Effects of simulated insect defoliation and annual weed competition on soybean and velvetleaf development

The separate and combined effects of velvetleaf competition and simulated green cloverworm (SGCW) defoliation were evaluated in a 3-year field study (1979 through 1981). Statistical documentation of velvetleaf competition stress (3 duration intervals x 2 weed densities plus a weed-free soybean control) was limited to soybeans in weed-proximate locations at the tested densities. Pre-harvest weed-induced reductions in nodes with unrolled leaves (1981 only) and certain component and total dry weight (DWT), mean crop growth rate (CGR),(\u27 )and mean relative growth rate (RGR) fractions were determined from samples collected 6 or more weeks after emergence. The SGCW defoliation procedure (4 or 5 densities were simulated through a temperature-dependent defoliation-developmental model by hole-punching) reduced soybean leaf area, height, lodging, certain component and total DWTs, CGRs,(\u27 )and RGRs. Nodal development lagged slightly in 1980 defoliated plots. Little evidence of compensatory growth has noted in plots where stresses were terminated by weed removal or SGCW pupation. None of the 28 or 35 treatment combinations altered soybean stand counts, branching, reproductive development, or gravimetric soil moisture determinations significantly. Statistical confirmation of velvetleaf and green cloverworm treatment interactions was not realized through the analysis of soybean growth and development;A standardized system of describing velvetleaf vegetative and reproductive development was proposed. An undescribed beetle of the family Bruchidae (Abutiloneus new species) emerged from velvetleaf seed. Monocropped velvetleaf eventually exceeded velvetleaf intercropped with soybeans in most growth characteristics. Soybeans began stressing intercropped weeds within 4 weeks of emergence. Few velvetleaf growth characteristics increased in response to linear increases in SGCW defoliation of adjacent full-blown soybeans. Simulated green cloverworm defoliation of adjacent soybeans did not alter weed heights significantly, but did affect the development of weed soybean/canopy height differentials (by stunting the crop). Furthermore, an analysis of weed/soybean canopy height differentials indicated that the temporal feasibility of certain late-season weed control devices (rope-wick applicators and recirculating sprayers) may vary, depending on the level of insect defoliation previously tolerated