Effect of Insecticide Usage on Parasitism of Chortoicetes terminifera (Walker) (Orthoptera: Acrididae)

Parasitism of adult Chortoicetes terminifera by Trichopsidea oestracea Westwood and Blaesoxipha rufipes (Macquart) was assessed to discern any effect of chemical control during an outbreak in the 1995–96 season in eastern New South Wales. In the Moree and Gunnedah regions the effects of ground control of bands and aerial control of bands and swarms on parasitism were compared with untreated controls. In the Hunter region only the effect of ground control of bands could be measured. Within both the Moree and Gunnedah regions there were no differences in parasitism rates by either T. oestracea or B. rufipes which could be attributed to chemical control. Parasitism by T. oestracea was significantly higher in the Hunter region than in all treatments in the Moree and Gunnedah regions, except untreated sites in the Gunnedah region. Parasitism by T. oestracea was significantly higher in the Gunnedah region than in the Moree region. Regional differences in parasitism by T. oestracea are attributed to intrinsic climatic and landuse differences between the regions rather than coincident regional differences in insecticide usage. There were no treatment or regional differences in parasitism by B. rufipes. Parasitism levels recorded during the study were atypically low, a fact attributed to a severe drought which preceded the outbreak. Emigration of C. terminifera took place during the study but is not considered to have affected trends in parasitism.     

Parasitism of the Wheat Stem Sawfly (Hymenoptera: Cephidae) in Montana

The wheat stem sawfly,Cephus cinctusNorton (Hymenoptera: Cephidae) was first found in wild grasses and soon became an economically important pest of wheat after cultivation began in the northern Great Plains. Of the species of parasitoids that attackC. cinctusin wild grasses, onlyBracon cephi(Gahan) andB. lissogasterMuesebeck (Hymenoptera: Braconidae) have been found in wheat. Levels of parasitism vary between wheat-producing regions in Montana. Parasitism levels were increased by releasing individuals from a region with high levels of parasitism into a region where parasitism levels were low. This suggests that there has been unequal rates of parasitoid adaptation from wild grasses to wheat in different regions of Montana.     

Evidence from a gelechiid leaf-tier on mesquite (Mimosaceae: Prosopis) that semi-concealed Lepidopteran biological control agents may not be at risk from parasitism in Australian rangelands

Parasitism can limit the effectiveness of weed biological control agents. In this study, we surveyed larval and pupal parasitoids of the leaf-tying moth, Evippe sp. #1 (Gelechiidae), a biological control agent released widely in rangeland Australia against mesquite (Mimosaceae: Prosopis spp.). This moth was expected to be vulnerable to parasitism, as it has a semi-concealed feeding habit, is a member of a well-represented family, and is a herbivore on a well-represented family of trees and shrubs. We conducted surveys in four regions across Australia to determine whether parasitoids were responsible for differential performance of the moth in rangeland Australia, and what the consequences might be for the release of further semi-concealed Lepidoptera. The parasitoid fauna was found to be diverse, but the composition was similar across the four regions surveyed. It included primary and hyperparasitoids of both larvae and pupae. Parasitism rates were generally low, rarely above a few percentage points in any one survey, and therefore, unlikely to threaten moth populations. There was also no relationship between parasitism rates and leaf-tie abundance. These results suggest that any new semi-concealed lepidopteran biological control agents in rangeland Australia might also be parasitised by a diverse fauna. However, a better understanding of what determines parasitism rates is required to predict whether it would be sufficient to threaten biological control.     

The biology of the phonotactic parasitoid, Homotrixa sp. (Diptera: Tachinidae), and its impact on the survival of male Sciarasaga quadrata (Orthoptera: Tettigoniidae) in the field

Abstract.

