VERTICAL-DISTRIBUTION IN SOIL, PERSISTENCE, AND EFFICACY AGAINST CITRUS ROOT WEEVIL (COLEOPTERA, CURCULIONIDAE) OF 2 SPECIES OF ENTOMOGENOUS NEMATODES (RHABDITIDA, STEINERNEMATIDAE, HETERORHABDITIDAE)

Experiments were conducted in a 6-yr-old citrus orchard to test the ef ficacy of different formulations of the nematodes Steinernema riobravi s (Cabanillas, Poinar & Raulston) and Heterorhabditis bacteriophora (P oinar) against a citrus root weevil, Diaprepes abbreviatus (L., and to measure nematode persistence at different soil depths. One hour after application, >55% of both nematode species that were recovered in soi l samples were at a depth of 0-1 cm and 33% of both species were recov ered between depths of 3 and 15 cm. Numbers of 8. riobravis at depths of 3-15 cm remained constant during the following 7 d, but numbers of H. bacteriophora declined 64%. During the same period, the numbers of both species recovered from depths of 0-1 cm declined by >99%. The dat a suggest that the lack of persistence reported for these nematodes in Florida citrus orchards is related to their vertical distribution in soil following application. Nine weeks after application, the numbers of D. abbreviatus at soil depths of 0-45 cm were reduced by 77 and 90% , respectively, in plots treated with liquid or granular formulations of S. riobravis. Neither formulation of H. bacteriophora affected the population density of D. abbreviatus.