Low Parasitism Rates Research Articles

Evaluation of the Community of Native Eulophid Parasitoids on <I>Cameraria ohridella</I> Deschka and Dimic in Urban Areas

The parasitoid complex associated with the exotic leafminer Cameraria ohridella Deschka and Dimic (Lepidoptera: Gracillariidae), which attacks horse chestnut (Aesculus hippocastanum L.), was studied in the urban environment of Turin (northern Italy). The studies were carried out over 5 yr after the first detection of the pest in our region in 1999. To evaluate parasitism, 438,029 leaf mines were examined over the 5-yr period, of which 29,033 were found to be parasitized (6.6%). Also, ornamental broadleaf trees attacked by other native gracillariid leafminers and located in the proximity of the target horse chestnut trees were sampled. A total of 11 parasitoid species (Hymenoptera: Eulophidae) were recorded on C. ohridella, and the most common species were Minotetrastichus frontalis (Nees), Closterocerus trifasciatus Westwood, and Pnigalio agraules (Walker). The first species accounted for >77.5% of all parasitoids collected. Cirrospilus talitzkii Boucek was found for the first time in 2005. The high population level of the pest and the low parasitism rate show that the parasitoid complex is currently inadequate to contain C. ohridella populations effectively. The most frequent parasitoids of the moth were also found on the most common broadleaf trees in the studied area, showing how native leafminer parasitoid species are able to switch to other hosts. These results show that both native and broadleaf plants species may potentially provide an important reservoir of parasitic wasps to help protect a simple biotope, such as the urban environment, from pests.

Why Does the Frequency of Nest Parasitism by the Cuckoo Differ Considerably Between Two Populations of Warblers Living in the Same Habitat?

AbstractThe rate of nest parasitism is a product of two interacting phenomena: host selection by cuckoos and defence by hosts. In our study area the rate of nest parasitism by cuckoos is significantly lower in the great reed warbler (GRW; Acrocephalus arundinaceus) than in the reed warbler (RW; A. scirpaceus), even though they breed in the same habitat and their reproductive biology is similar. We hypothesized that the difference in the proportion of parasitized nests may reflect a narrow selection of host by cuckoos (they prefer RW nests) or/and the relatively better alien egg discrimination in the GRW. In the egg discrimination experiment the GRW rejected the higher proportion of alien eggs than the RW. However, in both species the discriminative ability considerably varied in time, both within the day and within the breeding cycle. A logistic regression model suggests that the GRW would be a frequent host if only nest parasites could exploit the period of its lowest sensitivity to alien eggs. We conclude that the relatively low rate of nest parasitism in the GRW may reflect both its good discriminative ability and the low number of cuckoos that are specialized in dumping eggs to nests of this warbler. The adaptation of cuckoos to the particular host species may involve not only production of mimetic eggs, but also adjusting activity to temporal changes in sensitivity to alien eggs in the host.

Establishment and expansion of a Thaumetopoea pityocampa (Den. & Schiff.) (Lep. Notodontidae) population with a shifted life cycle in a production pine forest, Central-Coastal Portugal

In 1997, a temporally shifted population of Thaumetopoea pityocampa (Den. & Schiff.), was recorded for the first time in a restricted area of the oldest National Pine Forest of Portugal—the National Pine Forest of Leiria. This new population larval development takes place during the summer (summer population—SP), while in the normal population it takes place during the winter (winter population—WP). Between 2000 and 2003, we assessed the distribution and expansion of the SP, and quantified the differences in population densities, mortality and major life history traits with the WP within the Leiria Forest. The SP occurred at higher population levels (5–35% attacked trees) in a large continuous area of young plantations with high tree density, the same area where it was initially discovered. These densities were higher than the ones recorded for the WP in any part of the forest (0–12%), although both populations coexist geographically. The temporally shifted population preferably attacked young plantations. It also expanded south, following the direction of the dominant winds during adult flight. SP had a lower fecundity than the WP, however it benefited from lower egg mortality due mostly to a low rate of parasitism. It also benefited from a lower mortality in the early larval instars leading to low levels of colony extinction. SP larvae take half the time compared to the WP larvae, to reach the same final mass, probably due to the fact that they develop under higher temperature and global radiation. Separated emergence timings of adults lead to an allochronic isolation of the two populations. However, if this might represent an allochronic speciation event has to await further ecological and evolutionary studies of the system.

