Field persistence of ice-nucleating bacteria in overwintering Colorado potato beetles

Ice-nucleating bacteria are biological ice nucleators capable of elevating the temperature at which ice crystals form in the body fluids of insects, t ermed the supercooling point (SCP). In the freeze-intolerant Colorado potat o beetle, these bacteria reduce its cold tolerance and, consequently, incre ase the likelihood of mortality in overwintering adults exposed to subzero temperatures. In this held study, two ice-nucleating bacteria, Pseudomonas fluorescens and P. putida, were evaluated for their persistence and efficac y against over wintering adults. Both strains significantly elevated the SC P of treated beetles immediately after ingestion. However, only beetles fed P. fluorescens still had significantly elevated SCPs (-4.2 degreesC) versu s control (-6.4 degreesC) after overwintering in the field. Bacterial persi stence in beetle guts was confirmed by PCR assays that positively correlate d the presence of P. fluorescens with elevated SCPs. Despite the reduction of cold tolerance in overwintering adults with P. fluorescens, no significa nt difference was observed in the survival rates of treated versus control beetles during the winter. Because adults overwinter in the ground, the eff ect of this bacterium On beetle survival depends on the soil temperatures t hat overwintering adults experience, Nevertheless, recovery of most of the adults in the upper 15 cm of soil strata indicates that beetles with potent ice nucleating bacteria in their guts could be subject to critically low t emperatures during winter. Our results show that ice-nucleating P. fluoresc ens compromises the cold tolerance of Colorado potato beetles and suggest t hat by development of strategies to maximize bacterial efficacy in the fiel d, this bacterium offers a potential biocontrol system against overwinterin g populations. (C) 2001 Academic Press.