Corn Insects

By J.P. Michaud

Southwestern Corn Borer, Diatraea grandiosella.

Early instar larva.

Late instar larva.

Adult moth.

Life History

First-generation infestations begin in late June and consist of dark-spotted white worms that feed for five to 10 days on leaf tissue in the plant whorl, then move downward to begin tunneling within the stalk. Tunneling can extend far enough down to kill the growing point on small plants (deadheart), though deadheart injury does not usually occur on plants greater than 30 inches in height. While yield reduction can occur from damage caused by first-generation larvae, it is not considered serious unless the majority of the plants are infested and each of the infested plants contains several larvae.

The second (and most damaging) generation occurs in August. Adult moths begin emerging and laying eggs around July 15 to July 23, with oviposition reaching a peak somewhere between Aug. 1 to Aug. 15. The exact timing of oviposition is influenced by weather and geographic location. Eggs are deposited on leaves primarily in the ear region. Newly hatched larvae begin feeding on leaves, but prefer to feed on ear shoots, husks, and silks. Within 10 to 12 days, this generation also begins tunneling within the stalk, generally below the ear zone.

By mid-September the majority of the larvae are full grown and have tunneled down to the base of the stalk beneath the soil line to prepare for hibernation. By then, larvae begin to lose their dark spots and will be recognizable only as large, white larvae. In final preparation for overwintering, many larvae crawl back up the inside of the stalk and chew a ring around the interior, leaving only a thin layer of the exterior stalk intact. These girdled plants soon break and fall over, which contributes to extensive harvest losses. Girdling usually begins around Sept. 1 and is over by the first week of October.

Noticeable first-generation activity is apt to be more pronounced in the sandy regions of the southern portions of the state, particularly where tillage practices to expose the stubble during the winter are not practiced. Second-generation activity would be expected not only in those fields that experienced noticeable first-generation infestation, but also in other surrounding fields. This is because emerging moths that produce the second generation are capable of flying for miles in search of attractive corn in which to deposit eggs. Thus, second-generation damage can be severe in fields where first-generation activity was negligible or even absent.

Management

Treatment thresholds for first generation are not well established, but are assumed to be close to those for European corn borer, except that there may be more of a need to watch smaller plants because of the greater likelihood of larvae causing deadheart. Most insecticide treatments are directed at second-generation larvae. Insecticide applications should be considered on susceptible corn hybrids when 20 to 25 percent of the plants are infested with eggs or newly hatched larvae.

Growers in infested areas can reduce losses by using special management practices. These include the following:

• Harvesting earlier before girdling begins. (The amount of girdling usually increases rapidly after mid-September.) In order to harvest early, consider the following options: (a) plant early; (b) use a good-producing, short-season variety; (c) produce silage corn; (d) use high-moisture harvest.

• Fall tillage to break up root stubble and expose borers to natural enemies and winter hardships.

• On hard land, deep and clean plowing of corn stubble to a depth of 5 or more inches will bury larvae and/or pupae, and prevent a high percentage of moth emergence the following spring.

• Avoid late-planted corn, particularly extremely late-planted. Such fields are sometimes heavily infested with secondgeneration borers. Losses due to tunneling may be extensive. Harvesting before girdling begins may be harder to achieve, unless silage corn can be used.

Resistant Hybrids

The majority of corn hybrids grown in Kansas now contain a gene that expresses an insect-specific Bt toxin that makes them resistant to corn borer feeding. Insertion of the Bt gene into corn DNA is known as a transformation event. Each transformation event may contain a unique gene, gene promoter, gene marker, and/or gene location. The process is patented according to the gene(s) inserted, and where and how they were inserted. Thus the same gene can be inserted by different companies using slightly different processes. Bt corn hybrids were released commercially for the first time in 1996. This caused a virtual revolution in corn borer management. Seed companies continue to modify these Bt events for other common lepidopteran pests also. Bt corn acreage has increased from about 8 percent in 1997 to a little more than 63 percent in 2009. Some 'stacked' hybrids now have multiple genes targeting both lepidopteran pests (the corn borers) and rootworms. Refuges are still required for all of these Bt hybrids but the requirements differ depending upon the hybrid. Growers should read and understand refuge requirements for all hybrids planted. Conventional management is still recommended in refuge plots.  To get the latest information, it is best to consult with your local seed dealer.

Source : ksu.edu