• 1 Unlike most parasitoids, tachinid flies of the tribe Ormiini use sound to locate their hosts. Although thought to exert selection pressure on their host's calling behaviour, little is known about the biology of ormiines. Accordingly, this study reports the biology and impact of the ormiine Hornotrixa sp. upon calling males of the univoltine bushcricket Sciarasaga quadrata Rentz in south-western Australia.
• 2 Populations of adult S.quadrata were monitored in the field over two successive calling seasons. Females, which do not call, were not parasitized by Hornotrixa sp., but the risk of parasitism for males increased as the 3-month calling season progressed. Parasitism did not commence until c. 2 weeks into the calling season, but by the end of the season up to 87% of surviving males were parasitized.
• 3 Parasitized males lived for 14 days and were found singing until their penultimate evening before death in the field. Unparasitized males lived on average 69 days and a maximum of 119 days.
• 4 Multiparasitism of hosts was common, with up to sixteen fly larvae found within parasitized males. The number of fly larvae within hosts significantly increased at the end of the season. However, successful emergence of fly larvae from hosts, as well as pupal size, significantly decreased as more than one fly larva developed within the host.
• 5 Hornotrixa sp. has a long pupal duration of 30–31 days at 20°C. As a consequence, only one complete fly generation, which overwinters in the pupal stage, is likely within each host generation.
• 6 No evidence for differential (size-bias) mortality by Hornotrixa sp. on male S.quadrata was found. The size of parasitized and unparasitized males collected in the field was not significantly different.
• 7 It is concluded that Hornotrixa sp. is a significant mortality factor acting on the survival of adult male S.quadrata.

     

Seasonal variation in the effectiveness of the leaf-feeding beetle Zygogramma bicolorata (Coleoptera: Chrysomelidae) and stem-galling moth Epiblema strenuana (Lepidoptera: Tortricidae) as biocontrol agents on the weed Parthenium hysterophorus (Asteraceae)

Variation in the effectiveness of biocontrol agents on the weed Parthenium hysterophorus L. was evaluated at two properties (Mount Panorama and Plain Creek) in Queensland, Australia for four years (1996-2000) using a pesticide exclusion experiment. At Mount Panorama, higher levels of defoliation by the leaf-feeding beetle Zygogramma bicolorata Pallister and galling by the moth Epiblema strenuana Walker in 1996-97 coincided with an above average summer rainfall, but in the following three years with below average summer rainfall the defoliation and galling levels were significantly lower. Biocontrol had significant negative impact on the weed only in 1996-97 with no major impact in the following three years. At Plain Creek, galling by E. strenuana was evident in all the four years, but varied significantly between years due to non-synchrony between P. hysterophorus germination and E. strenuana emergence. At Plain Creek biocontrol had limited impact on the weed in 1996-97 and 1997-98, with no significant impact in the following two years. Over the 4-year period, defoliation and galling resulted in 70% reduction in the soil seed bank at Mount Panorama, but the reduction in the soil seed bank at Plain Creek due to galling was not significant. Effectiveness of Z. bicolorata and E. strenuana was dependent on weather conditions and as a result had only limited impact on the weed in three out of four years.     

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.     

Parasitism of the Nematode Heterodera glycines by the Fungus Hirsutella rhossiliensis as Influenced by Crop Sequence

The effect of crop sequence on parasitism of second-stage juveniles (J2) of Heterodera glycines by Hirsutella rhossiliensis was investigated. Data were collected from plots of a long-term crop rotation experiment established in 1982. Crop sequences included (i) continuous monoculture of corn and soybean; (ii) annual rotation of the two crops; and (iii) 1, 2, 3, 4, or 5 years of each crop following 5 years of the other crop. The nematode J2 density and percentage of J2 parasitized by the fungus were determined at planting, midseason, and end of season in 1997 and 1998. A significant effect of the crop sequence on parasitism of J2 was observed at midseason in both years and at end of season in 1998. In plots of first-year soybean following 5 years of corn, fungal parasitism increased from an undetectable level at planting to 2% and 4% of J2 parasitized by ends of season in 1997 and 1998, respectively. Fungal parasitism was similar in plots of second-through-fifth-year soybean after 5 years of corn and in plots of soybean monoculture. Parasitism of J2 in the soybean plots in annual rotation with corn increased from undetectable and 2% at planting to 6% and 23% at midseason in 1997 and 1998, respectively. The effect of crop sequence on the fungal parasitism of J2 may be attributed to a density-dependent relationship between the parasite and its host. Season also affected the fungal parasitism; percentage of J2 parasitized by the fungus was the highest at midseason and the lowest at planting.     

