In today’s world of corn products, most of the corn seed sold is protected from a myriad of above ground insects including common stalk borer (not to be confused with European corn borer) by Bacillus thuringiensis (Bt) insect protection traits. However, that doesn’t mean we shouldn’t be scouting for these insects.
In a large area of the United States, Bt insect protected corn products contain a refuge in the bag, which means a portion, often 5 to 10%, of the seed is not protected with a Bt insect resistant trait. In other areas of the United States, non-insect trait protected corn must be blocked in each field. Both options are required by the United States Department of Agriculture to help slow the resistance of insects to these Bt traits. It also means that the insect may feed on these plants and could be present in the average corn field. Aside from Bt insect protected corn products, a few growers plant conventional seed with no insect protection; therefore, those acres should be scouted for above ground insects such as common stalk borer.
Appearance: Young common stalk borer larvae tend to be dark purple in color (Figure 1). The back half of the larva has white stripes down the body while the front half tends to be more solid in color. The white stripes become distinctive (Figure 2) before fading as the larva matures. A mature stalk borer may be harder to identify compared to other larvae such as European corn borers and cutworms.
Scouting: Stalk borers tend to enter fields from grassy areas, fence lines, or terraced areas and often affect the outer rows of the field first as those grassy and weedy (especially giant ragweed) areas are preferred for egg laying. Eggs are laid in the fall by adult moths and hatch in late April to early May. Therefore, the outer corn rows near these areas are the most likely areas to find common stalk borers feeding.
Symptoms: Larvae tend to enter the plant by boring into the stem or by going down into the whorl of the plant during early to mid-vegetative stages (around June to early July).
The stalk borer tunnels upward through the interior of the stalk if it bores into the stem at ground level. This may cause twisting or stunting of impacted plants or in worst case scenarios, the plants may completely bend over.
If the stalk borer enters via the whorl, damage may appear as irregular rows of holes on corn leaves where the stalk borer may have chewed through the leaf while rolled up in the whorl. In severe cases newest leaves may be killed in the whorl leaving the older leaves intact.
Management: In-season rescue insecticide treatments are generally ineffective because stalk borers feeding inside the stem are protected from the insecticide. If the stalk borer is in the whorl and an applied insecticide reaches the inside of the whorl, it may be more effective.
Cultural control methods such as controlling grassways and weedy areas in or around the field especially during egg laying timeframe of mid-August may help control stalk borer.
Utilizing an above ground Bt insect protected corn product such as VT Double PRO® corn, Trecepta® corn, or SmartStax® corn may be the most effective control method for common stalk borer.