Annually, we conduct a survey to estimate Helicoverpa zea (corn earworm) infestation levels in field corn in mid- to late July. Corn is considered a nursery crop for corn earworm, allowing the pest to complete a lifecycle and then move on to other crops such as soybean, cotton, and peanut in August. Over 30 years of data show that there is a linear correlation between the infestation level in corn and the amount of soybean acreage that gets treated with insecticide for this pest.
To conduct the survey this year, the number of corn earworm larvae found in 50 ears of corn was recorded in 5 corn fields in each of 25 counties, totaling 6,250 ears and 125 fields sampled. 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 corn earworms, or if they had already exited the ears, was also recorded (data not shown). We greatly appreciate the help of Virginia Cooperative Extension Agriculture and Natural Resource (ANR) Agents, Virginia Tech faculty and staff, summer agricultural assistants, and interns in this effort. These cooperators are acknowledged in the attached Figure. We also would like to thank the many growers who graciously allowed us to inspect their fields for corn earworm larvae.
Results of the survey are provided in the Figure (CEW_survey_2015). Statewide, approximately 17.5% of ears were infested with corn earworm. This is even lower than the numbers reported in 2014 (20%) and 2013 (18%)—both years with very few cotton, peanut or soybean fields being infested with treatable numbers of worms. Regional averages for 2015 were 4% infested ears in the Northern Neck, 13.4% in Mid-Eastern, 9.5% in South-Central, and 27.7% in the Southeast.
So far, all indicators point to another year with few fields reaching thresholds. We will continue monitoring and posting updates.
This survey is intended to be a representative sample, not a complete picture. We always recommend scouting individual fields to determine exactly what is happening in terms of corn earworm as well as other pests and crop problems. Also, please check the black light trap reports on the Virginia Ag Pest and Crop Advisory and other reports posted weekly to keep up-to-date on the insect pest situation.
