Non-host plant extracts reduce oviposition of Plutella xylostella (Lepidoptera: Plutellidae) and enhance parasitism by its parasitoid Cotesia plutellae (Hymenoptera: Braconidae)

Botanical preparations, usually from non-host plants, can be used to manipulate the behaviour of insect pests and their natural enemies. In this study, the effects of extracts of Chrysanthemum morifolium, a non-host plant of the diamondback moth, Plutella xylostella (Linnaeus), on the olfactory and oviposition responses of this phytophagous insect and on levels of parasitism by its specialist parasitoid Cotesia plutellae (Kurdjumov) were examined, using Chinese cabbage Brassica campestris L. ssp. pekinensis as the test host plant. Olfactometer tests showed that volatiles of chrysanthemum extract-treated host plants were less attractive to P. xylostella females than those from untreated host plants; and in contrast, volatiles of the chrysanthemum extract-treated host plants were more attractive to females of its parasitoid C. plutellae than those from untreated host plants. Oviposition preference tests showed that P. xylostella females laid only a small proportion of their eggs on chrysanthemum extract-treated host plants, while ovipositing parasitoid females parasitized a much higher proportion of host larvae feeding on the treated host plants than on untreated host plants. These results suggest that certain non-host plant compounds, when applied onto a host plant, may render the plant less attractive to a phytophagous insect but more attractive to its parasitoids. Application of such non-host plant compounds can be explored to develop push-pull systems to reduce oviposition by a pest insect and at the same time enhance parasitism by its parasitoids in crops.     

Effects of specialist parasitoids on oviposition preference of phytophagous insects: encounter–dilution effects in a tritrophic interaction

Abstract.  1. Host plant preferences of the female diamondback moth Plutella xylostella were studied.
2. Female moths preferred conspecific-damaged cabbage plants over undamaged cabbage plants. The performance of P. xylostella larvae on conspecific-infested plants did not differ significantly from that of larvae on undamaged plants.
3.  Cotesia plutellae , the specialist parasitoid wasp of P. xylostella larvae, displayed equal preference for plants with differing levels of host-larvae damage, and the wasp attacked only one or two hosts on average before leaving an infested plant, irrespective of the number of hosts on the plant. It is hypothesised that the oviposition preferences of P. xylostella females for host plants already damaged by conspecific larvae demonstrate an encounter–dilution effect against C. plutellae .     

Body size as an indicator of parasitoid quality in male and female Anagyrus kamali (Hymenoptera: Encyrtidae)

The parasitoid Anagyrus kamali Moursi was recently introduced into the Caribbean as a biological control agent against the hibiscus mealybug, Maconellicoccus hirsutus Green. In the laboratory, parasitoid size, as measured by left hind tibia length, was positively correlated with several indicators of the parasitoid's fitness: longevity, mating preference, fecundity, reproductive longevity, progeny emergence and sex-ratio. When fed ad libidum with honey drops, large male parasitoids lived significantly longer (29.1 +/- 6.5 days) than small ones (18.4 +/- 5.7 days). Large females also lived significantly longer (35.4 +/- 10 days) than small females (27.9 +/- 9.6 days). Females showed no significant mating preference between large and small males. Lifetime fecundity was positively correlated with the size of adult females and ranged from 37 +/- 21 eggs for small females to 96 +/- 43 eggs for large ones. The reproductive longevity, daily oviposition rate, and number of progeny were also higher among large parasitoids. The sex ratio of progeny from small female parasitoids was higher (0.76 +/- 0.24) than that of large individuals (0.47 +/- 0.18).     

The role of volatiles from cruciferous plants and pre-flight experience in the foraging behaviour of the specialist parasitoid Cotesia plutellae

The braconid Cotesia plutellae is an important larval parasitoid of the diamondback moth (Plutella xylostella), a major pest of crucifers in the tropics and sub-tropics. The in-flight searching behaviour of C. plutellae was investigated in a wind tunnel and the close-range attack behaviour observed in cages. The relative importance of volatile stimuli emanating from the plant-host-complex, oilseed rape (Brassica napus) – P. xylostella, in the long-range attraction of C. plutellae was investigated. Plants that were mechanically damaged, or damaged by P. xylostella larvae, were attractive to the parasitoid. Host-damaged leaves remained attractive to the parasitoid after removal of the host larvae. These results indicate that C. plutellae predominantly uses plant derived stimuli in its in-flight searching behaviour. An oviposition experience or contact with a host-damaged leaf prior to the bioassay significantly increased the response to these volatile cues. The foraging behaviour of C. plutellae is compared with other braconid larval parasitoids attacking lepidopteran hosts on crucifers.     

