Mean Percentage Parasitism Research Articles

Aphid parasitism in winter wheat fields in a heterogeneous agricultural landscape.

The number, timing, and fitness of colonizing parasitoids in fields of ephemeral crops often depend on factors external to the fields. We investigated cereal aphid parasitism in 23 winter wheat fields using sentinel plants infested with bird cherry-oat aphids, Rhopalosiphum padi (L.) (Hemiptera: Aphididae), and we investigated the effect of parasitoids on cereal aphid population growth using exclusion and parasitoid-accessible cages infested with bird cherry-oat aphids. Lysiphlebus testaceipes (Cresson) (Hymenoptera: Braconidae), Aphelinus nigritus (Howard) (Hymenoptera: Aphelinidae),and Diaeretiella rapae (McIntosh) (Hymenoptera: Braconidae), in decreasing order of abundance, parasitized R. padi on sentinel plants. The mean percent parasitism in parasitoid-accessible cages was 5.2% in autumn and 35.0% in spring. Aphid population intensity was greater in complete exclusion than in parasitoid-accessible cages. Measures of landscape composition and configuration were quantified, and aphid parasitism in autumn by L. testaceipes and A. nigritus was positively associated with % landcover by summer crops and patch density. Parasitism by both species was negatively associated with contagion and % woodlands. Parasitism during spring was positively associated with % grassland and fractal dimension and negatively associated with % canola. The number of braconid mummies per sentinel plant was positively correlated to the number of braconid mummies on wheat stems from parasitoid-accessible cages. Results indicate that cereal aphid mortality caused by parasitoids and their ability to exert effective biological control is related to landscape structure. Comparing this study to an earlier study in the same agroecosystem demonstrated temporal stability of the landscape influence on aphid parasitism by L. testaceipes in winter wheat.

CHILO INFUSCATELLUS SNELLEN'S (LEPIDOPTERA: PYRALIDAE) BIOLOGY AND ITS MANAGEMENT

SUGARCANE stem borer, Chilo infuscatellus is the most devastating pest, causing huge economic losses in the sub-tropics and therefore requiring effective management strategies. The current research was conducted to study the biology and management of C. infuscatellus. Moreover, we checked the efficacy of Trichogramma chilonis and granular insecticides against this key pest. Theincubation period for C. infuscatellus was 2.30 days, with a total developing time of 23.30 days for the larvae and 5.90 days for the pupa. Male and female C. infuscatellus had a 36.80-day and 38.00-day total life span, respectively, with the female living longer (4.8 days) than the male (3.90 days). The total female fecundity was 315.90 eggs with a 90.87 egg hatchability rate. The mean percent parasitism (87.01%) and percentage of adult emergence (76.11%) of T. chilonis on C. infuscatellus were both very high, with a total developmental time of 8.74 days in laboratory environment. Additionally, there were notable differences between the findings on the effectiveness of granular pesticides and T. chilonis in the field. The plot treated with Fipronil had the least mean percent infestations (3.58%), followed by Carbofuran (4.26%), and T. chilonis (5.63%). The control plot had the highest mean incidence of infestations (13.34%). Future IPM initiatives should incorporate the introduction of Trichogramma or the utilization of Fipronil @ 16 kg ha-1 for the treatment of C. infuscatellus. This practice will play a critical role in environmental protection and natural resource conservation against insecticides.

Parasitism of Adult Pentatomidae by Tachinidae in Soybean in the North Central Region of the United States.

Stink bugs (Hemiptera: Pentatomidae) are agricultural pests of increasing significance in the North Central Region of the United States, posing a threat to major crops such as soybean. Biological control can reduce the need for insecticides to manage these pests, but the parasitism of stink bugs by Tachinidae (Diptera) is poorly characterized in this region. The objective of this study was to evaluate the rate of parasitism of stink bugs by tachinids over 2 yr from nine states across the North Central Region. Parasitism was assessed by quantifying tachinid eggs on the integument of stink bug adults. Parasitism rates (i.e., percent of adult stink bugs with tachinid eggs) were compared across stink bug species, states, stink bug sex, and years. The mean percent parasitism of stink bugs by tachinids was about 6% across the region and did not differ among stink bug species. Mean percent parasitism was significantly higher in Missouri than in northern and western states. In addition, male stink bugs had significantly higher mean percent parasitism than females. Stink bug species commonly found in soybean in the region showed some parasitism and are therefore potentially vulnerable to oviposition by these parasitoids. This is the first study to characterize the level of parasitism of stink bugs by tachinids across the North Central Region.

