Corn earworm survey shows slightly lower numbers than in 2007

By Ames Herbert

Virginia Extension Entomologist

August 5, 2008 - Annually, we conduct a survey to estimate Helicoverpa zea (corn earworm) infestation levels in field corn in late July. Corn is considered a nursery crop for earworm, allowing the pest to complete a lifecycle and then move on in August into other crops, such as soybean and cotton.

Statewide, 45% of ears sampled in our 2008 survey were infested.

This is down slightly from 48% in 2007. Regional averages ranged from less than 30% infested in the Northern Neck and northern areas to greater than 50% in the southeast and Eastern Shore.

See the accompanying table below for county and regional results.

Over 30 years of data show that there is nearly a one-to-one relationship between the infestation level in corn and the amount of soybean acreage that gets treated for this pest.

If 50% of corn ears are infested, we can expect about 48% of Virginia’s soybean acreage to be treated for earworm.

To conduct the survey, 50 ears are pulled from each of 5 fields in each county in the survey.

This year, the survey covered 27 counites, with a total of 135 fields and 6,750 ears. When fields were known to contain Bt or non-Bt corn, this was noted. Otherwise, samples were considered to be random and assumed to be representative of the actual Bt/non-Bt composition in each county. Age of earworm larvae was also recorded (although that data is not shown in the accompanying table).

Once earworms mature in the ear, they drop to the ground to pupate. After approximately 11 days they emerge as adult moths, ready to mate, fly to alternate crops, and lay eggs.

Now is the time to begin scouting soybean, cotton and other crops for earworm eggs and/or small larvae. We have already found egg threshold in the cotton fields at TAREC, and suspect that many others in the area are at threshold, as well.f course, this egg threshold applies only to non-Bollgard cotton varieties, but it indicates that Bollgard, Bollgard 2 and WideStrike varieties will need to be intensively scouted or treated for worms in 7 to 10 days.

We have not seen worms in soybeans yet but would expect to see them beginning in 10 days to 2 weeks. The soybean fields at the greatest risk to earworm infestation are those that:

• Are in drought stressed areas.

• That are flowering or setting pods.

• Have open canopies.

• Have already been treated with a pyrethroid. This is the greatest risk factor. Any fields where a pyrethroid was tank mixed with a fungicide and applied in July is at great risk to worm outbreaks because those earlier sprays would have eliminated most of the beneficial insects that would otherwise be active in eating corn earworm eggs and small larvae.

Scout at-risk fields first.

Another issue that has come to our attention is the possible problem we may have with pyrethroids for controlling corn earworms in soybean. Last year, with the heavy infestations and hot, dry spray conditions, we got reports of several cases where the normal low use rates of pyrethroids left a lot of live worms.

This year, we have been testing adult corn earworm moths for susceptibility to cypermethrin (a standard way to determine potential pyrethroid resistance) and have found some possibly alarming results.

We have two sample sites. Wes Alexander is working with us to collect moths in Southampton County, and we have a series of traps here at TAREC in Suffolk. To date, we have tested 384 moths from Southampton and found a 27.5% survival rate, which is a lot higher than anything previously recorded for Virginia.

We have tested 703 moths from TAREC and found a 16.0% survival rate. Most of these survivors occurred in late July. To us, this means we may see problems with pyrethroids. We suggest considering switching to non-pyrethroids like Larvin, Steward or Tracer.

We greatly appreciate the help of Virginia Cooperative Extension Agriculture and Natural Resource (ANR) Agents, Virginia Tech faculty and staff, and volunteers in this effort. These cooperators are acknowledged at the end of the results table (attached). We also would like to thank the many growers who graciously allowed us to inspect their fields for earworm.

Acknowledgments

Mike Arrington, Rebecca Arrington, Amanda Aston, David Owens (Tidewater AREC, Suffolk)

Wes Alexander (ANR Agent, Southampton Co.)

Keith Balderson (ANR Agent, Essex Co.)

Paul Davis (ANR Agent, New Kent Co.)

Cyndi Estienne (ANR Agent, Greensville Co.)

Tom Kuhar and Hélène Doughty (Eastern Shore AREC, Painter)

Watson Lawrence (ANR Agent, Chesapeake)

Matt Lewis (ANR Agent, Lancaster/Northumberland Cos.)

Kelly Liddington (ANR Agent, Richmond Co.)

David Moore (ANR Agent, Middlesex Co.), Susan Dunaway, and Andrew Grubbs

Nathan O’Berry (ANR Agent, Isle of Wight Co.)

Mike Parrish (ANR Agent, Dinwiddie Co.)

Mac Saphir (ANR Agent, Caroline Co.)

Glenn Slade (ANR Agent, Surry Co.)

(ANR Agents may provide service to multiple counties)