Incidence of the introduced parasitoids Cotesia kazak and Microplitis croceipes (Hymenoptera: Braconidae) from Helicoverpa armigera (Lepidoptera: Noctuidae) in tomatoes, sweet corn, and lucerne in New Zealand

Abstract Two parasitoids, Cotesia kazak (Telenga) and Microplitis croceipes (Cresson) have been successfully introduced into New Zealand to improve control of Helicoverpa armigera (Hubner), tomato fruitworm. C. kazak has spread throughout the North Island, but M. croceipes is restricted largely to its release areas in crops in the Gisborne and Hawke’s Bay regions in the East Coast of the North Island and in pine plantations in the central North Island. Rates of mortality of H. armigera from parasitism were studied in processing tomato crops where these parasitoids are a key component of an integrated pest management programme, and in sweet corn, lucerne and soybeans. C. kazak was the dominant parasitoid in tomatoes and soybeans. It emerged from small H. armigera larvae and usually killed the host before it caused major damage to fruit. M. croceipes emerged from larger host larvae than C. kazak . Death rates from parasitism of H. armigera larvae in tomatoes increased from less than 1% caused by native parasitoids prior to parasitoid introductions, to 25–45% in 1988 and 70–80% in 1996 following the establishment of both introduced parasitoids. M. croceipes was the most common parasitoid in lucerne and total mortality from all parasitoids was similar to that in tomatoes. Low rates of parasitism by C. kazak in sweet corn were attributed to the reduced penetration of the adult parasitoid into sweet corn fields and to the protection afforded by sweet corn when H. armigera larvae burrowed into corn cobs. The implementation of an IPM programme based partly on the effectiveness of these parasitoids has contributed to a decrease in insecticide applications to processing tomatoes.

Parasitism of canola aphids in France in autumn

In light of a recent increase in the aphid populations on young canola (Brassica napus) in autumn in Northwestern Europe, we carried out a survey of their parasitoid species during this season. The study was done in France from 1998 to 2001 using different sampling methods. Results highlighted the presence of two main species of Aphidiinae, Aphidius matricariae and Diaeretiella rapae (and to a lesser extent Aphidius ervi) on Myzus persicae, and of D. rapae on Brevicoryne brassicae. Nine other Aphidiinae species were found occasionally as well as some Aphelinidae parasitoids and hyperparasitoids. There was no difference in parasitoid species pattern between fields in Northern and Southern France. The principal parasitoid species found in the fields were reared in the laboratory to confirm their ability to develop on the canola aphids. Aphidius matricariae and D. rapae were reared successfully on M. persicae, but a low parasitism rate was obtained for A. ervi on this aphid. This study showed that A. matricariae and D. rapae could develop on aphids on canola and that they are present naturally in canola in autumn. However, the parasitism rate is low in autumn, so the options could be the use of these parasitoids in augmentative release biological control programs or in an IPM project on canola fields during this time of the year.

Biological characteristics of Evania appendigaster (L.) (Hymenoptera: Evaniidae) in different densities of Periplaneta americana (L.) oothecae (Blattodea: Blattidae)

Abstract Evania appendigaster (L.) is a solitary endoparasitoid of oothecae of the American cockroach, Periplaneta americana (L.). In this work we report our studies on the parasitism of P. americana oothecae by E. appendigaster and the biological characteristics of this egg parasite, as it is affected by host density and mating. Cockroach oothecae were presented for individual females at different densities (one, two, three, four, five and six oothecae) during 2 days of exposition time. Reproduction was determined to be biparental and arrhenotokous. Host density did not influence sex ratio and longevity. The greatest reproductive efficiency of the females occurred in the first 14 days of their lifetime. At higher densities low parasitism rates were accompanied by a high percentage of dead oothecae. The percentage of parasitism decreases with increase in host density.

