Issue No. 8, Article 2/May 27, 2011
Correct Identification Still the First Step in Pest Control
In the mad rush to finish what has been a challenging season for planting corn, speed may take priority over attention to detail. This can be especially true regarding crop scouting; thousands of acres need to be covered in a very short time, and the late planting date leaves little opportunity to replant any damaged, less-than-optimum stands.
Producers tend to err on the side of caution in these situations and to apply pest control measures at the earliest sign of a pest problem. High commodity prices make this reaction understandable, and even hard to argue against. It is still important, however, that a pest management practice be economical and return more dollars to the grower than what it costs to apply.
This is where correct pest identification becomes critical. Several pests of corn can cause very similar damage symptoms, and they are easily confused if one does not observe closely. Unfortunately only one of these pests, the black cutworm, is actually manageable once corn seed has been placed in the ground. Misidentification can thus lead to a pesticide application that is less than worthless.
So what are these "look-alike" problems, and how are they differentiated in the field?
One type of damage being observed in newly emerged corn is cutting of plants at the ground line. The instinctive reaction is to call in a sprayer to control a black cutworm infestation. However, redwing blackbirds and grackles also cut off corn seedlings as they tug and peck, trying to access the seed that is buried in the ground. This tugging and pecking often results in a small funnel-shaped hole in the ground at the base of the plant, a symptom not observed when the damage is caused by black cutworm. Also, when birds cause the damage, you will often find the discarded seed coat nearby. Bird damage is more common in fields where habitat is conducive to blackbird nesting: namely, adjacent to wheat fields, pastures, wetlands, and tree lines. An observant scout can actually hear and observe birds entering and leaving the field in these types of areas. Overhead power lines make excellent staging areas for birds to enter a field.
Blackbird damage on seedling corn. Note the funnel-shaped hole at the base of the cut plant along with the discarded seed coat lying nearby.
A black cutworm larva can often be found at the base of a damaged plant. Note the lack of a funnel-shaped hole, differentiating this from blackbird damage.
Adjacent wheat fields and power lines make excellent staging areas from which blackbirds may enter corn fields.
A second type of look-alike damage can occur when corn reaches the V3 to V5 growth stages. "Dead heart" or "dead whorl" can occur when a corn plant is too large, or rather when a black cutworm is too small to actually cut the plant off below ground. In this instance, the cutworm will often wrap itself around the plant and create a feeding hole until it consumes the underground growing point of the plant. The plant remains upright, but the whorl quickly wilts and deteriorates.
"Dead heart" caused by an insect tunneling into the plant’s growing point belowground.
The look-alike pest in this instance is often wireworm, which will also tunnel into the base of the plant and destroy the growing point. Commonly, both pests can be found in the same field at the same time. Unfortunately, there is no rescue treatment for wireworm infestations, so if wireworm and not black cutworm is the culprit, a pesticide application is useless.
Wireworm feeding injury. Note the small hole in the base of the plant.
Black cutworm feeding injury. Note that the diameter of the hole is bigger than with wireworm.
The most definitive method of differentiating wireworm from black cutworm as the cause of damage is to dig up the plant and find the offending larvae, but they aren't always polite enough to stick around at the scene of the crime. A backup diagnostic tool is the size of the hole bored into the base of the plant. Wireworm tunnels are normally no more than a couple of millimeters in diameter, while a cutworm tunnel will be up to 5 millimeters.--Robert C. Bellm, Extension educator, crop systems