闭弯尾姬蜂与菜蛾盘绒茧蜂寄生菜蛾幼虫时的种间竞争

在室内25℃下,以菜蛾3龄初幼虫作寄主,研究了菜蛾盘绒茧蜂Cotesia plutellae和半闭弯尾姬蜂Diadegma semiclausum的种间竞争。当寄主供2种蜂同时产卵寄生时,2种蜂各自的寄生率与其单独寄生时无显著差异,合计寄生率比一种蜂单独存在时有所提高,但差异不显著。2种蜂均能产卵寄生已被另一种蜂寄生了的寄主幼虫。当寄主被2种蜂寄生的间隔时间很短（少于10 h）时,所育出的蜂绝大部分（80%以上）为绒茧蜂;当寄主先被绒茧蜂寄生,并饲养2天以上再供弯尾姬蜂寄生时,所育出的全为绒茧蜂;当寄主先被弯尾姬蜂寄生,并饲养2天以上再供绒茧蜂寄生时,寄主幼虫绝大部分不能存活,只有少部分能育出寄生蜂,且多为弯尾姬蜂。当2种蜂的幼虫存在于同一寄主体内时,2种蜂的发育均受到另一种蜂的抑制;绒茧蜂1龄幼虫具有物理攻击能力,能将弯尾姬蜂卵或幼虫致死。这些结果表明,菜蛾盘绒茧蜂与半闭弯尾姬蜂在同一寄主中发育时,前者具有明显的竞争优势。     

Interspecific competition between Diadegma semiclausum Hellen (Hym., Ichneumonidae) and Cotesia plutellae (Kurdjumov) (Hym., Braconidae) in parasitizing Plutella xylostella (L.) (Lep., Plutellidea)

Abstract:  Interspecific competition between Diadegma semiclausum and Cotesia plutellae was investigated at 25°C in the laboratory, by exposing the third instar larvae of the diamondback moth, Plutella xylostella to both species together, either species alone or by exposing the host larvae already parasitized by one species, at different intervals, to the other. When host larvae were exposed simultaneously to two species in one arena, parasitism rates of the host by each species were not reduced by the presence of the other species; joint parasitism rate by two species was not significantly higher than that by either parasitoid alone. Both parasitoids could lay eggs into the host larvae which had previously been parasitized by the other species, leading to the occurrence of multiparasitized hosts. When host larvae were parasitized first by D. semiclausum and then being followed within 1–2 h by exposing to C. plutellae , or vice versa, ensuing parasitoid cocoons from the multiparasitized host larvae were nearly all C. plutellae . When host larvae were parasitized initially by C. plutellae and then being followed by D. semiclausum two or more days later, all parasitoids ensued from the multiparasitized hosts were C. plutellae . In contrast, when host larvae were parasitized initially by D. semiclausum and then being followed by C. plutellae two or more days later, most host larvae could not survive to prepupae and most of the ensuing parasitoid adults from the surviving hosts were D. semiclausum . Dissections of host larvae at various time intervals after parasitization by the two parasitoids showed that development of both parasitoids in multiparasitized hosts were somewhat arrested, and that the first instar larvae of C. plutellae could initiate a physical attack on the larvae of D. semiclausum and remove the latter.     

