Population dynamics and dispersal of two exotic biological control agents of spotted knapweed, Urophora affinis and U. quadrifasciata (Diptera: Tephritidae), in southwestern Virginia from 1986 to 2000

Abstract

Abstract A gall fly, Urophora affinis Frfld. (Diptera: Tephritidae), released at two sites in southwestern Virginia in 1986 for the biological control of spotted knapweed, Centaurea maculosa Lam., was monitored annually through 2000 to determine its population density and dispersal rate. Between 1986 and 1991, the mean number of U. affinis per knapweed capitulum increased 10-fold at these original release sites. By spring 2000, it was recovered at 44 of 55 sites sampled but its density at each original release site has not exceeded the 1991 level. Rate of spread averaged 1.3 km/year. A second species, Urophora quadrifasciata (Meigen) that was not released in Virginia, was first recovered in 1995 comprising 30% of the two Urophora species. From 1996 to 2000 it was recovered at all sites sampled. Its rapid increase made it the dominant species, comprising 87.5% of the two species within two years. Although U. quadrifasciata has spread more rapidly than U. affinis, the proportion of the two species has stabilized since 1998 with the population of U. quadrifasciata being three times that of U. affinis. Eighty-two percent of the capitula of spotted knapweed sampled were infested. Each infested capitulum contained 1–3 U. affinis or 1–6 U. quadrifasciata with 11.7% of the capitula being infested by both species. Sex ratio consistently showed a slightly higher number of males than females in both species. Comparison of both species present in 20 sites showed that despite a 9-year head start, U. affinis was the dominant species in only 2.6, 2.4, and 3.5% of the capitula in 1998, 1999, and 2000, respectively. The highest number of each species recorded per capitulum was a mean for all sampled sites of 1.68 U. quadrifasciata in 1997 and 0.4 U. affinis in 1991. Although U. quadrifasciata quickly became the dominant species in sites occupied by U. affinis, the latter has persisted, suggesting that the two species can coexist. Data derived from two sample sizes of 100 and 1000 capitula were not significantly different.