Boletín de la AeE

Species richness of stem-boring insects from 15 grass species (Poaceae) couid be related to the life history and architecture of their host plant. Herbivoreparasitoid assemblages were analyzed by dissection of grass shoots from several habitats and by rearing larvae.

Insect numbers on perennials increased with shoot length, whereas annuals were not attacked, both parameters explaining 97% of the variance. Results from chemical analyses and reciproca! transplant experiments helped to explain the fíndings.

Mean number of parasitoids per host did not change with grass shoot length, thereby suggesting that plant characteristics determined herbivore but not parasitoid diversity. Two-thirds of the on average 35 insect species per perennial grass were parasitoids, and half of the species were monophagous.

Number of parasitoid species centred upon the gall-inducing chalcid wasps of the phytophagous genera Telramesa spp. (Hym. Eurytomidae) and the gall midges (mostly of the genus Lasioplera and Mayetiola, Dipt. Cecidomyiidae) showed a great variability. Both wasps and midges were equally large (3-5 rnm) and equally immobile. Species could be characterized by their mode of galling (conspicuous gallers, inconspicuous gallers, or non-gallers), and by their number per gall or site of attack (solitary or gregarious larvae). The phytophagous chalcid wasps (14 spp.) suffered from three times more parasitoid species than the gall midges (24 spp.). ín the gall midges, but not in the chalcid wasps, species with complex galls and many larvae per gall supported significantly more parasitoid species. In these midges (but not in the chalcid wasps), physiological advantages of the galling habit appeared to be associated with the disadvantage of an increased mortality by parasitoids.