卵跳小蜂Ooencyrtus nezarae Ishii对豆缘蝽和豆璧蝽卵的选择性和密度反应(英文)

本文研究了卵跳小蜂对豆缘蝽 (R .clavatusThunberg)和豆璧蝽 (P .hybneriGmelin)卵的选择性和密度反应。在自由选择试验中 ,两种豆蝽卵中羽化的卵跳小蜂在不同的卵密度和比例中均选择豆缘蝽 ;而在非选择性试验中 ,从豆璧蝽中羽化的卵跳小蜂同时选择豆缘蝽和豆璧蝽卵 ,寄生率较高 ;然而 ,从豆缘蝽卵羽化的卵跳小蜂对豆璧蝽卵的寄生率非常低。嗅觉试验表明 ,从豆缘蝽卵上释放的气味对卵跳小蜂的搜寻和寄生行为影响很大。卵跳小蜂寄生的卵数量随着豆缘蝽和豆璧蝽卵密度的增加而提高 ;然而 ,卵跳小蜂对豆璧蝽的寄生率随着卵的密度增加而降低 ,尤其是从豆缘蝽卵中羽化的寄生蜂。研究还表明 ,卵跳小蜂在大田中对豆缘蝽卵的寄生存在着空间或时间上的障碍。     

Enhancement of Riptortus clavatus (Thunberg) (Hemiptera: Alydidae) egg parasitism by a component of the bug's aggregation pheromone

Egg parasitism of the bean bug, Riptortus clavatus (Thunberg) (Hemiptera: Alydidae), was surveyed using individual egg bags in a sweet persimmon orchard and at Gyeongsang National University (GNU) campus, Korea in 2006. The effect of (E)-2-hexenyl (Z)-3-hexenoate (E2HZ3H), one component of R. clavatus aggregation pheromone, on the parasitism enhancement was tested at GNU campus and in a soybean field in 2005 and 2006, respectively. In 2006, two egg parasitoid species, Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae) and Gryon japonicum (Ashmead) (Hymenoptera: Scelionidae), emerged from R. clavatus eggs. Parasitism by O. nezarae was 9.4–48.3% in mid August to mid September in GNU campus and 6.7–30.0% at the orchard. Total parasitism by G. japonicum (2.5%) at both sites throughout the experimental period was lower than that by O. nezarae (12.5%). This survey revealed nearly no activity of the two species after October at both sites. G. japonicum was solitary and O. nezarae could be either solitary or gregarious. From a single R. clavatus egg, one female or one male G. japonicum emerged. However, an average of 4.3 O. nezarae emerged from one host egg. It took 12.6 d for G. japonicum and 12.0 d for O. nezarae to emerge from R. clavatus eggs in the laboratory. Treatment with E2HZ3H increased parasitism by O. nezarae in both years, but did not increase parasitism by G. japonicum. This suggests that E2HZ3H can be used as a kairomone to reduce the density of R. clavatus in the fields where natural parasitism by O. nezarae is high.     

Ecotoxicological effect of insecticides on Ooencyrtus nezarae (Hymenoptera: Encyrtidae) reared from refrigerated and unrefrigerated Riptortus pedestris (Hemiptera: Alydidae) host

Due to increased field occurrence of Riptortus pedestris (Fabricius) (Hemiptera: Alydidae) on various crops including soybean, persimmon and apple in recent years in Korea, demand for insecticide applications to control the stink bug has increased. Acute toxicity of eight pesticides on Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae), a major egg parasitoid of R. pedestris, was compared in the laboratory. Fenitrothion, spinosad, cyfluthrin, etofenprox and carbosulfan caused 100% mortality of O. nezarae within 24 hours by topical application or exposure to residue. Fenitrothion was also highly toxic to the parasitoid when ingested orally. In a previous study, release of refrigerated inviable eggs of R. pedestris was found to increase field parasitism; therefore, we evaluated the sublethal effect of fenitrothion when O. nezarae parasitised refrigerated or unrefrigerated host eggs. Although parasitism rates on both kinds of eggs significantly decreased when O. nezarae were provided with host eggs sprayed with fenitrothion, no difference in parasitism rate, adult emergence, sex ratio, development time and longevity of O. nezarae was found between the refrigerated or unrefrigerated host eggs when the insecticide was treated either before or after oviposition. There was no significant sublethal effect when parasitised host eggs were treated with the insecticide. From these results, all the insecticides tested showed high, acute toxicity against O. nezarae with relatively lower sublethal effects. Refrigeration of host eggs did not affect the susceptibility of O. nezarae to insecticides.     

