Issue No. 4, Article 5/April 16, 2004
A Primer on Soybean Aphids
The planters are rolling across the east-central Illinois landscape, and many thousands of acres of corn have been planted. As we move into mid-April, warmer temperatures will fuel the frenetic corn-planting pace. Soybean planting can't be that far off, and with it the interest and curiosity in soybean aphids will intensify. Will the 2004 season bring a repeat of 2003 with respect to this new insect pest? Or will 2004 lull us into complacency regarding the management of soybean aphids? Predictions abound. Those of us familiar with agriculture know that ultimately we will have to experience another growing season to answer these questions.
In the most recent issue of the Annals of the Entomological Society of America (vol. 97, no. 2, March 2004), eight scientific articles were published concerning the biology and ecology of the soybean aphid. I've read through these intriguing articles and thought it might be helpful to share with the readers of the Bulletin some facts gleaned from these publications. Provided here is a primer of sorts concerning soybean aphids that may become more useful as the 2004 season unfolds.
The soybean aphid is an insect native to eastern Asia (northern China, Korea, Japan, the Philippines, Malaysia, Indonesia, and Australia). This pest was discovered in North America in July 2000. Initial infestations were reported in Wisconsin, and by summer's end entomologists in 10 states of the north-central region of the United States had confirmed pockets of soybean aphids in soybean fields. Presently, soybean aphids now occur in 21 states (at the conclusion of 2003) and three Canadian provinces.
Soybean aphids lead rather complex lives, with alternate hosts and asexual and sexual cycles. For the most part, the life cycle of soybean aphids in the United States closely resembles the life cycle in China and Japan. Overwintering hosts in these Asian countries are Rhamnus davurica Pallus and Rhamnus japonica Maxim. To date, there are two confirmed overwintering hosts in the United States--Rhamnus cathartica L. (common buckthorn) and Rhamnus alnifolia L'Héritier (native alderleaf buckthorn). In addition to these hosts, fall migrants (gynoparae--from soybean plants to overwintering host) and oviparous nymphs (reproducing by eggs laid by the female) have been found on glossy buckthorn (Frangula alnus).
David Voegtlin, an entomologist with the Center for Economic Entomology, Illinois Natural History Survey, could not confirm this species as a "primary host." Both common and glossy buckthorn species are exotic, whereas the alderleaf species is native to the United States. As the name suggests, common buckthorn is very abundant in the north-central region of the United States. Finding spring colonies of aphids on common buckthorn has proven very challenging. This had led to some speculation that there might be other hosts entomologists are yet unaware of. Because Rhamnus davurica and the native alderleaf buckthorn species are relatively uncommon in the north-central region, it seems unlikely that either serves as a significant production site each spring for aphids to subsequently infest soybean fields. Efforts to rear soybean aphids on kudzu, Pueraria lobata (Willd.), in Illinois have failed.
In 2003, soybean aphids were reported in soybean fields on June 3 in Minnesota and June 11 in Indiana. These very early sightings were approximately 1 month earlier than first observations in soybean fields the preceding two seasons. So predicting the first occurrences of soybean aphids in 2004 is anything but certain. It has been suggested that mild growing conditions in 2003 contributed to large densities of soybean aphids and widespread economic infestations in the north-central United States. The optimum temperature range for development of soybean aphids is reported as being 22 to 25°C (71.6 to 77°F).
If a mild June occurs in 2004, this could contribute to increasing densities of soybean aphids in some locations. Within soybean fields in the vegetative growth stages, soybean aphids are frequently discovered in colonies on the growing points. As plants reach the reproductive phases of development, aphids become more evenly distributed throughout the plant. It has been reported that host quality declines as soybean plants mature, potentially lowering the fecundity of soybean aphids. The phenology of soybeans may significantly affect the numbers of gynoparae (winged female fall migrants) and winged males that are able to successfully leave soybean fields prior to harvest and reach buckthorn. Early maturation and harvest of soybeans may interfere with a successful migration of winged females and males to buckthorn. Late-planted beans and double-cropped soybean fields may improve the chances for this fall migration to occur.
We will continue to learn much more about soybean aphids in the United States as we develop appropriate IPM programs for this interesting insect pest. Here are some interesting tidbits that I gleaned from the aforementioned articles.
- The number of generations of soybean aphids in China has been reported from 10 to 22 per year.
- In China, generation length ranged from 2 to 16 days.
- It has been reported that at 78.8°F, a mean (per female) of 58 and 38 nymphs were produced from wingless and winged soybean aphids.
- Odor eminating from soybean plants appears to be influential in attracting soybean aphids.
- The Chinese literature indicates that soybean aphids are important vectors of soybean mosaic virus. This disease is common in China.
- Soybean aphids have been reported as vectors of other diseases, such as soybean stunt virus, soybean dwarf virus, abaca mosaic, beet mosaic, tobacco vein-banding mosaic virus, bean yellow mosaic virus, mungbean mosaic virus, peanut mottle virus, peanut stripe poty virus, and peanut mosaic virus.
- In China, lady beetles are believed to be the most important predators of soybean aphids. The multicolored Asian lady beetle, Harmonia axyridis Pallas, is one of several lady beetle predators in China. Other lady beetle species include Propylaea japonica Thunberg and Coccinella septempunctata L.
- Entomologists at Purdue University have identified insidious flower bugs (Orius insidiosus Say) and multicolored Asian lady beetles (Harmonia axyridis Pallas) as "potentially key predators" of soybean aphids. They speculate that predators such as these that appear in greater densities earlier in the season in soybean fields may be more apt to prevent soybean aphid outbreaks than predators more commonly observed later in the season.
- In China, the density of overwintering soybean aphid eggs was strongly correlated with subsequent infestations during the upcoming growing season.
- Severe outbreaks resulted when the number of overwintering eggs was greater than 10,000 per 100 buckthorn branches. How's that for a predictive threshold?
- Host plant resistance research in China has shown that some varieties are highly resistant to soybean aphids. In the late 1980s, it was reported that two "highly" resistant varieties were found among 181 varieties that were screened. I suspect that we will eventually utilize this IPM approach extensively in the United States for this insect pest.
We will continue to learn more about soybean aphids over the years and hopefully offer an effective integrated management approach for this new and significant threat to soybean production.
I continue to be distressed about sales promotion tactics for certain insecticides targeted against soybean aphids. These tactics run completely counter to the IPM philosophy. In general, the sales campaigns promote the treatment of soybean fields for soybean aphids with little to no regard for aphid thresholds, aphid densities, or knowledge of predator densities. These promotional campaigns indicate that the cost of the product will be returned if yields don't pay for the treatment expenses. This is not a responsible approach to soybean aphid management nor is it an acceptable product stewardship philosophy. This issue will be addressed in greater depth as the season progresses.--Mike Gray