Satellite flies (Leucophora personata, Diptera: Anthomyiidae) and other dipteran parasites of the communal bee Andrena agilissima (Hymenoptera: Andrenidae) on the island of Elba, Italy

Solitary and presocial aculueate Hymenoptera are parasitized by a range of dipteran species in the families Anthomyiidae, Bombyliidae, Conopidae, Phoridae, and Sarcophagidae that are likely to impact on their hosts. We undertook a study over several years of a univoltine and communal bee, Andrena agilissima, and its main dipteran parasites, in particular the satellite fly Leucophora personata (Diptera: Anthomyiidae). Behavioural and ecological data were collected from one nesting aggregation of the host bee on the island of Elba, Italy, from 1993 to 2003, and from a foraging site of the bee, ca 5 km from the nesting aggregation. Other Diptera associated with A. agilissima at the field site were the bee fly Bombylius fimbriatus (Bombyliidae), the conopid fly Zodion cinereum (Conopidae), and the scuttle fly Megaselia andrenae (Phoridae). The phenology of the Diptera broadly overlapped with that of their host across the season of activity (end of April and all of May). Diurnal activity patterns differed slightly; L. personata in particular was active at the host's nesting site before A. agilissima. Female satellite flies also showed a range of behaviours in gaining entry to a host nest. We summarize published data on this and other Leucophora species that parasitize Andrena host bees. Host bees returning to their nests occasionally undertook zig‐zag flight manoeuvres if followed by a satellite fly that were generally successful in evading the fly. Satellite flies that entered a nest, presumably to oviposit, were less likely to remain therein if another host bee entered the same nest, suggesting that one advantage to communal nesting for this host is a reduction in brood cell parasitism by L. personata. We provide the first clear evidence for parasitism by a Zodion of any Andrena host. Both L. personata and M. andrenae concentrated their parasitic activities in the zone of the host nesting aggregation with highest nest densities. Three of the Diptera, L. personata, B. fimbriatus, and Z. cinereum, seemed to have extremely low rates of parasitism whilst that of M. andrenae appeared low. Though they have refined parasitic behaviour that allows them to gain entry into host nests (L. personata, B. fimbriatus, and M. andrenae) or to parasitize adults (Z. cinereum), these parasites seem not to impact upon the dynamics of the host A. agilissima at the nesting aggregation, and the host possesses traits to reduce parasitism.

Effect of temperature on the development and viability of Gryon gallardoi (Brethes) (Hymenoptera: Scelionidae) parasitizing Spartocera dentiventris (Berg) (Hemiptera: Coreidae) eggs

The development and viability of Gryon gallardoi (Brethes) (Hym.: Scelionidae) in Spartocera dentiventris (Berg) (Hem.: Coreidae) eggs were studied under four temperatures: 15, 20, 25, and 30 +/- 1 degree C, with a 12-h photophase. No parasitoid developed at 15 degrees C. Otherwise, viability reached 98.8% without varying significantly over the temperature range tested. The duration of development for males and females was inversely proportional to the temperature increase, varying respectively from 46.2 +/- 0.13 and 47.1 +/- 0.11 days (20 degrees C) to 13.3 +/- 0.07 and 13.4 +/- 0.06 days (30 degrees C). Males developed faster than females. The values estimated for the lowest thermic thresholds of development and the thermic constants were 15.5 degrees C and 185.19 DD for males and 15.6 degrees C and 192.31 DD for females, respectively. Given the average weather conditions in Porto Alegre, RS (30 degrees 01' S and 51 degrees 13' W), Brazil, G. gallardoi could annually produce 8.54 and 8.07 generations of males and females, respectively. The low rates of parasitism observed in the field during the first generation of its host may be due to the small number of G. gallardoi generations in this period.

