The influence of host plant species on parasitism of pollen beetles (Meligethes spp.) by Phradis morionellus

Field experiments were conducted at Ultuna in central Sweden in order to de termine if parasitism of pollen beetle larvae (Meligethes aeneus Fabr. and Meligethes viridescens Fabr. (Coleoptera: Nitidulidae)). by Phradis morione llus Holmgr. (Hymenoptera: Ichneumonidae) differed between the host plants spring rape, Brassica napus L., and white mustard, Sinapis alba L. Parasiti sm by P. morionellus varied between 25% and 53%. There was no consistent di fference between proportion parasitism on the two host plant species among the four studied fields. There was a significantly higher risk of parasitiz ation on S. alba than on B. napus in two fields. The opposite occurred in a third field and in a fourth held no difference between parasitism on plant species was found. Density of host larvae affected parasitization by P. mo rionellus on B. napus, with lower parasitism at higher pollen beetle densit ies. The next most common parasitoid species, after P. morionellus, encount ered on pollen beetle larvae was Diospilus capito Nees (Hymenoptera: Bracon idae). The level of total parasitism by these two species was between 31% a nd 66%. Interspecific competition of P. morionellus with D. capito through multiparasitism was independent of both held location and plant species. Th e level of multiparasitism was, in all cases, less than 6.3%.