Refrigeration of Riptortus clavatus (Hemiptera: Alydidae) eggs for the parasitization by Gryon japonicum (Hymenoptera: Scelionidae)

Gryon japonicum (Ashmead) (Hymenoptera: Scelionidae) is a solitary egg parasitoid of Riptortus clavatus (Thunberg) (Hemiptera: Alydidae) one of the most important pests in soybean in Korean and Japan. Refrigeration of R. clavatus eggs for up to 120 days was evaluated as a method of mass production of G. japonicum. The refrigeration reduced the eclosion of eggs, thus no nymph emerged after 30 days of egg refrigeration. Increased duration of host acceptance behaviors by G. japonicum and some detrimental effects on parasitism rate, developmental time, longevity, and adult size of the parasitoid were found in refrigerated host eggs. However, all the biological parameters of G. japonicum were unaffected by the refrigeration of up to 30 days. Gryon japonicum parasitized 16 and 14 host eggs daily that were refrigerated for 15 and 30 days, respectively, which did not differ from parasitization of fresh host eggs. Furthermore, refrigeration of host eggs did not reduce the reproduction of the emerged adult parasitoids and emergence and sex ratio of their progeny. Gryon japonicum also parasitized 14 and 13 refrigerated host eggs per day kept at 26.3°C and 78.7% RH for 2 and 4 days of post-refrigeration without significant reduction, respectively. These results show that refrigeration of R. clavatus eggs can be a good method for mass rearing of the parasitoid, and the host eggs killed by cold storage can be supplemented in the field to boost field parasitism.     

Interspecific larval competition between two egg parasitoids in refrigerated host eggs of Riptortus pedestris (Hemiptera: Alydidae)

In a previous study, we found that soybean fields could be supplemented with refrigerated eggs of Riptortus pedestris (Fabricius) (Hemiptera: Alydidae) to enhance parasitism. As a part of a study to evaluate the effect of host egg refrigeration on parasitism, host acceptance behavior and interspecific larval competition between Gryon japonicum (Ashmead) (Hymenoptera: Scelionidae) and Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae) were studied in multiparasitized unrefrigerated and refrigerated eggs. O. nezarae showed complete host acceptance behavior when offered refrigerated host eggs that were preparasitized by G. japonicum. Adult emergence rate of O. nezarae was 43 and 74% when the interval between the first and second oviposition was 0 and 4 days, respectively, and was not different between refrigerated and unrefrigerated eggs. Refrigeration did not change host acceptance behavior of G. japonicum, but adult emergence declined from 80% in unrefrigerated eggs to 37% in refrigerated eggs that were pre-parasitized by O. nezarae on the same day. No negative effects of refrigeration on sex ratio, adult longevity, and adult size of the both parasitoids were found. Generally host egg refrigeration did not negatively affect host acceptance behavior of the both parasitoids on preparasitized eggs or larval competition between the two parasitoids in multiparasitized host eggs with exceptions in the development time and emergence rate of G. japonicum. Therefore, host egg refrigeration may not interrupt interactions between the parasitoid populations in the field.     

