Allocation of searching time within the soybean canopy by parasitoids attacking the green cloverworm

We examined the searching behavior of a guild of primary parasitoids which attack the green cloverworm,Plathypena scabra (Fabricius), as well as that of an associated hyperparasitoid. We hypothesized that self-superparasitism is an important constraint on the foraging behavior of primary parasitoids, and therefore these parasitoids should avoid portions of the soybean canopy where parasitized caterpillars accumulate. Conversely, we hypothesized that the hyperparasitoid preferentially searches parts of the canopy where parasitized caterpillars accumulate. In a greenhouse experiment, we found that exposure to parasitoids [eitherCotesia marginiventris (Cresson) orDiolcogaster facetosa Ashmead] resulted in the accumulation of caterpillars lower in the canopy. In a field experiment, we measured the amount of time parasitoids spent searching in each of three strata (upper, middle, bottom) of the soybean canopy. Leaf area in each stratum was used to calculate expected values for search effort. The time spent searching each of the strata was proportional to leaf area for all primary parasitoids, exceptD. facetosa, which spent significantly more time searching the top stratum of plants than predicted by leaf area in that stratum. The hyperparasitoidMesochorus discitergus (Say) tended to search the bottom stratum of the canopy. Thus only one of the three primary parasitoids appears to search in a manner that would reduce its rate of encounter with previously parasitized green cloverworms. The hyperparasitoid searching pattern may increase its probability of encountering parasitized caterpillars, thereby increasing its foraging success.     

A survey of Lygus parasitoids in Sweden

Lygus spp. (tarnished plant bugs) are generalist herbivores and occur as pests on a wide range of crops. In the development of conservation biological control strategies for Lygus spp. in Sweden, more information is needed on the impact of different natural enemies. In this study, we determined the occurrence and the degree of parasitism on adults and nymphs of the most common Lygus species in alfalfa, barley, red clover and oilseed rape in Uppsala in Central Sweden and in Umeå in Northern Sweden. Nymphs and adults of Lygus spp. were collected by sweep netting for estimation of their parasitism level (by dissection) and identification of parasitoid species (from reared Lygus specimens). The dominant Lygus species in both locations was L. rugulipennis (75–99%). Parasitism by Phasia obesa (Tachnidae) on overwintering Lygus adults was recorded in almost every field sampled at both locations. The parasitism level was low at overwintering sites but increased in arable fields during summer. Lygus nymphs collected in Umeå were parasitised by Peristenus pallipes (Braconidae) and in Uppsala by P. relictus, P. pallipes and P. varisae. The hyperparasite Mesochorus globulator (Ichneumonidae) was recorded in Umeå.     

Intrinsic competition between primary hyperparasitoids of the solitary endoparasitoid Cotesia rubecula

1. In nature, competitive interactions occur when different species exploit similar niches. Parasitic wasps (parasitoids) often have narrow host ranges and need to cope with competitors that use the same host species for development of their offspring. When larvae of different parasitoid species develop in the same host, this leads to intrinsic and often contest competition. Thus far, most studies on intrinsic competition have focused on primary parasitoids. However, competition among primary hyperparasitoids, parasitic wasps that use primary parasitoids as a host, has been little studied. 2. This study investigated intrinsic competition between two primary hyperparasitoids, the gregarious Baryscapus galactopus and the solitary Mesochorus gemellus, which lay their eggs in primary parasitoid larvae of Cotesia rubecula, while those in turn are developing inside their herbivore host, Pieris rapae. The aims were to identify: (i) which hyperparasitoid is the superior competitor; and (ii) whether oviposition sequence affects the outcome of intrinsic competition. 3. The results show that B. galactopus won 70% of contests when the two hyperparasitoids parasitised the host at the same time, and 90% when B. galactopus oviposited first. When M. gemellus had a 48 h head start, the two hyperparasitoids had an equal chance to win the competition. This suggests that B. galactopus is an intrinsically superior competitor to M. gemellus. Moreover, the outcome of competition is affected by time lags in oviposition events. 4. In contrast to what has been reported for primary parasitoids, we found that a gregarious hyperparasitoid species had a competitive advantage over a solitary species.     

Hyperparasitism by Mesochorus spp. (Hymenoptera: Ichneumonidae) in Peristenus sp. (Hymenoptera: Braconidae) and development of PCR primers for hyperparasitoid detection

Peristenus sp. pupae collected from Lygus spp. nymphs in 2001 and 2002 were over-wintered in the laboratory. In both years, more than 30% of adults emerging from over-wintering pupae were identified as ichneumonid hyperparasitoids, Mesochorus curvulus Thomson and Meschorus sp. (Hymenoptera: Ichneumonidae). At the end of the over-wintering period, Peristenus sp. males emerged first followed by Peristenus sp. females and finally Mesochorus spp. The male:female ratio in emerging Peristenus sp. adults was skewed towards males. The Internal Transcribed Spacer (ITS) region and the cytochrome oxidase I (COI) gene from Mesochorus spp. were sequenced. ITS sequences were used to develop PCR primers to detect Mesochorus spp. hyperparasitism in the primary host, Lygus spp. PCR analysis of field-collected Lygus spp. nymphs gave similar estimates of Mesochorus spp. hyperparasitism to the rearing protocols (25–28%). Sequence analysis of COI and ITS regions and subsequent restriction endonuclease analysis of ITS PCR products from Mesochorus spp. indicate the presence of two genotypes in the population. The possibility that these two genotypes represent separate or cyrptic species is discussed.     

Larval parasitoids of Lilioceris lilii (Coleoptera:Chrysomelidae) in Sweden and potential for biological control

A survey of the parasitoid complex of lily leaf beetle (Lilioceris lilii) was conducted in Sweden from 2006 to 2007. Total parasitism varied between 21 and 57%. The most abundant parasitoids were the ichneumonids Lemophagus errabundus and L. pulcher and the eulophid Tetrastichus setifer. The potential for biological control is discussed.