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

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 discussed in relation to our own observations. The breakdown of a mass propagation seems to be caused by the interaction of several factors of varying importance, such as predators, crowding and exhaustion of the substratum, decrease in vitality, and climatic influences. This requires further investigation. Zusammenfassung Ernährungsphysiologische, bionomische und ökologische Unterschiede erfordern für die drei von Börner (1952) unterschiedenen mitteleuropäischen Weißtannenläuse der Gattung Dreyfusia eine weitere vorläufige Aufteilung. Ökologisch läßt sich der Vertilgerkreis dieser Arten wie folgt gliedern: 1. Vorherrschende Arten mit meist hoher Abundanz, Konstanz und oft enger Wirtsbindung; 2. Zurücktretende Arten mit geringer Abundanz, oft hoher Konstanz und lockerer Wirtsbindung und 3. Gelegentlich auftretende Arten mit geringer Abundanz und Konstanz sowie großem Wirtskreis. - Die für diese drei Gruppen festgestellten Vertilgerarten werden namhaft gemacht und über ihre Nahrungswahl, Wirtsspezifität, Erscheinungszeit, Fraßdauer sowie Diapause Einzelheiten mitgeteilt. Einige der Vertilgerarten treten noch bei geringer Wirtsdichte auf, andere sind an Massenbefall der Läuse gebunden. - Der Zusammenbruch einer Massenvermehrung der Stammläuse ist das Ergebnis voneinander abhängiger und zugleich von klimatischen Einflüssen überlagerter Vorgänge (Vertilgerkreis, Raum- und Nahrungskonkurrenz, Vitalitätsverminderung, Klima). Jede dieser Faktorengruppen kann von Fall zu Fall in verschiedenem Ausmaß wirken. Eine ausführlichere deutsche Zusammenfassung der Ergebnisse unserer „Neueren Untersuchungen über die Weißtannenläuse der Gattung Dreyfusia C. B. u. ihren Vertilgerkreis” erscheint im „Anzeiger für Schädlingskunde” 1956.

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