What’s new with soybean aphid?
The soybean aphid (Aphis glycines), which is native to China, was accidentally introduced into Iowa in 2000. Shortly after, it became the most economically damaging insect threat in soybeans in northeast Iowa. It has now been found throughout the Upper Midwest.
Aphid feeding can cause stunted soybean plants and reduced pods and seeds. It may also transmit viruses that distort leaves, reduce seed set and discolor soybean seeds. Soybean aphid may have up to 18 generations per year, beginning with overwintering eggs on the alternate host — buckthorn.
In spring, winged aphids migrate from buckthorn to nearby emerged soybeans. Generations advance in these fields, and then another winged migration occurs in summer spreading from these fields to others. A third migration occurs in fall with aphids moving back to buckthorn.
Depending on the season, soybean fields’ proximity to buckthorn and soybean aphid migration patterns, populations of aphids have usually peaked in soybeans between late July and late August.
First a review of the past
By 2003, university entomologists established an economic threshold for soybean aphid of 250 aphids per plant. A second scouting method called “speed scouting” was also developed. It is just as effective as regular scouting, and as its name implies, it is a quicker scouting tool.
If a foliar insecticide application is warranted, research has not found any particular labeled product heads above any other. The emphasis of a foliar application needs to be placed on proper application method, nozzle choice, spray pressure, coverage throughout the canopy and timing of the treatment.
Insecticide seed treatments are very effective against soybean aphid for about 50 days following planting, but have limited value since soybean aphid activity persists through August. Some of the most effective early-season control of soybean aphid is from its natural enemies, including lady beetles, lacewings, syrphids, orius, nabids, spiders, and predatory wasps and fungi.
This is an important reason to not treat soybeans with an insecticide application if there is just a minimal aphid presence. The insecticide would crash the beneficial insect population. Consecutive days of hot or wet weather can also negatively affect aphid growth and reproduction.
Now, for the ‘What’s new?’
Following are a few recent developments in the management of soybean aphid:
• University entomologists initiated efforts to introduce into the north-central region species of predatory wasps, called parasitoids, which provide natural control against soybean aphid in their native land of China. Entomologists have identified multiple species of these parasitoids, brought them back to the U.S., and worked with them under quarantine for several years to identify any possibility of non-target impacts.
Finding no non-target impacts, entomologists have conducted multiple releases of these wasps in the last few years to see if they will adapt and survive in the U.S. to combat soybean aphid. This process is still in its initial stages, but at this time these parasitoids are not yet a dominant predator in soybean fields.
• University plant breeders have discovered “resistant aphid genes” that can be naturally bred into soybeans to impart protection against soybean aphid. So far four genes have been identified, with the first of these genes called Rag1. Rag1 is now available in a few commercial soybean varieties at no additional cost to the seed.
Recent research has shown this resistance to be very effective, but it’s not yet “bulletproof.” At this time, researchers have identified a few different biotypes of soybean aphid in the north-central region of the U.S.
This is important because some Rag genes may only offer resistance against some aphid biotypes. Added to the problem is the distribution of these aphid biotypes across the region is not known.
In the north-central region, 14 states are cooperating in a large soybean aphid sampling project in an attempt to map this distribution. Plant breeders are also involved in breeding soybean varieties with multiple Rag genes to provide broader protection against multiple aphid biotypes.
• A look at aphid trials over the last 10 years in northeast Iowa suggests we may be seeing a shift in seasonal population patterns of soybean aphid. Only time will tell if this is real or not.
The ISU trials show weekly average aphid counts for the time periods of 2002-07 and 2008-11. No doubt soybean aphid problems were worse in some years versus others within these time periods, but the population trends were similar. This suggests that major aphid activity in northeast Iowa has shifted more to August.
Is this really true? Does this increase or decrease the overall pest threat? Might this alter management tactics of “a last pass herbicide-insecticide tank mix,” or require an adjustment in the economic threshold? It will likely take a few more years of aphid trials to sort this out.
The bottom line: We currently have very effective tools for soybean aphid management. However, this pest is still a bit of a “moving target.” Research continues, and new advances are being made to make it even easier to deal with and manage this pest in the future.
Lang is the ISU Extension field agronomist at Decorah in northeast Iowa.
This article published in the December, 2011 edition of WALLACES FARMER.