Parasitoids of the African wild silkmoth,Gonometa postica (Lepidoptera: Lasiocampidae) in the Mwingi forests,Kenya

Gonometa postica Walker produces silk of high quality, but it is affected by parasitoids attack. A study on the parasitism of G. postica larvae and pupae on host and non‐host plants were undertaken for the first and second generations, corresponding to the long (March–May) and short (October–December) rainy seasons in 2006 at six field sites, three each in the Imba and Mumoni forests of Mwingi, eastern Kenya. All freshly spun cocoons of G. postica were sampled at each site from a total of 100 trees of host plants and other non‐host plants where they have migrated before pupation. The cocoons were kept individually in fine net‐sealed plastic vials to determine percentage parasitism. Two dipterans and four hymenopteran larval–pupal parasitoids were identified from the two forests. The most common parasitoids were Palexorista sp. (Diptera: Tachinidae) and Goryphus sp. (Hymenoptera: Ichneumonidae) with parasitism ranging from 1.8 to 32.7% and 2.2 to 7.5%, respectively. Parasitism varied significantly according to host or non‐host plants, seasons and sites. This study indicates that, of the six parasitoid species recovered, only two had a significant impact in reducing the quality of the cocoons.     

Parasitism and ant protection alter the survival of the lycaenid Hemiargus isola

Abstract. 1. Although the majority of lycaenid–ant associations is facultative, few studies have documented the protection benefits provided by ants to lycaenids that are tended facultatively (Pierce & Easteal, 1986; Peterson, 1993).
2. Larvae of Hemiargus isola (Lepidoptera: Lycaenidae) feeding on Dalea albiflora are tended facultatively by several species of ant. In 1999 and 2000, the levels of parasitism and the identities of attendant ants were determined for larvae of H. isola . In addition, the presence of ants was manipulated experimentally to determine the efficacy of protection provided by attendant ants to H. isola .
3. Lycaenids were parasitised by a braconid wasp, Cotesia cyaniridis (Riley), and a tachinid fly, Aplomya theclarum (Scudder). In 1999 and 2000, 62 and 65% of larvae were parasitised; the percentage of the population parasitised did not differ significantly between years. In both 1999 and 2000, parasitism by the braconid wasp C. cyaniridis accounted for > 99% of all parasitism events.
4. Four species of ant, Crematogaster sp., Dorymyrmex sp., Forelius sp., and Formica sp., were associated with 88–99% of the tended lycaenids collected in both 1999 and 2000. For both years, there was a single, numerically dominant species associated with >80% of the tended larvae collected, but the identity of this numerically dominant ant differed between years.
5. Experimental exclusion of ants from D. albiflora plants resulted in 78% larval mortality as a result of parasitism, nearly twice that of larvae that were tended by ants on unmanipulated plants.     

