Biology, ecology, and impact of Cryptonevra nigritarsis Duda, a potential biological control agent against the giant reed Arundo donax

Abstract Arundo donax (L.), commonly known as giant reed, is an invasive weed in riparian habitats worldwide. The chloropid fly Cryptonevra nigritarsis Duda infests young shoots of A. donax in its native range and has been considered for a long time as a potential biological control agent. This study was designed to investigate the biology and field ecology of C. nigritarsis and document its impact on A. donax. Populations of A. donax were monitored in ten field sites for a full year and sticky traps were used to assess C. nigritarsis abundance. Young A. donax shoots with signs of infestation were dissected monthly, and a DNA-based diagnosis method was developed to assess the prevalence of C. nigritarsis among the larvae and pupae collected. Finally, a manipulative experiment was performed to evaluate the impact of water stress on A. donax vulnerability to C. nigritarsis. Results showed that C. nigritarsis is likely to be a multivoltine species overwintering mainly as pupa. Only 6.3% of the A. donax shoots monitored were infested and the overall prevalence of C. nigritarsis among the larvae and pupae collected was 54.3%. C. nigritarsis was found in shoots of all diameters and was the predominant species in very thin and very thick shoots. Thicker shoots had a greater probability of infestation by chloropid flies in general. The manipulative experiment showed that mortality of A. donax shoots in the presence of C. nigritarsis was higher is water-stressed plants, but overall shoot mortality and infestation were low. These results shed light on some important aspects on the biology and ecology of C. nigritarsis and help in evaluating its potential as a biological control agent.