We tested how nitrogen availability affects suitability of
ectomycorrhizal (ECM) and non-mycorrhizal Pinus
sylvestris (L.) for a polyphagous insect herbivore. Seedlings
grown on sterile agar at two N levels, were infected
or not with the ECM fungus Cenococcum geophilum (Sow.) Ferd. &
Winge.
Ultrastructural observations showed
that C. geophilum formed well-developed ectomycorrhizas in pine
roots. After mycorrhizal establishment,
oviposition preference of the polyphagous tarnished plant bug
(Lygus rugulipennis Popp.) was determined in three
different experiments. The duration of these experiments and
the amount of irradiance were different. Nitrogen
level or mycorrhizal treatment did not have significant effects
on oviposition preference. However, mycorrhizal
seedlings were favoured as an oviposition site at the lower
N level, but only in expt 1. Total or individual terpene,
total resin acid and total or individual sugar concentrations
in pine shoots were not altered by treatment. Total
phenolics concentration in pine shoots and roots was also the
same in different treatments. Of individual resin
acids, the concentration of dehydroabietic acid was increased
after mycorrhizal treatment. Total resin acid, abietic
acid and sandaraco pimaric acid concentrations correlated
negatively with the number of Lygus eggs, but the
correlations were only marginally significant. Of individual
terpenes, β-pinene and bornylacetate concentrations
had a significant positive correlation with the number of Lygus
eggs. Greater amounts of N predictably increased
total N concentration in pine seedlings in expts 1 and 2, but
mycorrhizal infection increased total nitrogen
concentration only in expt 3. Total N concentration did not
correlate with the number of Lygus eggs in expts 1
and 2. An early stage of mycorrhizal development did not
provide mycorrhizal benefits to the host plant as a higher
nutrient concentration or as increased growth. These results
suggest that host–plant quality might be involved in
selection of oviposition site by L. rugulipennis, but
ectomycorrhizal infection at this level does not seem to be an
important mediator.