Comparison of Diadegma insulare (Hymenoptera: Ichneumonidae) and Microplitis plutellae (Hymenoptera: Braconidae) as biological control agents of Plutella xylostella (Lepidoptera: Plutellidae): field parasitism, insecticide susceptibility, and host-searching

Parasitism of Plutella xylostella (L.) third and fourth instars was evaluated in a cabbage field in Geneva, NY, in 1999. Over the entire season, average parasitism was 33.6% for third instars and 53.6% for fourth instars, and the main parasitoids were Diadegma insulare (Cresson) and Microplitis plutellae Muesbeck. In the early season, total parasitism was low, and mainly caused by D. insulare. However, later in the season, parasitism reached >80% for the fouth instars and 50% for the third instars. Our survey indicated that M. plutellae heavily parasitized P. xylostella, and provided higher parasitism rates than D. insulare in the late season. Comparison of these two species in laboratory bioassays indicated there were no significant differences in susceptibility to four insecticides commonly used in crucifer fields. For both parasitoids, an experience with P. xylostella on a damaged leaf increased their host-searching efficacy. Compared with M. plutellae, D. insulare was a better host-searcher both for the naive and the experienced adults. Although both parasitoids can cause high mortality rates of P. xylostella, D. insulare may be more suitable to be released in fields to enhance natural control against P. xylostella.     

The influence of host distribution, sex, and size on the level of parasitism by Itoplectis conquisitor (Hymenoptera: Ichneumonidae)

Abstract. 1. Parasitism of the bagworm, Thyridopteryx ephemeraeformis Haw. (Lepidoptera: Psychidae), on red cedar was studied at Manassas, Va., U.S.A.
2. Fifteen parasitoids, five of which were hyperparasitoids, attacked the pupal stage of the bagworm. The Ichneumonid Itoplectis conquisitor alone accounted for 58% of the parasitized bagworms.
3. Parasitism by I conquisitor (Say) was inversely related to host size, but independent of host distribution within a tree. Male bagworms experienced disproportionately higher levels of parasitism than females. We conclude that it is small size that renders bagworms susceptible to parasitism rather than sex per se.
4. Ovipositor lengths of I.conquisitor were insufficient to penetrate the larger bagworm hosts (>57 mm); and in fact, as bagworm size increased, the proportion of the I.conquisitor population capable of penetrating the pupa declined abruptly. We suggest that the mechanical difficulties with oviposition, and perhaps the defensive capabilities of larger hosts, are responsible for the relationship between host size and per cent parasitism.
5. I.conquisitor could be an important selective agent for bagworm size at pupation, but it is not likely to act as a significant control agent of population density.     

Biological control of house flies Musca domestica and stable flies Stomoxys calcitrans(Diptera: Muscidae) by means of inundative releases of Spalangia cameroni(Hymenoptera: Pteromalidae)

The efficacy of the pupal parasitoid Spalangia cameroni Perkins as a biological control agent was tested against house flies Musca domestica Linnaeus and stable flies Stomoxys calcitrans (Linnaeus) in one dairy cattle and two pig installations in Denmark. Weekly releases of S. cameroni from April through to September-October 1999 and 2000 resulted in significant suppressions of house fly populations to below nuisance level, whereas no effect on stable flies was found. Parasitism was significantly higher in the release years compared to the control years, but was below 25% averaged over the fly season for each farm. A statistical model based on a functional relationship between the innate capacity of increase of the two fly species and three explanatory variables (air temperature, fly density and parasitism) provided a fairly good fit to data with the abundances of house flies and stable flies explained mostly by temperature, but intra- and interspecific competition, and parasitism had a significant effect as well. Overall, the model was capable of explaining 14% and 6.6% of the total variation in data for house fly and stable fly, respectively. Spalangia cameroni was the predominant parasitoid to emerge from exposed house fly pupae, but from mid summer onwards Muscidifurax raptor Girault & Sanders (Hymenoptera: Pteromalidae) was also quite common. The study indicated that biological control of house flies can be an efficient alternative to chemical control.     

