Natural Mortality of Lepidopteran Eggs and Larvae in Missouri Soybeans

Abstract

Eggs and each instar of the cabbage looper, Trichoplusia ni (Hubner), were introduced weekly into Missouri soybeans to assess natural mortality. Percent recovery 24 h later demonstrated high mortality due to predation, which peaked between flowering and pod fill of the soybeans. First and 2nd instars were most susceptible, with seasonal average mortalities of 51.9 and 43.5%, respectively; 3rd and 4th instars were least susceptible, with averages of 30.1 and 25.0%, whereas mortality of 5th instars, 34.9%, was intermediate. Few of the recovered eggs or larvae were parasitized, but disease caused significant mortality of larvae, due especially to a late-season epizootic of the entomopathogenic fungus, Nomuraea rileyi (Farlow) Samson. Naturally occurring green cloverworms, Plathypena scabra (F.), were attacked by several parasites and diseases. Parasitism, mostly by Rogas nolophanae (Ashmead) and Cotesia marginiventris (Cresson), peaked at 36.3% in July. Mortality due to disease reached a peak of 70.0% late in the season, again due to the epizootic of N. rileyi .