Papaipema nebris Guenee
Appearance and Life History
Stalk borers are distributed throughout the United States east of the Rocky Mountains. They attack many species of crop and weed plants. In conventional tillage, stalk borer damage is usually confined to field margins and corn plants along grassy waterways and terraces. Eggs laid on grassy-type weeds in the field are usually destroyed by tillage. Stalk borers can cause damage throughout a field where reduced tillage is practiced and/or where cover crops are used. When grass-type plants are not disturbed by tillage, stalk borer eggs may be present throughout the field and subsequent damage will not be limited to field margins.
The larva is easily recognized by the prominent longitudinal white stripes on the anterior and posterior ends. The stripes are interrupted by a dark purple area on the third thoracic segment and the first three abdominal segments. Full grown larva are 1-1/4 inches (3.8 cm) long. The last larval instar does not have the prominent white stripes and dark purple area. Because it is dirty white in color, it is more difficult to recognize. The larvae pupate in late July, usually just below the soil surface or sometimes inside the stem of the host plant. The grayish moth, with or without silver spots on the forewings, appears in mid-August and September.
Eggs are laid from late August until frost on grasses and some broadleaf weeds. Grassy-type plants or weeds in the field, waterways, or fence rows are typical oviposition sites. Giant ragweed, a broadleaf weed, has also been associated with infestations. The eggs hatch in May and the larvae bore into the stems of nearby weeds or crop plants. Larvae are found throughout the summer with pupation occurring in August.
Corn plants attacked by this insect will show round to irregular shaped rows of holes through the unfolded leaves which may become tattered with time. Many plants will have an unnatural growth, being twisted or bent over, presenting a stunted appearance and will not produce an ear. The larvae may enter corn and other plants at the side of the stem and burrow upward, causing the center leaves of the plant to die ("dead heart"). They may also enter small corn plants from the whorl and work downward, causing part of the plant to wilt and die. Larval damage occurs from May through July.
This insect is very difficult to monitor. Egg hatch and subsequent larval activity may occur throughout May and into June. In addition, some larvae will first infest the stems of almost any grass or grassy-type weed plant for one to several weeks. Later, when the larvae become too large for the stem, or the plants are destroyed, they will abandon the plant. These partially-grown larvae move to plants such as corn and bore into their new host during June and early July.
One method that can assist in determining time of sampling for stalk borer is a degree day accumulation model developed by Iowa State University. Using a base threshold of 41° F (5° C) from January 1, egg hatch and initial infestation will take place around 600 degree days. Scouting grasses at the edge of the field at this time may help one identify a potentially damaging infestation. Of greater importance is the migration of the larvae from initial host to corn. This normally occurs at about 1,400 degree days.
Scout corn near field edges or former weedy areas for damaged corn. Degree day accumulations are usually available in state extension pest newsletters. When scouting conventionally tilled fields, examine corn plants along fence rows, ditch banks, grass waterways, or where weed patches existed the previous year. Examine 20 corn plants in 5 random locations to determine percent infestation. In reduced tillage fields, fields with a cover crop, or weedy (grasses) fields, inspect 100 plants in 5 random locations. Record the number of plants infested and determine the location of the worms, such as in the whorl, stalk, etc. When cover crops are destroyed (i.e., burned down with a herbicide, etc.), stalk borers may be noted moving to corn. Sample for stalk borers a few days after the cover crop is destroyed.
Corn Insect Control Recommendations: E-series 219-W (PDF)
There are no rescue treatments for stalk borers once they enter the plant. In reduced tillage or weedy situations, treatment may be advisable if the cover crop or weeds in the field are infested prior to planting. A broadcast application of a contact insecticide applied with or after a burn-down herbicide may be effective. With fast-acting burn-down herbicides, the insecticide, if compatible, can be tank-mixed with the herbicide or applied 1 to 2 days following the application of the herbicide. When a translocated, slow-acting herbicide is used, the larvae will not be flushed from their grass hosts for approximately two weeks. Thus, the insecticide should be applied once the grass begins to die.
Early instar stalk borers can be controlled before they enter the plant if a sufficient number of plants show leaf feeding. Control of populations moving into a field from field borders is also possible with a broadcast treatment along the edge of the field and in infested areas within a field. In some cases the elimination of winter cover crops, or good weed control, may be necessary to prevent stalk borer damage. If seedlings are being killed by stalk borers, the application of an insecticide with good residual activity may be beneficial to control the worms as they move from dying plants to new hosts. However, timing of such treatments are critical to increase the probability of success.
If control is necessary, contact your state Cooperative Extension Service or click here for control materials and rates.