The Natural Control of Population Balance in the Knapweed Gall-Fly (Urophora jaceana)

Abstract

All use subject to JSTOR Terms and Conditions * Until I937 the knapweed gall-fly was known in this country as Urophora solstitialis (L.), but it had long been known that continental U. solstitialis was usually a gall-fly of thistles. However, gall-flies bred from the continental knapweed Centaurea jacea were found to differ from the thistle species, and were described as new by Hering (I935) under the name jaceana. Collin (1937) has found that the British specimens from knapweed are in fact jaceana, and not solstitialis. The generic name of the knapweed gall-fly is a point of dispute. Collin (I937) follows Hendel (I927) in accepting

Eurytoma robusta (Hymenoptera: Eurytomidae), a local key factor in the population dynamics of Urophora cardui (Diptera: Tephritidae): a comparative analysis

“…E. robusta has a broad host range, unlike the monophagous parasitoid E. serratulae, and its oviposition period is often not well synchronized with the gall development of U. cardui (Zwölfer et al, in prep. ), as previously reported by Varley (1947) for an E. robusta population attacking galls of Urophora jaceana Hering in knapweed flower heads. E. robusta females prefer U. cardui galls containing thirdinstar larvae, but are unable to assess the optimal U. cardui gall sizes for oviposition (Zwölfer et al, in prep.…”

Seasonal Changes in Oak Leaf Tannins and Nutrients as a Cause of Spring Feeding by Winter Moth Caterpillars

“…described here, suggested by Professor G. C. VarThe population dynamics of oak-feeding Lepi-ley, provides evidence in favor of this hypothesis. doptera in Wytham Wood have been studied for several years by Varley andGradwell (1958a, 1963b). They conclude that the fluctuations in density from year to year, largely synchronous for all early-feeding species, are determined by the density-independent action of weather, but that the relative population levels of the different species are largely governed by differing efficiencies of the specific parasites, acting in a delayed densitydependent manner (Varley 1947).…”

“…doptera in Wytham Wood have been studied for several years by Varley andGradwell (1958a, 1963b). They conclude that the fluctuations in density from year to year, largely synchronous for all early-feeding species, are determined by the density-independent action of weather, but that the relative population levels of the different species are largely governed by differing efficiencies of the specific parasites, acting in a delayed densitydependent manner (Varley 1947). While parasitism may be sufficient to control at low level the less common Lepidoptera species, which seem to suffer from more efficient parasites, parasitism of the more abundant species is unable to prevent the host density from occasionally reaching the food limit.…”

The Evolution and Ecology of Closely Related Species Living in the Same Area

“…(c) Populations controlled by interaction with predator or parasite populations (a view supported by observations on insect populations, e.g., Varley (1947». A classic demonstration that predators may control the balance of species in a plant community is found in the experiments of Jones (1933) who showed that various agriculturally desirable and undesirable combinations of species could be created from an initial mixed population by controlling the time and intensity of grazing by a predator (the sheep). Many comparable examples can be found in the agricultural literature, but studies on natural vegetation are extremely rare, badly documented, or derive from uncontrolled experiments.…”