Seasonal abundance of the parasitoid complex associated with the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae) in Hangzhou, China

An investigation of insect parasitoids of the diamondback moth, Plutella xylostella (Linnaeus), in brassica vegetable crops in the suburbs of Hangzhou was conducted during five periods from 1989 to 1997. Eight species of primary parasitoids were recorded: Trichogramma chilonis Ishii, Cotesia plutellae Kurdjumov, Microplitis sp., Oomyzus sokolowskii Kurdjumov, Diadromus collaris (Gravenhorst), Itoplectis naranyae (Ashmead), Exochus sp. and Brachymeria excarinata Gahan. Seven species of hyperparasitoids were also collected. Rates of parasitism of eggs of P. xylostella were usually very low. However, rates of parasitism of larvae and pupae were substantial and showed two peaks each year, around June-July and September-November respectively. Rates of parasitism during peaks were usually 10-60% and reached over 80% on a few occasions. Cotesia plutellae, O. sokolowskii and D. collaris were the major larval, larval-pupal and pupal parasitoids respectively. In the field, C. plutellae was active throughout the year. Oomyzus sokolowskii was active from May to October, entered a quiescent pupal stage in October-November to overwinter and did not emerge until next April-May. Diadromus collaris was recorded from April to July and October. Rates of parasitism of P. xylostella in radish and mustard fields were usually higher than those in cabbage and Chinese cabbage fields in the same locality. Negative correlations of parasitism rates between C. plutellae and O. sokolowskii indicate a competitive relationship for host larvae between these two larval parasitoids.     

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.     

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.     

Population dynamics of Plutella xylostella (Lep., Yponomeutidae) and its parasitoids in the region of Brasilia

Abstract: The diamond back moth Plutella xylostella (L.) is the most serious pest of Brassicaceae in the world. It is resistant to many insecticides which has led to a search for alternative techniques of control, principally biocontrol-based pest management. The impact of P. xylostella and its parasitoids was analysed in a 6-month study of cabbage crops in the Brasilia region of Brazil, from June to November 1998. The population of the pest and its guild of parasitoids were quantified on two experimental plots at the Embrapa-Hortaliças research station by weekly samplings. In the first cropping (June to early September), populations of P. xylostella followed a logistic growth curve leading to the destruction of the crops. In the second cropping, rains prevented the development of P. xylostella . Seven species of parasitoids were observed, Diadegma leontiniae (Brethes) and Apanteles piceotrichosus (Blanchard) being dominant. Cotesia plutellae (Kurdjumov) and Actia sp., previously more abundant, became very minor parasitoids. Significant differences were observed in specific parasitism rates between the plots. The combined parasitism rates were identical on both plots (around 23%) and were insufficient to regulate the populations of P. xylostella . Six species of hyperparasites were reared from Diadegma leontiniae and Apanteles piceotrichosus , showing a high diversity of natural enemies in this region of recent colonization by P. xylostella .     

Influence of floral resources on sugar feeding and nutrient dynamics of a parasitoid in the field

Abstract.  1. To investigate how floral resources impact feeding and nutrient dynamics of parasitoids, the ichneumonid Diadegma insulare (Cresson) was monitored in cabbage fields with and without buckwheat Fagopyrum esculentum Moench borders. Biochemical analyses of field-collected wasps determined whether sugar feeding occurred, and whether it enhanced nutrient levels and increased parasitism of diamondback larvae Plutella xylostella (L.) from 2000 to 2003. The effects of experimental spatial scale on sugar feeding dynamics were also evaluated in 2001.
2. The majority of D. insulare foraging on cabbage had fed upon sugar, but floral borders did not consistently increase the proportion of sugar-fed wasps or their levels of glycogen and lipid over all 4 years. Floral borders increased sugar storage by females in 2001. More females fed on sugar when experimental plots were separated by at least 800 m compared with 67 m.
3. Feeding on buckwheat nectar vs. honeydew produced by the soybean aphid Aphis glycines Matsumura in surrounding fields was distinguished using the ratio of fructose to total sugars, which was higher in nectar- than in honeydew-fed parasitoids. Floral borders increased the probability that females collected from adjacent crops were nectar-fed in 2002, demonstrating that floral resources were utilised by foraging parasitoids. Diadegma insulare that fed upon any sugar source had higher levels of sugar and glycogen than unfed wasps, and feeding on honeydew appeared to enhance nutrient storage.
4. The prevalence of sugar or nectar feeding by female D. insulare was not correlated with parasitism rates over the 4 years from all plots, but nectar feeding was positively correlated with parasitism rates within floral plots.     

