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Life Cycle and Biology
Summer = Soybean
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Winter = Buckthorn  |
Overview of Life Cycle
Like many aphids, the life cycle of the soybean aphid is complex (see below). Soybean aphid has two primary hosts: a woody shrub or understory tree called buckthorn that serves as a host in the fall, winter and spring; and soybean, its primary summer host. Prominent features of the life cycle include:
Common
buckthorn – a publication of the Alien Plant Working Group |
Fall, Winter and Spring Host Range
Various species of Rhamnus are reported as the woody host of the soybean aphid. In the U.S., several native and a few introduced species of Rhamnus occur.
Summer Host Range
While soybean is the primary summer host, soybean aphids have been shown to survive and even reproduce on a limited number of alternative hosts.
Detailed Life Cycle
Seasonal Appearance of Soybean Aphid |
Soybean Aphid Seasonal Pattern in China (Wang et al. 1962) Image adapted from Wang, C.L., Siang, N.I., Chang, G.S. and Chu, H.F. (1962) Studies on the soybean aphid Aphis glycines Matsumura. Acta entomologica Sinica 11: 31-44 |
Colonization, Local Dispersal and Long-Distance Migration
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Winged soybean aphids appear at several times during the year as they colonize summer hosts, redistribute as the summer progresses, disperse over long distances, and return to their winter host, buckthorn in the fall. The seasonal appearance of nymphs developing wings and winged adults in soybeans in Minnesota is depicted to the left for 2003.
After egg hatch, soybean aphids began feeding and reproducing on the newly emerging leaves of buckthorn. With each generation an increasing proportion of the young develop wings, leave buckthorn and begin the colonization of soybean and other summer hosts in late May and June. Spring colonizing flights appear to be low-level, relatively short distance flights to nearby emerged soybean. Soybeans as young as the VC stage (cotyledons plus unifoliate leaves just opening) were colonized in 2003. Heavy, early infestations tend to appear in relatively protected areas near abundant overwintering sites. In Minnesota, early infestations are generally heavier in southeast Minnesota and occasionally along wooded river valleys and streams to the west and north.
Ants and Aphids on Soybean |
Initial infestation is often spotty and may be more severe near field edges, but this effect is transitory as colonization and midsummer dispersal continue. Ants may also assist in this process by protecting and transporting aphids to other nearby uninfested plants. Building populations on heavily infested young plants also begin to produce winged young that redistribute to other uninfested plants within the field or to nearby later-emerging fields. Usually this local dispersal results in more uniform infestations within a field as populations intensify.
Wingless females |
Winged females |
By mid-July as infestations continue to build a change takes place in dispersal. Whether in response to crowding, declining soybean quality or possibly daylength triggers, the production of winged females becomes more common. In fact, most of the young in a field may become winged within a week. Their departure often creates the impression that populations in a field collapsed from other factors. The aphids locally disperse to relatively uninfested fields, such as double-cropped soybeans after peas or small grains, and initiate long-distance dispersal.
Soybean Aphid Captures in Rosemount Suction Trap, 2001 |
These newly winged females are more likely to engage in longer-distance, higher level flights. For the first time, aphid flight occurs at high enough levels to be picked up in suction traps. Note that both the colonizing flight activity and the local redistribution and dispersal occur at a low enough altitude that they’re not picked up by the 8 ft tall suction traps.
Aphids in Toronto
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Since aphids are relatively weak fliers, long-distance dispersal is often at the mercy of prevailing winds. Aerial applicators frequently report having to stop to clean their windshields from flying into clouds of these aphids above heavily infested fields. In 2001, the influx of winged soybean aphids into the open dome of the Toronto Blue Jays even caused an early end to a Toronto Blue Jays game. It’s these flights that lead to rapid, progressive colonization of soybean, almost like a wave moving across the countryside. Under favorable conditions for aphid infestations, the settling of winged aphids into uninfested fields has been described as “aphid rain.”
The ability of soybean aphid to disperse and utilize soybeans throughout the agricultural landscape is one of the major challenges to its management. The different types of winged aphid activity (colonization, local redistribution, long-distance dispersal) are reflected in the changing geographical picture of aphid infestations in Minnesota in 2003. (see maps below) Note how the geography and intensity of the infestation change during these respective phases.
Click for larger image of maps
Eggs |
Overwintering of the Soybean Aphid
Soybean aphid eggs are often laid under bud scales on buckthorn. This position, probably above snow cover except for shorter buckthorn, leaves the eggs relatively exposed to intensely cold air temperatures compared to insects that lay eggs in the soil or under leaf litter. Understanding the effects of overwintering temperatures on egg survival provides crucial insight to the ultimate geographical range of the insect and its success in Minnesota.
Temperature studies on soybean aphid longevity and reproductive success suggest that this introduction of the soybean aphid probably came from a north temperate area. Supercooling studies conducted by Dr. Rob Venette indicate soybean aphid eggs can survive temperatures down to ??oF before freezing occurs. Obviously these eggs are fairly winterhardy… but Minnesota climate can be fairly challenging as well.
 Click for larger image |
The maps to the left indicate how predictions of winter survival, modeling by Dr. Venette, compare to aphid detections (yellow dots) by MDA’s Plant Pest Survey the following spring. The potential overwintering range of soybean aphid contracts as winter severity increases; compare the mild winter of 2001-2 with the more severe winter of 2002-3. However, as indicated by the previous dispersal discussion, the soybean aphid can rapidly re-colonize any geographical range lost during a severe winter if conditions favor aphid infestations.
Temperature Effects on Soybean Aphid
 Click for larger image |
Temperature plays a major role in the longevity and reproduction of soybean aphid, according to recent work by McCornack et al. (2004). As you might expect with an insect, soybean aphid longevity is directly related to temperature. Longevity declines as temperature increases dropping from 36 days at 68oF to only 11 days at 95oF. Expressed on a physiological basis (i.e., using degree days above 47oF), longevity is about the same over the broad range of temperatures from 68oF to 86oF. By 95oF, however, it’s obvious that soybean aphids are under stress; mortality is quicker and no young are produced.
 Click for larger image |
Birth of live young begins 4 to 5 days after birth for most temperatures. Under laboratory conditions (e.g., constant temperature, young soybean plants [V1], no crowding), a single aphid female produces a maximum of 9.5 offspring per day during peak reproduction. Reproduction started 2 days earlier at 77oF and 86oF than at 68oF. The total number of offspring produced per female peaked at cooler temperatures, ?? at 68oF, ?? at 77oF, dropped to ?? at 86oF, and as stated earlier, no young were produced at 95oF.
 Click for larger image |
Population growth is affected by a number of factors, including offspring production, survivorship, departure / arrival of winged females, and maturation. The rate of population growth reflects the interaction of longevity and reproduction. For soybean aphid, the development rate peaks and the doubling time is fastest when temperatures near 80oF.
Knowledge of temperature effects on adult survival and reproduction has profound implications for management. If temperatures are projected to be hot (in the 90s oF), 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 70s, population growth will reach its maximum.
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URL: http://www.soybeans.umn.edu/crop/insects/aphid/aphid_biology.htm
Last Modified 3/25/11 2:27 PM
efans@umn.edu
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