Parasitoids of Gangara thyrsis (Fabricius) (Lepidoptera: Hesperiidae) with description of a new species of Agiommatus Crawford, 1911 (Hymenoptera: Pteromalidae) from India with notes on biology.

In rearing of Gangara thyrsis (Fabricius) (Lepidoptera: Hesperiidae) from Karnataka and Goa, India, six species of parasitoids were observed. One new species of parasitic wasp is described and illustrated: Agiommatus thyrsisae n. sp. (Hymenoptera: Pteromalidae), a solitary parasitoid reared from the egg of G. thyrsis on the natural host plant Dypsis lutescens (H. Wendl.) Beentje & J. Dransf. Three additional species of parasitic wasps were also reared: Anastatus ramakrishnai (Mani, 1935) (Hymenoptera: Eupelmidae), a solitary hyperparasitoid of A. thyrsisae n. sp.; Sympiesis thyrsisae Gupta, Gawas & Bhambure (Hymenoptera: Eulophidae), a gregarious parasitoid reared from the caterpillar of G. thyrsis on the host plant Cocos nucifera L., and Brachymeria lasus (Walker) reared from pupa of G. thyrsis on the host plant D. lutescens. Additionally, two species of tachinid flies were also reared from the pupae of G. thyrsis: Exorista sorbillans (Wiedemann, 1830) and an innominate species close to Blepharella spp. Gangara thyrsis is a new host record for the genus Agiommatus and for A. ramakrishnai and B. lasus. The mean percent parasitism in G. thyrsis eggs was 26.58% with an incubation period of 6-7 days. Amongst the egg parasitoids, 57.14-73.08% were females and 23.08% were males. Hyperparasitism ranged from 3.85 to 42.86%. Dypsis lutescens, a member of Arecaceae, is a new host plant record for G. thyrsis.

Range expansion and increasing impact of the introduced wasp Aphidius matricariae Haliday on sub-Antarctic Marion Island

Despite the significance of biological invasions in the Antarctic region, understanding of the rates of spread and impact of introduced species is limited. Such information is necessary to develop and to justify management actions. Here we quantify rates of spread and changes in impact of the introduced wasp Aphidius matricariae Haliday, which parasitizes the invasive aphid Rhopalosiphum padi (L.), on sub-Antarctic Marion Island, to which the wasp was introduced in ca. 2001. Between 2006 and 2011, the wasp had colonised all coastal sites, with an estimated rate of spread of 3–5 km year−1. Adult abundance doubled over the period, while impact, measured as mean percentage parasitism of R. padi, had increased from 6.9 to 30.1 %. Adult wasps have thermal tolerances (LT50s) of between −18 and 33.8 °C, with a crystallization temperature of −22.9 °C, and little tolerance (ca. 37 h) of low humidity at 10 °C. Desiccation intolerance is probably limiting for the adult wasps, while distribution of their aphid host likely sets ultimate distributional limits, especially towards higher elevations where R. padi is absent, despite the presence of its host grass on the island, Poa cookii (Hook. f.). Rising temperatures are benefitting P. cookii, and will probably do the same for both R. padi and A. matricariae. Our study shows that once established, spread of introduced species on the island may be rapid, emphasizing the importance of initial quarantine.

Parasitism, emergence, and development of Spalangia endius (Hymenoptera: Pteromalidae) in pupae of different ages of Bactrocera cucurbitae (Diptera: Tephritidae).

