Comparison of Single and Multiple Releases of Heterorhabditis bacteriophora Poinar (Nematoda: Heterorhabditidae) for Control of Cylas formicarius (Fabricius) (Coleoptera: Apionidae)

Single and multiple releases of the entomopathogenic nematode, Heterorhabditis bacteriophora Poinar (=H. heliothidis Khan, Brooks, and Hirschmann) (HP88 strain), were evaluated for controlling populations and damage of the sweetpotato weevil, Cylas formicarius (Fabricius), during two consecutive growing seasons in southern Florida. Sweet potato, Ipomoea batatas (L.) Lam., plants were treated with one, two, or three applications of H. bacteriophora in a Krome very gravelly loam soil. Weevil population reduction and damage in nematode-treated sweet potato plots were compared with that in plots treated with monthly applications of chemical insecticides and in nontreated plots. Persistence of nematodes in soil was assessed at various times after release by baiting soil samples with greater wax moth larvae, Galleria mellonella (L.). Applications of H. bacteriophora did not consistently reduce weevil abundance, but did consistently reduce damage to sweet potato storage roots compared with nontreated plants. Weevil densities and damage did not differ between plants treated with one, two, or three applications of H. bacteriophora, suggesting that a single, early season release of this nematode was adequate for managing this weevil. Weevil damage on plants treated with chemical insecticides was intermediate to that on nematode-treated and nontreated plants. H. bacteriophora persisted in soil for over 130 and 250 days after application in the first and second experiment, respectively. (C) 1991 Academic Press, Inc.