Reproductive attributes of the eulophid Oomyzus sokolowskii,a biological control agent of diamondback moth,Plutella xylostella (Lepidoptera: Plutellidae)

Oomyzus sokolowskii (Kurdjumov) is a gregarious larval-pupal parasitoid of the diamondback moth, Plutella xylostella L. Under laboratory conditions, we explored the reproductive attributes including the functional response of O. sokolowskii and the impact of mutual interference among female parasitoids on progeny production and progeny sex ratio. In the functional response experiment, groups of 1–50 late instar larvae were exposed to groups of five female parasitoids for 48 h. Both Holling's disc equation (type II response) and Williams and Martinez's model (type III response) fit significantly better than the null-model assuming density-independent parasitation efficiency. Williams and Martinez's model was only marginally better than Holling's disc equation which explained 82% of the variation in the number of parasitized hosts. According to Holling's disc equation, the estimated maximum number of hosts parasitised during 48 h was 17 per group of five parasitoids, the estimated attack efficiency of a single female parasitoid (a) was 0.0024 larvae per hour, and the estimated handling time of a single female (T _h) was 14.4 h per host. In the mutual interference experiment 50 host larvae were exposed to different numbers of female parasitoids (P=5, 10, 20, 30, 40 and 50) for 48 h. With increasing numbers of conspecifics, the proportion of male progeny increased and total number of progeny per parasitoid decreased, yielding an estimated mutual interference constant (m) (±SE) of 0.95±0.14. The total number of wasps emerging from individual host pupae increased with increasing number of conspecifics. The age-specific fecundity of O. sokolowskii was investigated by providing 10 host larvae daily to individual female parasitoids. After 3 days of oviposition, 90% of the females were still alive and had achieved 76.4% of their lifetime fecundity. The proportion of female progeny decreased with female age. No relationship was found between female longevity and lifetime fecundity. The results suggest that parasitoid and host densities influence the progeny production and sex allocation strategy of O. sokolowskii. The information obtained from this study would help to develop a mass-rearing protocol for O. sokolowskii.     

Differential parasitism of Plutella xylostella (Lepidoptera: Plutellidae) larvae by the parasitoid Cotesia plutellae (Hymenoptera: Braconidae) on two host plant species

Laboratory experiments were conducted to examine host selection by Cotesia plutellae Kurdjumov when larvae of its host, Plutella xylostella (Linnaeus), fed on Chinese cabbage, Brassica campestris L. ssp. pekinensis and those fed on common cabbage, Brassica oleracea L. var. capitata were provided simultaneously, and to investigate the roles of plant and host volatiles in mediating host selection. When C. plutellae were provided with equal numbers of host larvae on plants of the two species in one arena, the parasitoid parasitized 4- to 15-fold more host larvae on Chinese cabbage than on common cabbage. This preference changed little with host density. However, an experience of searching coupled with an oviposition in a host larva on a leaf of the less-preferred plant, common cabbage, significantly increased the preference for parasitizing host larvae on this plant and resulted in twice as many host larvae parasitized on this plant than on Chinese cabbage. Dual choice tests with a Y-tube olfactometer showed that plant volatiles from Chinese cabbage were more attractive to female C. plutellae than those from common cabbage when plants of both species were either intact or infested. In parallel to the increased parasitism on common cabbage following experience, oviposition in a host larva on this less-preferred plant significantly increased the response to volatiles emanating from that plant. These results indicate that host plants may strongly influence the foraging behaviour of C. plutellae, but their differential attractiveness to the parasitoid may be altered by experience of the parasitoid.     

Mutual Interference Among Female Anagyrus kamali Moursi (Hymenoptera: Encyrtidae) and its Impact on Fecundity, Progeny Production and Sex Ratio

The solitary endoparasitoid Anagyrus kamali Moursi was introduced to the Caribbean to control populations of Hibiscus Mealybug (HMB) Maconellicoccus hirsutus Green. As part of a biological control programme, mass rearing of A. kamali should produce a maximum of good quality female wasps, because only female parasitoids attack the mealybug. In laboratory experiments conducted at 27 +/- 2°C, mutual interference between female parasitoids on the total oviposition, progeny production and sex ratio was assessed at five parasitoid densities: i.e. 1, 2, 5, 10 and 20 female wasps per 50 mealybugs. Oviposition rates of females decreased with increasing female density, without significantly affecting the sex ratio (average across densities of 0.49 +/- 0.322) (male/female). Under mass-rearing conditions, an increase of the female density from 25 to 75 individuals per cage resulted in an increase of the progeny production from 266 +/- 70.1 to 877.5 +/- 393.3 parasitoids. From 75 to 100 females released per cages, the progeny production was not significantly different with 877.5 +/- 393.3 and 965.3 +/- 608.3 parasitoids produced, respectively. Regardless of the female density, progeny sex ratio remained stable at an average of 0.45 +/- 0.095. As a result, the best efficiency of the mass-production system was obtained at a density of 75 females per cage. However, parasitoid production at this density was inferior to singly caged females by about 50%.     

