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Soybean Aphid Growth Estimator: The SAGE Model To use the SAGE model, enter the average aphid density (aphids/plant) and today's high and low temperature. Make sure to include the zeroes (no aphids on a plant) when you calculate the average. As you enter the forecasted high and low temperatures for the next 6 days, the model will predict an average rate of population build up (expressed as the time required for populations to double). The model will also predict when the population is expected to reach a threshold of 250 aphids per plant. The model is likely to over-estimate aphid populations in the field. Use this model as a guide to determine when to resample a field (preferably at least 2-3 days before threshold is reached). Your entries into SAGE can be saved using the "Save As..." function in Excel. To track populations more carefully, replace forecasted temperatures with observed temperatures as time progresses since you last sampled a field. If you need to move temperatures within the spreadsheet, use Copy and Paste. DO NOT use Cut and Paste; SAGE won't like it. |
Highlights from McCornack et al (2004)
Under laboratory conditions (e.g., constant temperature, V1 plants, no crowding), a single soybean aphid produced, on average, 9.5 offspring per day during peak reproduction. Reproduction started 2 days earlier at 77 and 86°F than at 68°F. At 95°F, no offspring were produced.
As temperatures increase, soybean aphids, like all insects, essentially age more quickly. The longest an aphid will live at 68°F is 36 days; at 77°F, 31 days; and at 86°F, 24 days. When age is measured on a physiological basis (i.e., using degree-days above 47°F), these differences in survivorship disappear. At 95°F, however, soybean aphids experience significant stress. Survivorship is much less (11 days, maximum) than at any other temperature studied. The rapid decline in populations is not explained by physiological age.
Population growth is affected by offspring production, survivorship, and maturation. The optimal temperature for soybean aphid maturation is 80°F. At 68°F, soybean aphid populations can double in less than 2 days. At 77°F, populations can double in 1.5 days. At 86°F, populations double in 2 days. This result may seem counter-intuitive because offspring production and survivorship are reduced at this temperature. However, at 86F, offspring mature rapidly (within 5 days). This slight advantage in the timing of reproduction compensates for the lower number of offspring that are produced in total. It is also important to note that at 95F, populations are no longer increasing. In fact, they are decreasing over time.
Future directions
We are currently studying how exposure to a few very warm days (95F) might be expected to adversely affect aphid population growth. These results will be used in the SAGE model.
Robert Venette, Brian McCornack, and David Ragsdale
Created 2/4/04
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URL: http://www.soybeans.umn.edu/crop/insects/aphid/aphid_sagemodel.htm
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