Plant population size and isolation affect herbivory of Silene latifolia by the specialist herbivore Hadena bicruris and parasitism of the herbivore by parasitoids

Habitat fragmentation can affect levels of herbivory in plant populations if plants and herbivores are differentially affected by fragmentation. Moreover, if herbivores are top-down controlled by predators or parasitoids, herbivory may also be affected by differential effects of fragmentation on herbivores and their natural enemies. We used natural Silene latifolia populations to examine the effects of plant population size and isolation on the level of herbivory by the seed predating noctuid Hadena bicruris and the rate of parasitism of the herbivore by its parasitoids. In addition, we examined oviposition rate, herbivory and parasitism in differently sized experimental populations. In natural populations, the level of herbivory increased and the rate of parasitism decreased with decreasing plant population size and increasing degree of isolation. The number of parasitoid species also declined with decreasing plant population size. In the experimental populations, the level of herbivory was also higher in smaller populations, in accordance with higher oviposition rates, but was not accompanied by lower rates of parasitism. Similarly, oviposition rate and herbivory, but not parasitism rate, increased near the edges of populations. These results suggests that in this system with the well dispersing herbivore H. bicruris, habitat fragmentation increases herbivory of the plant through a behavioural response of the moth that leads to higher oviposition rates in fragmented populations with a reduced population size, increased isolation and higher edge-to-interior ratio. Although the rate of parasitism and the number of parasitoid species declined with decreasing population size in the natural populations, we argue that in this system it is unlikely that this decline made a major contribution to increased herbivory.

Phenology and parasitism rates in introduced populations of Pseudacteon tricuspis, a parasitoid of Solenopsis invicta

We documented patterns of seasonal abundance and rates of parasitism in introduced populations of Pseudacteon tricuspis Borgmeier, a phorid parasitoid of the red imported fire ant, Solenopsis invicta Buren. Adult P. tricuspis populations were censused at monthly intervals for 1 year at three sites in northern Florida. Censuses were conducted by aspirating phorids attracted to disturbed S. invicta mounds. Pseudacteon tricuspis adults were present in every month at all sites, although abundances varied greatly among sites and over time. The highest densities of flies (up to 453 censused at 10 disturbed S. invicta mounds in 30 min) were observed in November, and changes in abundance over time were positively correlated among sites. Sex ratios were usually male biased. Parasitism rates were evaluated by collecting workers from field colonies and monitoring them in the laboratory for evidence of parasitism. Parasitism rates were very low – always less than 1%. The average parasitism rate per colony over 16 colonies and 2 years was 0.058%. No pupariation occurred within the first 8 days of collection, suggesting parasitism by P. tricuspis induced behavioral changes in parasitized workers that precluded such workers from our collections. If so, true field parasitism rates may be several times higher than measured here, yet still low in an absolute sense. These low parasitism rates can be reconciled with observed adult phorid densities by considering the large number of host ants present at the study sites.

Overwintering Biology, Food Supplementation and Parasitism of Eggs of Homalodisca Coagulata (Say) (Homoptera: Cicadellidae) by Gonatocerus Ashmeadi Girault and Gonatocerus Morrilli (Howard) (Hymenoptera: Mymaridae)