In vitro rearing of Ooencyrtus nezarae (Hymenoptera: Encyrtidae), an egg parasitoid of Riptortus pedestris (Hemiptera: Alydidae)

Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae) is a polyphagous egg parasitoid of various true bugs, including Riptortus pedestris (Fabricius) (Hemiptera: Alydidae), a major pest of soybean and fruit trees in northeastern Asia. This study was conducted to develop artificial host eggs containing insect haemolymph for mass rearing of O. nezarae. The haemolymph of Chinese oak silkworm (Antheraea pernyi Guérin-Méneville) pupae was found to be most suitable for artificial eggs for the complete development of O. nezarae. Among 764 parasitoid eggs laid in artificial eggs, 49.2% successfully developed to adult parasitoids. The developmental time in artificial eggs was delayed by 3–5 days relative to that observed in natural host eggs; however there was no significant difference in the length of hind tibia between adults that emerged from artificial and natural host eggs. Moreover, new generations that emerged from artificial eggs reproduced on all egg types offered (either natural or artificial eggs). We propose that the artificial eggs developed herein can therefore be an effective means for mass rearing O. nezarae.     

Winter survival and oviposition before and after overwintering of a parasitoid wasp, Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae)

Winter survival and oviposition before and after overwintering in Ooencyrtus nezarae, an egg parasitoid of phytophagous heteropterans, were examined in Osaka, Japan. Eggs of Riptortus clavatus parasitized by O. nezarae were kept under natural photoperiod and temperature. When honey was supplied, some female adults emerging from early September to late November overwintered. The percentage of overwintering individuals increased as the date of adult emergence advanced. Most female adults supplied with honey and hosts oviposited soon after emergence, then stopped laying eggs. Female adults emerging in mid‐October and early November laid eggs and then overwintered. The induction of diapause in the field seems to vary greatly depending on host availability. Without honey, the survival time of female adults was very short, whether host eggs were supplied or not. After overwintering, most females began to lay eggs in early May if host eggs were supplied, and they produced both male and female progeny. In the study area, a legume field in Osaka, parasitization by O. nezarae was observed from early July to November.     

Biological attributes of Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae) reared on refrigerated eggs of Riptortus pedestris (= clavatus) Fabricius (Hemiptera: Alydidae)

Storing host eggs at low temperatures has been used to mass rear parasitoids of stink bugs, including Riptortus pedestris Fabricius (Hemiptera: Alydidae), a major soybean pest in Korea and Japan. However, no information on the effect of cold storage on parasitization by Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae), one of the major parasitoids of R. pedestris, has been published. In this study, we examined biological attributes, including parasitism rate, development time, sex ratio, adult size, and longevity, of O. nezarae when the adult parasitoids were provided with host eggs refrigerated for 0, 15, 30, 60, and 120 days at 2.0 °C. None of the attributes of the first or second generation of O. nezarae was negatively affected by host egg refrigeration up to 30 days. In addition, O. nezarae could parasitize refrigerated host eggs successfully for the first four days of post-refrigeration period when they were kept at 26.3 °C and 78.7% RH conditions. Therefore, refrigeration of R. pedestris eggs can be a good method to mass rear O. nezarae.     

Flight orientation behavior of Ooencyrtus nezarae (Hymenoptera: Encyrtidae), an egg parasitoid of phytophagous bugs in soybean

We conducted field experiments to examine flight orientation behavior of Ooencyrtus nezarae Ishii, an egg parasitoid of soybean feeding bugs, using sticky traps with synthetic aggregation pheromones of the host, Riptortus clavatus (Thunberg). Experiments were conducted in plots either containing a soybean field or not containing one. When the traps were suspended at heights of 0.5, 1.5 and 3.0 m above a soybean field in Kobe, O. nezarae females were most frequently caught in the 0.5 m traps. In an open field on the same campus where the vegetation was predominantly tall goldenrod, females were more frequently trapped at 1.5 or 3.0 m. In a soybean field in Kumamoto, females were most frequently trapped in the 0.8 m traps, compared to traps placed at 2.0, 4.0 and 6.0 m. In an oat field in the same area, females were trapped most frequently at heights of 0.8 and 2.0 m. Unlike females, only a small number of males were trapped in the fields in both areas. The result that O. nezarae females were caught in traps at higher positions in non‐host habitats than in soybean fields probably reflects differences in host searching behavior in or movement between the habitats. In host habitats, females mainly walk on the plants or fly within the plant canopy to search for hosts. In non‐host habitats, females may not intensively search for hosts within the plants, and directly respond to host pheromones in the traps at heights equal to or a little higher than the plant canopy.     