The wasp Spalangia endius Walker (Hymenoptera: Pteromalidae) is a major parasitoid of the pupae of fruit flies, which are a common agricultural pest. An understanding of this intricate host–parasitoid interaction could provide basic information necessary for the sustainable integrated biological control of fruit flies. In this study, we investigated the effect of S. endius on different-aged pupae of the melon fly Bactrocera cucurbitae Coquillett by using choice and nonchoice tests under laboratory conditions. We showed that S. endius females oviposited, and their progeny successfully developed, in different-aged pupae of B. cucurbitae regardless of the method of exposure. There was an oviposition preference for 3–5-d-old pupa. The highest mean percentage parasitism occurred on 4- and 5-d-old hosts, followed by 2- and 3-d-old hosts. The average development time for both males and females was significantly longer in 6–7-d-old hosts than in the younger host stages. Adult females that developed from younger host pupae (2–5-d old) were significantly heavier than those from older host pupae (6–7-d old), and they also lived longer. The sex ratio (proportion of females) of the parasite progeny decreased with an increase in host age. Host mortality also decreased gradually as the pupal age increased. The differences in development time, body weight, and longevity between females and males were significant. These results suggest that S. endius is a good candidate for the biological control of B. cucurbitae.

Sentinel eggs underestimate rates of parasitism of the exotic brown marmorated stink bug, Halyomorpha halys

Native to eastern Asia, the brown marmorated stink bug (BMSB), Halyomorpha halys (Stål), has become a serious invasive pest in North America. Consequently, accurate assessment of parasitism rates under field conditions is critical for determining baseline parasitism rates of native egg parasitoids on BMSB, and for future evaluations of native or exotic parasitoid biological control release strategies and impacts. BMSB sentinel (laboratory-laid) egg masses have typically been used for this purpose, even though they could be providing misleading estimates of parasitoid activity. Accordingly, we compared the use of BMSB sentinel (laboratory-laid) and wild (naturally field-laid) egg masses in 2012 and 2013 to examine rates of parasitism and the parasitoid community composition of indigenous egg parasitoids in outdoor ornamental nurseries. Wild egg masses consistently had higher rates of parasitism than sentinel egg masses. In 2012, wild egg masses had a mean percent parasitism of 28.4% compared to 4.6% in sentinel egg masses, while in 2013 the difference between the two methods increased even further with a mean percent parasitism of 55.3% in wild egg masses compared to 0.8% in sentinel eggs. Furthermore, we found greater total numbers of parasitoids (889, 42) and greater species richness (seven, five), when using wild egg masses compared to sentinel egg masses, respectively. While sentinel egg masses provide a rapid and convenient way to assess the presence of natural enemies, our findings indicate that using sentinel egg masses could dramatically underestimate actual rates of parasitism and provide inaccurate estimates of parasitoid community composition. Future studies should address potential mechanisms underlying these patterns such as lack of certain host cues required by parasitoids in sentinel compared to wild egg masses.

Assessment of naturally occurring parasitism of diamondback moth in field using recruitment method

The recent major outbreaks of diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae), in cruciferous plants in Iran has led to attempts for sustainable pest management strategies, mainly based on the natural enemies. The present study aimed to investigate naturally occurring parasitism of P. xylostella in cabbage and cauliflower fields of central Iran. For this purpose, field studies were performed to identify parasitoids of P. xylostella, and to evaluate percentage parasitism of P. xylostella using the recruitment method in main cabbage-growing areas of Isfahan province in 2009. In present study, seven species of parasitoid wasps (five larval and two pupal parasitoids) and two species of hyperparasitoid wasps were determined. The parasitoids included the braconids Cotesia vestalis (Kurdjumov), Bracon hebetor Say and Apanteles sp., the ichneumonid Diadegma semiclausum (Hellen), and the eulophid Oomyzus sokolowskii (Kurdjumov) as larval parasitoids, and the ichneumonids Diadromus collaris (Gravenhorst) and Diadromus subtilicornis (Gravenhorst) as pupal parasitoids. In addition, the pteromalids Mokrzeckia obscura Graham and Pteromalus sp. were identified as the hyperparasitoids, which in turn parasitise C. vestalis. The most predominant species were C. vestalis and D. semiclausum with the proportional abundance of 0.43 and 0.42, respectively. Percentage parasitism varied significantly between host plants, but not between areas; the parasitised proportion of P. xylostella larvae fed on common cabbage was significantly greater than that on cauliflower (0.42 vs. 0.34). The mean percentage parasitism varied between 14.5 and 68.4 for different fields, and accounted for 37.4% of P. xylostella population on an average. The greatest parasitism was achieved by C. vestalis, D. semiclausum and O. sokolowskii, with a parasitism of 21.0, 12.9 and 3.5% of field populations of P. xylostella, respectively. These findings illustrated the important role of parasitoids for sustainable management of diamondback moth.