The efficiency of parasitism of Gonatocerus ashmeadi Girault and G. morrilli (Howard) (Hymenoptera: Myramidae) on glassy-winged sharpshooter, Homalodisca coagulata (Say) (Homoptera: Cicadellidae), eggs of varying age was documented. Excised leaves with H. coagulata eggs of age 1 to 9 d were presented to mated female parasitoids in Petri dishes as well as on intact foliage. Parasitism rates for both species were high with 3 females in Petri dishes parasitizing virtually all eggs in 3 egg masses, and 8 mated females finding and parasitizing almost all eggs from 5 egg masses in small shrubs of Ilex opaca cv cornuta. For both species, parasitism efficiency exceeded 95% within 24 h. Parasitism rates declined for older eggs (8 and 9 d) on excised leaves. Because this did not occur on live plant material, hardening of excised foliage likely contributed to lower rates of parasitism. No cases of superparasitism were observed: Overwintering of the parasitoids was examined by monitoring the fate of eggs of H. coagulata deposited in November. We documented the first case of H. coagulata overwintering as eggs under North Florida conditions. In addition, both G. ashmeadi and G. morrilli were able to overwinter as undeveloped larvae within parasitized eggs under similar conditions. Overwintering parasitoids took approximately 85 d for emergence, which slightly exceeded the minimum predicted for emergence by existing degree day models. Feeding experiments conducted with G. ashmeadi and G. morrilli as food sources are limited during winter/spring; however, our results confirmed that honeydew from Dialeurodes citri (Ashmead) (available in North Florida in late winter/early spring) significantly increased the longevity of both parasitoid species when compared to honey and water.

Life history pattern, host plants, and habitat as determinants of population survival of Pieris napi oleracea interacting with an introduced braconid parasitoid

Abstract Pieris napi oleracea Harris is a native pierid butterfly whose range in New England has decreased since the 1869 invasion of an exotic congener, Pieris rapae (L.). Populations have disappeared from all but a few locations in western Massachusetts, but persist more widely in northern Vermont. Parasitism by the introduced braconid wasp Cotesia glomerata (L.) has been postulated as responsible for this decline. Field exposures of cohorts of P. napi larvae in 2002 showed that, on the summer host plant Sisymbrium officinale (L.) Scoop., parasitism rates of second generation butterfly larvae were very high (100% in Massachsusetts and 66% in Vermont). Direct observations of host plant and habitat use patterns by P. napi females in northern Vermont in 2000 showed that this species changes habitats between the first and second generation, with over 95% of eggs of the first generation of P. napi being laid in wooded habitats on the crucifer Cardamine (formerly Dentaria) diphylla (Michx.) Wood. Second generation butterflies fly in meadows and oviposit on S. officinale and other crucifers in full-sun habitats. The second flight of P. napi is, however, only a partial generation, with many first generation pupae entering diapause. The proportion of butterflies entering diapause increases if eggs are laid on C. diphylla plants at or past the flower bud stage. Of first instar P. napi reared outdoors on pre-flower-bud C. diphylla, 63% (n=64) entered diapause in Massachusetts, but 86% (n=119) did so in northern Vermont. Higher rates of diapause in early larval groups in northern Vermont, plus lower rates of parasitism of second generation larvae in meadows, as compared to Massachusetts, indicate that the impact of C. glomerata on total population survival is lower in Vermont and is likely the reason P. napi has persisted in northern Vermont, but not in western Massachusetts. This case illustrates how non-target effects of parasitoids may be mediated by habitat and life history features of populations of affected insects, which may differ geographically.

Effects of mass releases of Trichogramma brassicae on predatory insects in maize

AbstractWe investigated whether mass releases of Trichogramma brassicae against the European corn borer will have detrimental effects on populations of other natural enemies in maize. In a tiered approach, experiments investigated the host acceptance of T. brassicae towards eggs of Chrysoperla carnea, Episyrphus balteatus, Adalia bipunctata, and Coccinella septempunctata under laboratory, caged, and field conditions. The factitious host, Ephestia kuehniella, and in one case also Mamestra brassicae were used as control. Under laboratory conditions, 53% of the T. brassicae females parasitised C. carnea eggs, 70% parasitised E. balteatus eggs, while 91% of the females parasitised E. kuehniella eggs, with T. brassicae emerging from C. carnea, E. balteatus, and E. kuehniella at rates higher than 81%. No parasitoid offspring emerged from eggs of A. bipunctata and C. septempunctata. However, we observed significantly increased mortality on A. bipunctata eggs, compared to the control, and also found young instars of T. brassicae inside A. bipunctata eggs. In a second experiment where the host acceptance behaviour of the parasitoid female was directly observed for 10 min, 10% of T. brassicae females were found to oviposit in eggs of A. bipunctata, but the development of parasitoid offspring failed. No C. septempunctata eggs, 13% of E. balteatus eggs, 23% of C. carnea eggs, 33% of E. kuehniella egg clusters, and 83% of M. brassicae eggs were accepted for oviposition. In the greenhouse under caged conditions, the parasitism rate of C. carnea eggs (7%) and E. balteatus eggs (0.4%) were significantly lower than of E. kuehniella eggs (21 and 27%, respectively). In the final tier, 3.1% of C. carnea eggs were parasitised by T. brassicae under field conditions. This was significantly less than the observed parasitism rate of E. kuehniella egg clusters (64%). From direct observations of the parasitoids’ host acceptance behaviour and the low parasitism rates observed under caged and field conditions, we conclude that detrimental effects of the mass release of T. brassicae on populations of natural enemies in maize are unlikely to occur.

