Conditioning the entomopathogenic nematodes Steinernema carpocapsae and Heterorhabditis megidis by pre-application storage improves efficacy against black vine weevil, Otiorhynchus sulcatus (Coleoptera: Curculionidae) at low and moderate temperatures

Entomopathogenic nematodes (Heterorhabditis spp. and Steinernema spp.) are effective biocontrol agents for several insect pests, but their use is restricted by environmental constraints such as temperature and by their high cost. Use of nematodes against vine weevil (Otiorhynchus sulcatus) is restricted by low temperatures prevailing at the time when control is required. Here we investigate the potential for “conditioning” - storing nematode infective juveniles at 9 °C for at least three weeks prior to application - to improve efficacy, both at low and moderate temperatures. Conditioned Heterorhabditis megidis were previously shown to give improved control of vine weevil when applied to potted plants at constant 9 °C. Here we show similar results for Steinernema carpocapsae conditioned for 3-6 weeks at 9 °C. We also show that conditioning (3 weeks pre-application storage at 9 °C) improved efficacy of both species against vine weevil in strawberries grown in bags in an unheated glasshouse at 0-12 °C. In a final experiment, we also test whether improved performance of 9 °C-stored H. megidis compensates for a reduced application rate at a more permissive temperature, 15 °C. Conditioned H. megidis gave control equal to that of unstored H. megidis applied at double the rate. Commercial product based on the cold-active species Steinernema kraussei was used as a reference treatment in all experiments. At constant 9 °C, S. kraussei gave control superior to that of both unstored and conditioned H. megidis and S. carpocapsae, while at constant 15 °C, both conditioned and unstored H. megidis applied at half the recommended rate gave superior control to S. kraussei applied at full rate. Repetition of experiments with vine weevil larvae of increasing age indicated that success of S. kraussei is more influenced by larval age than that of the other two species, and that this species is more effective against younger (2nd-3rd) than older (4th-7th) instars.