Interaction Between the Entomopathogens Beauveria bassiana , Metarhizium anisopliae and Paecilomyces fumosoroseus and the Mycoparasites Clonostachys spp., Trichoderma harzianum and Lecanicillium lecanii U. KRAUSS 1 , E. HIDALGO 1 , C. ARROYO 1 AND S. R. PIPER 1,2 1 CABI/CATIE/USDA Project, c/o Centro Agrono ´mico Tropical de Investigacio ´n y Ensen ˜anza (CATIE), 7170 Turrialba, Costa Rica; 2 Scottish Agricultural College, West Mains Road, Edinburgh, EH9 3JG, UK (Received 30 March 2003; returned 29 May 2003; accepted 13 July 2003) Biocontrol agents of numerous insect pests and fungal pathogens exist but virtually nothing is known about their interaction if used simultaneously. Our objective was to investigate the compatibility of the entomopathogens Beauveria bassiana, Metarhizium anisopliae and Paecilomyces fumosoroseus , and the broad host-range mycoparasites Clonostachys spp., Trichoderma harzianum and Lecanicillium lecanii. In vitro host-range tests revealed that M. anisopliae was highly susceptible to all mycoparasites tested. B. bassiana was attacked by Clonostachys rosea, and P. fumosoroseus. was resistant to mycoparasites. M. anisopliae but not P. fumosoroseus killed nymphs of Bemisia tabaci in bioassays. B. bassiana and M. anisopliae proved lethal to Cosmopolites sordidus , Diatraea saccharalis and Sitophilus oryzae. Coapplication of mycoparasites with entomopathogens did not affect their biocontrol efficacy in vivo , although the reisolation success of entomopathogens could be significantly reduced, especially from smaller insect species. Trichoderma spp. were reisolated from mycoparasite-treated insects more frequently than C. rosea. The coapplication of the highly susceptible M. anisopliae generally enhanced mycoparasite recovery. Mycofungicide prepara- tions caused some insect mortality but less than a copper hydroxide fungicide which is still permissible in organic agriculture. We concluded that the tested entomopathogens and mycoparasites are compatible elements of integrated pest management. Keywords: Beauveria bassiana, Bemisia tabaci, Clonostachys spp., Cosmopolites sordidus , Diatraea saccharalis , entomopathogen, Metarhizium anisopliae, mycoparasite, Paecilomyces fumosoroseus , Sitophilus oryzae, Trichoderma harzianum, Lecanicillium lecanii Correspondence to: U. Krauss. Tel.: / 506 556 1632; Fax: / 506 556 0606; E-mail: ukrauss@catie.ac.cr Biocontrol Science and Technology (June 2004), Vol. 14, No. 4, 331  /346 ISSN 0958-3157 (print)/ISSN 1360-0478 (online)/04/040331-16 # 2004 Taylor & Francis Ltd DOI: 10.1080/09583150410001665196