Susceptibility of ectomycorrhizal and non-mycorrhizal Scots pine (Pinus sylvestris) seedlings to a generalist insect herbivore, Lygus rugulipennis, at two nitrogen availability levels

Abstract

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.