Origin of a Mediterranean fruit fly, Ceratitis capitata (Wiedemann), outbreak determined by DNA analysis

Sterile male Mediterranean fruit flies, Ceratitis capitata (Wiedemann), have been released in the Hex River Valley since 1999 in a sterile insect technique (SIT) programme to suppress this pest on export table grapes. The source of these sterile fruit flies was the Mediterranean fruit fly rearing facility in Stellenbosch, where a genetic sexing strain of C. capitata made it possible to release only male fruit flies in the SIT field operation. In December 2001 an outbreak of fruit flies in export table grapes in the Hex River Valley was reported. Growers in the Hex River Valley ascribed this infestation to the presence of unsterilized C. capitata females in deliveries of sterile C. capitata males from the rearing facility, and blamed the facility for the resulting crop losses. Fruit fly specimens collected from grapes in the infestation area were subjected to mitochondrial DNA analysis to compare the distribution of haplotypes of the infesting flies from the SIT area with that of the laboratory strain. None of the analysed flies showed the genetic fingerprints present in the genetic sexing strain in the rearing facility. This demonstrated that the origin of the infestation was not the rearing facility, but most probably arose from infestations of wild C. capitata in uncontrolled commercial, domestic and wild fruit fly host plants in the SIT area.