Can intra‐specific genetic variation in arbuscular mycorrhizal fungi ( Glomus etunicatum ) affect a mesophyll‐feeding herbivore ( Tupiocoris notatus Distant)?

1. Arbuscular mycorrhizal fungal (AMF) infection can have negative, positive or neutral effects on insect herbivore populations, but patterns are difficult to predict. 2. Intra‐specific genetic variation in nutrient uptake ability between fungal isolates may also have indirect effects on insect herbivores due to changes in plant quality. In preliminary studies mirid ( Tupiocoris notatus ) populations were significantly reduced on tobacco ( Nicotiana rustica ) colonised by AMF but it was unknown if same‐species fungal isolates differed in their effect. 3. An experiment was performed as a first test of the effect of intra‐specific genetic variation in the mycorrhizal fungus Glomus etunicatum on mirid nymphal population structure, dynamics, and growth rate. 4. Mirid nymphal populations were lower on mycorrhizal fungal‐infected plants. Population size, however, did not differ between the mycorrhizal isolates. While no statistical difference in population between isolates was found, one isolate consistently had 1.7–2.4 times lower mirid populations compared with the controls, indicating that the magnitude of effect is different between mycorrhizal isolates. 5. The significantly negative effect of AMF on mirid populations likely resulted from AMF‐induced changes in plant quality (e.g. increased defence). This study lends further support to recent demonstrations that below‐ground symbionts significantly influence above‐ground processes. In addition, mycorrhizal fungi can affect insect population structure, which may have consequences for future herbivory.