Observations on the Biology of Spurgia capitigena Bremi on Leafy Spurge in North Dakota

The gall midge, Spurgia capitigena, was released in North Dakota for the biological control of leafy spurge, Euphorbia esula, S. capitigena induces a rosette-type gall formation on the stem tip. Calling destroys the stem tip, which prevents flowering and seed production. S. capitigena is a multivoltine insect and individual generations can be separated by obtaining counts of larvae and pupae inhabiting leafy spurge galls. In North Dakota, three generations were delineated in 1992 and four generations in 1993. The number of adult S, capitigena collected from galls was highest in the first generation and declined by 50% in the second. The sex ratio of emerged adults favored females by approximately 2:1. S. capitigena produced between 70 and 110 eggs per female. Counts of S. capitigena larvae and pupae per gall were highest in the second generation and declined dramatically in the third and fourth generations. The decrease in larvae and pupae per gall in generations three and four was due to increased mortality and exit of larvae from the gall to overwinter in the soil.