Conflicts between a fungal entomopathogen, Zoophthora radicans, and two larval parasitoids of the diamondback moth

Zoophthora radicans (Zygomycetes: Entomophthorales), Diadegma semiclausum (Hymenoptera: Ichneumonidae), and Cotesia plutellae (Hymenoptera: Braconidae) are all natural enemies of the diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae). Adult C. plutellae are not susceptible to Z. radicans infection but the pathogen can infect and kill adult D. semiclausum. Infection of adult D. semiclausum prior to exposure to P. xylostella host larvae significantly reduced the number of parasitoid cocoons subsequently developing from the host larvae. Although Z. radicans infection of P. xylostella larvae prior to parasitism by D. semiclausum or C. plutellae always resulted in the death of the immature parasitoids, neither species discriminated between healthy and Z. radicans-infected host larvae in an oviposition choice experiment. However, host larvae recently killed by Z. radicans were always rejected by D. semiclausum but sometimes accepted by C. plutellae. At 20 degrees C, egg to pupa development took 6.7 and 7.8 days for D. semiclausum and C. plutellae, respectively. C. plutellae parasitism significantly increased host instar duration but D. semiclausum parasitism did not. Cadavers of P. xylostella larvae parasitized 1 day prior to fungal infection showed no reduction in Z. radicans conidia yield. However, cadavers of larvae parasitized 3 days prior to fungal infection demonstrated a marked decrease in Z. radicans conidia yield. Z. radicans infection of P. xylostella larvae < or = 4 days after parasitism resulted in 100% parasitoid mortality; thereafter, the reduction in parasitoid cocoon yield decreased as the time between parasitism and initiation of fungal infection increased. The extended duration of the host larval stage induced by C. plutellae parasitism increased the availability of the parasitoid to the pathogen. Estimates of interspecific competition indicated a similar pattern for the interaction between Z. radicans and each species of parasitoid.     

Daily progeny production and thermal influence on development and adult longevity of the leafminer parasitoid, Gronotoma micromorpha (Hym., Eucoilidae)

Abstract:  To evaluate the potential of the parasitoid Gronotoma micromorpha (Perkins) as a biological control agent for the serpentine leafminer Liriomyza trifolii (Burgess) in greenhouses, we determined daily progeny production and thermal influence on both total development (oviposition to adult emergence) and adult longevity of this wasp in the laboratory in the host L. trifolii . The mean total number of progeny produced was 75.6 per female at 25°C under a 15-h light/9-h dark (15L : 9D) photoregime. The developmental rate of G. micromorpha significantly correlated with constant temperature between 18 and 30°C. The lower thermal threshold and thermal constant for total development of this wasp were 11.7°C and 333.3 heat degree-days, respectively. The results indicate that adult female G. micromorpha do not oviposit at 15°C and that the lower threshold for oviposition is approximately 18°C. In heated greenhouses in Japan, leafminers and their parasitoids are exposed to both high temperatures and a short photoregime from late autumn to winter, but these conditions (25°C, 10L : 14D) did not significantly affect the total development of G. micromorpha . When wasps were not exposed to hosts, the mean adult longevity at 15 or 20°C was significantly longer than that at 25°C. When wasps were allowed to oviposit into hosts at 25°C, the mean adult longevity was significantly longer than when wasps were not exposed to hosts. As adult female G. micromorpha do not feed on hosts and are pro-ovigenic, suppression of oviposition probably decreased adult longevity. The biological traits of G. micromorpha elucidated by this and previous studies indicate that this wasp is suitable as a biological control agent for L. trifolii in greenhouses.     