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.     

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.     

Interactions between the Parasitoid Rhopus nigroclavatus (Ashmead) (Hymenoptera: Encyrtidae) and its Mealybug Hosts Tridiscus sporoboli (Cockerell) and Trionymus sp. (Homoptera: Pseudococcidae)

This research investigated age-class-specific parasitism rates of the buffalograss mealybugs Tridiscus sporoboli (Cockerell) and Trionymus sp. by Rhopus nigroclavatus (Ashmead) (Hymenoptera; Encyrtidae), size class preference of this parasitoid, and mealybug–parasitoid interactions through choice and no-choice studies. In the no-choice studies, the mean rates of parasitism by R. nigroclavatus were 45, 20, 0, and 0%, respectively, for mealybugs adult female, third and fourth instars, first and second instars, and eggs. Choice studies indicated that rate of parasitism increased with host size. The mean rates of parasitism on mealybugs in the choice studies were 100% for adult females, 24% for third and fourth instars, 0% for first and second instars, and 0% for eggs. A second set of choice studies investigating mealybug/parasitoid behavior revealed that R. nigroclavatus oviposits in all post-egg mealybug age classes, but first and second instars were less often parasitized than older mealybugs.     

Phenological dynamics of Dasineura mali (Diptera: Cecidomyiidae) and its parasitoid Platygaster demades (Hymenoptera: Platygasteridae) in apple orchards

The midge Dasineura mali Kieffer (Diptera: Cecidomyiidae) is an important pest of apple (Malus domestica Borkh.) and a potential fresh fruit contaminant, causing quarantine concerns. The phenological dynamics of D. mali and its egg parasitoid Platygaster demades Walker (Hymenoptera: Platygasteridae) were studied in the field in Palmerston North, New Zealand, for 2 yr. Both shoot infestation rate by D. mali and D. mali density per shoot sharply increased in the second generation, reaching approximately 65% and 100-200 eggs, respectively. However, although the infestation rate in the third generation remained as high as in the second generation, the pest density per shoot significantly decreased to 40-60 eggs in the third generation. In the fourth generation, both infestation rate and pest density per shoot decreased to approximately 30% and 10 eggs. Due to the simultaneous decline of the apple shoot number and D. mali density in the third and fourth D. mali generations, the absolute number of D. mali in the orchard also has declined proportionally during the same period. The parasitism and superparasitism rates significantly increased as the season progressed, from 45 to 55 and 37% in the first generation to 87 and 82% in the fourth generation, respectively. Our results suggest that P. demades contributes to the continuous decline of D. mali numbers in the field; it is a good searcher, particularly when its hosts become increasingly scarcer over the season, and it avoids overshooting the host population later in the season by increasing superparasitism. The frequency of P. demades aestivation increases from late spring to midsummer and then decreases during the late summer and early autumn. Although the emergence of P. demades was approximately 2 to 3 wk behind that of D. mali in each generation, the increasing parasitism rates from the first to the fourth generations indicate that P. demades is synchronized with D. mali in the field.     

