Toxicity of imidacloprid in the German cockroach (Dictyoptera: Blattellidae), and the synergism between imidacloprid and Metarhizium anisopliae (Imperfect Fungi: Hyphomycetes)

German cockroaches, Blattella germanica (L.), orally exposed to solid imidacloprid baits exhibited toxic symptoms rapidly (within 30 min). However, cockroaches recovered from this toxicity. The percentage of knockdown was positively correlated and the rate of recovery was negatively correlated with concentration. In the liquid bait tests, imidacloprid produced rapid moribundity (within minutes) in all exposed cockroaches, but this moribundity was transient. Topically applied imidacloprid also produced rapid but transient moribundity in both the laboratory-susceptible (JWax-S) and the field-collected resistant (Muncie '86) strains. In the topical application bioassay, recovery time was dependent on cockroach strain and presence of piperonyl butoxide (PBO). Resistant cockroaches recovered faster than susceptible cockroaches; cockroaches pretreated with PBO recovered faster than those not pretreated. The JWax and Lafayette (field-collected) strains were killed significantly faster with the entomopathogenic fungus, Metarhizium anisopliae alone, or Metarhizium + solid imidacloprid bait than imidacloprid bait alone in both the choice and no-choice bioassays. Combinations of direct contact to the conidia of Metarhizium and feeding on imidacloprid bait indicated synergy when compared with the activity of the individual components. We found no differences in mortality when the JWax cockroaches fed on either 500 or 1,000 ppm imidacloprid baits. The presence of alternate food slowed the lethal time for imidacloprid against the JWax and Lafayette strains. Both strains were killed significantly faster when cockroaches fed on imidacloprid after a topical application of spore suspensions of Metarhizium than when insects fed on imidacloprid bait alone, indicating possible synergistic interaction (i.e., greater than expected mortality) between imidacloprid bait and Metarhizium. The mortality of cockroaches inoculated topically with spore suspensions of Metarhizium increased significantly with an increase in the number of fungal spores. No significant differences in daily cockroach mortality were detected among imidacloprid concentrations for each Metarhizium dose. Imidacloprid bait alone has a limited potential for use in controlling cockroaches, but it may have a potential if used in combination with a fungal pathogen such as Metarhizium.