Project Overview

The soybean aphid, Aphis glycines, on the leaf and petiole of a soybean plant

The soybean aphid, Aphis glycines Matsumura, a native of Asia, was first discovered in North America in July 2000 in Wisconsin (Ragsdale et al. 2004). Since then, the soybean aphid has spread to 22 U.S. states and 3 Canadian provinces, putting over 60 million acres of soybean at risk to crop injury caused by this exotic insect.

The aphid reduces soybean yield directly through feeding on the plant, and through the transmission of several pathogens. While damage is done mainly through direct aphid feeding, the potential for aphid-transmitted disease outbreaks exists, as soybean pathogens now have a widespread and abundant vector they lacked in the past. Recent estimates of losses to the aphid in the Midwest have suggested a 14% average yield loss, resulting in hundreds of millions of dollars in damage and control costs. In 2003, an estimated 300 million bushels were lost to the aphid, and approximately 7 million acres of soybeans were sprayed in Minnesota, Iowa, Michigan, Wisconsin, Illinois and Indiana (2004 USB "Soybean Aphid Special Report"; Landis 2004). These estimates make the soybean aphid the most costly insect pest ever to infest US soybean production.

Early discoveries in aphid ecology

Orius (pirate bug) nymph preying on a soybean aphid. Orius is a common predator in midwestern fields.

In response to the threat posed by the soybean aphid, teams of Midwestern scientists have collaborated to investigate the aphid's ecology and management options. Among key findings is the significant impact natural enemies have on soybean aphid population growth and pest status. In many soybean fields in the North Central region, natural enemies provide a significant level of control, and without these natural enemies our soybean aphid problem would be far worse. However, it is clear from the outbreak 2003 that the natural enemies we currently have in U.S. soybean fields do not consistently control the soybean aphid.

Prospect for importation biological control
Fortunately, there is a well-tested and proven method of control for invasive species like the soybean aphid. This method adds new natural enemies from the pest's homeland to re-unite old enemies to lower pest levels below economic thresholds. The approach, termed "importation biological control", has been used successfully against insects for over 120 years.

Approximately 400 species of insects have been targeted with importation biological control. Of these, about 75 have been brought under complete control, requiring no further control interventions. For another 90 species there have been significant reductions in control efforts. The approach has been successfully used against aphids in alfalfa, cabbage, cereals, carrots, and corn.

In the Midwest, importation biological control has successfully controlled exotic pests such the alfalfa weevil and alfalfa blotch leafminer in alfalfa, and the cereal leaf beetle in small grains. Importation biological control can be used as a "stand alone" approach or it can be integrated into a larger pest management strategy.

The return to producers in protected yield and reduced management costs can be substantial. For example, since its initial success in 1958, the spotted alfalfa aphid biological control program has saved producers $3 million annually (Bellows and Fisher 1999). As a return on research investment, importation biological control has averaged $30 of benefits to producers for every $1 invested in research (Tisdell 1999). Finally, once natural enemies have been released, the control they provide is free to producers. Importation biological control is one of the most cost-effective and ecologically responsible methods of pest management in use today.

Impact of natural enemies

Figure 1. Mean (+ SEM) A. glycines population (log scale) over time in open (upper line) and exclusion (lower line) cages. The arrow indicates when open and exclusion plot cages were switched. One randomly selected replicate of each cage type was left unmanipulated (dashed lines) to show the trend if cage treatments remain unaltered. (Fox et al. 2004)

When the aphid invaded US soybean fields, it faced attack by endemic natural enemies in the growing season, and in its overwintering sites. Midwestern researchers have identified over 45 species of natural enemies that attack the soybean aphid, including several species of parasitoids and pathogens, and a great diversity of predatory ground beetles, flies, true bugs, ladybeetles and lacewings (Rutledge et al. 2004).

Studies in the Midwest, have shown significant differences in population size and growth rates for aphids protected from natural enemies versus those exposed to attack by them. For example, in a 2002 Michigan study, aphids protected from natural enemies reached a population level nearly 100-fold higher than aphids exposed to attack (Figure 1). Furthermore, exposing previously protected aphids to attack by natural enemies resulted in significantly lower aphid densities, suggesting we can reduce aphid numbers through manipulation of their exposure to attack by natural enemies.

Research in Indiana in 2004, showed that aphid populations protected from attack by natural enemies grew at significantly faster rates than aphids exposed to natural enemies (Fig. 2). In most cases exposed aphids failed to show positive population growth, whereas protected aphids typically quadrupled their population size in a week. These results help explain why we saw few economically-infested fields in the Midwest in 2004 even though aphids arrived early in the season and temperature conditions were near optimal for their growth (Fox et al. 2004; McCornack et al. 2004; Rutledge et al. 2004).

Figure 2. Frequency histogram of weekly soybean aphid population growth inside cages (black) and in the field. Ten aphids were placed in both locations. Aphid growth indicated by finding > 10 aphids in the following week's sample. Desneux and O'Neil, unpublished data.

Unfortunately, the outbreak of the aphid in 2003 showed that the natural enemies we currently have in soybeans do not provide a consistent level of economic control. However, using importation biological control, we could add new natural enemies to soybeans they may consistently reduce soybean aphid populations to provide the needed level of control. Logically those natural enemies should be secured from the homeland of the aphid (Asia), where its enemies may have evolved strategies that result in lower aphid densities and reduced impact on soybean yield (Heimpel et al. 2004, Wu et al. 2004).

The soybean aphid in Asia
Because the soybean aphid is from Asia, several of the co-investigators (O'Neil, Voegtlin, Ragsdale and Heimpel) traveled there to assess the aphid's pest status, learn about its ecology, and discover new natural enemies for possible use in the United States. Of the 3 countries surveyed, China, Japan and South Korea, the soybean aphid is considered a non-pest in Japan and Korea. In China, the aphid is a sporadic pest in northeast provinces, but is largely seen as a non-pest in most other areas (Wu et al. 2004).

Our surveys of Asian soybean fields confirmed that there are dozens of natural enemies species that attack the soybean aphid. Importantly, our Asian colleagues have shown that natural enemies are key to the non-pest status of the soybean aphid (Liu et al. 2004). Thus, we saw first-hand that although soybean aphids do colonize soybean fields in Asia, they are not a pest in most of its native range. If we add new natural enemies from Asia to Midwestern soybean fields, the soybean aphid could potentially become a non-pest here, as it is in most Asian soybean fields.

What needs to be done
To manage the soybean aphid using biological control we must add new (Asian) natural enemies to complement the ones currently in US soybean fields. To maximize the potential for success and minimize the potential risks of introducing new species, importation biological control uses a rigorous set of protocols and approaches. Among these are pre-release screening of candidate natural enemies to define their specificity to the target pest, ecological studies to identify critical pest-enemy relationships, and post-release evaluation to determine the level of success and needs for further management actions.

Critical to the success of any biological control project is the education of growers, who may not be familiar with biological control, and whose crop management can effect how natural enemies are working in their fields.