Erin Hodgson, Department of Entomology and Adam Sisson, Corn and Soybean Initiative
The stalk borer, Papaipema nebris, is a native insect to North America and has a wide host range (more than 175 plant species). Female moths prefer to lay eggs in narrow-leaved perennial grasses like tall fescue, giant foxtail and quackgrass. When eggs hatch, the larvae burrow into the grass and kill the top of the plant, which is sometimes referred to as leaving a "dead head" plant. Eventually the larvae get too big for the grass and migrate to larger plants like corn, sunflower and sometimes soybean.
Stalk borer larvae have three pairs of true legs and four pairs of fleshy prolegs. The body is creamy white and dark purple with brown stripes. Often there is a creamy white stripe running down the back. A distinctive feature is an orange head with two dark lateral stripes (Figure 1). The adults are dark grey and brown colored moths, with jagged white lines and two to three clusters of white spots.
Figure 1. Stalk borer larva. Photo credit Marlin E. Rice.
Begin scouting when 1,300-1,400 growing degree days (base 41 F) have accumulated. About 10 percent of stalk borer larvae movement has occurred when 1,400 degree days accrue. The map below (Figure 2) shows the estimated dates to begin scouting for stalk borer. The earliest scouting date, June 11, occurs in southeast Iowa. The date to begin scouting in south central and southwest Iowa are June 14 and June 12, respectively. Scouting should begin in east central Iowa June 17, central Iowa on June 20 and west central Iowa on June 18. The northwestern and north central climate divisions should begin scouting June 23 and finally, June 24 for the northeastern division.
Figure 2. Estimated date to begin scouting for stalk borer in Iowa climate divisions. Estimates are based on accumulated growing degree days (base 41°F). Begin scouting around 1,300-1,400 degree days.
Look for larvae inside the whorls to determine the number of plants infested. The larvae are not highly mobile and typically only move into the first four to six rows of corn. Look for new leaves with irregular feeding holes or for small larvae resting inside the corn whorls. Larvae will excrete a considerable amount of frass pellets in the whorl or at the entry hole in the stalk. Young corn is particularly vulnerable to severe damage, but plants are unlikely to be killed once reaching V7 (seven true leaves).
Corn is infested when stalk borer larvae move to find bigger host plants, typically adjacent to grassy edges of emerging corn. The most susceptible stages of infestation are at V1-V5, or about 2-24 inches in plant height. Larvae can damage corn by defoliating leaves and burrowing into stalks. Stalk borers do not typically cause economic damage when feeding on the leaves, but can clip newly emerging plants and cause death. More often, larvae kill corn plants by entering the stalk and destroying the growing point (i.e., flagging or dead heart). A dead heart plant will have outer leaves that appear healthy, but the newest whorl leaves die and can cause a barren plant.
Stalk borer infestations are more likely in corn surrounded by giant ragweed. Although minimizing weeds in and around corn will discourage egg-laying females, using herbicides to kill weeds can force stalk borer larvae to find new host plants. Long term management requires controlling grass edges so that mated females will not lay eggs in that area during the fall. Mowing grassy areas adjacent to corn fields the second week of August will make borders unattractive to adults.
Fields with persistent stalk borer infestations should be monitored every year. Applying insecticides to infested corn is not effective because the larvae are protected once tunneled into the stalk. Instead, target foliar applications to larvae as they migrate from grasses to corn.
To prevent stand loss, scout and determine the percent of infested plants. The use of an economic threshold (Table 1), first developed by ISU entomologist Larry Pedigo, will help determine justifiable insecticide treatments based on market value and plant stage. Young plants have a lower threshold because they are more easily killed by stalk borer larvae.
Table 1. Economic thresholds (percent infested plants) for stalk borer in corn, based on plant stage, expected yield and market value.
If an insecticide is warranted, some products can be tank-mixed with a fast burndown herbicide, or applied seven days after a slow burndown herbicide. Border treatment should be considered if infestations are localized. Insecticides must be well-timed so that products are reaching exposed larvae before they burrow into the stalk. Make sure to read the label and follow directions, especially if tank-mixing with a herbicide, for optimal stalk borer control.
For more information on stalk borer biology and management, read a recent Journal of Integrated Pest Management article by Rice and Davis (2010), "Stalk borer ecology and IPM in corn."
Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at email@example.com or phone 515-294-2847. Adam Sisson is a program assistant with responsibilities with the Corn and Soybean Initiative. Sisson can be contacted by email at firstname.lastname@example.org or by calling 515-294-5899.