Effect of spinosad on the predator Podisus nigrispinus and its lepidopterous prey

Laboratory studies were conducted to determine the toxicity of the microbial insecticide spinosad (480 g active ingredients per liter) to the predator Podisus nigrispinus (Heteroptera: Pentatomidae) in relation to its prey Spodoptera frugiperda (Lepidoptera: Noctuidae) and Tuta absoluta (Lepidoptera: Gelechiidae). The LC50- and LC90-values for adults and fifth instars of the predator were determined by topical exposure and by ingestion of treated drinking water; lethal concentrations to third-fourth instars of the lepidopterous prey were determined by ingestion of treated host plants. Both prey were very susceptible with LC50- and LC90-values of 0.67 and 3.84 mg a.i. per l for third instars of S. frugiperda and 1.34 and 26.78 mg a.i. per liter for 3rd-4th instars of T. absoluta, respectively. For adult predators, these values were respectively 145.29 and 527.32 mg a.i. per l by direct topical exposure and 53.08 and 135.94 mg a.i. per liter by ingestion. For predator nymphs exposed to spinosad by ingestion, LC50- and LC90-values were 44.64 and 87.02 mg a.i. per l, respectively. Topical exposure of nymphs to insecticide concentrations ranging from 134.4 to 960 mg a.i. per l yielded 10-15/ of mortality at the highest concentration. Differential selectivity indices (DSI) and relative toxicity indices (RTI) were determined based on the LC50- and LC90-values. The DSI based on LC90 indicated that adult predators were about 5 times less susceptible to the insecticide than T. absoluta larvae and 35 times less so than S. frugiperda larvae by ingestion exposure. The RTI based on LC90 showed that ingestion was about 4 times more toxic to P. nigrispinus adults than topical exposure. Further, spinosad was 7 times more toxic to S. frugiperda than to T. absoluta. The effect of topically applied spinosad at the LC90-value for S. frugiperda or of feeding on caterpillars treated with the same concentration of the compound on survival of P. nigrispinus adults was also studied. After 5 days, males suffered 16.9/ and 26.0/ mortality and females suflered 4.9/ and 16.1/ mortality in the topical and ingestion treatment, respectively. These data suggest that the predatory pentatomid P. nigrispinus is less susceptible to spinosad than its lepidopterous prey and that the compound thus may be a valuable asset for the integrated control of caterpillar pests.