WW-03935     Revised 2001  

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Copyright ©  2001  Regents of the University of Minnesota. All rights reserved.

Significance

The soybean cyst nematode (SCN), Heterodera glycines, is one of the most destructive pests affecting soybeans in the United States as well as in the other top ten soybean-producing countries of the world. Annual yield losses in soybeans due to SCN have been estimated at about $1.5 billion in the U.S. alone. SCN was first reported in North America in North Carolina in 1954, and since then has spread to 28 soybean-producing states and Canada. In Minnesota, SCN was first detected in 1978 near Frost in Faribault County. By 2000, its presence had been detected in 52 counties in the state (Figure 1).

Figure 1. Minnesota Map showing known distribution
of the soybean cyst nematode by county and  the
decade in which it was discovered.

The soybean cyst nematode infects soybean, dry edible bean, and snap bean, and not rotation crops such as corn, small grains, and alfalfa (Table 1). Because the nematode can be present in fields without causing obvious above-ground symptoms, yield losses caused by SCN are often under-estimated. Yield losses incurred by susceptible crops growing in SCN infested fields can be expected to vary from year to year. In heavily infested fields of fertile silt loam soil, SCN can cause yield losses of more than 30%. In Minnesota, crop failures during warm and droughty growing seasons have occurred with soybean planted in non-irrigated, heavily-infested, sandy soils on the Anoka Sand Plain.

Development and Life Cycle

SCN is a microscopic roundworm that attacks roots of soybean and a limited number of other host plants (Table 1). Developmental stages of the nematode include the egg and four juvenile stages (Figure 2). The first-stage juvenile develops within the egg and molts to form a second-stage juvenile (J2). The J2 hatches from the egg, moves through soil pores in the film of water surrounding soil particles, is attracted to actively growing roots, and infects by penetrating host plant roots usually near the root tip. After penetrating the root, the nematode establishes a feeding site in the vascular tissue where it becomes sedentary, enlarges to become sausage-shaped, and molts three more times before becoming an adult. The adult female is lemon-shaped and, when fully developed, is visible on the root surface without magnification. The adult male undergoes a metamorphosis during the last molt to become a slender, motile worm. The male stops feeding and exits the root.

Figure 2. Life cycle of the soybean cyst nematode. (sketch by Dirk Charlson, Iowa State University.)

A pheromone released by the female attracts the male for mating. The female exudes a gelatinous matrix from the posterior portion of its body into which it deposits a small portion of the total eggs that it will produce. The gelatinous matrix containing eggs is referred to as an egg mass. Eggs in the egg mass hatch and the resulting larvae infect soybean roots the same year they are produced (Figure 2). Several hundred additional eggs are retained inside the female body. As the female matures, its lemon-shaped body changes color from white to yellow. When the female dies the body (now referred to as the cyst), becomes dislodged from the root and undergoes a tanning process during which the cyst changes color to a dark brown. The cyst protects the eggs from damage by environmental stresses and serves as the over-wintering and long-term survival structure for the nematode eggs. In addition to the protection afforded by the cyst, the egg itself is durable and resistant. Some eggs within the cyst have been shown, under laboratory conditions, to be able to survive for more than 9 years before hatching.

The life cycle for SCN typically takes 3 to 4 weeks depending on geographic location, soil temperature, and nutritional conditions. Optimal soil temperatures for the various phases of the SCN life cycle are 75 degrees F for egg hatch, 82 degrees F for root penetration, and 82-89 degrees F for juvenile and adult development. Little or no development takes place below 59 degrees F or above 95 degrees F. In southern Minnesota, SCN can complete three to four generations during a soybean-growing season. In central Minnesota, the nematode probably can complete only two to three generations.

Produced by Communication and Educational Technology Services, University of Minnesota Extension.

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