Biology and Rearing of <I>Cleruchoides noackae</I> (Hymenoptera: Mymaridae), an Egg Parasitoid for the Biological Control of <I>Thaumastocoris peregrinus</I> (Hemiptera: Thaumastocoridae)

Cleruchoides noackae Lin and Huber (Hymenoptera: Mymaridae) is a solitary egg parasitoid of Thaumastocoris peregrinus Carpintero and Dellapé (Hemiptera: Thaumastocoridae). The parasitoid was first described in 2009 and its biology and rearing are poorly understood. A key obstacle to the use of C. noackae as a biological control agent has been the ability to consistently rear it under quarantine conditions. This study reports on a series of experiments conducted in quarantine to rear C. noackae and to examine the effects of diet on longevity, per capita reproduction, and progeny sex ratio, as well as to determine development time, and preference and suitability of host eggs of different ages. When supplemented with honey solution, the longevity of C. noackae females increased significantly by 2.4 d and that of males by 1.7 d, relative to the unfed adults. Mean per capita reproduction for the honey-fed wasps was 7.7 offspring per female, with progeny sex ratio slightly skewed toward males. Mean percentage parasitism was 32.2%. C. noackae was capable of parasitizing and completing development from oviposition to adult eclosion within 15.7 d in host eggs between 0 and 5 d old. The ability of C. noackae to parasitize a wide range of host egg ages increases the period of vulnerability of T. peregrinus to attack, increasing its potential efficacy as a biological control agent. The methods and results reported here represent a crucial step in the ongoing efforts to develop this potential biological control system.

Host suitability of UV‐irradiated eggs of three Lepidoptera species for rearing Trichogrammatoidea lutea Girault (Hymenoptera: Trichogrammatidae)

AbstractTo develop a rearing programme for the egg parasitoid Trichogrammatoidea lutea for augmentative biological control, the effects of ultraviolet (UV) irradiated host eggs were evaluated on the number of eggs parasitized, development time, number of progeny and sex ratio of T. lutea, using eggs of the lepidopteran species Helicoverpa armigera, Chilo partellus and Cadra cautella as hosts. UV‐irradiation of 13 min was sufficient to kill embryos of all three hosts. The highest mean percentage parasitism on UV‐irradiated and un‐irradiated eggs occurred on those of H. armigera (47.8% and 33.3%) and C. cautella (33.2% and 55.5%), followed by C. partellus (6.6% for both). The mean number of individuals emerged per single parasitized host of UV‐irradiated and un‐irradiated eggs was highest for H. armigera (2.5 and 2.1) and lower for C. partellus (1.3 and 1.4) and C. cautella (0.9 and 0.8). The sex ratio of T. lutea was female‐biased on UV‐irradiated eggs of H. armigera, on UV‐irradiated and un‐irradiated eggs of C. cautella, and male‐biased for C. partellus eggs and un‐irradiated eggs of H. armigera. The developmental time of T. lutea from egg to adult male and female differed slightly between UV‐irradiated and un‐irradiated eggs. Overall, UV‐irradiation of host eggs had negligible effects on percent parasitism, number of progeny per parasitized egg and sex ratio of T. lutea. Of the three lepidopteran species evaluated, H. armigera appears to be the most suitable host for mass‐rearing of T. lutea. The results indicate that UV‐irradiation of Lepidoptera eggs is suitable for mass rearing of T. lutea.