Regulation of host diapause by an insect parasitoid

Abstract. 1. The interaction between larval development and parasitism by the braconid wasp Cotesia koebelei (Riley), was investigated in a population of the butterfly Euphydryas editha (Boisduval) (Nymphalidae). In this population, the butterfly host has an obligatory overwintering larval diapause.
2. It was found that E. editha larvae harbouring parasitoids were more likely to pass through an extra feeding instar before entering diapause than were non-parasitized conspecifics.
3. In addition, some individuals that were experimentally exposed to multiple parasitoid attacks bypassed diapause completely; these larvae passed through five or six feeding instars, reaching sizes typical of final instar post-diapause larvae.
4. The observed effect of superparasitism occurred regardless of whether the host larvae subsequently produced mature parasitoids, suggesting that parasitoid attack is sufficient to invoke the response.
5. It is proposed that the parasitoid C.koebelei regulates the number of pre-diapause feeding instars of its insect host E. editha, and that some component of the female venom, injected at oviposition, is responsible for this regulation.     

Biological aspects of Cirrospilus neotropicus Diez & Fidalgo (Hymenoptera: Eulophidae), parasitoid of Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae)

The biology of Cirrospilus neotropicus Diez & Fidalgo reared on third instar Phyllocnistis citrella Stainton larvae having Citrus limonia Osbeck as host plant, was evaluated under controlled conditions (25 +/- 1 masculineC; 12h photophase). The survival, immatures development, longevity, sex ratio, host feeding and oviposition ratio of C. neotropicus, were registered. Two groups of parasitoid females were evaluated: one, with parasitoids obtained from P. citrella pupae collected in citrus orchards and the other group came from laboratory rearing. These females, after mating, were maintained individually on gerbox containers with honey and pollen as food source. At each 48h, 12 P. citrella larvae were changed. The average biological cycle of the female progenies from orchard and laboratory generation groups were 11.8 and 11.6 days, respectively. The males progenies had biological cycles of 11.6 and 10.9 days, as well. The shortest immature survival period, for both studied groups was the larval (57.3% - orchard group and 57.4% - lab group). The mated females average longevity (21.8 days) was superior than the non mated ones (9.1 days) for the orchard group. The average daily oviposition rate for this last group was also significantly superior (2.8 eggs /day) than the lab group (1.6 eggs / day). The same trend was observed for the average daily host feeding rate (1.7 larvae /day for the orchard group and 0.9 larvae/ day for the lab group). These data suggest that C. neotropicus have potencial as a biological control agent of P. citrella.     

Parasitism-induced Effects on Host Growth and Metabolic Efficiency in Tobacco Hornworm Larvae Parasitized by Cotesia congregata

Parasitism by the braconid wasp Cotesia congregata affects the growth of Manduca sexta larvae in a parasitoid 'dose-dependent' fashion. Following parasitization of fourth-instar larvae, more heavily parasitized larvae grew larger compared to those containing fewer parasitoids due to an increase in host dry weight. The differences in host mass appeared to arise after oviposition. A 'dose-dependent' enhancement of host dry weight would appear nutritionally beneficial for the parasitoids developing in more 'crowded' hosts. The efficiencies of conversion of ingested and digested food to body mass and the approximate digestibility of the diet ingested by the host caterpillar did not vary significantly with clutch size although parasitoids took slightly longer to develop in the more heavily parasitized hosts. Larval parasitoids developing in the presence of many competitors weighed up to 50% less than those developing in hosts with fewer endoparasitoids, although the weight of adult female parasitoids did not vary significantly with wasp clutch size. The maximum number of emerging wasps was 200 parasitoids, possibly representing the host's 'carrying capacity' for larvae parasitized in the fourth-instar. The ratio of emerging to non-emerging parasitoids decreased as parasitoid clutch size increased, with few or none emerging from very heavily parasitized hosts containing more than 400 parasitoids. Copyright 1997 Elsevier Science Ltd. All right reserved     

Hymenopteran parasitoids of diamondback moth (Lepidoptera: Ypeunomutidae) in northern Thailand

