Performance of the thistle gall fly, Urophora cardui, in relation to host plant nitrogen and mycorrhizal colonization

Abstract

SUMM.'VRYMany hypotheses have been developed to explain the adaptive nature of insect galls. One of these, the nutrition hypothesis, states that gall formers have advantages over other insects because gall tissue provides a better (higher quality) food source than unmodified tissue. However, this has rarely been experimentally tested. In a test of this hypothesis, we grew plants of Cirsium arvense (L.) Scop, tn a factorial design with two main treatments: the addition of nitrogen (to enhance foliar N levels) and of fungicide (to reduce colonization of roots by arbuscular mycorrhizal fungi). Mycorrhizal fungi have been shown previously to reduce the X concentration of host plants. Plants were exposed to adult gall flies, Urophora cardui L., and maintained through one season to allow maturation of galls.Reduction of the percentage mycorrhizal colonization by fungicide resulted in an elevation of total stem N comparable to that achie\'ed by X addition, but gall N concentration remained unchanged in all treatments. Nitrogen application elevated stem N levels when mycorrhizal fungi were present, but apphcation of both compounds together did not result in any increase over either single treatment. Fungicide application resulted in larger galls, which contained more lar\-al chambers, with more live, and heavier, larvae. However, the main effects of N were not significant, as N addition only increased fly performance on plants where mycorrhizas were not reduced.It is suggested that U. cardui gall inhabitants can manipulate N at an optimal level and thus might conform to a modified version of the nutrition hypothesis. Mycorrhizal colonization might reduce gall fly performance by delaying the appearance, or impairing the quality, of secondary nutritive tissue in the gall. Future tests of the nutrition hypothesis should include a consideration of the plant's mycorrhizal status.

Arbuscular mycorrhizal fungi influence life history traits of a lepidopteran herbivore

“…Some leaf-quality variables which are important for herbivores and which are influenced by AMF are summarised in Table 1. Several studies have shown that the growth of herbivores on mycorrhizal and non-mycorrhizal plants differs; some herbivore species performed better on mycorrhizal plants (Gange and West 1994;Gehring and Whitham 1994) while others performed better on nonmycorrhizal plants (Rabin and Pacovsky 1985;Gange and West 1994;Gehring and Whitham 1994;Gange and Nice 1997).…”

Positive effects of an arbuscular mycorrhizal fungus on aphid life history traits

“…More recently, Gange and West (1994) found that reducing mycorrhizal levels of ®eld-grown Plantago lanceolata L. resulted in higher levels of damage by chewing insects, while a laboratory trial demonstrated that performance of the generalist chewing insect Arctia caja L. was lower on highly mycorrhizal plants. Meanwhile, Gange and Nice (1997) demonstrated that the performance of the thistle gall¯y, Urophora cardui (L.) Scop., was increased if natural levels of mycorrhizal colonization of its host plant were reduced by fungicide application. Root-feeding insects may also be negatively aected by AM fungi, as found that survival and growth of black vine weevil, Otiorhynchus sulcatus (Fab.…”

Three‐Way Interactions among Mutualistic Mycorrhizal Fungi, Plants, and Plant Enemies: Hypotheses and Synthesis

“…However, additional studies with two of these species-Plantago lanceolata (Gange and West 1994) and soybean (Rabin and Pacovsky 1985)-demonstrated a decline in herbivore fitness on host plants associated with mycorrhizal fungi, suggesting a need for further study. Studies found that in five out of nine perennial species examined-cottonwood (Gehring and Whitham 2002), Scots pine (Manninen et al 1998(Manninen et al , 1999(Manninen et al , 2000, Cirsium arvense (Gange and Nice 1997), oak (Rieske 2001), and perennial rye grass (Vicari et al 2002)-herbivore fitness declined on host plants associated with mycorrhizal fungi. However, in two out of five of these species-cottonwood (Gehring and Whitham 2002) and Scots pine (Manninen et al , 2000-different herbivores also saw an increase in fitness when feeding on mycorrhizal hosts.…”