Two summers ago, Gary Dillion headed to his soybean fields with sticky yellow traps. During the next four weeks, the Chrisman, IL, farmer checked his traps and wasn't surprised to find them populated with western corn rootworm beetles. Even though he used the traditional corn-soybean rotation, he had spotted evidence of adult beetles in the soybeans earlier.

So last summer he took action and treated his corn with a soil insecticide at planting. On a 40-acre test plot in which every other 16 rows were treated with an insecticide and 16 rows were left untreated, Dillion found that the insecticide produced results. The treated rows yielded 10 to 16 bu./acre more than the untreated corn.

Dillion was one of the early users of the sticky yellow traps for scouting beetles. Last summer, university tests with the sticky traps were conducted with help from 450 farmers in Illinois and Indiana. The University of Illinois released guidelines on deciding when to apply an insecticide according to beetle numbers on the traps. This spring farmers in the test will apply an insecticide if it is needed based on the number of beetles scouted.

Rotation no defense. The high incidence of the western corn rootworm in the Corn Belt is evidence that crop rotation is no longer an effective defense. Some rootworm beetles have lost their instinctive preference for only laying their eggs in corn. They now feed and lay eggs in soybeans, oats and alfalfa. When the soybean fields are rotated to corn the following year, the larvae hatch and feed on corn roots.

"The best way for a grower to determine potential risk in first-year corn is to scout soybeans the previous July and August," reports Kevin Steffey, entomologist with the University of Illinois.

Based on tests using Pherocon AM traps, the university researchers recommend that growers distribute 12 unbaited traps evenly throughout the interior of each soybean field, regardless of size. They suggest placing the traps in the field the last week of July through the third week in August. Attach the traps to metal fence posts and position the traps just above the soybean canopy. Every week count the male and female beetles found on each trap and then replace the traps.

When insecticide is needed. Determine the beetle density of each field scouted. This means taking the total number of beetles for one field during the four weeks and dividing by the number of traps, 12. Divide the result by the number of trapping days, 28. This will equal the number of rootworm beetles per trap per day. (Ex-ample: 2,353 total beetles divided by 12 traps divided by 28 days equals 7.)

The university says that at 2 to 7 beetles/trap/day, plants will show root injury. At 2 beetles/trap/day, for example, several roots will be eaten off to within 11/2 in. of the plant, and at 7 beetles, one node of roots will be destroyed. At this level and higher, a soil insecticide may be economically warranted.

Growers just wanting to determine the presence or absence of the western corn rootworm may distribute only four traps. But the university entomologists report that this modified procedure should not be used to determine whether an insecticide is needed.

One source for the Pherocon AM sticky yellow traps is Gempler's. A package of 25 with ties costs $42. Ask for part #T3306 at Gempler's, Dept. FIN, 100 Countryside Dr., Belleville, WI 53508, 800/382-8473.