Demonstration of interspecific competition between two sympatric egg parasitoids of Riptortus pedestris (Fabricius) in laboratory condition

Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae) acts as a facultative hyperparasitoid of Gryon japonicum (Ashmead) (Hymenoptera: Platygastridae) sympatric parasitoid of Riptortus pedestris (Fabricius) (Hemiptera: Alydidae). A longer period of host egg exploitation by both parasitoid species would be beneficial for O. nezarae, while G. japonicum tends to be successful when the parasitoids have only a short exploitation period. We demonstrated the interspecific competition by measuring parasitism in nine combinations of host densities (10, 20, and 40 eggs) and exploitation times (1, 3, and 5 days). To reflect the gregarious-solitary dichotomy of the two species, three O. nezarae and one G. japonicum mated females were compared in addition to a one-to-one competition design. We found that O. nezarae was the better competitor when exploitation time was longer than 1 day, irrespective of host density. Total parasitism rate and progeny emergence of O. nezarae were 1.6–2.8 and 4.7–7.3 times higher than for G. japonicum in three-to-one competition design, respectively. Although G. japonicum females were more effective in host finding (as shown by their higher per capita rate of parasitism when exploitation time was short), their progeny suffered high mortality from the larval interspecific competition inside multiparasitized host eggs. These results suggest that gregarious O. nezarae is the superior competitor when host eggs are available for longer period of time while solitary G. japonicum is superior when host resources are available for only a limited time.     

Host discrimination in the parasitoid Ooencyrtus nezarae: the role of the egg stalk as an external marker

Laboratory experiments were conducted to examine the role of the egg stalk in host discrimination by Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae), an egg parasitoid of the bean bug, Riptortus clavatus Thunberg (Hemiptera: Alydidae). These experiments showed that females that have oviposited in unparasitized hosts within 1 h before the test discriminated between parasitized and unparasitized hosts. When protruding parts of the parasitoid's egg stalks were removed from hosts, the latter were accepted by experienced females. This suggests that the protruding portion of the parasitoid egg stalk functions as an external marker. This part of the egg stalk was responsible for host discrimination up to 8 days after parasitism. Internal discrimination was also observed on hosts 3 or 8 days after parasitism.

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Résumé Des expériences de laboratoire ont permis d'examiner l'influence du pédoncule de l'oeuf dans la sélection des hôtes par O. nezarae Ishii (Hym.: Encyrtidae), parasite d'oeufs de Riptortus clavatus Thunberg (Hemip.: Alydidae). Les femelles qui avaient pondu dans un hôte vierge dans l'heure qui avait précédé l'expérience ont été capables de choisir entre des hôtes parasités ou non. Quand la partie saillante du pédoncule de l'oeuf de parasitoïde avait été extraite de l'hôte, ce dernier avait été accepté par les femelles. Ceci laisse supposer que la partie saillante du pédoncule de l'oeuf fonctionne comme marqueur externe. Cette partie du pédoncule de l'oeuf a été responsable de la discrimination des hôtes jusqu'à 8 jours après qu'ils aient été parasités, montrant une plus longue efficacité comme marqueur externe que les phéromones externes observées chez d'autres parasitoïdes.

     

Effect of induced tolerance to Bt toxin on the egg size of Helicoverpa armigera and parasitism by Trichogramma pretiosum