Variation in susceptibility of Diadegma insulare (Hymenoptera: Ichneumonidae) to permethrin

Six field populations of the parasitoid Diadegma insulare (Cresson) were collected from Mexico and the United States and tested for their susceptibility to permethrin by using a glass-vial residue bioassay. Significant differences existed in susceptibility, with one population significantly more susceptible than the other five. Susceptibility of D. insulare increased when they were reared for several generations in the greenhouse in the absence of selection. D. insulare was less suceptible to permethrin than the adult stage of its host, Plutella xylostella (L.), from a laboratory colony, but more susceptible than those hosts collected from a commercial field. Our data suggest that a field population of D. insulare appeared to increase its tolerance to permethrin much more slowly than P. xylostella.     

Effect of insecticides on the diamondback moth (Lepidoptera: Plutellidae) and its parasitoid Diadegma insulare (Hymenoptera: Ichneumonidae)

Studies were conducted to evaluate the toxicity of insecticides to adult Diadegma insulare (Cresson) and its host the diamondback moth, Plutella xylostella (L.). Leaf-dip and direct-dip bioassays for diamondback moth larvae and residual bioassays for adults of diamondback moth and D. insulare were used to assess mortalities. Larval mortalities at field rates were significantly higher with carbaryl, permethrin, spinosad, and tebufenozide when compared with Bacillus thuringiensis, or imidacloprid in the larval-dip bioassay 72 h after treatment. In the leaf-dip and residual bioassays, both permethrin and spinosad caused 100% mortalities to diamondback moth larvae and adults, respectively, 72 h after treatment. Of all the materials tested, only B. thuringiensis and tebufenozide were not toxic to D. insulare 24 h after treatment. Spinosad was not toxic to D. insulare 30 min after treatment. However, 100% mortality was observed 8 h after treatment.     

Suitability of Bemisia tabaci (Hemiptera: Aleyrodidae) instars for the parasitization by Encarsia bimaculata and Eretmocerus sp. nr. furuhashii (Hymenoptera: Aphelinidae) on glabrous and hirsute host plants

The host instar preferences of Encarsia bimaculata and Eretmocerus sp. nr. furuhashii parasitizing Bemisia tabaci and their development on four host plants, collard, eggplant, cucumber and tomato, were studied in the laboratory. Both of the parasitoids accepted all nymphal stages of B. tabaci, but E. bimaculata preferred third and fourth instars while Er. sp. nr. furuhashii preferred second and third instars under both choice and no choice conditions. Regardless of host stage parasitized, adults of parasitoids emerged only from fourth instars. When given the simultaneous choice of all instars, E. bimaculata reduced parasitization of first and second instars (3.73 and 4.76%, respectively) while increasing parasitization of third and fourth instars (5.44 and 6.93%, respectively), in contrast Er. sp. nr. furuhashii increased its parasitization of second and third instar nymphs (1.27 and 3.17%, respectively) and decreased that of first and fourth instars (7.0 and 3.06%, respectively). Host plants did not significantly influence instar preference for either parasitoid. Developmental periods of both the parasitoids from egg to adult emergence were longest when first instars were parasitized and shortest when fourth instars were selected. Parasitoid developmental time was generally shorter on glabrous plants than on hirsute plants.     

Development of the parasitoid,Cotesia rubecula (Hymenoptera: Braconidae) in Pieris rapae and Pieris brassicae (Lepidoptera: Pieridae): evidence for host regulation

Several recent models examining the developmental strategies of parasitoids attacking hosts which continue feeding and growing after parasitism (=koinobiont parasitoids) assume that host quality is a non-linear function of host size at oviposition. We tested this assumption by comparing the growth and development of males of the solitary koinobiont endoparasitoid, Cotesia rubecula, in first (L1) to third (L3) larval instars of its preferred host, Pieris rapae and in a less preferred host, Pieris brassicae. Beginning 3 days after parasitism, hosts were dissected daily, and both host and parasitoid dry mass was determined. Using data on parasitoid dry mass, we measured the mean relative growth rate of C. rubecula, and compared the trajectories of larval growth of the parasitoid during the larval and pupal stages using non-linear equations. Parasitoids generally survived better, completed development faster, and grew larger in earlier than in later instars of both host species, and adult wasps emerging from P. rapae were significantly larger than wasps emerging from all corresponding instars of P. brassicae. During their early larval stages, parasitoids grew most slowly in L1 P. rapae, whereas in all other host classes of both host species growth to pupation proceeded fairly uniformly. The growth of both host species was markedly reduced after parasitism compared with controls, with the development of P. brassicae arrested at an earlier stage, and at a smaller body mass, than P. rapae. Our results suggest that C. rubecula regulates certain biochemical processes more effectively in P. rapae than in P. brassicae, in accordance with its own nutritional and physiological requirements. Furthermore, we propose that, for parasitoids such as C. rubecula, which do not consume all host tissues prior to pupation, that parasitoid size and host quality may vary independently of host size at oviposition and at larval parasitoid egression.     