<I>Psyttalia</I> cf. c<I>oncolor</I> (Hymenoptera: Braconidae) for Biological Control of Olive Fruit Fly (Diptera: Tephritidae) in California

The larval parasitoid, Psyttalia cf. concolor (Szépligeti), reared on Mediterranean fruit fly, Ceratitis capitata (Weidemann), by the USDA-APHIS-PPQ, Guatemala City, Guatemala, was imported into California for biological control of olive fruit fly, Bactrocera oleae (Gmelin), in olives, Olea europaea L. Mean percentage parasitism of olive fruit fly third instars infesting fruit in field cages ranged from 7.0 in Grapevine to 59.7 in Santa Barbara and in free releases ranged from 0 in Grapevine to 10.6 in Santa Barbara after 4- to 6-d exposures. In the laboratory, more parasitoids developed to adults in olive fruit fly larvae that were 11-13 d old than in larvae 8-10 d old. Adult parasitoids lived significantly longer when provided with water than adults without water in environmental chambers at 5 degrees C, 85% RH; 15 degrees C, 65% RH; 25 degrees C, 25% RH; and 35 degrees C, 25% RH. Adult parasitoids lived for 48 d with honey for food and water and 32 d with food and sugar solution at 15 degrees C and 65% RH. Survival of adult parasitoids without food and water in greenhouse tests was approximately 4 d in a simulated coastal climate and 1 d in a simulated inland valley climate and was significantly increased by providing food and water. The parasitoid did not develop in the beneficial seedhead fly, Chaetorellia succinea (Costa), in yellow star thistle. The rate of parasitism of walnut husk fly, Rhagoletis completa Cresson, larvae in green walnut husks was 28.4% in laboratory no-choice tests. In choice tests, the rate of parasitism of walnut husk fly versus olive fruit fly larvae in olives was 11.5 and 24.2%, respectively.

Testing the impact of laboratory reared indigenous leafroller (Lepidoptera: Tortricidae) parasitoids (Hymenoptera: Ichneumonidae, Braconidae) on sentinel hosts in controlled orchard releases

Leafrollers can experience high levels of indigenous parasitism in organically managed apple orchards and the augmenta- tive release of specific parasitoid species to suppress these secondary pests may be advantageous in orchards converting to non- chemical pest management. Caged and uncaged releases of two ichneumonid (Apophua simplicipes (Cresson) and Glypta variegata Dasch) and two braconid (Macrocentrus linearis Nees and Apanteles polychrosidis Viereck) koinobiont endoparasitoids of the obliquebanded leafroller, Choristoneura rosaceana (Lepidoptera: Tortricidae) on host-infested potted apple trees were conducted to assess the parasitoids' abilities to find and successfully parasitize sentinel hosts under orchard conditions. Seasonal timing of the trials varied for each parasitoid species, based on their relative performance under simulated summer/fall conditions in laboratory tri- als. After the release of five or fifty parasitoid females, the mean percent parasitism of leafroller larvae collected from infested trees ranged from 0 to 75% depending on the parasitoid species involved. Although caged releases tended to increase the percentage of live parasitized hosts in release treatments, uncaged releases provided a more realistic assessment of the parasitoid's ability to seek and find hosts within an infested area over a longer period. Release of the large, solitary A. simplicipes, had the most significant impact on the host population density.