Insect parasites of scarabs from sugarcane fields in southern Queensland (Coleoptera: Scarabaeidae)

Data on parasitism of scarabs were collected by sampling sugarcane in the Bundaberg region of southern Queensland during 1993–1997. The scoliid Campsomeris tasmaniensis (Saussure) parasitised third instars of the melolonthines Antitrogus parvulus Britton, Lepidiota crinita Brenske, Lepidiota negatoria Blackburn and Lepidiota noxia Britton, and of the ruteline Anoplognathus porosus (Dalman). Levels of parasitism were low (< 2%). Third instars of A. porosus were also parasitised by the dexine tachinid Rutilia sp. Puparia of Prosena sp. (Tachinidae: Dexiinae) were found in soil with larvae of L. noxia . Larvae of Prosena sp. are parasites of scarab larvae elsewhere, and are probably parasites of scarabs in southern Queensland. Adults of A. parvulus , L. crinita and L. noxia were parasitised by the tachinine tachinid Palpostoma sp. at levels of 2–18.4%.     

Natural parasitism of Romanomermis iyengari (Welch) (Nematoda: Mermithidae) on various species of mosquitoes breeding in rice fields in Pondicherry,India

Natural habitat of Romanomermis iyengari, an entomophilic mermithid nematode, is rice field. Parasitism of this nematode on mosquito larvae breeding in rice fields in Pondicherry, India, has been studied in 1979. The present study was to find out its persistence in the same habitat, after many years of agricultural practices. Evaluation of parasitism on mosquito larvae was carried out during two rice cultivation seasons at a schedule of 1 week prior, and 1 and 2 weeks after transplantation of paddy. Larval stages of mosquito species such as Culex tritaeniorhynchus, Culex vishnui, Anopheles vagus, and Anopheles subpictus showed parasitism of R. iyengari. Parasitism was the highest on C. tritaeniorhynchus with a maximum of 15.38% during season I and 17.85% during season II. Followed by this, it was on A. vagus (10.82% and 9.43%), C. vishnui (7.37% and 8.69%), and A. subpictus (3.70% and 6.36%). The overall density of mosquito larvae and level of parasitism of R. iyengari on them during season II was significantly higher than that of season I. The parasitism was maximum at 1 week before transplantation and showed a decreasing trend towards 2 weeks after transplantation, during both the seasons. The level of natural parasitism of R. iyengari during the present investigation was higher than that observed in the same ecosystem almost 35 years ago. This is clearly an indication towards the capability of the nematode to withstand several years of agricultural practices, including pesticide and agrichemical application. Hence, if applied at sufficient level and frequency, this nematode could act as a successful biocontrol agent against mosquitoes breeding in rice fields.     

Natural enemies of the maize cob borer,Mussidia nigrivenella (Lepidoptera: Pyralidae) in Benin,West Africa

Mussidia nigrivenella Ragonot is a pest of maize cobs in West Africa. It significantly reduces maize yields and grain quality, with quantitative losses of 2-25%at harvest, and up to 10-15% indirect losses due to an increase in storage pest infestation levels. Infestation by M. nigrivenella also significantly increased the susceptibility of maize to Aspergillus flavus infection and subsequent aflatoxin contamination. Surveys conducted in different agro-ecological zones of Benin on cultivated and wild host plants during 1994-1997 revealed one egg parasitoid, three larval parasitoids and one pupal parasitoid attacking M. nigrivenella. Egg parasitism was scarce on all host plants sampled and in all four agro-ecological zones. Parasitism by larval and pupal parasitoids was usually less than 10%, and varied with host plant species. Both larval and pupal parasitoids were rare or absent in cultivated maize fields. The solitary chalcidid pupal parasitoid, Antrocephalus crassipes Masi, was the predominant species, contributing approximately 53% of the observed mortality. Logistic regression analysis indicated that this parasitoid was more prevalent on fruits of Gardenia spp. (Rubiaceae) than on the other host plant species including maize used by M. nigrivenella, and was most abundant between February and September. The differences in parasitoid diversity and parasitism between Benin and other regions suggest that there are opportunities for biological control through introduction of exotic parasitoids or using the 'new association' approach, which uses natural enemies of closely related host species that occupy similar ecological niches to the target pest.     