Parasitism by Cotesia plutellae alters the hemocyte population and immunological function of the diamondback moth, Plutella xylostella

Cotesia plutellae, a solitary endoparasitoid wasp, parasitizes the diamondback moth, Plutella xylostella, and induces host immunosuppression and lethality in the late larval stage. This study focused on changes of cellular immunity in the parasitized P. xylostella in terms of hemocyte composition and cellular functions. In third and fourth instar larvae of nonparasitized P. xylostella, granular cells represented the main hemocyte type (60-70%) and plasmatocytes were also present at around 15% among the total hemocytes. Following parasitization by C. plutellae, the relative proportions of these two major hemocytes changed very little, but the total hemocyte counts exhibited a significant reduction. Functionally, the granular cells played a significant role in phagocytosis based on a fluorescence assay using fluorecein isothiocyanate-labeled bacteria. The phagocytic activity of the granular cells occurred as early as 5 min after incubation with the bacteria, and increased during the first 40 min of incubation. The parasitism by C. plutellae significantly inhibited phagocytosis of the granular cells. Plasmatocytes also exhibited minor phagocytic activity. Moreover, plasmatocyte phagocytosis was not inhibited by parasitism. On the other hand, hemocyte-spreading behavior in response to pathogen infection was significant only for plasmatocytes, which exhibited a characteristic spindle shape upon infection. A significant spreading of the plasmatocytes was found as early as 5 min after pathogen incubation and their ratio increased during the first 40 min.An insect cytokine, plasmatocyte-spreading peptide 1 (PSP1) from Pseudoplusia includens, was highly active in inducing plasmatocyte-spreading behavior of P. xylostella in a dose-dependent manner. P. xylostella parasitized by C. plutella was significantly inhibited in plasmatocyte-spreading in response to an active dose of PSP1. An in vivo encapsulation assay showed that the parasitized P. xylostella could not effectively form the hemocyte capsules around injected agarose beads. This research demonstrates that the parasitism of C. plutellae adversely affects the total hemocyte populations in number and function, which would contribute to host immunosuppression.     

Diadegma mollipla parasitizing Plutella xylostella: host instar preference and suitability

Oviposition decisions (i.e., host selection and sex allocation) of female parasitoids are expected to correspond with host quality, as their offspring fitness is dependent on the amount and quality of resources provided by a single host. The host size model assumes that host quality is a linear function of host size, with larger hosts believed to contain a greater quantity of resources, and thus be more profitable than smaller hosts. We tested this assertion in the laboratory on a solitary larval–pupal parasitoid Diadegma mollipla (Holmgren) (Hymenoptera: Ichneumonidae) developing on three instars (second–fourth) of one of its hosts, the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae). In a no‐choice test, parasitism levels and sex ratio (i.e., proportion of female progeny) were significantly high in hosts attacked in the second instar followed by third then fourth instars. However, the few parasitoids that completed a generation from the fourth instars did so significantly faster than conspecifics that started development in the other two instars. In direct observations, however, the parasitoids (i) randomly attacked the various host instars, (ii) spent a similar period examining the various host instars with their ovipositors, (iii) subdued all three host instars with about the same effort, and (iv) no statistical differences were observed in the attack rates on the three host instars. In a choice test, the females parasitized significantly more third instars followed by second then fourth instars. However, total parasitism in this experiment was 43% lower compared to parasitism of only second instars in the no‐choice test. No significant differences were detected in progeny sex ratios. In both choice and no‐choice tests, significantly more fourth instars died during the course of the experiments than second instars, while third instars were intermediate. The higher parasitism of third than second instars in the choice test indicates that the females perceived larger hosts as higher quality than smaller hosts, despite their lower suitability for larval development.     

