The Feeding Behaviour of Gall Midge Larvae and Its Implications for Biocontrol of the Giant Reed: Insights from Stable Isotope Analysis

SIMPLE SUMMARY: We used stable isotope analysis of carbon and nitrogen to investigate the feeding relationships between the giant reed Arundo donax, larvae of the gall midge Lasioptera donacis and the saprophytic fungus Arthrinium arundinis, which grows in the reed’s internodes. We also evaluated the effects of the parasitic nematode Tripius gyraloura on midge larval feeding behaviour. The giant reed is one of the most invasive plant species in the world and the gall midge is a potential candidate for its biological control. It is currently unknown whether the larvae can feed directly on the reed or only the fungus growing in the reeds. Furthermore, it is not clear whether midge larvae infected by the parasitic nematode have different feeding behaviour. Our results indicated that the larvae feed on both the reed and the fungus in variable proportions depending on reed quality. We also observed that parasitised and non-parasitised larvae have the same diet, indicating that infection by nematodes does not change the midge larval trophic preferences. ABSTRACT: The gall midge Lasioptera donacis, whose larval stage interferes with the reed’s leaf development, is a potential candidate agent for the biological control of Arundo donax. Reed infestation is always associated with the presence of a saprophytic fungus, Arthrinium arundinis, which is believed to provide food for the larvae. Larvae also interact with a parasitic nematode, Tripius gyraloura, which can be considered its natural enemy. To deepen our knowledge of the plant–fungus–insect trophic interactions and to understand the effects of the nematode on midge larval feeding behaviour, we applied stable isotope analysis, one of the most effective methods for investigating animal feeding preferences in various contexts. The results showed that on average the fungus accounted for 65% of the diet of the midge larvae, which however consumed the reed and the fungus in variable proportions depending on reed quality (expressed as the C:N ratio). No differences in feeding behaviour were observed between parasitised and non-parasitised midge larvae, indicating that nematodes have no effect in this regard. Due to its trophic habits, L. donacis could be an effective control agent of A. donax and these results need to be considered when implementing biological control measures.