Non-target parasitism of the endemic New Zealand red admiral butterfly (Bassaris gonerilla) by the introduced biological control agent Pteromalus puparum

The New Zealand red admiral butterfly, Bassaris gonerilla (F.) (Lepidoptera: Nymphalidae), has been known as a non-target host for the introduced biological control agent Pteromalus puparum (L.) (Hymenoptera: Pteromalidae) for at least 35 years, but the level of parasitism has never been quantified. Pre-imaginal mortality in B. gonerilla was assessed over the southern summer of 2000/01 at six field sites in the Christchurch area of the South Island, New Zealand. Individual eggs and larvae were identified by tagging the stem of the Urtica ferox Forst.f. plant on which they were found and the fate of these individuals was checked weekly. These data were used to construct a partial life table for B. gonerilla. Egg mortality was very high (95%), with parasitism by an unidentified Telenomus sp. Haliday (Hymenoptera: Scelionidae) causing 57% mortality. Mortality in the larval and pupal stages increased at a constant rate with age and the major mortality factor was disappearance, which was assumed to be a result of predation and dispersal of larvae. The introduced biological control agent P. puparum parasitized 14% of B. gonerilla pupae sampled. However, parasitism by another exotic parasitoid, the self-introduced Echthromorpha intricatoria (F.) (Hymenoptera: Ichneumonidae), was even higher at 26%. A survey of pupal parasitism in three regions of New Zealand (Wellington, Christchurch, and Dunedin) revealed overall parasitism levels of 67% by E. intricatoria and 8% by P. puparum, but due to the difference in emergence times of B. gonerilla and its parasitoids, these are likely to be overestimates of percent parasitism. It is concluded that P. puparum has permanently enhanced mortality in B. gonerilla, but the level of mortality is low relative to egg parasitism by Telenomus sp., larval disappearance mortality, and pupal mortality due to E. intricatoria parasitism. To determine if this level of pupal parasitism has had population effects will require more data and the development of a population model for B. gonerilla.     

Endoparasitoids of larval Anomis privata (Lepidoptera: Noctuidae), major pest of Hibiscus syriacus (Columniferae: Malvaceae)

Endoparasitoids of Anomis privata larvae include five species in three families of two orders. In this work, two species of Hymenoptera Braconidae (Cotesia sp., Microplitis sp.), one species of Ichneumonidae (Mesochorus vittator) and two species of Diptera Tachinidae (Exorista (Podotachina) sorbillans, Timavia amoena) were investigated. Of the 261 larvae of A. privata examined, 32 had a parasite, so the rate of parasitism was 12.26%. Parasitism by taxon was the highest, at 10.35% (27 individuals), in Cotesia sp. in Hymenoptera Braconidae. Parasitoids of Braconidae and Ichneumonidae were larval parasitoids. A parasitic insect of Tachinidae was a larva–pupal parasitoid. Solitary parasitoids included Microplitis sp. in Braconidae and E. sorbillans in Tachinidae. Gregarious parasitoids included Cotesia sp. in Braconidae, M. vittator in Ichneumonidae and T. amoena in Tachinidae. There was also a multiparasitoid (T. amoena) and two superparasitoids (Cotesia sp., M. vittator). A larva of A. privata sought feed even after it was parasitized every parasitoid investigated in this study, so five species of parasitoids were all koinobiont.     

