Evaluation of spinosad as a grain protectant on three Kansas farms

The persistence and insecticidal activity of a commercial insecticide, spinosad, based on the fermentation products of the actinomycete bacterium Saccharopolyspora spinosa Mertz and Yao (Actinomycetales: Actinomycetaceae), was evaluated on three Kansas farms between July 2002 and January 2003. Spinosad was applied to newly harvested hard red winter wheat at 1 mg (a.i.)/kg of grain at the time of storage in round, steel bins with capacity of 60-125 metric tons. Insect populations in grain samples collected monthly from bins receiving spinosad treatment were compared with populations in bins receiving no treatment (control), bins receiving 3 mg (a.i.)/kg of chlorpyrifos-methyl, and bins receiving 1 mg (a.i.)/kg spinosad+3 mg (a.i.)/kg chlorpyrifos-methyl treatment. The actual spinosad residue on wheat immediately after treatment was 30% less than the application rate of 1 mg (a.i.)/kg, but there was no significant degradation of these residues during the 6-month test period. None of the July samples from any of the four treatments had live adults of stored-product insects. No live adults of the lesser grain borer, Rhyzopertha dominica (F.), and very low densities (⩽3 live adults/kg of sample) of the red flour beetle, Tribolium castaneum (Herbst); rusty grain beetle, Cryptolestes ferrugineus (Stephens); and sawtoothed grain beetle, Oryzaephilus surinamensis (L.), were found in grain samples collected between August and January from bins treated with spinosad or spinosad+chlorpyrifos-methyl. The density of all four insect species combined in untreated grain samples steadily increased from 0.5 adults/kg in August 2002 to a high of 22.1 adults/kg in January 2003. In grain samples from bins treated with chlorpyrifos-methyl, the density of R. dominica adults increased from 0.4 adults/kg in September 2002 to a maximum of 10.2 adults/kg in January 2003, whereas densities of the other three species were ⩽1 adult/kg. Laboratory bioassays with monthly grain samples collected from the field applications showed that spinosad, alone or in combination with chlorpyrifos-methyl, was effective in killing adults of R. dominica and C. ferrugineus, but not T. castaneum. Progeny production of these three species was suppressed on spinosad-treated grain, irrespective of the sampling month. Results show that a single application of spinosad at 1 mg (a.i.)/kg is effective for managing common stored-grain insects, including R. dominica, for at least 6 months.