The effect of intraspecific competition on progeny sex ratio in Gonatocerus spp. for Homalodisca coagulata egg masses: Economic implications for mass rearing and biological control

The eVect of the number of simultaneously ovipositing females on progeny sex ratio of three Homalodisca coagulata egg parasitoids, Gonatocerus ashmeadi, G. triguttatus, and G. fasciatus was investigated in the laboratory. When one female Gonatocerus parasitoid was present, progeny production was strongly female biased producing »1 male:8 females, 1:14 and 1:9 for G. ashmeadi, G. triguttatus and G. fasciatus, respectively. Increasing the number of simultaneous ovipositing females from 1 to 3, signiWcantly increased percentage of male oVspring by up to 61% for all three Gonatocerus species. Overall percentage male oVspring ranged from 31–34%, and did not signiWcantly diVer among species. Overall mean percentage parasitism by G. ashmeadi was up to 16% higher compared with G. triguttatus and G. fasciatus, whereas, average clutch size and mean progeny production was more than twofold higher for G. fasciatus when compared with G. ashmeadi and G. triguttatus. Additional laboratory trials with only G. ashmeadi demonstrated that when H. coagulata egg masses consisted of 2–13 eggs, 67–100% of females allocated one male oVspring per egg mass. When one egg mass was presented to Wve female G. ashmeadi searching concurrently, male oVspring was up to 22% higher compared with male oVspring from host eggs exposed to one female G. ashmeadi. Experiments indicated that allocation of male eggs was not inXuenced by constant female-to-female contact for 19 h prior to host exposure, and female G. ashmeadi did not alter allocation of male eggs when sibling and non-sibling conspeciWcs were contesting host egg masses. The economic signiWcance for mass rearing is substantial. For G. ashmeadi at 2003 production costs and under optimal conditions, estimated cost per female was $1.42, under suboptimal conditions, cost per female increased to $4.51; a 218% increase in production cost per female parasitoid. © 2006 Elsevier Inc. All rights reserved.

Effect of cold storage on the performance of Telenomus busseolae Gahan (Hymenoptera: Scelionidae), an egg parasitoid of Sesamia nonagrioides (Lefebvre) (Lepidoptera: Noctuidae)

Cold storage of both pupae and adults of Telenomus busseolae Gahan (Hymenoptera: Scelionidae) was investigated. Firstly, the effect of storage at three temperatures (4, 8, 12 ± 1 °C), for seven time durations (1, 2, 4, 6, 8, 10, and 12 weeks) and four preimaginal ages of T. busseolae pupae (9, 11, 13, and 15 day old parasitized eggs) in the eggs of Sesamia nonagrioides (Lefebvre) (Lepidoptera: Noctuidae) was studied. Laboratory performance of emerged parasitoids was measured and compared to that of the unstored (control) parasitoids. Storage had a significant adverse effect on mean adult emergence. T. busseolae could be stored for 4 weeks at 4 and 8 ± 1 °C but only 2 weeks storage was possible at 12 ± 1 °C. Furthermore, mean percentage parasitism of host eggs by emerged parasitoids was reduced and F1 progeny sex ratio of the parasitoid became more male biased with increasing length of storage treatment. There was no effect of cold storage on the mean adult emergence of F1 progeny. Secondly, storage of adult T. busseolae was investigated at two temperatures (8 and 12 ± 1 °C) and six different storage times (1, 2, 4, 8, 12, and 16 weeks). Percent survival was reduced with longer storage times. Even though parasitoids survived for 12 weeks at both 8 and 12 °C temperatures, storage times influenced percent parasitism. Parasitoids stored for 8 weeks or longer parasitized fewer eggs than parasitoids stored for shorter times. Neither percent emergence nor sex ratio of F1 progeny was affected by storage temperatures and storage times. The current study suggests that T. busseolae adults were successfully stored up to 12 weeks although some reduction in percent parasitism occurred at both temperatures. The ideal storage temperature for T. busseolae adults was 12 ± 1 °C. Results are discussed in relation to potential mass production and release of T. busseolae for biological control of S. nonagrioides.

Distribution, host range, and climatic constraints on Centistes gasseni (Hymenoptera: Braconidae), a South American parasitoid of cucumber beetles, Diabrotica spp. (Coleoptera: Chrysomelidae).