Inverse density-dependent and density-independent parasitism in a solitary ground-nesting bee in Southeast Brazil

Aspects of the population dynamics of a solitary ground-nesting oligolectic bee, Diadasina distincta (Holmberg 1903) (Hymenoptera Anthophoridae), were studied in relation to the number of parasitized nests in aggregations of different sizes. inversely density-dependent parasitism by bombyliid flies and density- independent mortality by fungi together explained most of the variation in population size. There was a significant negative correlation between the number of nests in an aggregation and parasitism by bombyliid flies. Bombyliids accounted for 19.4% of mortality and two fungal species for an additional 19%. It is hypothesized that male patrolling in larger aggregations accounted for lower parasitism rates by bombyliid flies.

Superparasitism and sex ratio adjustment in a wasp parasitoid: results at variance with Local Mate Competition?

Anaphes nitens is a solitary parasitoid of the egg capsules of the Eucalyptus snout beetle, Gonipterus scutellatus. Some traits of its natural history suggest that Local Mate Competition (LMC) could account for sex ratio adjustment in this species. We tested whether males emerged early, a prerequisite for fully local mating, and investigated the occurrence and effect of superparasitism on adult size and pre-emergence mortality, factors that might influence sex ratio adjustment. We found in field-collected egg capsules that males emerged first. To investigate the effects of superparasitism on adult size, we compared the sizes of parasitoids that emerged early and late from egg capsules collected in the field, and from egg capsules parasitized and superparasitized in the laboratory. Superparasitism reduced parasitoid size, affecting females more strongly than males, and increased pre-emergence mortality. We estimated A. nitens sex ratio and parasitism rate in the field during 2 years in five localities and during 4 years in a sixth. Following LMC we expected an increase in sex ratio (proportion of males) with increasing parasitism rate (assumed to reflect parasitoid density). We found that sex ratio decreased from 0.38 when the parasitism rate was low (0-20%) to 0.21 when parasitism was high (80-100%). In contrast with field results, a laboratory experiment showed that: (1) at a low parasitism level sex ratio was clearly female biased (0.28+/-0.04), (2) at a high parasitism level sex ratio increased (0.40+/-0.07), (3) male larval survivorship was not lower than female survivorship, and (4) low-quality hosts (i.e. superparasitized) were allocated more males. We conclude that LMC cannot explain the sex ratio adjustment observed in the field, even at low parasitism rates, and alternative explications for highly female-biased sex ratios must be found. One such alternative is female-biased dispersal.

