Identification of ice nucleating active Pseudomonas fluorescens strains for biological control of overwintering Colorado potato beetles (Coleoptera: Chrysomelidae).

Abstract

Laboratory studies were conducted to identify ice-nucleating active bacterial strains able to elevate the supercooling point, the temperature at which freezing is initiated in body fluids, of Colorado potato beetles, Leptinotarsa decemlineata (Say), and to persist in their gut. Adult beetles fed ice-nucleating active strains of Pseudomonas fluorescens, P. putida, or P. syringae at 10(6) or 10(3) bacterial cells per beetle had significantly elevated supercooling points, from -4.5 to -5.7 degrees C and from -5.2 to -6.6 degrees C, respectively, immediately after ingestion. In contrast, mean supercooling point of untreated control beetles was -9.2 degrees C. When sampled at 2 and 12 wk after ingestion, only beetles fed P. fluorescens F26-4C and 88-335 still had significantly elevated supercooling points, indicating that these strains of bacteria were retained. Furthermore, beetle supercooling points were comparable to those observed immediately after ingestion, suggesting that beetle gut conditions were favorable not only for colonization but also for expression of ice-nucleating activity by these two strains. The results obtained from exposure to a single, low dose of either bacterial strain also show that a minimum amount of inoculum is sufficient for establishment of the bacterium in the gut. Persistence of these bacteria in Colorado potato beetles long after ingestion was also confirmed using a polymerase chain reaction technique that detected ice-nucleating active bacteria by virtue of their ina genes. Application of these ice-nucleating active bacteria to elevate the supercooling point of this freeze-intolerant insect pest could significantly reduce their winter survival, thereby reducing local populations and, consequently, crop damage.