The genus Diabrotica includes a large number of pest species, including some of the most important crop pests of the Americas. The parasitoid Centistes gasseni Shaw is the first braconid to be described parasitizing Diabrotica in South America, and high natural infestations are reported. Field and experimental observations on the host range, distribution and biology of this parasitoid are described. Centistes gasseniwas collected in southern Brazil, eastern Paraguay and northeastern Argentina, in a region comprising humid lowlands and highlands, and cool temperate to warm subtropical climates, with regular rainfall in excess of 1300 mm. Three Diabroticaspecies, D. limitata (Sahlberg), D. speciosa (Germar) and D. viridula (Fabricius) were found to host the parasitoid, with mean percent parasitism of 5.4, 2.0 and 1.0%, respectively. Diabrotica speciosa and D. viridula are the two most important pest Diabroticaspecies in South America. Laboratory experiments with field-collected beetles and parasitoid cocoons indicated that C. gasseni overwinters in adult host beetles, remaining dormant in its live host below developmental temperatures. A potential distribution of C. gasseni in North America is proposed based on its known climatic range and the distribution of the main pest species of adult overwintering North American Diabrotica.

Effect of plant structure on host finding capacity of lepidopterous pests of crucifers by two Trichogramma parasitoids

Generalist parasitoids are confronted by different plant structures and differences in host abundance and distribution within and among plants. These differences may affect host finding success. In this study, we investigated the effect of plant structure on host finding success of two Trichogramma species using three species of Lepidoptera and three structurally, but closely related crucifers (cabbage, broccoli, Brussels sprouts). We also determined the searching pattern of female parasitoids by characterizing the distribution pattern of parasitized eggs according to leaf surface and plant height. Neither species of parasitoid showed a preference for any of the three plant varieties. We found significant effects of plant variety, pest species, and parasitoid species on parasitism and only the parasitoid × plant structure interaction was significant. Mean percent parasitism generally tended to decrease with an increase of plant structural complexity and was greatest with cabbage, lowest with Brussels sprouts, and intermediate with broccoli. On simple plant structures, both wasp species performed well on the three varieties of plants. Females parasitized more eggs at the base of plants and on the inferior side of leaves. This study demonstrates that plant structure can constrain parasitoids while they are foraging for hosts.

Parasitoid wasps on Epinotia granitalis (Lepidoptera: Tortricidae) larvae

Parasitism by parasitoid wasps on the larvae of Epinotia granitalis (Butler), which is a wood-injuring pest in coniferous plantations, was observed for 6 years in two plantation forests of Chamaecyparis obtusa (Sieb. et Zucc.) Endl. and Cryptomeria japonica (L.f.) D. Don that were adjacent to each other and consisted of trees of almost the same size. E. granitalis larvae were collected from early April to mid-May, then reared in glass bottles, and finally examined to determine whether or not the larvae had been parasitized. Five species of parasitoids were recorded; two of them were classified in Ichneumonidae, Campoplex spp. A and B, and others in Braconidae, Iconella repleta Papp, Bassus cingulipes Nees von Esenbeck, and Bracon spp. In all 6 years, parasitism was first observed around 20 April. Species composition of parasitoids differed greatly among years and between forests. In 1 year in each forest the percentage parasitism increased significantly with the sampling date, indicating that not all parasitoids attack the same developmental stage of E. granitalis. The mean percentage parasitism per year was significantly higher in the forest of C. obtusa [11.5(%) ± 5.7(SD)] than in the forest of C. japonica (3.0 ± 4.1). These results suggest that the efficiency of parasitoid adults in finding hosts differed between the two forests.

Spatially aggregated parasitism on pea aphids, Acyrthosiphon pisum , caused by random foraging behavior of the parasitoid Aphidius ervi

