Influence of Larval Age and Temperature on Effectiveness of a Nuclear Polyhedrosis Virus in the Soybean Looper, Pseudoplusia includens (Lepidoptera: Noctuidae) on Soybean

The effects of temperature and larval age of the soybean looper, Pseudoplusia includens (Walker), on nuclear polyhedrosis virus (PiNPV)-induced mortality on soybeans were studied in the laboratory and field. Leaflets collected from soybeans sprayed with the virus at a rate of 6 X 10(11) polyhedral inclusion bodies (PIB)/ha were fed to larvae in the laboratory. A significant interaction between larval instar and temperature for PiNPV-induced mortality was not detected. Mortality was significantly higher (P <= 0.05) at 30 degrees C (75.8%) than at 25 (69%) or 35 degrees C (65.2%). LT50 values were higher in late instars, than in early instars. LT50 values were also higher at 25 degrees C than at 30 or 35 degrees C. The virus-induced mortality increased with an increase in larval age. In the field study, soybean looper mortality on soybean plants sprayed with virus was significantly higher in late instars than in early instars. The PiNPV rates of 6 X 10(11) and 1.5 X 10(12) PIB/ha resulted in similar virus-induced mortalities with a mean of 33.6 and 35.7%, respectively, across all instars. (C) 1991 Academic Press, Inc.