, solitary, larval endoparasitoid which lays its eggs on the surface of gypsy moth larvae. Field collection of the host larvae (2nd through 5th instar) from an artificially established gypsy moth population were made to compare stage specific parasitism between larvae without and with P. silvestris tachinid eggs. The tachinid oviposition rate detected was highest in second instar larvae, and then decreased as larvae developed toward full maturity. The opposite was true for tachinid parasitoid emergence which had no emergence from second through third host instar larvae. Fourth instar gypsy moth larvae, however, experienced significantly higher parasitism by P. silvestris in the larvae with eggs than those without the eggs. The braconid wasp Cotesia melanoscelus caused significantly higher parasitism in early instar larvae with P. silvestris eggs than in those without the eggs. The tachinid prefers to lay more eggs on parasitised larvae by the braconid even though the braconid is a superior competitor to the fly during multiparasitism. Factors influencing parasitism rates by P. silvestris such as host-parasitoid synchronisation and the multiparasitism interaction with C. melanoscelus are discussed.     

Predicting the impacts of climate change on the biological control of Plutella xylostella by Diadegma semiclausum

1. The parasitoid Diadegma semiclausum Hellén (Hymenoptera: Ichneumonidae) has been widely adopted as a biological control agent for Plutella xylostella L. (Lepidoptera: Plutellidae) over the last 80 years. Earlier studies have found differential responses to temperature between the host and its parasitoid and demonstrated the multiple ways in which the parasitoid is more susceptible to elevated temperatures.
2. Using data from experimental studies, the modelling package CLIMEX was used to investigate the suitability of current climates for the host and its parasitoid and the effects on their potential global geographical distributions. The study was then extended to investigate possible changes to these distributions that might result under different climate change scenarios by 2080. The models predict that the global distributions of both the host and parasitoid will be reduced. These changes will not be proportionate and many areas in tropical, sub-tropical, and temperate regions that are currently suitable for D. semiclausum are predicted to become unsuitable for the parasitoid, whereas retaining suitability for P. xylostella. The seasonal dynamics of both the host and parasitoid are also predicted to be significantly reshaped under climate change.
3. Analysis of associations between annual P. xylostella outbreaks and weather conditions in three provinces in China with field data collected between 1995 and 2017 indicated significant effects of temperature on P. xylostella outbreaks at the beginning of the peak season in warmer provinces where P. xylostella can overwinter; such associations were not found in the colder provinces where it is unable to survive harsh winters.

     

Investigation on flight and walking activity of Oomyzus sokolowskii Kurdjumov (Hymenoptera: Eulophidae) at different generations under laboratory conditions

The flight and walking activities of second, third, fourth, fifth, sixth and seventh generations of Oomyzus sokolowskii reared on Plutella xylostella L. pupas were compared under laboratory conditions. The parasitoid wasps were obtained by collecting pupa masses of diamondback moth from cabbage fields. The experiments of flight and walking activities were carried out using black PVC tubes and the percentages of inactive, flying and walking wasps to light were calculated. Results showed a reduced flight and walking activities and increased percentage of inactive wasps with the increasing number of generations. In second- third- and fourth-generation average, 52.1 ± 1.27 and 29.6 ± 1.51 of population had flight and walking ability, respectively, and 17.1 ± 1.55 of them were not able to fly and walk. The highest percentage of inactive wasps was observed at seventh generation, whereas wasps showed significantly better flight and walking activities at second, third and fourth generations. Based on the results, flight and walking abilities of O. sokolowskii gradually decreased at continuous laboratory rearing.     

Offering honey containing a selective insecticide as food for pests and parasitoids: another effective use

We investigated the effects of the presence of a selective insecticide, pyridalyl, in aqueous solutions of honey as food for adults of diamondback moth (DBM) Plutella xylostella (L.) and its larval parasitoid Cotesia vestalis (Halliday) on their performances. We used a commercial formulation of pyridalyl which contained 10% pyridalyl. Survival times of DBMs reared with honey solution with pyridalyl at 10 000‐fold dilution were not significantly different from those of DBMs reared with pure honey solution. However, at 1000‐fold and 100‐fold dilutions of pyridalyl in honey solution, survival times were significantly shorter than those with honey solution alone. By contrast, survival times of C. vestalis reared with honey solution with pyridalyl at 1000‐fold and 100‐fold dilution were not significantly different from those of C. vestalis reared with pure honey solution. Offering honey solution with pyridalyl at 100‐fold dilution to C. vestalis did not affect its parasitization ability or offspring sex ratio. The novel aspects of the use of selective pesticides to control DBMs using C. vestalis are discussed.     

