Other Aphid Biological Control Work

NCSRP-Funded Soybean Aphid Project 2002-2006
Our previous North Central Soybean Research Program (NCSRP) funded project , Management of Soybean Aphid in the North Central States, made significant contributions to our understanding of soybean aphid biology in the North Central states. Among our accomplishments were the following:

1. Development of an economic threshold of 250 aphids per plant with more than 80% of plants infested.
2. Testing and release of Speed Scouting, a sequential sampling plan for scouting soybean aphid.
3. Defining the role temperature plays in soybean aphid biology, both during the summer (reproduction) and in the winter (survival).
4. Determining the role native natural enemies play in aphid population dynamics.
   Conducting initial foreign exploration and host range testing of natural enemies of soybean aphid found in China, Japan, and Korea.

Economic Threshold
The development of the economic threshold (ET) was a coordinated effort of 6 North Central states that associated aphid population density with yield loss at 19 location-years. These data show that a simple linear regression explains nearly 70% of the variation.

Sensitivity analysis of the ET for varying control costs, soybean price, and yield potential were calculated. On average, the aphid population that will result in a yield reduction equal to the cost of control or the economic injury level (EIL) is 654 aphids per plant. The ET was calculated at 264 aphids per plant and was not significantly different from earlier estimates of 250 aphids per plant. The assumption is that the aphid population is growing and within 7 days will reach the EIL.

The ET has proved effective in preventing loss caused by soybean aphid and is sufficiently high to allow natural enemies to respond to an aphid outbreak. The ET of 250 aphids/plant has been widely adopted throughout the North Central Region. This ET remains the only threshold based on field data of aphid population growth over a broad range of agronomic and environmental conditions.

Scouting
Counting aphids is tedious and time consuming. We know that it takes more than an hour in a single field to accurately count aphids when the population is at threshold densities. Speed Scouting, a binomial sampling plan based on the current economic threshold, was developed and validated through the regional project. In 2005, Speed Scouting was shown to be as reliable as enumerative sampling, but instead of taking more than an hour to count aphids, crop professionals could make a treatment decision in about 10 minutes per field. Printable Speed Scouting worksheets (pdf file) have directions you can use to make a treatment decision for soybean aphid. More information and FAQ can be found at the University of Minnesota Just for Growers website.

Temperature
Soybean aphid, like many other aphids, is adapted to a temperate climate. Optimal temperature for growth and reproduction of soybean aphid is 82 ºF. At this temperature aphid populations can double in as few as 1.3 days. Under field conditions we have observed populations doubling in a little as 2 days.

A 4 degree rise in temperature to 86 ºF will reduce lifetime reproduction of each female by more than 70%, while a 5 degree cooling to 77 ºF reproduction drops less than 10%. Clearly, soybean aphid is better adapted to moderate summer temperatures.

Survival in the egg stage on the soybean aphid's primary host, common buckthorn, is not hindered by normal winter temperatures. The supercooling point of soybean aphid eggs has been measured at minus 29 ºF. Observations by NCSRP partners in North Dakota and northern Minnesota confirm that soybean aphid can overwinter successfully in the northern most soybean production areas. Winter temperatures do not appear to limit the survival of soybean aphid.

Natural enemies
The soybean aphid is attacked by a variety of natural enemies in both Asia and North America. In Asia, initial exploration led to the discovery of 7 species of parasitoids and a number of predaceous arthropods. Initial testing in the laboratory has provided an appreciation of the host ranges of the parasitoid species, as well as data on their reproductive rates on soybean aphids.

Research has also documented the significant impact of natural enemies on soybean aphid dynamics in the United States. Soybean aphids are attacked by a wide variety of natural enemies, including parasitoids, predators and fungal pathogens. Studies in several North Central states have shown that arthropod predators in particular are important in the aphid's population growth and in many fields and years prevent the aphid from reaching pest status.

Submitted by Dr. David Ragsdale, Department of Entomology, University of Minnesota, October 2006