Larvae of Helicoverpa armigera (Hübner) can develop a form of Bt tolerance after exposure to sub‐lethal doses of Bt‐toxin subclass Cry1Ac. Increasing levels of tolerance are produced over generations of larval exposure, which are not related to DNA sequence changes, and are largely maternally transmitted. The characteristic of maternal transmission, combined with the importance of egg parasitoids to cotton pest management, raises questions about the effects of Bt tolerance/exposure on the eggs of H. armigera and on some key metrics of egg parasitism. In the present study, the effect of inducible tolerance on eggs of H. armigera and parasitism by Trichogramma pretiosum (Riley) is investigated. First, the volumes of eggs laid by susceptible and tolerant H. armigera females are compared. In addition, the effect of inducible tolerance on egg parasitism is determined by comparing parasitism success, the number of adult wasps emerged per host egg, and the proportion of male and female offspring emerged per host egg. The results obtained suggest that Cry1Ac‐tolerance is associated with increased egg volume, even after just one generation of sub‐lethal exposure. When tolerant H. armigera are freed from ongoing sub‐lethal exposure, a corresponding decrease in egg volume is not detected. Although there is no difference in the percentage of eggs parasitized, there is an increase in the number of emergent parasitoids, especially males, from eggs laid by tolerant H. armigera. These results confirm that maternally‐transmitted Bt tolerance is reflected in the phenotype of the eggs of tolerant offspring, which affects egg parasitism.     

茭白害虫长绿飞虱与稻田缨小蜂关系的研究

茭白害虫长绿飞虱 Saccharosydne procerus是稻虱缨小蜂Anagrus nilaparvatae 在稻田外的主要寄主之一。茭白田可为稻田提供大量的稻虱缨小蜂,从而提高对稻田稻飞虱的控制能力。从褐飞虱和长绿飞虱卵中羽化的稻虱缨小蜂均明显地选择寄生褐飞虱卵,对长绿飞虱卵的寄生率也较高。长绿飞虱在水稻上不能完成其生活史。因此,稻田和茭白田的合理布局可有效地提高稻田缨小蜂的数量,从而提高该寄生蜂对有关害虫的生防效益。     

Trichogramma pretiosum parasitism and dispersal capacity: a basis for developing biological control programs for soybean caterpillars

In order to succeed in biological control programs, not only is it crucial to understand the number of natural enemies to be released but also on how many sites per area this releasing must be performed. These variables might differ deeply among egg parasitoid species and crops worked. Therefore, these trials were carried out to evaluate the parasitism (%) in eggs of Anticarsia gemmatalis and Pseudoplusia includens after the release of different densities of the egg parasitoid Trichogramma pretiosum. Field dispersal was also studied, in order to determine appropriate recommendations for the release of this parasitoid in soybean fields. The regression analysis between parasitism (%) and densities of the parasitoid indicated a quadratic effect for both A. gemmatalis and P. includens. The maximum parasitism within 24 h after the release was reached with densities of 25.6 and 51.2 parasitoids per host egg, respectively, for the two pests. Parasitism of T. pretiosum in eggs of P. includens decreased linearly as the distance of the pest eggs from the parasitoid release sites increased. For P. includens, the mean radius of T. pretiosum action and the area of parasitoid dispersal in the soybean crop were 8.01 m and 85.18 m2, respectively. We conclude that for a successful biological control program of lepidopteran pests using T. pretiosum in soybean fields, a density of 25.6 parasitoids per host egg, divided into 117 sites per hectare, should be used.     

Egg parasitoids of Thaumetopoea pityocampa (Den. & Schiff.) (Lep., Thaumetopoeidae) and their impact in SW Turkey

