1. Data mostly from the published literature were used to assess the effect of galling on the number of parasitoid species per host species in the phylogeny of nematine sawflies from free external feeders (colonial and solitary) to leaf gallers and shoot gallers.2. The strongest effects of galling were the total elimination of the species-rich cocoon-attacking guild of parasitoids, and eonymphal parasitoids, from the parasitoid community on shoot gallers, all of which are in the genus Euuru.3. All tachinid larval parasitoids were also eliminated by the galling habit. 4.The cumulative effects of these exclusions resulted in a decline in mean number of parasitoid species per host species from almost sixteen species on external colonial feeders to 4.0 species on shoot gallers. 5 . General patterns in per cent parasitism by non-tachinid and tachinid larval parasitoids, eonymphal and cocoon parasitoids, on exposed feeders t o shoot gallers, showed declines in non-tachinid attack and elimination of tachinid, eonymphal and cocoon parasitoids. But leaf gallers tended to be attacked more than exposed feeders by non-tachinid larval parasitoids.6. The galling habit had a long-term impact by reducing the number of parasitoid species attacking nematine sawfly gallers and per cent mortality inflicted, so that natural enemies may have been important as a selective factor in the evolution of galling nematine sawflies.

No abstract

Summary The white fir woolly aphids of Europe are provisionally divided into: 1 a) Dreyfusia piceae form typica, an euryoekous (i. e. ‐ with wide ecological tolerances), obligatory multivoltine form, living on the trunks of Abies pectinata (= alba), and on A. balsamea in Canada; 1b) D. piceae form aggressiva, a facultative multivoltine form, on seedlings, young trees and on trunks of older firs, which shows morphological differences, and is somewhat intermediate between piceae and nüsslini; 2 a) D. nüsslini (= nordmannianae) form typica, mainly univoltine, on needles, twigs and trunks of younger trees in warmer locations; 2 b) D. nüsslini form schneideri, mainly univoltine, on the trunks of old firs in various stands, corresponding morphologically with typica. The predator complex attacking these aphids includes three groups: a) species with regular occurence, great abundance and host specificity, including Aphidoletes thompsoni, Cremifania nigrocellulata, Leucopomyia obscura, Cnemodon dreyfusiae, Laricobius erichsoni, and Pullus impexus; b) species of regular occurence, but with lower abundance and less host specificity, including Aphidecta obliterata, Cnemodon latitarsis, Syrphus arcuatus, Chrysopa ventralis, and Trombidiid mites; c) a number of incidental species. Table 1 shows the choices of food and the prey preferences of the main predators. All species, except the Trombidiids (preying upon adults only), feed on eggs and crawlers, and in later instars also on adult aphids. Neosistentes in diapause are hardly attacked. The number of species comprising the predator complex associated with D. piceae is considerably larger than that associated with D. nüsslini. Fig. 3 illustrates the influence of temperature on the spring activity of Laricobius and Cremifania. Fig. 4 shows the sequence of predators in relation to prey development. The adaptation of predator development to the cycle of the prey is dependend on diapause, and is accomplished in different ways. Egg diapause is found in Pullus only; diapause in young larval instars is present in Chrysopa, and in Leucopomyia; diapause in fullgrown larvae is found in both Cnemodon species, prepupal diapause occurs in Cremifania and Aphidoletes; diapause in adult stages is shown by Laricobius and Pullus, and in the latter species is a second precaution for hibernation. D. piceae typica is present in low densities (ten to several hundred aphids per 10 sq. yd.) in practically all stands examined, even where few firs are to be found. However, outbreaks are more restricted ecologically. Only the Trombidiids, and to a certain extent, the larvae of Pullus (only at densities greater than 500 aphids per 10 sq. yd.) are found at normal (i. e.‐lower) host densities. Laricobius, Aphidecta, Chrysopa, and especially the dipterons, prefer denser prey infestations. Investigations on the predator‐prey relationships were handicapped in 1955 by an great reduction in numbers of D. piceae typica and its uniform replacement by D. nüsslini schneideri. Earlier studies in the literature are d...

Interspecific competition between the 3 principal larval parasitoids of the pine sawfly, Neodiprion sertifer, is of common occurrence when total larval parasitism, and hence multiple attack rates, are high. At the intrinsic level, the ectoparasitoid Exenterus abruptorius is superior to the 2 endoparasitoids, Lophyroplectus luteator and Lamachus eques, respectively, whereas L. luteator is superior to L. eques only. During mass outbreaks of the host competition between Exenterus and Lophyroplectus is most intense, whilst Lamachus fails to build up in the presence of its 2 competitors (Fig. 1). However, due to its superior host finding ability, Lamachus dominates in areas of low host densities where the 2 other species are less efficient (Table 1). L. luteator frequently sustains heavy losses in direct competition with E. abruptorius, but these can be largely compensated in the next generation because its fertility is about 5-times higher than that of Exenterus.

No abstract