Rapid change in the settling behavior of the arrowhead scaleUnaspis yanonensisas an avoidance mechanism against introduced parasitoids,Aphytis yanonensis and Coccobius fulvus

AbstractParasitoids are thought to exert immense selection pressures that shape the traits of herbivores. We examined whether two species of parasitoid wasps,Aphytis yanonensisDeBach et Rosen andCoccobius fulvusCompere et Annecke (Hymenoptera: Aphelinidae), affect the settling behavior of the arrowhead scaleUnaspis yanonensisKuwana (Hemiptera: Diaspididae), in order to demonstrate the evolution of antiparasitism behavior of herbivores using parasitoids in the field. We used the following five methods: a comparison of parasitism risk in different settling behaviors‐parasitoid introduction into a parasitoid‐free population; a comparison of the settling behavior between parasitoid‐present and parasitoid‐free populations; a common garden experiment, in which scales were transferred from parasitoid‐present and parasitoid‐free populations into the same garden; and a laboratory observation of the settling behavior of the first instars derived from the two population categories. Both parasitoids were introduced into a parasitoid‐free population in Wakayama in 1987, and the settling mode of the scales was examined in 1987, 1994, and 1995. The introduction of parasitoids modified the scale‐settling mode so that more crawlers settled under another scale (called burrowing), which was consistent with the results observed in parasitoid‐present (including South China) and parasitoid‐free populations. Moreover, only the burrowing scale exhibited a lower parasitism rate compared to scales settling singly and being burrowed. The common garden experiment demonstrated that scales introduced from the parasitoid‐present population had a greater proportion of burrowers than the parasitoid‐free population, even in the same field cage under parasitoid‐free conditions. Laboratory observations demonstrated that the population difference in parasitism rate was principally due to intrinsic differences in the settling behavior of nymphs; some first‐instar nymphs derived from the parasitoid‐present population burrowed under another scale settled. These results strongly suggest that the natural selection pressure imposed by the parasitoids modified the settling behavior of the arrowhead scale.

Influência da idade dos ovos de Oxydia vesulia no parasitismo de Trichogramma maxacalii

Trichogramma é um dos gêneros de insetos mais importantes no controle biológico. Este trabalho avaliou o potencial de parasitismo de Trichogramma maxacalii em ovos de Oxydia vesulia, em diferentes estágios embrionários. Vinte e cinco ovos de O. vesulia, com um, três, cinco e sete dias de idade foram oferecidos a uma fêmea de T. maxacalii, em ensaio com 15 repetições, por um período de 24 horas. Não houve influência da idade dos ovos de O. vesulia na porcentagem de parasitismo, na viabilidade, na razão sexual e na porcentagem de fêmeas que parasitaram. Foram encontradas baixas taxas de parasitismo (de 6% a 10%) de T. maxacalii em ovos de O. vesulia, embora mais de 70% das fêmeas desse parasitóide tenham realizado o parasitismo em ovos de até cinco dias de idade.

Parasitism of non-target butterflies by Trichogramma brassicae Bezdenko (Hymenoptera: Trichogrammatidae) under field cage and field conditions

Abstract The egg parasitoid Trichogramma brassicae Bezdenko was introduced from Moldavia to western Europe some 30 years ago and since then has been inundatively released to control the European corn borer, Ostrinia nubilalis Hubner, in maize. We investigated parasitism of non-target butterfly eggs by T. brassicae in field cages and under field conditions in Switzerland. Parasitism of eggs (exposed for 24 h) of six non-target species (Papilio machaon L., Clossiana selene Denis and Schiffermuller, Maniola jurtina L., Melanargia galathea L., Hipparchia alcyone Denis and Schiffermuller, Cyaniris semiargus Rottemburg) in field cages of 2×2×2 m ranged between 2.5 and 18.7% when based on a relative high number of released parasitoid females ( 780±273 SD/cage ). This number is 16-fold more compared to agricultural practices (120,000/ha). As a control, Ephestia kuehniella Zeller eggs were glued on the same plant. A difference in parasitism rates between the non-target species and E. kuehniella was only observed for M. galathea. During inundative releases of T. brassicae in maize, sentinel eggs of two suitable non-target hosts and of E. kuehniella were exposed for 72 h. Parasitism rates of E. kuehniella egg masses inside maize fields averaged 40% compared to significantly lower parasitism rates of 26.2 and 12.6% for eggs of H. alcyone and M. galathea. At 2 m distance from the maize field, parasitism rates decreased to 2.3% for M. galathea, 6.1% for H. alcyone, and 9.8% for E. kuehniella while no single egg was found parasitized at 20 m distance from the release fields. We conclude that, although the tested non-target butterflies were all attacked under semi-field and field conditions, effects on non-target butterflies due to mass released T. brassicae are minimal.

