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Studies of the interactions of SCN with the soybean aphid

Investigators: Gregory L. Tylka, Matthew E. O’Neal, Felicitas Avendano
Funding: Iowa Soybean Association

The soybean aphid, Aphis glycines, was first discovered feeding on soybeans in Iowa and other Midwestern states in 2000. Since then, it has spread throughout the state and region. Currently, soybean aphids are found in every county in Iowa and the insect has become a serious yield-reducing pest of soybeans in the state.

Distribution of SCN in Iowa and occurrence of HG types

Investigator: Gregory L. Tylka, David Soh
Funding: Iowa Soybean Association

SCN was first discovered in Iowa in Winnebago County, in extreme north central Iowa, in 1978. Currently, the nematode is known to exist in 97 of 99 Iowa counties (all except Allamakee and Ida Counties). A random survey of Iowa was conducted in 1995-1996 to better define the distribution of SCN in the state. SCN was found in 74% of Iowa fields sampled in that survey. SCN was found much less frequently in no-till fields than in tilled fields sampled in the survey.

Extraction of SCN eggs from cysts

Wash the cyst/sediment suspension (from the cyst extraction method) from the 100 ml beakers into the small PVC #60 sieve (250 micron diameter pores). Eggs of SCN average 47 microns by 100 microns in size.

Place a #200 sieve (75 micron diameter pores) over a #500 sieve (25 micron diameter pores) into the funnel below the rubber stopper, and start the drill press using the foot pedal. Bring the spinning rubber stopper into contact with the sieve surface.

Staining and counting SCN eggs

After extracting the eggs from the cysts  in the prepared soil sample, place 4 to 5 drops of 1 M HCl and approximately one eye-dropper full of acid fuchsin stain into each beaker of egg solution. Heat beakers containing eggs and stain in the microwave until the suspension is about to boil (approximately 3 minutes for 12 samples). Remove beakers from microwave and allow to cool. Place beakers in the refrigerator until they are counted.

Staining roots for SCN

Place the root samples in separate 100 or 150 ml glass beakers. Prepare 600 ml of a water-bleach solution by adding 100 ml household bleach (5% sodium hypochlorite or NaClO) to 500 ml distilled water. Add about 50 ml of water-bleach solution to each beaker containing a root sample.

Incubate roots in the water-bleach solution for 4 minutes, stirring with a metal spatula periodically. Rinse roots for 45 seconds or so in running tap water, then soak the roots in tap water for 15 minutes.

In-vitro hatching experiments

Construct microsieves from 18-mm and 20-mm test tube caps and 25 micrometer pore nylon mesh.
UV-sterilize all microsieves and hatch trays.

Arrange the hatching trays in replicated boxes using a pre-generated random order. Fill each hatching tray with 12 ml of the appropriate test or control solution.

Pipette between 5,000 and 10,000 surface-disinfested SCN eggs (depending on experimental details) onto each microsieve.