We investigated the role of the foraging,behavior of the parasitoid wasp Aphidius erui in producing nonrandom spatial patterns of parasitism among pea aphids, Acyrthosiphon pisum. We measured spatial variability in percent parasitism by determining the number of aphids and percent parasitism in 40 sampling plots (0.65-m 2 circles) located within a homogeneous alfalfa field. In one replicate of this experiment, mean parasitism of aphids was 18.7%, and percent parasitism showed density-independent aggregation (i.e., greater than random variability in percent parasitism among sampling plots). In the other replicate, mean parasitism was 56.3%, and percent parasitism was not aggregated among plots. We used a combination of field observations of parasitoid foraging and mathematical models to explore these results. In particular, we asked whether the presence or absence of density-independent aggregation at different mean percent parasitism can be explained even if parasitoids forage randomly, without changing their behavior in response to encounters with aphids. Observations show that parasitoids tend to move short distances between nearby alfalfa stems (mean = 10.8 cm), and the turning angle between successively visited stems was uniformly distributed. We incorporated this behavior into both simulation and analytical models of parasitoid foraging. The models show the same pattern as that observed in the field: parasitism is aggregated in a density-independent fashion when mean percent parasitism is low but not when mean percent parasitism is high. Therefore, density-independent aggregation in percent parasitism does not necessarily imply behavioral responses of parasitoids to host encounters and previously parasitized hosts.

Effect of Landscape Structure on Parasitoid Diversity and Parasitism in Agroecosystems

The armyworm (Pseudaletia unipuncta) was used as a model host insect to explore the influence of agricultural landscape structure at two spatial scales on larval parasitoid species richness and rates of larval parasitism in southern Michigan. First, within fields, we compared parasitoid communities in maize fields near, and distant from, a hedgerow edge. Second, we replicated these studies within a complex landscape (agricultural fields of small size embedded in a landscape with abundant hedgerows and woodlots) vs. a simple landscape (agricultural fields of large size embedded in a landscape with few hedgerows and woodlots). The structural differences between the simple and complex agricultural landscapes were characterized by analysis of aerial photographs and digital land‐use data. After landscape analysis, three maize fields from each area were selected for the experimental studies. The complex landscape contained fields that were 75% smaller, had 63% more perimeter of wooded field edge per hectare of field area, and had 81% more field edge in wide hedgerow than fields in the simple landscape. Fields in the simple landscape had 74% and 53% more field edge in herbaceous roadside and crop‐to‐crop interfaces, respectively, than did fields in the complex landscape. In the six selected maize fields, third and fifth instar P. unipuncta were released individually onto maize plants 5 m and 90 m from a hedgerow edge. Larvae were recovered after 4‐5 d and reared in the laboratory to record parasitoid emergence. Parasitoid species diversity was similar in both landscape types (simple landscape: four species; complex landscape: five species). Mean percentage parasitism was significantly higher in the complex landscape than in the simple landscape (13.1% vs. 2.4%) but was not affected by the location within fields (near hedgerows vs. distant from hedgerows) in either landscape type.

Field Studies on the Effect of Pesticides on the Diamondback Moth (Lepidoptera: Plutellidae) and Parasitism by Diadegma insulare (Hymenoptera: Ichneumonidae)

The effects of pesticides on the diamondback moth, Plutella xylostella (L.), and parasitism by Diadegma insulaTe (Cresson) was studied at the Michigan State University Entomology Research Farm during July and August 1990. The range of diamondback moth parasitism from three different locations in Michigan was 60.8–83.3%. Pesticides used were Bacillus thuringiensis var. Kurstaki (Berliner), thiodicarb, thiodicarb + B. thuringiensis , chlorpyrifos, permethrin, ICIA 0321 (pyrethroid), and esfenvalerate. Diamondback larval moth mortality was not Significantly different among the treated plots but was significantly higher than in untreated plots 3 d after spraying. The change in number of diamondback moth larvae (0 versus 3 d), and diamondback moth and D. insulare pupae (0 versus 6 d) was similar in all treatments. Parasitism by D. insulare in treated plots was not affected 6 d after spraying with any pesticide tested. Mean percentage parasitism by D. insulare in guard rows was also not different. However, the percentage parasitism in cages 3 d after treatment with permethrin, azinphosmethyl, or B. thuringiensis or untreated was 7.8, 13.3, 81.5, and 79.4%, respectively. Parasitism by D. insulare of the diamondback moth larvae seems to be influenced by the vegetation or refuge plants present outside the experimental plots. Diamondback moth parasitism by D. insulare may not be affected severely if pesticides are judiciously used.