THE PEANUT GROWER, June, 2003
Know Your Rootworm Risk
by Jina Martin

Southern corn rootworms (SCRs) can cause havoc in a peanut field. SCRs have been a pest of North Carolina and Virginia for many years, causing pod damage and economic loss.

Now growers have an easy way to determine if their field is at risk and whether they should treat for rootworms - the SCR Advisory.

Researchers and Extension faculty at Virginia Tech and North Carolina State University (NCSU) developed the advisory.

"The new advisory is the end product of several years of field validation work that followed several years of research," says Ames Herbert, Extension entomologist with Virginia Tech.

SCR Facts

  • Eggs and small rootworms cannot survive dry soil conditions.
  • Irrigation and wet weather make rootworm problems worse.
  • Treatment late in the season following significant rainfall may be too late to effectively prevent root worm injury. Late-season treatments may also encourage spider mite breakouts.
  • Rapid growth after rain can cause short splits or creases to occur in the outer pod wall that can be confused with SCR damage.
  • High loam content in the soil increases the risk of pod damage.
  • Poor drainage and high soil moisture content increase the risk of pod damage.
  • Late planting date and delayed rate of peanut pod maturity increase the risk of pod damage.
  • Treating only the high-risk fields will save you time and money.
  • University research shows that the majority of commercial peanut fields that are treated for SCR do not need to be treated.

Source: Virginia Cooperative Extension

The Problem

The SCR is the larvae of the spotted cucumber beetle. The beetles don't damage peanuts; it's the larvae that are the problem.

The larvae may feed on the roots of plants, but mostly on the pegs and pods. Often, the only signs of a rootworm infestation are the tiny holes they leave in the pods after feeding.

The SCR is slender, white to cream colored and reaches a length of .25 to .5 of an inch in length when mature. It has a wrinkled body with three pairs of legs. The head and the last segment of the body are dark brown to black.

NC 6 is a variety that provides good rootworm resistance and reduces the risk of pod damage. The variety, VA 98 R, is not as susceptible to rootworm attack during the peak pressure in July and early August.

Also, early planting reduces SCR risk because pods tend to mature before rootworm feeding.

The pest is more commonly found on heavier, poorly drained soils, says Mike Linker, professor of crop science and entomology and Integrated Pest Management Extension specialist at NCSU. However, if soil moisture is adequate, such as in irrigated areas, rootworms will infest almost any field.

Beginning in late July and continuing through August, the spotted cucumber beetle lays eggs in the field. Egg laying and egg survival depend on the soil being moist.

"We do not think spring weather has much effect on summer populations. The soil has plenty of time to dry out, if rainfall quits," Herbert says. "Soil moisture conditions during July through early September have more impact."

A Solution

Researchers at Virginia Tech and NCSU conducted research on 436 commercial peanut fields over a span of 12 years. They found that the majority of fields do not need to be treated for SCR. This research led to the development of the SCR Advisory, as a way for growers to identify those fields not at risk for pod damage or economic loss from the SCR.

The SCR Advisory takes soil texture, drainage class, field history of rootworm damage, planting date and cultivar resistance into account. Each variable in the category is assigned a point value. For example, under the drainage class category of the advisory, growers are asked to select whether the soil in their field is well drained, moderately well drained, somewhat poorly drained or poorly drained. Each of these variables is given a score.

Growers fill out the entire advisory score sheet, and find their total score. Lower scores indicate less of a risk of rootworms. Higher scores indicate that the field is a high-risk field for the SCR.

And, the SCR Advisory works. Growers who followed the recommendations of the SCR Advisory were protected from economic damage 98.5 percent of the time.

Efforts to help growers use the Advisory have been stepped up this year by Extension agents and the Center for Agricultural Partnerships (CAP), a North Carolina-based organization.

Agents are providing educational materials that make it easy for growers to use the Advisory. CAP is also making posters and cards available to the agents for use in working with farmers to accurately identify what insects are actually damaging their crops.

"The SCR Advisory is our attempt to make sure the use of insecticides is efficient as possible," says Rick Brandenburg, professor of entomology at NCSU. "This helps make sure fields that need a treatment get treated, and those that probably won't be infested, don't get treated."

 

ARE YOUR PEANUTS AT RISK FROM SOUTHERN CORN ROOTWORM?

2-Minute SCR Field Advisory Score Sheet
Soil Texture   Your Score
Loamy Sand 5  
Fine Loamy Sand 10  
Loam 15  
Drainage Class
Well drained 5  
Moderately well drained 10  
Somewhat poorly drained 15  
Poorly drained 20  
Field History of Rootworm Damage (if unknown, assume moderate)
None 0  
Low 5  
Moderate 10  
High 15  
Planting Date
Before April 25 5  
April 25 - May 15 10  
After May 15 15  
Cultivar Resistance
NC 6 5  
VA 98R 10  
NC10C, NC12C, NC7, NC9, NC-V11, VA93B, runners, other variations 20  
Total Score  

To use the Peanut Southern Corn Rootworm Advisory, identify the variable under each major heading that best describes your peanut field. Add the points to determine a total score for that
field. This calculation should be done for each of your fields.

ADVISORY TREATMENT RECOMMENDATIONS

Score of 50 or less = Low-risk Field

  • Do not treat for southern corn rootworm.
  • Congratulations! You saved as much as $15/acre on this field!
  • Scout for leafhoppers and, if needed, treat with foliar spray.

Score Between 55 and 65 = Moderate-risk Field

  • Rootworm damage MAY not reach economically damaging levels.
  • Treatment decision should consider weather conditions and land-lease requirements.
  • Consider treating low areas of fields.
  • In wet years, treating before rain (even in late July) may prevent severe pod damage.
  • Scout for leafhoppers and, if needed, treat with foliar spray.

Score of 70 or higher = High-risk Field

  • Treat all high-risk fields for southern corn rootworm.
  • Treat all irrigated fields,
  • Scout for spider mites.

What are the keys to fields with low scores?

  • Resistant cultivars. NC 6 provides good rootworm resistance and greatly reduces risk to pod damage. The early maturing pods of VA 98R are not as susceptible to rootworm attack during the peak pest pressure in late July and early August.
  • Good drainage and sandy soils. SCR larvae prefer moist soils, irrigation, high loam content and poor drainage increase the risk of damage. Always treat irrigated fields.
  • Early planting. Early planting reduces risk because pods tend to mature before rootworm feeding.
  • Known history. Base your estimate on experience in previous years with damage levels in areas of the field not treated with insecticide. If fields have always been treated, estimate a moderate level of damage.

Source: 2003 North Carolina Peanut Production Guide.