Parasitism of different larval stages of Cameraria ohridella

About 20 species of parasitic Hymenoptera havebeen reported from the horse chestnutleafminer, Cameraria ohridella Deschka & Dimic (Lepidoptera: Gracillariidae). Generally,parasitism is low compared to other closelyrelated leafminers and the parasitism levelvaries considerably. Among other reasons,parasitism rates depend on the developmentalstage of the moth and therefore vary with thesampling date. In the current study,investigations on the parasitism of the moth'sfirst generation were carried out in order todetermine which preimaginal stages areparasitized by the most abundant parasiticwasps. Minotetrastichus frontalis (Nees),Pnigalio agraules (Walker) and Chrysocharis nephereus (Walker) (allHymenoptera: Eulophidae) developed as larval orpupal parasitoids and preferred later larvalinstars of the moth. Egg parasitism did notoccur. Overall, the last two of the six larvalinstars (the spinning instars) of the leafminersuffered the heaviest attack. In concordancewith the fairly low parasitism rates, theeffect of the four most abundant chalcidoids onthe leafminer population was negligible, havingno significant influence on the mortality ofC. ohridella. Although the speciescomposition of the parasitoid complex of C. ohridella shows similarities with thesituation found in other closely relatedleafmining moths, it is unlikely that thenaturally occurring chalcidoids will be able toprevent the horse chestnut leafminer fromdeveloping epidemic population densities in thenear future.

IS ATTRACTION FATAL? THE EFFECTS OF HERBIVORE-INDUCED PLANT VOLATILES ON HERBIVORE PARASITISM

We investigated the relationship between parasitoid attraction to herbivore-induced plant volatiles and larval parasitism rates of two closely related heliothine, noctuid moths. Heliothis subflexa Guenee is a specialist on plants in the genus Physalis, while Heliothis virescens Fabricius is an extreme generalist. In North America, these species serve as the only known hosts for the specialist parasitoid Cardiochiles nigriceps Vierick; oviposition into Helicoverpa zea, a non-host, does occur but results in lethal encapsulation of C. nigriceps' eggs. Heliothis virescens larvae are parasitised by C. nigriceps far more frequently than are H. subflexa larvae. Parasitoid attraction to volatiles emitted by tobacco in response to herbivory by H. virescens has previously been demonstrated. Using field experiments, we examined the possibility that pre-detection defenses against parasitoid attraction to herbivore-induced plant volatiles are responsible for H. subflexa's relatively low rates of parasitism by C. nigriceps. Herbivore-damaged plants were significantly more attractive to C. nigriceps than were larvae, larval frass, larval saliva, or damaged leaves alone. Plant species affected parasitoid attraction: tobacco was the most preferred plant species, followed by Physalis angulata, and then cotton. The parasitoid was also more attracted to host species (H. subflexa and H. virescens) than to the non-host species, H. zea. There was an interaction between plant species and herbivore species: each plant species was most attractive when infested by its typical herbivore (e.g., H. virescens on tobacco). We compared these data with those of a previously published experiment on field parasitism of H. virescens and H. subflexa, conducted at the same time and place. Our results indicate that differences in parasitoid attraction to herbivore-induced plant volatiles do not contribute to H. subflexa's relatively low parasitism rate. For the generalist, H. virescens, rates of attraction correspond with rates of parasitism; for the specialist, H. subflexa, they do not. Attraction to H. subflexa on P. angulata is greater than to H. virescens on P. angulata, yet parasitism of H. subflexa is much less than parasitism of H. virescens. These results indicate that pre-detection defenses are not responsible for H. subflexa's low parasitism on P. angulata.