BioControl 47: 23–31, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. Parasitoid diversity and impact on populations of the diamondback moth Plutella xylostella (L.) on Brassica crops in central México M. MARTÍNEZ-CASTILLO 1 , J.L. LEYVA 1∗ , J. CIBRIÁN-TOVAR 1 and R. BUJANOS-MUÑIZ 2 1 Colegio de Postgraduados, Instituto de Fitosanidad, Montecillo, Estado de México, México; 2 InstitutoNacional de Investigaciones Forestales, Agrícolas y Pecuarias (INIFAP), CEBAJ-CIR-Centro, Celaya, Guanajuato, México ∗ author for correspondence; e-mail: jlleyva@colpos.colpos.mx Received 23 June 2000; accepted in revised form 29 January 2001 Abstract. Three experimental plots were established and maintained during one year at two sites in the state of Querétaro, México, in order to identify species of parasitoids attacking the diamondback moth (DBM) Plutella xylostella (L.) (Lepidoptera: Plutellidae), and to gather information concerning their relative importance and patterns of population fluctuation. At both sites, the plots were planted with broccoli, cabbage and cauliflower during three cropping seasons. Parasitoid species identified were: Diadegma insulare Cresson, Diadromus (= Thyraeella) collaris Gravenhorst (Hymenoptera: Ichneumonidae); Habro- bracon sp. (Hymenoptera: Braconidae); Oomyzus (= Tetrastichus) sokolowoskii Kurdjumov (Hymenoptera: Eulophidae); and Spilochalcis (= Conura) sp. (Hymenoptera: Chalcididae). This last species is a hyperparasitoid of D. insulare. The most abundant and frequently occur- ring species was D. insulare, it occurred in both localities during all three cropping seasons. The highest levels of parasitism caused by D. insulare on DBM were registered in the spring- summer season of 1996 at ‘La Soledad’ farm with averages of 42.7, 45.0 and 44.5% on cabbage, broccoli, and cauliflower, respectively. Because D. insulare was detected attacking the pest at very low population densities during the initial stages of the crop cycle, it is assumed that the parasitoid has a high searching capacity. Correlation (r) between DBM and D. insulare population numbers was positive and significant. The other species identified occurred sporad- ically and had little impact on pest populations. The identification of D. collaris represents the first record of this species in North America. Key words: biodiversity, biological control, crucifer pests, natural enemies, natural control Introduction The diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutel- lidae), is a cosmopolitan insect that has a marked capacity to adapt to different climatic conditions. It is considered as the pest of greatest economic