Stem galling of Ageratina adenophora (Asterales: Asteraceae) by a biocontrol agent Procecidochares utilis (Diptera: Tephritidae) is elevation dependent in central Nepal

Gall inducers are widely used as biocontrol agents to suppress the vegetative and reproductive growth of invasive weeds. Procecidochares utilis Stone (Diptera: Tephritidae) is a gall forming fly that was released as a biocontrol agent against the noxious invasive weed Ageratina adenophora (Sprengel) R. King and H. Robinson (Asterales: Asteraceae). However, the effectiveness of P. utilis in controlling A. adenophora has been reported to vary according to geographic regions, with very low effectiveness in the Himalaya. In this study, we measured the abundance of P. utilis stem galling on A. adenophora along the elevation gradient (240-2965 m asl) in central Nepal. We found that elevation had a significant effect on gall abundance as well as gall size. Gall abundance and size peaked at mid-elevation (1940-2000 m asl). Stem galling by the fly reduced the stem diameter of the weed and all the three nutrients (N, P and K) in galls were significantly higher than in ungalled stems. Our study indicates that elevation is an important abiotic factor that strongly influences the P. utilis stem gall abundance, as well as gall size, and galls formed by P. utilis act as nutrient sinks. These findings provide insights in insect gall ecology, which may help the successful biocontrol of A. adenophora.