Abstract: Egg parasitoids (Hym., Chalcidoidea) and egg parasitism were studied in 132 egg batches of the pine processionary moth collected from stands of Pinus nigra Arnold and P. brutia Ten. in SW Turkey, at four places in heights of 800–1010 m a.s.l. in October 1998, beyond the caterpillars’ hatch. The egg batches deposited on P. nigra were high‐significantly smaller than those found on P. brutia. In this mountainous regions, Baryscapus servadeii (Dom.) and Ooencyrtus pityocampae (Mercet) were the most abundant egg parasitoids of Thaumetopoea pityocampa (Den. & Schiff.). Considering the different sampling places, Trichogramma sp. was found maximally in 3.1% of the parasitized eggs. Additionally, the hyper‐parasitoid Baryscapusransversalis Graham (mean infestation 1.8%) and Pediobius bruchicida (Rond.) (0.1%), both occurring in both sexes, and Anastatus bifasciatus (Fonsc.) (0.1%), only females, were established. The primary parasitoid O. pityocampae dominantly parasitized the eggs placed near the top of the batches, and B. servadeii was found mainly in eggs near the base. The rate of parasitism of the egg batches was very variable. The impact of egg parasitoids and predators varied from 24 to 35.9% depending on the sampling place. The hatching rate of the caterpillars was reduced to 50.7–65.6%. Under laboratory conditions, the emergence dynamics of the parasitoids showed a peak of B. tranversalis in December of the year, in which the host eggs were laid. Baryscapus servadeii emerged mainly in March–May of the following year, when the second part of O. pityocampae appeared. A large part of both parasitoids emerged before sampling of the batches. In O. pityocampae 1.65% and in B. servadeii 2.23% males developed under laboratory conditions. A rate of 15.7% of O. pityocampae and 7.9% of B. servadeii died in the eggs. The polyembryonic Trichogramma sp. (see also Tsankov et al., 1996a ) attacked 1.8% of the eggs. Up to 20 (mean 12) dead pupae were found in one egg. Minor parts of eggs were undeveloped (2.9%) and 0.6% totally empty.     

Timing of Spawning and Host-nest Choice for Brood Parasitism by the Japanese Minnow,Pungtungia herzi,on the Japanese Aucha Perch,Siniperca kawamebari

A field study of the breeding ecology of the Japanese aucha perch, Siniperca kawamebari, and brood parasitism by the Japanese minnow, Pungtungia herzi, on nests of the perch was carried out from 1989 to 1991. Observations of perch nests under natural conditions in 1990 showed that brood parasitism by the minnow was concentrated on host nests in which nest owners had just begun their nesting cycle. When spawned in a perch nest with recently spawned perch eggs, parasite eggs always hatched earlier than host eggs. An experiment with imitation perch eggs in 1991 confirmed that changing colour of host eggs was the cue for the parasites to distinguish between different developmental stages of host eggs. Parasite eggs rapidly disappeared without guarding by a host male (Baba et al. 1990). This loss was caused by predation by fishes. Parasite fry left the nest immediately after hatching, so parasite eggs spawned in a host nest in an early stage should be well guarded until they hatch. In the field, minnows deposited their eggs in perch nests which had larger numbers of newly spawned perch eggs. Since the perch males always deserted their nests when their own eggs disappeared, the parasite's choice of host nests with larger numbers of host eggs may ensure survival of the parasite eggs. The timing of egg deposition and choice of host nest by the minnow appear to be adaptive in terms of brood parasitism on nests of the perch.     

Reciprocal influences and costs of parasitism on the development of Corcyra cephalonica and its endoparasitoid Venturia canescens

Many endoparasitoids develop successfully within a range of host instars. Parasitoid survival is highest when parasitism is initiated in earlier host instars, due to age-related changes in internal (physiological) host defences. Most studies examining fitness-related costs associated with differences in host instar have concentrated on the parasitoid, ignoring the effects of parasitism on the development of surviving hosts that have encapsulated parasitoid eggs. A laboratory experiment was undertaken examining fitness-related costs associated with encapsulation of Venturia canescens (Hymenoptera: Ichneumonidae) eggs by fifth (L5) instar larvae of Corcyra cephalonica (Lepidoptera: Pyralidae). Growth and development of both host and parasitoid were monitored in C. cephalonica larvae containing 0, 1, 2, or 4 parasitoid eggs. Adult size and fecundity of C. cephalonica did not vary with the number of eggs per host. However, there was a distinct increase in host mortality with egg number, although most parasitoids emerged from hosts containing a single egg. The most dramatic effect on the host was a highly significant increase in development time from parasitism to adult eclosion, with hosts containing 4 parasitoid eggs taking over 2.5 days longer to complete development than unparasitized larvae. The egg-to-adult development time and size of adult V. canescens did not vary with egg number per host, as demonstrated in a previous experiment using a different host (Plodia interpunctella). The results described here show that there are fitness-related costs to the host associated with resistance to parasitism.     

