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An eye on the bugs

Though there are still a lot of corn and soybean acres left to plant in parts of the country, the seed that's in the ground is starting to take off. Now, before you know it, your attention will have to turn to pest protection and insect scouting in your fields.

But, 2011 is a whole new ballgame -- each bushel of corn and beans is worth about twice what it was just a couple of years ago. That could change your approach to pest protection.

"Remember that, with high-priced corn, the damage threshold for treatment is lower," says Iowa State University (ISU) Extension field agronomist Jim Fawcett.

Here are a few pests common to the Corn Belt in the first stages of the growing season and what their economic treatment thresholds are today and how they have changed because of higher grain prices.

Already discovered at near record levels in Indiana earlier this month, the most dangerous corn pest in Fawcett's area of eastern Iowa right now is Black Cutworms. The economic threshold for this bug is when the average larvae in the field is around 3/4 inch long. If you have 2% or 3% damage (in the form of wilting or cutting), you have a strong case to spray an insecticide, says ISU Extension Entomologist Jon Tollefson.

"If cutworms are longer, treatment should be applied if 5 percent of the plants are cut. If the field has a poor plant population, (20,000 or less) these thresholds should be lowered," he says. "With the black cutworm, the infestations are sporadic and uncertain. Don’t forget to check the rest of the fields if you find infestations in the 'sentinel' fields."

Corn Earworm

A pest that can hit both corn and soybeans, Corn Earworm can start showing up as early as June in the Midwest after migrating northward from the South. In Wisconsin, for example, there are typically 2 generations each year, according to a report from University of Wiscosin-Madison Extension and Research. The largae are typically white in color with a black head. And, they're cannibalistic, meaning only one is found per ear.

"Pheromone trap catches of 5 to 10 moths per night for three consecutive nights indicate that moths are likely laying enough eggs to warrant treatment of fields, if corn is in the vulnerable stage between the brush stage and silk browning," according to a university report. "Blacklight traps can also be used to monitor populations, though they collect fewer corn earworms. Consider treatment when blacklight traps catch 3 to 5 moths per night for three consecutive nights if nearby fields are in the vulnerable stage."

Western Bean Cutworm

You have a little bit more time before you need to start watching for this pest, but when that time comes, be ready to act, specialists advise. Western Bean Cutworm moth flights typically don't happen until early- to mid-July, but scouting needs to happen very soon after they're flying, says University of Nebraska Extension entomologist Bob Wright.

"Scouting for western bean cutworm should begin in field corn when the first moths are caught. Control decisions should be made shortly after the moth flight peaks," Wright says. "The moth flight usually peaks in early to mid-July."

When you're scouting Western Bean Cutworm, Wright advises checking about 20 plants in 5 different areas of each field. " Look for eggs on the top surface of the upper most leaf or look for larvae in the tassel. If 8% of field corn plants, 5% of seed corn plants or 5% of popcorn plants have egg masses or larvae, consider applying an insecticide," he says.

When treating, make sure of a couple things: First, at least 95% of the corn plants need to be tasseling. And, check the pre-harvest interval for what you spray.

"Western bean cutworm moths prefer to lay eggs in corn plants that are in the late whorl stage compared to those that have completely tasseled. Pay particular attention to later planted fields or those with uneven development," Wright says. "Western bean cutworm eggs that hatch when corn plants are in the late whorl stage of growth have a high rate of survival. The larvae are well protected in the whorl or tassel."

Corn Rootworm

Stock up on sticky traps to keep tabs on this bug, at least in your soybean fields, says University of Illinois Extension crop scientist Michael Gray. That's the best way to get a good grasp on how much damage potential is in your fields.

"Growers located in high-risk and moderate-risk areas in Illinois should monitor for [Western Corn Rootworm] (WCR) in soybean fields where corn will be planted the following year," Gray says, adding that farmers should start setting pheremone traps, evenly distributed, in their fields starting in July.

"If WCR adults are detected, you should determine densities by monitoring your fields with 12 traps," Gray adds. Then, use those trap numbers to show potential damage in first-year corn fields.

"If trap captures in a soybean field average 3 beetles per trap per day, roots in a first-year cornfield may average 1 or 2 on the injury rating scale the following season. An economic threshold of 10 beetles per trap per day may result in a root injury of 4.0 (one node of roots destroyed," he says.

When pinpointing economic loss, don't extrapolate too much between fields and varieties. Rootworm damage varies quite a bit between different hybrids, making it important to scout closely.

"The level of root injury that may result in economic loss varies according to the growing season and the hybrid selected. In general, economic injury occurs with root injury ratings between 3.0 and 4.0," Gray says. "However, for certain corn hybrids during poor growing conditions, recent research has shown yield losses can occur with root ratings between 2.0 and 3.0 (minor root scarring to minor root pruning)."

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