What is the soybean cyst nematode? What is its life cycle?
The soybean cyst nematode is a microscopic roundworm that attacks roots of soybean and a number of other host plants. The life cycle of SCN includes the egg, four juvenile stages, and adult stage (Fig. 3).
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 the host plant root, usually near the root tip. After penetrating the root, the nematode establishes a feeding site in the vascular tissue, where it becomes sedentary. It then enlarges to become sausage-shaped, and molts three more times before becoming an adult.
The adult female is lemon-shaped. When fully developed, the female's body protrudes outside of the root and is visible without magnification. The adult male undergoes a metamorphosis during the last molt to become a slender, motile worm. The mature male stops feeding and exits the root.
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 juveniles infect soybean roots the same year they are produced.
Several hundred additional eggs are retained inside the female body. As the female ages, its body changes color from white to yellow. When the female dies, the body (now referred to as 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 length of the SCN life cycle is typically about 4 weeks depending on geographic location, soil temperature, and nutritional conditions. Optimal soil temperatures are 75 °F for egg hatch, 82 °F for root penetration, and 82-89 °F for juvenile and adult development. Little or no development takes place either below 59 °F or above 95 °F.
In southern Minnesota, SCN can complete three to four generations during a soybean-growing season. In central to northern Minnesota, the nematode probably completes only three generations.