Mississippi State University
Date of Degree
Dissertation - Open Access
Insect defoliation thresholds were reevaluated to determine their effectiveness during vegetative and reproductive stages of soybean. Field experiments were planted with maturity group IV soybeans and subjected 17, 33, 66, and 100% defoliation during R3, R5, and R6 growth stages. In addition to different amounts of defoliation for each growth stage, defoliation occurring within different portions of the plant canopy was also evaluated. Results of this experiment confirmed that soybeans during R3 and R5 stages are more susceptible to yield loss at high levels (>57%) of defoliation than R6 growth stage. However, yield loss was not significantly different below 57% defoliation. No significant yield difference was observed from defoliation occurring in the top or bottom part of the canopy. Yield loss from various levels of defoliation during the vegetative stages was significant at V6. No yield loss was observed from defoliation during the V3 growth stage. Both maturity group IV and V soybeans behaved similarly to each level of defoliation. Results from these experiments were used to determine dynamic economic injury levels for each growth stage based on yield loss equations, value of the crop, and cost of control and can be used to make insecticide application recommendations based on the amount of defoliation at a particular reproductive growth stage. Soybean looper, Chrysodeixis includens (Walker), is an economic pest of soybeans that has developed resistance to several insecticide classes. New insecticides have recently been labeled for control of lepidopteran pests in soybeans, including soybean loopers. Field reference strains were collected in 2010 and 2011 from soybean fields in Mississippi and Louisiana and subjected to insecticide incorporated diet treated with flubendiamide, chlorantraniliprole, and methoxyfenozide. Susceptibility of soybean loopers to flubendiamide and chlorantraniliprole did not differ. However the overall susceptibility to methoxyfenozide was greater than chlorantraniliprole. Diet incorporated assays determined a 9.4 fold variation in susceptibility to flubendiamide among the seven soybean looper populations tested. Variation to chlorantraniliprole was 6.25 fold and variation for methoxyfenozide was 5.37 fold. These data can be used as a benchmark for referencing future soybean looper populations in Mississippi and Lousiana.
Owen, Lucas Neil, "Effects of Defoliation in Soybeans and Susceptibility of Soybean Loopers to Reduced Risk Insecticides" (2012). Theses and Dissertations. 1731.