Of all the chomping creatures that can invade a corn or soybean field, not many are slimier than slugs. And if enough of them ooze onto young plants, yield potential can slide away.
The good news is that slug populations are normally not found across an entire field. The bad news is they can still cause significant economic losses, says Bobby Clark, Virginia Tech (VT) Extension agronomist and agricultural agent in northwest Virginia.
“Damage to corn is typically limited to defoliation of emerging leaves,” adds Brian Jones, another VT agronomist from Verona, noting that his area witnessed abnormal slug damage to corn and beans in 2009. “Corn may often outgrow slug injury, however, slugs in soybeans can cause more damage because the growing point is above ground at an early stage and slug feeding typically causes plant death.”
THE CRAWLERS CRAVE moist dusk-till-dawn conditions. Damage last year escalated due to the extended cool, wet spring and summer weather. “We had as much as 80% stand loss in fields hit by slugs,” Jones says. “We saw even more prevalent damage in many soybean fields.”
In the slug cycle, adults lay their eggs in the fall. They overwinter until eggs hatch in early spring, usually just after corn is planted in mid-April. Juveniles begin feeding in May through June, when they are even less than ¼ in. long. They grow and feed all summer.
“They usually hatch after corn is planted,” says Clark, “but have already hatched when soybeans are planted. Control is sometimes achieved through using a slug bait, or by manipulating the planting date.”
Slugs like temperatures in the 65° F range, but can still feed when it's dry and warm. They can thrive under popular conservation-tillage conditions, when a previous crop's residue or cover crop provides a favorable environment, says Clark.
Jones says the economic threshold of slugs is currently unknown, “but this is something we are working on. Populations are spotty and variable from year to year, even within the same field. But where slug populations are heavy, they can quickly decimate young corn and soybean stands.”
Control can be expensive, and is typically granular. Clark says cost is $20-22/acre for a 10-12-lb. application of Deadline M-Ps bait. He says application rate is very low and proper bait dispersal is essential for control. Since infestation is often patchy and not uniform across an entire field, application can be made “from a four-wheeler with a spinner on the back to broadcast the bait,” he says.
He recommends using blue granules, to assess spread pattern. “We hope to get three-days' kill out of one application,” he says, noting that ½ in. of rain will render most of the bait useless.
Clark likes to spread the bait in the late afternoon just before slugs begin feeding in hopes of getting “at least a night or two use out of the bait before it rains.”
IN SHENANDOAH VALLEY, VA, replicated trials, there were statistically significant yield responses three times, he says. “There was, however, a numerical yield response in nine out of the 10 plots,” he adds.
The median yield response in six corn-grain plots and three soybean plots was 3.2 and 2.6 bu./acre, respectively. Clark is working with VT Extension specialists to determine economic threshold.
Jones says replanting may be needed, but growers should estimate the costs of replanting first. “Even if the yield from replanting would be greater than that from the damaged field, the cost of replanting may exceed the value of the additional yield gained from replanting,” says Jones.
“Growers should try to fill in within two weeks of the original planting date and obtain a uniform plant spacing within the rows between the old and new plants,” he says.
For more on slug control in corn and soybeans, go to http://tinyurl.com/SlugMgmt.