Incidence of Anagrus obscurus (Hymenoptera: Mymaridae) egg parasitism on Calopteryx haemorrhoidalis and Platycnemis pennipes (Odonata: Calopterygidae: Platycnemididae) in Italy

Very little is known about the incidence of egg parasitoids in odonates, perhaps because Odonata eggs are well protected by stems or leaves, sometimes below water. In Central Italy (Pontecorvo, Frosinone Province) two damselflies, Calopteryx haemorrhoidalis and Platycnemis pennipes, occur in high densities. In August 2007 we collected 30 stems of the aquatic plant Potamogeton sp. used as substrate for oviposition and incubated eggs in the laboratory. Most stems (24 for C. haemorrhoidalis and 23 for P. pennipes) contained Odonata eggs. Parasitoids emerged from 12 stems, with a mean parasitism of 2% for C. haemorrhoidalis and 6% for P. pennipes, and a maximum of 14% and 50%, respectively. Furthermore, we observed egg‐laying of 19 females of C. haemorrhoidalis and 11 of P. pennipes, and marked the stems where oviposition was observed. Clutches remained in the river for five days and were then collected and incubated. Parasitoids emerged from 11 of 30 stems, with an average parasitism of 8% for C. haemorrhoidalis and 3% for P. pennipes (maximums of 50% and 29%, respectively). All parasitoids belonged to the family Mymaridae, and were identified as Anagrus (Anagrus) obscurus Förster, 1861, sensu Soyka, 1955 . This is the first time that this species is described as an egg parasitoid of odonates, and that the egg parasitoid of C. haemorrhoidalis and P. pennipes is identified. Our data suggest that egg parasitism might be a significant selective factor for both odonates in the studied locality, affecting female oviposition behavior.     

Induction of adult diapause and of low and high reproductive states in a parasitoid wasp,Ooencyrtus nezarae, by photoperiod and temperature

Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae) was reared on eggs ofRiptortus clavatus (Thunberg) (Heteroptera: Alydidae) at various temperatures under long-day (L16:D8) or short-day (L10:D14) conditions. There was no diapause during egg, larval or pupal stages under any set of conditions examined. However, at 15°C under short-day conditions, vitellogenesis was arrested in all adult females and they entered diapause. At 15°C under long-day conditions, or at 20°C under short-day conditions, some adult females entered diapause. Under the latter set of conditions, the adult females laid eggs but they laid fewer eggs than under long-day conditions, Even at 25°C, under short-day conditions, adult females laid fewer eggs than under long-day conditions, and this low rate of oviposition was attributed to the retarded development of ovaries. Diapause adults reared at 15°C were more resistant to low temperature than nondiapause adults reared at 25°C.     

Density dependence of Anaphes sordidatus (Hymenoptera: Mymaridae) parasitism on eggs of Listronotus oregonensis (Coleoptera: Curculionidae)

Listronotus oregonensis (LeConte) (Coleoptera: Curculionidae) oviposits in carrot leaves and the larvae feed in carrot roots. Its eggs are parasitized by Anaphes sordidatus (Girault) (Hymenoptera: Mymaridae) throughout its oviposition period. This parasitoid is the major biotic mortality factor for L. oregonensis. Parasitism by A. sordidatus was studied for three years in sequentially sown plots of carrots, Daucus carota var. sativa. Over the scason, significantly fewer L. oregonensis eggs were oviposited in later sown carrots than in earlier sown carrots because oviposition started later in late sown plots of carrots. A positive temporal density-dependent relationship was observed each year between parasitism rates and host densities. This positive density dependence occurred in early and mid-summer for earlysown carrots where host density reached 1–2 host eggs per plant but disappeared in late summer when host density decreased while parasitism remained high. Latesown carrots had low host egg density (0.2 host egg per plant) and contributed little to the total number of eggs. In these late sown plots, parasitism increased rapidly to over 80% but no density dependence was observed. Spatially, few statistically significant regressions were found but all indicated a positive spatial density-dependence. Most non-significant regressions occurred because the range of egg density was too small between plots for a given date.