Parasitoid Diversity and Biological Control of Bactericera tremblayi and Bactericera trigonica (Hemiptera: Psylloidea) in Horticultural Crops in Spain

Biological control using parasitoids is a key strategy for managing psyllid pests in horticultural crops. This study investigated the parasitoid complex associated with Bactericera tremblayi and Bactericera trigonica, two economically important psyllid pests of leek and carrot, respectively, in Castile and León (Spain). Nymphs of both psyllid species were collected and reared to assess parasitism; emerging parasitoids were identified morphologically and by DNA barcoding. A total of 969 parasitized nymphs were collected, from which 291 parasitoids emerged, including Tamarixia pronomus, Tamarixia tremblayi, Syrphophagus herbidus, Syrphophagus cf. taeniatus, Cheiloneurus sp. and Encyrtidae sp. Tamarixia pronomus is reported here for the first time as parasitoids of B. tremblayi while T. tremblayi, S. herbidus, and S. cf. taeniatus are newly recorded as parasitoids of B. trigonica. Moreover, the two Syrphophagus species represent the first records of this genus for Spain. The COI barcode sequences were generated for T. pronomus, T. tremblayi and S. cf. taeniatus. The parasitism rate of S. herbidus on B. trigonica reached a maximum of 23.18%. No significant differences in parasitism by Tamarixia species were observed between the two Bactericera hosts. Maximum parasitism rates recorded were 32.43% for T. pronomus on B. trigonica, and approximately 11–12% for T. tremblayi on both host species. Total parasitism of B. tremblayi in leek crops reached up to 70%, whereas parasitism of B. trigonica in carrot remained generally low and poorly synchronized with pest population dynamics. These results highlight a diverse parasitoid complex with substantial natural control of B. tremblayi and variable but lower impact on B. trigonica, underscoring the relevance of habitat and temporal factors for conservation biological control in these agroecosystems.