Bemisia tabaci (Homoptera: Aleyrodidae) instar effects on rate of parasitism by Eretmocerus mundus and Encarsia pergandiella (Hymenoptera: Aphelinidae)

Studies were conducted to compare preference among Bemisia tabaci Gennadius, biotype B instars for parasitization by Eretmocerus mundus Mercet and Encarsia pergandiella Howard when provided one instar only, two different instars, and four different instars simultaneously. In the single‐instar no choice treatment, Er. mundus was more successful in parasitizing the younger host instars, while En. pergandiella parasitized a greater proportion of the older instars. Similar results were observed when parasitoids were provided a choice of two instars in six different pair combinations. When all four instars were provided simultaneously, the numbers of first, second, and third instars parasitized by Er. mundus were not significantly different from each other (range 10.3–16.4%), but all were significantly higher than parasitism of fourth instar nymphs (2.1%). The highest percentage parasitization by En. pergandiella was in third instar (17.2%), and the lowest in first instar (2.8%).     

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.     

Characteristics Of Parasitism Of Plutella Xylostella (Lep., Plutellidae) By Oomyzus Sokolowskii (Hym., Eulophidae)

Laboratory and greenhouse studies were conducted on Oomyzus sokolowskii Kurdjumov, a parasite of the crucifer pest Plutella xylostella ( L.). O. sokolowskii preferred the 3^rd and 4^th instar P. xylostella larvae over fresh pupae for parasitization. It is thus a larval parasite. Within the range of 10?C to 35?C, parasitism was positively correlated with temperature. High parasitism at temperatures of 30?C and 35?C indicates that this insect is suitable for the control of P. xylostella in the tropical lowlands. In a no- choice test where only fresh pupae of Cotesia plutellae Kurdjumov, another potentially competing larval parasite of P. xylostella, were offered, O. sokolowskii failed to parasitize pupae of C. plutellae. In a choice test where the 4^th instar P. xylostella larvae and fresh C. plutellae pupae were offered, O. sokolowskii parasitized only P. xylostella larvae. This parasite, therefore, is not a hyperparasite of P. xylostella. When C. plutellae-ovvposited P. xylostella larvae were offered at intervals, O. sokolowskii, parasitized only freshly oviposited host larvae. The longer the period that elapsed after C. plutellae oviposition of P. xylostella larvae, the lesser was the parasitism of these larvae by O. sokolowski.     

小菜蛾主要寄生性天敌——菜蛾绒茧蜂与菜蛾啮小蜂间的相互作用

在28℃下,以小菜蛾3龄幼虫作寄主,研究了菜蛾绒茧蜂与菜蛾啮小蜂间的相互关系．当寄主供2种蜂同时产卵寄生时,与只供1种蜂时相比。绒茧蜂的寄生率无显著变化,而啮小蜂的寄生率则显著下降;2种蜂的合计寄生率与任一种蜂单独存在时相比无显著差异．当寄主先供绒茧蜂寄生,再供啮小蜂寄生时,绒茧蜂的成功寄生率不受影响,而啮小蜂的寄生率仅为8％～13％;啮小蜂能寄生在寄主体内的绒茧蜂高龄幼虫．绒茧蜂能寄生已被啮小蜂寄生的寄主幼虫,其子代部分个体能正常发育至成虫羽化．当已被绒茧蜂寄生和未被寄生的寄主同时存在时,啮小蜂主要寄生未被寄生的寄主．表明绒茧蜂具有竞争优势。但这种优势可因啮小蜂的寄生而被削弱．