Larvae of the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Ypeunomutidae), cause severe economic damage to cabbage, Brassica oleracea L. variety capitata (Brassicaceae) and related vegetables in Thailand. Overuse of broad-spectrum insecticides for diamondback moth control is a serious problem and has obscured the contributions of indigenous parasitoids. Our objectives were to identify indigenous diamondback moth parasitoids in northern Thailand and to assess their potential for natural control. Six parasitoid species were reared from diamondback moth larvae and pupae collected in 1990 and in 2003-2004. These included the larval parasitoid Cotesia plutellae Kurdjumov (Braconidae), a larval-pupal parasitoid Macromalon orientale Kerrich (Ichneumonidae), and pupal parasitoids Diadromus collaris Gravenhorst (Ichneumonidae) and Brachymeria excarinata Gahan (Chalcididae). Single specimens of Isotima sp. Forster (Ichneumonidae) and Brachymeria lasus Walker (Chalcididae) also were reared from diamondback moth hosts. C. plutellae was the dominant larval parasitoid and was often reared from host larvae collected from fields sprayed regularly with insecticides; parasitism ranged from 14 to 78%. Average parasitism by M. orientale was only 0.5-6%. Parasitism of host pupae by D. collaris ranged from 9 to 31%, whereas B. excarinata pupal parasitism ranged from 9 to 25%. An integrated pest management (IPM) protocol using simple presence-absence sampling for lepidopterous larvae and the exclusive use of Bacillus thuringiensis (Bt) or neem resulted in the highest yields of undamaged cabbage compared with a control or weekly sprays of cypermethrin (local farmer practice). IPM programs focused on conservation of local diamondback moth parasitoids and on greater implementation of biological control will help alleviate growing public concerns regarding the effects of pesticides on vegetable growers and consumers.     

Determination of the optimal parasitoid‐to‐host ratio for efficient mass‐rearing of the parasitoid,Sclerodermus pupariae (Hymenoptera: Bethylidae)

Sclerodermus pupariae Yang et Yao (Hymenoptera: Bethylidae) is used as a potential biocontrol agent for several buprestid and cerambycid larvae. This study aimed to enhance the efficiency of mass‐rearing of this parasitoid by investigating the fitness gain of this bethylid wasp, including the proportion of successful parasitism and development, brood size, sex ratio, proportion of winged female offspring, body size and longevity of female offspring, under eight different maternal parasitoid density treatments using Thyestilla gebleri Faldermann as host in the laboratory. The results indicated that the foundress densities did not affect the parasitism or emergence rate of this parasitoid. Brood size of the parasitoids increased significantly when the number of maternal wasps ranged from one to four. However, further increases in foundress number did not affect the parasitoid brood size. The sex ratios of S. pupariae were always female‐biased. The proportions of male in the progeny colonies were <10% throughout all experimental treatments. The percentage of winged female progeny was not significantly influenced by the density of adult maternal parasitoids. Body sizes of parasitoids significantly declined with increasing maternal parasitoid densities. Although the parasitoid body size reduced when maternal wasp number was higher, it could be compromised by the relatively higher number of female offspring produced. Further, more than 70% of the parasitoids remained alive when they were stored at 12°C for four months throughout the experiments. These findings suggest that exposure of four female wasps to a single host larva would result in the highest fitness of S. pupariae. Our findings might provide a new approach to enhance the efficiency of mass‐rearing of this bethylid wasp.     

Influence of parasitoid-host density and host availability on the laboratory propagation ofMicroctonus aethiops [Hym.: Braconidae] parasitoid ofHypera postica [Coleop.: Curculionidae]

Laboratory studies of the arrhenotokous braconid wasp,Microctonus aethiops (Nees) were conducted relative to maximizing parasitoid production without adversely affecting sex ratios of the progeny. The influence of hostHypera postica (Gyllenhal) and parasitoid densities on parasitoid productivity was evaluated. Maximum production or a mean 25.8M. aethiops progeny were obtained per female when the host parasitoid ratio was 50∶1. More than one parasitoid per rearing cage resulted in fewer parasitoid progeny per female and increased host mortality. Oviposition significantly increased when female parasitoids were exposed to 25 different weevils every 24 hours. The most intense ovipositional activity usually occurred during the first 48 hours of exposure. However, superparasitism occurred if exposure to 25 hosts continued for more than 24 hours. Continuous and discontinuous oviposition patterns did not affect the numbers of parasitoid progeny produced. However, fewer females were produced when new weevils were provided every 24 hours as in the continuous oviposition group.     