泽兰实蝇寄生状况及其对紫茎泽兰生长与生殖的影响

泽兰实蝇(Procecidochares utilis)作为天敌控制紫茎泽兰(Eupatorium adenophorum)已经在国内外得到广泛的应用,但关于它对紫茎泽兰控制的有效性和防治现状的研究尚不深入。该文通过对攀西地区紫茎泽兰入侵危害严重的路域生态系统中泽兰实蝇寄生状况的抽样调查,初步研究了泽兰实蝇对紫茎泽兰生长,特别是生殖能力的影响。寄生率的调查采用随机取样法,分别从植株寄生率和枝条寄生率两方面进行,并比较不同生境下及不同年龄植株枝条寄生率的差异;选取相同数量、相同部位的各年龄植株寄生和非寄生枝条(对照),分别调查其花枝量、头状花序数及结实量,进行分析比较。研究结果表明:1)植株寄生率与枝条寄生率有显著差异(p<0.05),分别为71.67%和17.30%,前者显著高于后者;样方调查结果,成熟群落中枝条寄生率为17.48枝·m^-2;1虫瘿·枝条^-1的枝条占所有寄生枝条的92.30%;2)湿润生境下紫茎泽兰的枝条寄生率为20.27%,显著高于干旱生境下的枝条寄生率(9.33%)(p<0.05);3)不同年龄植株枝条寄生率有差异,0~1年生植株枝条寄生率分别为36.36%和21.56%,显著高于2~4年生的植株枝条寄生率,分别为13.50%、8.82%和12.16%(p<0.05);4)在目前的寄生强度下,泽兰实蝇对紫茎泽兰枝条的直径、花枝量、头状花序数及结实量均无显著影响(p>0.05)。     

Cuterebrid parasitism (Rogenhofera bonaerensis) on the shrubland mouse (Akodon molinae), in Argentina.

We evaluated the influence of temperature, rainfall, and host relative abundance on Rogenhofera bonaerensis (Diptera, Cuterebridae) parasitism prevalence in shrubland mouse (Akodon molinae) populations in central Argentina, from February 1983 to December 1987. Parasitism did not vary significantly with host age: juvenile-subadults (32%), adults (26%), and old adults (29%). Females were more frequently parasitized (36%) than were males (20%). There was no correlation between parasitism and reproductive activity. Infested hosts were recorded most commonly in summer (January to March, 19%), and in fall (April to June, 30%). During the dry season, July through November, cuterebrid parasitism averaged only 3%. The monthly prevalence of parasitism throughout the year was not associated either with monthly precipitation nor with mean monthly temperature at the time of sampling. But a 2 to 3 month time-lag effect of both climatic variables on parasitism was recorded. Bot fly prevalence was correlated to an index of host density. We propose climate and host availability as important factors affecting bot fly parasitism in the semiarid shrubland of central Argentina.     

Field parasitism of nontarget weevil species (Coleoptera: Curculionidae) by the introduced biological control agent Microctonus aethiopoides Loan (Hymenoptera: Braconidae) over an altitude gradient

The parasitoid, Microctonus aethiopoides Loan (Hymenoptera: Braconidae) was introduced into New Zealand in 1982 to control the alfalfa pest, Sitona discoideus Gyllenhal (Coleoptera: Curculionidae). Studies have shown that a number of nontarget weevil species are attacked in the field by this parasitoid. A field study was carried out to investigate nontarget parasitism by M. aethiopoides over an altitudinal sequence from the target host habitat (alfalfa) into native grassland. Three locations were selected for the study, and at each, the alfalfa growing in the valley floor was sampled annually for parasitism of the target pest that ranged between 17 and 78%. At progressively higher altitudes, three further grassland sites at each location were sampled monthly during spring to autumn for up to 6 yr. Weevil densities were estimated, species identified, and dissections carried out to determine reproductive status and parasitism. Almost 12,000 weevils were collected during the study, which were identified as 36 species in total from the three locations. Eight weevil species were found to be parasitized, including S. discoideus, the target host that was found at all sites. Parasitism of nontarget species was approximately 2% overall but varied with location, site, and season. Substantial nontarget parasitism was found at only one of the locations, with up to 24% parasitism of a native weevil, Nicaeana fraudator Broun (Coleoptera: Curculionidae), recorded. Another species, Irenimus egens (Broun) (Coleoptera: Curculionidae), was also found at this location at similar population densities but was attacked far less by M. aethiopoides. Results are discussed in relation to weevil phenology.