Title: DOES STARTER N FOR SOYBEANS AFFECT BEAN LEAF BEETLE POPULATIONS? Authors
Submitted to: State University Ag Report
Publication Type: Experiment Station
Publication Acceptance Date: February 24, 2006
Publication Date: March 15, 2006
Citation: Riedell, W.E., Lundgren, J.G., Osborne, S.L., Pikul Jr, J.L. 2006. Does starter n for soybeans affect bean leaf beetle populations? Soil/Water Research Reports, Plant Science Department, South Dakota Ag Experiment Station. Interpretive Summary: The bean leaf beetle (Cerotoma trifurcata) is a soybean insect pest of increasing in economic importance in South Dakota. Bean leaf beetle larvae feed on soybean roots and nodules. Because soybean root nodules are an important food source for bean leaf beetle larvae, it is possible that soil management tactics that affect soybean nodulation, such as starter N application, could have an negative effect on bean leaf beetle biology. If this were the case, then fertilizers could be used as a means of non-insecticidal control of this insect pest. We found that bean leaf beetle adults first emerged when soybean reached the R3 development stage and that emergence continued beyond the R8 (full maturity) development stage, for a total time period of about 70 days. The cumulative number of adults as well as the rate at which adults emerged appeared to be greater in the high N input treatments than the other treatments. Our findings also suggest that the soil N management treatments imposed during the 2005 growing season had little effect upon soybean root nodulation. Thus it was surprising to find that there was an increased rate of bean leaf beetle adult emergence from the high N treatment as compared to the other treatments. These findings seem to confirm a previously-proposed ecological relationship that, with increasing soil insect emergence levels, the emergent individuals tend to be smaller in size. Additional data are needed to confirm these speculations.
Technical Abstract: Our objective was to investigate the potential relationships between soil N management, soybean N relations, and bean leaf beetle biology. Experimental plots used in this study were established in 1990 on Barnes clay loam soil at the Eastern South Dakota Soil and Water Research Farm near Brookings SD. Soil N input treatments were: corn fertilized for a yield goal of 8.5 Mg ha-1 (high N input), 5.3 Mg ha-1 (medium N input), or corn not fertilized (no N input). Corn and soybean plots were treated with banded starter fertilizer (112 kg ha-1) as 14-16-11, 7-16-11, or 0-16-11 elemental N-P-K on the high N, medium N, and no N input treatments, respectively. For the soybean phase of the experiment, seeds were planted (76 cm row spacing) 3.7 cm deep at a population of 489,000 live seeds ha-1 on 27 May 2005. Soybean shoot samples were taken on a time-course basis and were analyzed for ureide-N and NO3-N. After field cultivation (8 July 2005), bean leaf beetle adults that emerged from the plots were captured in emergence cages (0.89 m long, 0.46 m wide, 0.15 m tall; 4 cages per plot). Adult head capsule widths were measured using a dissecting microscope fitted with a micrometer ocular lens. We found that the cumulative number of bean leaf beetle adults as well as the rate at which adults emerged appeared to be greater in the high N input treatments than the other treatments. The head capsule width of first generation beetles that emerged from medium N input treatments was significantly larger than the other treatments. It is difficult to speculate as to the reasons why we obtained these results for adult emergence. Additional data are needed.