Novel bacterial isolates of B. thuringiensis are having enhanced toxicity with respect to previously resistant or insufficiently susceptible insect species, including, but not limited to, Plutella xylostella, Spodoptera frugiperda and Spodoptera exigua, as well as certain secondary pests such as Trichoplusia ni. Such isolates may be characterized by their possession of a particular subset of the genes coding for the various B. thuringiensis delta -endotoxin proteins and by a characteristic plasmid profile, or array, known to be associated therewith. Also disclosed are a method for the efficient identification of such bacterial isolates utilizing generalized or, alternatively, gene-specific DNA probes; cloned or synthesized nucleotide sequences which are useful in the preparation of those probes; compositions containing an insecticidally effective amount of a bacterial isolate of the invention in combination with an acceptable carrier; and a method for the use thereof in the control or eradication of insect pests.