No. 10 Article 3/June 1, 2007

Scouting for Stalk Borers Should Be Underway in Central and Northern Illinois: Time for Treatment Decisions in Southern Illinois

Because of the increasing use and effectiveness of Bt hybrids against several lepidopteran pests, such as the European corn borer, many producers may neglect to scout for stalk borers. Non-Bt corn planted in refuge acres is susceptible to injury by stalk borer larvae. Even YieldGard Bt hybrids provide only suppression against stalk borers. Producers are encouraged to begin scouting for stalk borers in central and northern Illinois. In southern Illinois, treatment decisions for this insect pest should be underway.

Stalk borer larva.

Stalk borers are one of the most challenging insect pests to manage effectively. Stalk borers overwinter as eggs on weed hosts and pass through one generation per year. Fields infested with ragweed, wirestem muhly, quackgrass, and giant foxtail are attractive egg-laying sites for stalk borer moths in late summer. Fencerows infested with these weeds offer excellent staging sites for stalk borer larvae to move from and into adjacent border rows of corn. If your Bt refuge has been planted as a perimeter, these non-Bt plants are susceptible to stalk borer damage.

Stalk borer eggs hatch from late April through early June. Larvae begin to feed on their weed host but eventually outgrow it and emerge to search for a new host plant, which most often includes seedling corn. Newly hatched larvae appear to be brown, but they have prominent white and purple stripes running lengthwise along their bodies. At about the midsection of the body is a conspicuous dark purple band. The last larval instar is often dirty gray or whitish and may be more difficult to identify. Stalk borers complete 7 to 10 instars and actively feed for 8 to 10 weeks.

Damage occurs most often in corn rows adjacent to fencerows, road ditches, grass conservation waterways, and terraces, or in spots within the field that had weed infestations the previous year. There are no effective rescue treatments for controlling stalk borers after they have entered the corn stalk. Destroying weed hosts will reduce egg-laying sites in late summer. Postemergence insecticide treatments are effective when larvae are moving from weed hosts to corn plants. It can be very difficult to pinpoint the exact time that movement from weeds to corn plants occurs.

Plant injury occurs in two ways. If larvae enter the plant through the lower stalk and tunnel upward, the feeding injures the growing point and the damage appears as "dead heart." Larvae that enter the top of the plant tunnel downward into the stalk. Injury first appears on newly emerging leaves as ragged holes that increase in size as the leaves expand. Later, the growth of the tassel may be affected and the upper part of the plant may be deformed.

Corn damaged by stalk borer larvae.

Stalk borers first begin to move into corn when 1,100 heat units have accumulated above a base temperature of 41°F since January 1; 50% movement occurs when about 1,400 to 1,700 degree-days have accumulated. When about 1,300 to 1,400 degree days have accumulated, scout corn to verify the presence of stalk borers in weeds (dead stems, larvae inside) or border rows of corn. For more information about degree-day accumulations for this insect pest, refer to the degree-day calculator. By May 28, approximately 1,100 to 1,300 degree days (base 41°F) had accumulated across northern Illinois. For central and southern Illinois, the ranges were 1,300 to 1,500 and 1,600 to 1,700 degree days (base 41 F), respectively.

Insecticides labeled for stalk borer rescue treatments are *Ambush 25W, 6.4 to 12.8 ounces of product per acre; *Asana XL, 5.8 to 9.6 ounces of product per acre; *Baythroid 2, 1.6 to 2.8 ounces of product per acre; *Capture 2EC, 2.1 to 6.4 ounces of product per acre; *Lorsban 4E, 2 pints of product per acre; *Mustang Max, 2.72 to 4 ounces of product per acre; *Pounce 3.2 EC, 4 to 8 ounces of product per acre; *Proaxis, 2.56 to 3.84 ounces of product per acre; and *Warrior, 2.56 to 3.84 ounces of product per acre. Products preceded by an asterisk are restricted-use insecticides and may be used only by certified applicators.

Economic injury levels for stalk borers have been developed and published by Iowa State University. These thresholds (Table 1) are based on six corn-leaf stages, three corn prices, control costs of $13 per acre, and a control level of 80%. With the current favorable commodity prices, economic injury may result with very low levels of a stalk borer infestation. It may be well worth the investment to scout fields this spring for stalk borers.--Mike Gray

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