Reply to: Comment on “Soybean Aphid Population Dynamics, Soybean Yield Loss, and Development of Stage-Specific Economic Injury Levels” by M.A. Catangui, E.A. Beckendorf, and W.E. Riedell, Agron. J. 101:1080-1092 (2009)

Authors: CATANGUI, MICHAEL - South Dakota State University; Beckendorf, Eric; Riedell, Walter

Submitted to: Agronomy Journal
Publication Type: Other
Publication Acceptance Date: 11/9/2009
Publication Date: 11/12/2009
Citation: Catangui, M.A., Beckendorf, E.A., Riedell, W.E. 2009. Reply to: Comment on “Soybean Aphid Population Dynamics, Soybean Yield Loss, and Development of Stage-Specific Economic Injury Levels” by M.A. Catangui, E.A. Beckendorf, and W.E. Riedell, Agron. J. 101:1080-1092 (2009). Agronomy Journal. 102:56-58.

Interpretive Summary: Thank you for the opportunity to respond to a letter to the editor concerning the paper entitled “Soybean Aphid Population Dynamics, Soybean Yield Loss, and Development of Stage-Specific Economic Injury Levels” published in the September-October 2009 issue of the Agronomy Journal. Our paper provided a robust foundation and framework on the subject of soybean aphid population dynamics, soybean yield loss due to soybean aphids, and the development of plant stage-specific economic injury levels of the soybean aphid in soybean. The mathematical models used were based on well-established theories. The “general theory of population dynamics” was used to describe soybean aphid population dynamics. For the first time in integrated pest management, it has been proposed that the different phases of the symmetrical logistic curve be used to guide soybean aphid population management in the field. The fact that soybean aphid populations followed the logistic curve as well as the symmetrical bell-shaped curve with very high fidelity suggests that soybean aphid populations inside field cages were under regulation by certain density-dependent factors. We speculated that the main density-dependent population regulatory factor was intra-specific soybean aphid competition for limited and transient N in the soybean sap. Thus, we were aware from the beginning that our paper was just but one contribution to the understanding of the soybean aphid as a pest of soybean. We welcome criticisms, positive comments, collaborations, and future refinements to the models and recommendations advanced to the scientific community by our paper. The letter to the editor author should have no reason to find our paper “disconcerting” because we did not include certain publications; we would have, if the main topic of our paper was about soybean aphid natural enemies. Although the concepts advanced in our paper may be useful for studying soybean aphid natural enemies, the title of our paper clearly states that soybean aphid natural enemies was not the subject matter. The author of the letter to the editor believes that our paper will “grossly distort the need to apply insecticide and result in significant over-treatment, which would facilitate the selection of insecticide resistant soybean aphid populations.” For the record, nowhere in our paper did we advocate “over-treatment.” In fact, if the logistic curve and its distinct phases are followed, soybean aphid population regulation can be effected more judiciously regardless of whether the regulatory tactic being deployed is resistant soybean cultivars, organic insecticides, synthetic insecticides, or even natural enemies.

Technical Abstract: As this manuscript is a response to a Letter to the Editor of the Agronomy Journal, no technical abstract exists. Presented below is the technical abstract for the paper in question. Stage-specific economic injury levels form the basis of integrated pest management for soybean aphid (Aphis glycines Matsumura) in soybean (Glycine max L.). Experimental objectives were to develop a procedure for calculating economic injury levels of the soybean aphid specific to the R2 (full bloom), R4 (full pod), and R5 (beginning seed) soybean development stages using the law of the diminishing increment regression model. Soybean aphid population growth over time appeared to follow the symmetrical bell-shaped and logistic growth curve models. Peak soybean aphid population levels and rates of increase occurred at the R5 development stage and then declined sharply thereafter. Highest peak soybean aphid populations were 21,626 aphids per plant for infestations starting at V5, and 6446 aphids per plant for infestations starting at R2. Highest maximum aphid-days per plant recorded were 537,217 for V5-introduced soybean aphids and 148,609 aphid-days per plant for R2-introduced soybean aphids. On average, the calculated maximum possible yield loss was 75 % for soybean aphid infestations starting at the V5 (five node) stage and 48 % for soybean aphid infestations starting at the R2 stage. Interrelationships among the current or predicted market value of soybean, cost of soybean aphid control, and the yield potential of the soybean field were considered in the calculations of the stage-specific economic injury levels. Practical examples for calculating stage-specific economic injury levels are presented. Economic injury levels were calculated both as soybean aphids per plant and soybean aphid-days per plant. Use of these stage-specific economic injury levels may enable growers to manage soybean aphids more accurately.