The Predator Complex of the White-Fir Woolly Aphids (Genus Dreyfusia, Adelgidae)

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...

Cases of biological control restoring natural systems

“…The failure of several species can be attributed to poor climate matching and not vetting specificity to the target species. It should not be surprising, how ever, that the predators did not bring the adelgid below damaging thresholds, since in Europe the popu lation dynamic of the entire predator complex is largely inversely density dependent and not regulative (Eichhorn, 1969a), and tree resistance and weather both have strong influences on the pest's population dynamics (Franz, 1956;Pschorn-Walcher and Zwölfer, 1956). None of the 15 species from India and Pakistan that were released in the southern Appalachians established (Amman and Spears, 1971).…”

“…The ineffectiveness of the predators exported from Europe has also been attributed to poor synchronization with one or both adelgid generations, and predator preferences for adelgids on the stem versus on the twigs (Pschorn-Walcher and Zwölfer, 1956;Eichhorn, 1968). A main weakness of the predaceous beetles L. erichsonii, A. obliterata, and S. impexus is that they are univoltine, whereas the adelgid has two to three generations per year.…”

Dreyfusia nordmannianae in Northern and Central Europe: potential for biological control and comments on its taxonomy

“…While most species are probably native predators of European Adelgidae, it cannot be ruled out that some came more recently from the area of origin of D. nordmannianae . Pschorn‐Walcher and Zwölfer () and Eichhorn () state that the predator complex of D. nordmannianae in Europe is less diverse than that of the closely related species D. piceae and D. merkeri , and that the twig‐form of D. nordmannianae has an even poorer complex than the stem‐form. Eichhorn () suggested that the poorer predator complex may be due to the lack of a pronounced aestivosistens generation, which could be an obstacle for bi‐ and multivoltine predatory species.…”

“… References: a: Eichhorn (); b: Eichhorn (); c: Eichhorn (); d: Eichhorn (); e: Kucera (); f: Pschorn‐Walcher and Kraus (); g: Pschorn‐Walcher and Zwölfer (); h: Varty (); i: Wylie (). …”

“…(), Neoleucopis obscura seems to be another, rare species, preying specifically on D. prelli . The biology and phenology of N. atratula is described in the study by Pschorn‐Walcher and Zwölfer () and Eichhorn (). It is well synchronized with the phenology of D. piceae , because larvae are active during the egg‐laying period of both hiemosistens in spring and the aestivosistens in late summer.…”