With soybean planting well underway for 2005 there is a lot of concern about potential pest problems, both rust and aphids this year. Following a year in which the soybean aphid was widely distributed in the Midwest but rarely common enough to generate concern, the question is what about 2005?
Guest Article Feature: Planning for the 2005 Soybean Aphid Population
David Voegtlin, Center for Ecological Entomology, Illinois Natural History Survey
Robert O’Neil, Department of Entomology Purdue University
The following article is featured in this week’s Wisconsin Crop Manager as a guest-authored article by Midwestern entomologist colleagues and project collaborators David Voegtlin and Robert O’Neil. This article was published recently in The New Agriculture Network, Vol. 2, No. 2 – May 12, 2005. It is available at http://www.ipm.msu.edu/new-ag.htm -Eileen Cullen.
With soybean planting well underway for 2005 there is a lot of concern about potential pest problems, both rust and aphids this year. Following a year in which the soybean aphid was widely distributed in the Midwest but rarely common enough to generate concern, the question is what about 2005?
There are a number of factors that come into play that determine aphid numbers. The soybean aphid survives the winter as eggs on common buckthorn (scientific name: Rhamnus cathartica), an exotic, weedy, shrub common in much of the Midwest north of I-80. In the spring these eggs hatch and several generations on buckthorn produce winged aphids that migrate in search of soybeans. For the past four years a suction trap network has been operating in Illinois. While we have been unable to monitor the spring movement of aphids from buckthorn to soybean, we have been able to monitor the fall flight of aphids from soybean to buckthorn.
In 2001 and 2003 we had low fall flights and the next growing year the soybean aphid was not a problem. In 2002 we had a large fall flight and we experienced a major and widespread outbreak of the aphid in the 2003 production season. The fall flight in 2004 was the highest yet, begging the question: "Does the large fall flight of 2004 mean that 2005 will be a repeat of 2003?”
This question is difficult to answer because many factors can influence the size of the following year's aphid population in soybeans. To start with, large numbers of multi-colored Asian lady beetles can be found on shrubs and trees in the latter part of September and into October. An abundance of these predators on buckthorn can effectively prevent the deposition of over-wintering eggs, and the subsequent production of spring migrants that fly into soybeans. Those eggs that are successfully deposited must survive the winter and after successfully hatching, they need to survive the spring weather. Heavy rains and sub-freezing temperature in early spring can eliminate young colonies on buckthorn. In the spring, lady beetles (andother predators) become active and will feed on soybean aphid colonies on buckthorn. Timing of planting can also impact the success of the spring migration. Outdoor cage tests and field observations in central Illinois have shown that many winged aphids leave buckthorn in late April and early May. This migration often precedes the planting and emergence of much of the soybean crop. (This gap between the migration of winged aphids from buckthorn and the availability of soybeans decreases in the northern growing regions and increases in the south.).
Finally, a spring migrant successfully finding soybeans does not necessarily guarantee the development of a colony. Studies by various university researchers have shown that a number of natural enemies, both predators and parasites, are present in soybean fields. When these natural enemies are eliminated or excluded in field experiments, soybean aphid colonization is more successful and population growth is rapid. So while the aphid can outbreak, the factors that lead to an outbreak are many, and all along the way lay pitfalls that prevent or ameliorate aphid outbreaks in any given year.
Spring 2005 field observations -There have been a number of reports from throughout the Midwest of aphids on Rhamnus. Dr. Ho Jung Yoo of Purdue has been monitoring colonies in Indiana. Dr. Ron Hammond at The Ohio State University documented egg hatch around the last week of March. Dr. Christina DiFonzo from MichiganStateUniversity noted egg hatch in early April. In Illinois, we have found colonies in the Quad cities and in the Rockford area as well as in Champaign.
But how do we interpret these findings? It seems inevitable that there will be more aphids than in 2004, but will populations reach the pest levels seen in 2003? The best we can say now is maybe. We now have an economic threshold (250 aphids/plant), and farmers are now well aware of the damage potential of the soybean aphid. Unlike in 2003 when farmers did not realize the impact the aphids were having on their plants (thus allowing their populations to grow), it is unlikely that growers will tolerate a similar build up of aphids this year. The local control of aphids may, therefore, limit the production of large numbers of winged aphids that would fuel a widespread outbreak. However, on the flip-side, chemical control measures against populations reaching the threshold level can wipe out the local predators so that any winged aphid arriving in one of these sprayed fields will experience less resistance to successful colonization. Likewise the aphids surviving an application would have few natural enemies left to prevent aphid population build-up and the field may experience a "rebound" effect leading to faster aphid population growth and pest damage.
So what can farmers do? Farmers should know that there are lots of people looking for the aphid and they are networked. News of infestations travel fast and farmers should have good lead-time, particularly if they keep in touch with their local Extension Service, fellow farmers and state Land Grant Universities.
Second, farmers should be scouting for aphids. A weekly visit to their fields to examine even a few plants will help them keep abreast of aphid population growth in their crop. Third, farmers should use the threshold of 250 aphids per plant. To use the threshold properly, it's important to know if the aphid population is increasing and that's why farmers need to visit fields regularly to look for aphids. And fourth, farmers should not panic! Just because they have some aphids in their field does not mean they will reach the threshold of 250 aphids per plant. The natural enemies in their field are providing free pest control in most fields and in most years. Spraying fields before the threshold only helps to disrupt the control these natural enemies provide and does not guarantee economic control. The temptation to "tank-mix" a little insecticide with an herbicide (or fungicide) application "to get those aphids before they get me" is to be avoided! Looking for aphids, checking the threshold, and then deciding if you need to do something, is the key for this year's aphid pest management.
Organic production -And what about organic producers or those that just don't want to spray their crop? Well, the good news is that soybean fields, particularly those in more diversified farms or areas, have lots of natural enemies that really help in aphid control. In most fields and in most years these beneficials are sufficient to keep the aphid at bay. Unfortunately, we don't have many biological control options on the shelf, and farmers should be leery of untested methods that promise aphid control. Many researchers in the Midwest are working on finding additional options, including work being done in Asia to identify the natural enemies there that prevent the aphid from being a problem in its native home. If we can find a safe and effective natural enemy, we may be able to make the soybean aphid a pest of the past, and save farmers from having to manage this major, yet occasional pest of soybeans.