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Jun
23
Posted by:
WCM Staff
6/23/2005 12:44 PM
Much of Wisconsin is forecasted to experience high temperatures. As we are all in the field experiencing temperatures in the upper 80s and approaching 90s or higher heat index ratings, how will this affect population growth of the soybean aphid?
Soybean Aphids and High Temperatures
Eileen Cullen, Extension Entomologist UW Entomology Department
Much of Wisconsin is forecasted to experience high temperatures. As we are all in the field experiencing temperatures in the upper 80s and approaching 90s or higher heat index ratings, how will this affect population growth of the soybean aphid? You may recall that laboratory studies have been performed recently on the relationship between temperature and soybean aphid longevity and reproduction by entomologists at the University of Minnesota (McCornack et al. 2004). You can find this information on the University of Minnesota MN Soybean Production web page at the following link to Soybean Aphid Life Cycle and Biology.
Click on the link below, and scroll down to the section titled “Temperature Effects on Soybean Aphid”, this is near the bottom of the page.
http://www.soybeans.umn.edu/crop/insects/aphid/aphid _biology.htm
The University of Minnesota team (McCornack, Venette and Ragsdale) found that temperature plays a major role in the longevity and reproduction of soybean aphid. The studies were performed in the laboratory in growth chambers set at a range of respective constant temperatures so that researchers could examine soybean aphid response across a range of temperatures. Soybean host plants were V1 and the soybean aphids were distributed on plants in growth chambers so that they were not overcrowded. In this way, the researchers controlled for other effects such as host plant quality, crowding, etc. and focused exclusively on soybean aphid developmental rate at the different temperatures.
First, the U of MN team found that soybean aphid longevity declines as temperature increases. At 68º Fahrenheit each soybean aphid female survived an average of 36 days. However, soybean aphids in the growth chamber set to 95º F survived for only about 11 days. Comparing developmental rates of soybean aphid over a broad range of temperatures on a physiological basis (i.e., using degree days above 47º F), they showed that soybean aphid longevity is about the same (36 day longevity) from 68ºF to 86ºF. However, it is clear that by 95º F, soybean aphids are under stress and they have a shorter life span.
Second, the U of MN team found that birth of live young (aphid nymphs) begins 4 to 5 days after birth for most temperatures. Remember that during the summer soybean aphids are all female and do not mate to reproduce. They are born “pregnant”, that is with a daughter nymph already developing inside of them. This process is called “parthenogenesis”. McCornack et al. (2004) found that a single soybean aphid female produces a maximum of 9.5 offspring per day during peak reproduction. Within the optimal soybean aphid longevity (life span) range of 68ºF to 86ºF, reproduction started 2 days earlier at 77ºF and 86ºF than at 68ºF. However, at 95ºF no young were produced.
Soybean aphids in the field are exposed to fluctuating night and day temperatures, and are of course not exposed to a controlled constant temperature environment as they are in laboratory studies. Field conditions vary with localized departure and arrival of winged soybean aphid females, natural enemy activity, potential for entomopathogenic fungal infection of aphid as influenced by humidity, and rainfall patterns (e.g., moisture stress or conversely lack of moisture stress and/or driving rains that may subdue aphid populations). Many factors interact to influence soybean aphid population growth in the field. The U of MN study is important because it demonstrates that rate of soybean aphid population growth reflects the interaction between longevity and reproduction. In summary, for soybean aphid, the development rate peaks and the generation doubling time is fastest when temperatures near 80ºF.
As explained on the U of MN website above, thisknowledge of how temperature affects soybean aphid adult survival and reproduction has profound implications for management. If temperatures are projected to be hot (in the 90º F range and above), soybean aphid population growth will slow, stop and may even decrease as temperature stress shortens longevity and stops reproduction. Conversely, if temperatures are projected for the mid to upper 70º F range, soybean aphid population growth will reach its maximum.
At the time of writing, June 22nd, no major changes to aphid number reports of last week. We are still below threshold in our West Madison and Arlington studies that I described in last week’s issue Field Notes: Soybean Aphid on Seedling and Early Vegetative Soybeans http://ipcm.wisc.edu/wcm/pdfs/2005/0515insect2.html. We recommend you continue to keep an eye out and begin regular scouting now if you haven’t already done so. This article on the relationship between soybean aphid population development and temperatures should help you to develop expectations for development of soybean aphid populations you are already aware of and monitoring.
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