Many hypotheses have been developed to explain the adaptive nature
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. in 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 N 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 achieved by N addition, but gall N concentration
remained unchanged in all treatments.
Nitrogen application elevated stem N levels when mycorrhizal fungi were
present, but application of both
compounds together did not result in any increase over either single
treatment. Fungicide application resulted in
larger galls, which contained more larval 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