Tritrophic Choice Experiments with Bt Plants,the Diamondback Moth (Plutella xylostella) and the parasitoid Cotesia plutellae

Parasitoids are important natural enemies of many pest species and are used extensively in biological and integrated control programmes. Crop plants transformed to express toxin genes derived from Bacillus thuringiensis (Bt) provide high levels of resistance to certain pest species, which is likely to have consequent effects on parasitoids specialising on such pests. A better understanding of the interaction between transgenic plants, pests and parasitoids is important to limit disruption of biological control and to provide background knowledge essential for implementing measures for the conservation of parasitoid populations. It is also essential for investigations into the potential role of parasitoids in delaying the build-up of Bt-resistant pest populations. The diamondback moth (Plutella xylostella), a major pest of brassica crops, is normally highly susceptible to a range of Bt toxins. However, extensive use of microbial Bt sprays has led to the selection of resistance to Bt toxins in P. xylostella. Cotesia plutellae is an important endoparasitoid of P. xylostella larvae. Although unable to survive in Bt-susceptible P. xylostella larvae on highly resistant Bt oilseed rape plants due to premature host mortality, C. plutellae is able to complete its larval development in Bt-resistant P. xylostella larvae. Experiments of parasitoid flight and foraging behaviour presented in this paper showed that adult C. plutellae females do not distinguish between Bt and wildtype oilseed rape plants, and are more attracted to Bt plants damaged by Bt-resistant hosts than by susceptible hosts. This stronger attraction to Bt plants damaged by resistant hosts was due to more extensive feeding damage. Population scale experiments with mixtures of Bt and wildtype plants demonstrated that the parasitoid is as effective in controlling Bt-resistant P. xylostella larvae on Bt plants as on wildtype plants. In these experiments equal or higher numbers of parasitoid adults emerged per transgenic as per wildtype plant. The implications for integrated pest management and the evolution of resistance to Bt in P. xylostella are discussed.     

Organic and conventional fertilizer effects on a tritrophic interaction: parasitism,performance and preference of Cotesia vestalis

Interest in sustainable farming methods that rely on alternatives to conventional synthetic fertilizers and pesticides is increasing. Sustainable farming methods often utilize natural populations of predatory and parasitic species to control populations of herbivores, which may be potential pest species. We investigated the effects of several types of fertilizer, including those typical of sustainable and conventional farming systems, on the interaction between a herbivore and parasitoid. The effects of fertilizer type on percentage parasitism, parasitoid performance, parasitoid attack behaviour and responses to plant volatiles were examined using a model Brassica system, consisting of Brassica oleracea var capitata, Plutella xylostella (Lepidoptera) larvae and Cotesia vestalis (parasitoid). Percentage parasitism was greatest for P. xylostella larvae feeding on plants that had received either a synthetic ammonium nitrate fertilizer or were unfertilized, in comparison to those receiving a composite fertilizer containing hoof and horn. Parasitism was intermediate on plants fertilized with an organically produced animal manure. Male parasitoid tibia length showed the same pattern as percentage parasitism, an indication that offspring performance was maximized on the treatments preferred by female parasitoids for oviposition. Percentage parasitism and parasitoid size were not correlated with foliar nitrogen concentration. The parasitoids did not discriminate between hosts feeding on plants in the four fertilizer treatments in parasitoid behaviour assays, but showed a preference for unfertilized plants in olfactometer experiments. The percentage parasitism and tibia length results provide support for the preference–performance hypothesis.     

Efficacy of entomopathogenic nematode,Steinernema carpocapsae against the diamondback moth,Plutella xylostella (L.) in laboratory condition

In vitro studies were carried out on the diamondback moth, Plutella xylostella larvae using an insect entomopathogenic nematode isolate, Steinernema carpocapsae obtained from the Koppert company, the Netherlands. Larvae of P. xylostella were collected from cabbage farms around Mashhad city of Iran. During the study, the responses of larvae at 25?°C for three periods of 24, 48 and 72?h with different concentrations of 0, 5, 10, 20, 40, 80, 160 and 320 third instar larvae of nematode (infective stage?=?IJs) per insect into 10?cm Petri dishes containing filter paper soaked with 1?ml of nematodes suspension were compared. Maximum mortality caused by S. carpocapsae nematode was 88% at 24?h, and it was 100% at 48 and 72 h. With increasing nematode population level and exposure time (ET in hour), mortality of P. xylostella larvae was increased. Based on probit analysis, LC₅₀ values of S. carpocapsae nematode in three test periods were 45.61, 12.02 and 40.80 IJs per insect, respectively. Initial ANOVA was performed for S. carpocapsae nematode. The effect of both nematode population levels (IJ) and ET on third instar larvae of the diamondback moth, P. xylostella and interaction between IJ and ET were significant. In general, it is recommended to apply this nematode in suitable condition for controlling diamondback moth.     

Genome size estimation of an endoparasitoid wasp,Cotesia plutellae,using quantitative real-time polymerase chain reaction

An endoparasitoid wasp, Cotesia plutellae, is a natural enemy against the diamondback moth, Plutella xylostella, which is the most destructive insect pest of cruciferous crop plants. The wasp genome contains genetic information of several parasitic factors, such as its symbiotic virus (C. plutellae bracovirus), venom, teratocyte as well as the parasitoid itself. These parasitic factors interfere with physiological processes of the immature stages of P. xylostella and need to be analyzed concerning their genetic components. A full genome sequence of C. plutellae would be highly informative to determine functional genes associated with these parasitic factors. Before a full genome sequence analysis of C. plutellae can be undertaken, an estimate of genome size is needed. In this study, we used a strategy using a quantitative real-time polymerase chain reaction (qPCR) to measure the wasp genome size. qPCR determined the number of a single copy gene hexokinase in a total mass of genomic DNA. The resulting molecular weight of the DNA sample was used to calculate the genome size in base pair (bp). This estimation approach indicated a genome size of C. plutellae of 186.06 ± 1.21 Mb.