Bean leaf beetles, Cerotoma trifurcata Forster, were sampled biweekly from soybean emergence until harvest 2001-2002 near Brookings, SD, with plans to repeat 2002 sampling in 2003. Observation was limited to one soybean field in 2001, but 2002 sampling included two tillage and two nitrogen fertilization treatments in soybean plots 30.5 x 30.5 m rotated annually with maize since 1990. Treatments in 2002 were replicated three times in a factorial design. Sampling consisted of visual counts until plants had 3 or 4 trifoliate leaves; older beans were swept with a 38-cm diameter net. Sub-sample examination determined age (teneral or not), sex, and beetle reproductive status based on ovarian maturity and sperm in spermathecae. Preliminary 2001-2002 results showed two adult emergence peaks annually in July and September but few if any reproductive females after the beginning of August. These data suggest two generations yearly but production of the second by only the earliest emerging first generation beetles. If so, late planting could not only reduce soybean colonization by overwintered beetles, but also directly reduce beetle feeding during susceptible later crop stages (R4-R6). Plots treated with 15.7 kg/ha starter fertilizer yielded about 1/3rd fewer beetles than did untreated plots, perhaps because of the tendency for nitrogen to suppress development of nitrogen-fixing root nodules where bean leaf beetle larvae aggregate. Numbers also ran higher in chisel- than ridge-tilled plots despite identical timing of soybean germination. Data from 2003 will determine whether tillage and fertilization trends observed in 2002 are reproducible.
Species 1: Coleoptera Chrysomelidae Cerotoma trifurcata (bean leaf beetle)
Species 2: Fabales Fabaceae Glycine max (soybean)
Keywords: seasonal reproductive status, nitrogen and tillage management
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