Effects of Bt plants on the development and survival of the parasitoid Cotesia plutellae (Hymenoptera: Braconidae) in susceptible and Bt-resistant larvae of the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae)

A range of crops have been transformed with delta-endotoxin genes from Bacillus thuringiensis (Bt) to produce transgenic plants with high levels of resistance to lepidopteran pests. Parasitoids are important natural enemies of lepidopteran larvae and the effects of Bt plants on these non-target insects have to be investigated to avoid unnecessary disruption of biological control. This study investigated the effects of Cry1Ac-expressing transgenic oilseed rape (Brassica napus) on the solitary braconid endoparasitoid Cotesia plutellae in small-scale laboratory experiments. C. plutellae is an important natural enemy of the diamondback moth (Plutella xylostella), the most important pest of brassica crops world-wide. Bt oilseed rape caused 100% mortality of a Bt-susceptible P. xylostella strain but no mortality of the Bt-resistant P. xylostella strain NO-QA. C. plutellae eggs laid in Bt-susceptible hosts feeding on Bt leaves hatched but premature host mortality did not allow C. plutellae larvae to complete their development. In contrast, C. plutellae developed to maturity in Bt-resistant hosts fed on Bt oilseed rape leaves and there was no effect of Bt plants on percentage parasitism, time to emergence from hosts, time to adult emergence and percentage adult emergence from cocoons. Weights of female progeny after development in Bt-resistant hosts did not differ between plant types but male progeny was significantly heavier on wildtype plants in one of two experiments. The proportion of female progeny was significantly higher on Bt plants in the first experiment with Bt-resistant hosts but this effect was not observed again when the experiment was repeated.     

Interaction of a solitary larval endoparasitoid, Microplitis mediator, with its host, Mamestra brassicae: host acceptance and host suitability

Abstract:  Microplitis mediator (Haliday) (Hym., Braconidae) is an important parasitoid of early instar larvae of the European cabbage moth, Mamestra brassicae L. (Lep., Noctuidae). In the laboratory, we examined attack responses of female M. mediator to the first three larval instars of M. brassicae . Females were presented with M. brassicae larvae either one individual at a time in a no-choice experiment, or three individuals, one from each instar, simultaneously in a choice experiment. Whether or not there was choice, naïve female parasitoids attacked a high proportion of larvae and did not discriminate among instars. In the no-choice experiment, attacked larvae were reared, and parasitoid cocoons were produced from about 76% of larvae attacked as first and second instars, but from only 19% of larvae attacked as third instars. Dissections of attacked larvae from the choice experiment showed that about 79% of attacks on first and second instars resulted in oviposition compared with only 49% for third instars. When given choice, frequency and number of attacks on first instar larvae increased with increasing parasitoid experience. Our results suggest that first and second instar larvae of M. brassicae are suitable hosts for M. mediator , but that third instar larvae are suboptimal both because oviposition attempts were frequently unsuccessful and because immature parasitoids failed to complete development. Nevertheless, naïve attacking parasitoids exhibited minimal discrimination among instars, although experienced parasitoids most frequently attacked first instar larvae. The host selection behaviour of M. mediator is discussed in the context of optimal foraging theory and implications for biological control.     

Larvicidal activity of lectins onLucilia cuprina: mechanism of action

Foraging behaviour and host-instar preference of young and old females of the solitary aphid parasitoid,Lysiphlebus cardui Marshall (Hymenoptera: Aphidiidae), were studied in the laboratory. The analysis of interactions between parasitoids and different stages ofAphis fabae cirsiiacanthoidis Scop. (Homoptera: Aphididae) revealed that encounter rates between aphids and parasitoid females and defence reactions of the aphids influenced the degree to which a particular aphid age class is parasitized. Encounter rates between hosts and parasitoid females depended on the foraging pattern of the parasitoid, which varied with age. In mixed aphid colonies patch residence time increased with parasitoid age. Furthermore, younger parasitoids (≦1 day old) laid more eggs into second and third instars, while older parasitoids (≧4 days old) did not show distinct host instar preferences. It is suggested that the oviposition behaviour ofL. cardui is influenced by the physiological state, i.e. the age of the wasp.