No abstract

Arbuscular mycorrhiza (AM) formed between plants and AM fungi (AMF) can alter host plant quality and thus influence plant-herbivore interactions. While AM is known to affect the development of generalist chewing-biting herbivores, AM-mediated impacts on insect behavior have been neglected until now. In this study, the effects of Rhizophagus irregularis, a generalist AMF, on phenotypic and leaf metabolic traits of Plantago major plants were investigated. Further, the influence of AM-mediated host plant modifications on the development and on seven behavioral traits of larvae of the generalist Mamestra brassicae were recorded. Tests were carried out in the third (L3) and fourth (L4) larval instar, respectively. While shoot water content, specific leaf area, and foliar concentrations of the secondary metabolite aucubin were higher in AM-treated compared to non-mycorrhized (NM) plants, lower concentrations of the primary metabolites citric acid and isocitric acid were found in leaves of AM plants. Larvae reared on AM plants gained a higher body mass and tended to develop faster than individuals reared on NM plants. However, plant treatment had no significant effect on any of the behavioral traits. Instead, differences between larvae of different ages were detected in several behavioral features, with L4 being less active and less bold than L3 larvae. The results demonstrate that AM-induced modifications of host plant quality influence larval development, whereas the behavioral phenotype seems to be more fixed at least under the tested conditions.

1. Life‐history traits of herbivores are shaped by the combination of various extrinsic and intrinsic variables. However, studies investigating the impact of both variables on insect behavioural phenotypes are rare, and research including the modulation of host plant quality by arbuscular mycorrhiza (AM) (extrinsic variable) and the herbivore developmental stage (intrinsic variable) on aphid behaviour is lacking. 2. To study the influences of extrinsic and intrinsic variables on aphid performance and behaviour, individuals of the generalist aphid pest Myzus persicae (Sulzer) were reared on Plantago lanceolata and Poa annua plants that were either non‐mycorrhized (NM) or mycorrhized (AM). Aphid performance was monitored over 2 weeks. Behavioural traits of nymphs were recorded twice during development after removing individuals from their feeding sites and placing them in an unknown environment (‘open field’), causing disturbance. Plant quality was determined by measuring the water content and leaf mass per area (LMA). 3. Differences in body mass of nymphs at day 6 dependent on plant species and treatment were compensated by the individuals within 4 days. Behavioural traits, considered as activity and exploration, were affected by the interaction between extrinsic and intrinsic variables (activity and exploration) and changed over ontogeny (activity). Independent of AM, a lower water content and LMA were recorded in P. annua leaves. 4. Overall, this study demonstrates that extrinsic and intrinsic variables need to be considered in combination and behavioural traits should be studied repeatedly during insect ontogeny to understand the complexity of microbe‐modulated plant–herbivore interactions.