Soil Water Potential: Effects on Soybean Looper Feedin on Soybean Leaves

Abstract

Defoliation by soybean looper, Pseudoplusia includens (Walker), often is uniformly high over portions of soybean [Glycine max (L.) Merr.l fields but is uniformly low, or progresses more slowly, over other portions of the same fields. Through the use of insect bioassays the effect of soil water potential (SWP) and two soil types were investigated to determine if they are factors associated with observed soybean looper defoliation patterns. Tests were conducted using excised leaves from greenhouse‐grown plants and laboratory‐reared insects. In Test I, a significant (P ≤ 0.01) reduction in 10‐d larval weights and an increase in larval development periods was caused by plants grown at reduced SWP for 15 d before bioassay initiation. No differences in 10‐d larval weights or development periods associated with Dubbs silt loam (fine‐Silty, mixed, thermic Typic Hapludalf) or Sharkey clay (very‐fine, montmorillonitic, nonacid, thermic Vertic Haplaquept) soils occurred in Test I. In Test II, a large, significant reduction occurred in 10‐d larval weights and an increase occurred in development periods that was associated with plants grown at reduced SWP for 27 d before bioassay initiation. A small, significant decrease in 10‐d larval weights and an increase in larval development periods under reduced SWP was associated with Sharkey clay in Test II. In Test III, a small, significant reduction in 10‐d larval weights and an increase in development periods was associated with plants grown for 24 d at moderately reduced SWP. The effects observed in these tests are great enough to be taken into consideration when conducting host‐plant resistance research and when making insect control decisions.