WEST LAFAYETTE, Ind. -- To many farmers, the only good bug is a dead bug.

That philosophy can cost producers extra dollars in unnecessary pesticide applications, as well as the loss of beneficial insects to their crops, according to a Purdue University entomologist.

John Obermeyer, who also supervises Purdue's Integrated Pest Management program, said farmers often mistake harmless insects for crop-eating pests. A better understanding of good and bad bugs would save producers time and money.

"There are hundreds upon thousands of animal species that may utilize the soil of a cornfield for all or a short period of their life. Most have nothing to do with the growing of corn," Obermeyer said. "But if a producer has crop damage he's still going to look for a culprit, and often blame the first critter found."

One farmer with damaged corn found millipedes in the soil, Obermeyer said. Thinking they were corn-devouring wireworms, the producer applied pesticide to his entire field. "Those kind of mistakes can add up," Obermeyer said. Treating an average acre of land with pesticide costs from $10 to $15, he said.

With any pest control program, producers should be careful not to terminate their insect allies. Certain species feed on harmful bugs or unwanted matter.

An example is the ground beetle, a light brown or black insect that appears bright green to blue when sunlight reflects off its shiny outer wings. The beetle usually grows from a quarter inch to 1 inch long.

Ground beetles feast on several field pests, Obermeyer said. "They seek out soft-tissued animals to feed on, such as cutworms. And we suspect they have a lot to do with feeding on rootworm eggs in the soil," he said.

Another farm-friendly insect is the cranefly larva, commonly known as a "leatherjacket." The pale gray, legless maggot often is confused with black cutworm, a major corn pest. Cranefly larvae eat decaying plant matter in the soil.

Juvenile earthworms also are frequently misidentified, Obermeyer said. The skinny, almost translucent worms are about half the length of a penny. Some farmers think the worms are overgrown nematodes, one of the deadliest soybean pests. In fact, earthworms are welcome news to any farmer.

"Where there is one, there are often many," Obermeyer said. "Their presence is a sign of good soil health."

To reduce extraneous pesticide applications, Obermeyer recommends that farmers:

- Scout fields each year for insects and keep a record of pest damage risk.

- Learn to identify problem insects. A good resource is Purdue's "Field Crops Pest Management Manual," a 550-plus page reference guide. The manual is $80 and can be ordered through Purdue's Media Distribution Center by calling toll-free (888) 398-4636. Ask for publication IPM-1.

- Recognize a pest's crop damage stage. "An insect's life cycle begins as an egg. Then it becomes a worm or larva, then it pupates, then emerges as a moth," Obermeyer said. "The only crop damage stage is larva."