CORN INSECTS (continued)

Insecticide Quick Reference Chart for: Corn, Sorghum

WHITE GRUBS

White grubs that are destructive to field crops in North Dakota have a three year life cycle. In southeast North Dakota, the most common white grub pest occurs in continuous cropping situations at sites where willow and cottonwood trees are present. In other areas of the state, white grubs are most likely to be found when rotation from grassland, pasture, or grassy weed sites occur. Most root feeding occurs in the second year of the life cycle. In most cases, the number of second year grubs will only be great enough to justify control once every three years.

Thresholds:

Treatment is recommended when sampling indicates an average of one or more white grubs per square foot are found. The following sampling procedure provides treatment decisions based on this guideline.

Soil sampling . . . Sampling in late summer or early fall, before a freeze, provides a more reliable estimate of populations than spring sampling just before planting. Take soil samples, one square foot in size to a depth of 8 inches. Begin taking samples 45 yards from shelterbelts. A total of 30 samples per field, randomly spaced along the shelterbelts are necessary. If at least a single grub is found in less than 40% of the samples, treatment may be required only out 20 yards from the tree line. If 40 to 60% of the samples are infested, treatment is needed to this distance and maybe as far as 65 yards. If greater than 60% of the samples are infested, treatment may be needed out to 90 yards from the tree line.

RUP - Restricted use pesticide

WIREWORMS

Wireworms are most likely to be problems when corn follows pasture or grassland. Continuous corn has developed problems in the past, also. Infestations often are found in coarse textured soils (sandy loam) where moisture is abundant, perhaps in low spots of fields.

Thresholds:

There is no easy way to estimate wireworm infestations. Two methods are currently used.

Soil Sampling . . . Sample 20, well spaced, one square foot sites to a depth of 4 to 6 inches for every 40 acres being planted. If an average of 1 wireworm per square foot is found, treatment would be justified.

Solar Baiting . . . In September, establish bait stations for 2 to 3 weeks before freeze. Place bait stations randomly through the field, but representing all areas of the field. There should be 10 - 12 stations per 40 acre field. Place one cup wheat and one cup shelled corn in a 4 to 6 inch deep hole. Cover grain with soil and then an 18-inch square piece of clear plastic. Dig up the grain. If an average of one or more wireworm larvae are found per station, treatment would be justified.

Seed Treatment . . . Seed treatments available for use on corn for managing wireworm are listed on page 7.

RUP - Restricted use pesticide

EUROPEAN CORN BORER - Field corn, Popcorn and Sweet corn

Managing corn borer in North Dakota is a challenge due to the lengthy emergence interval of the moths from overwintering. In ND, borers have the potential for one or two generations during the season. The two generation borers are present in the southern region of the state. They begin emerging in early June and represent the first flush of larval feeding. The single generation borer is present throughout ND, emerging from mid-June to August. The challenge of the crop manager is to distinguish when egg laying and larval populations can be tolerated or if they need to be controlled. Corn should be monitored weekly for at least five weeks once plants exceed an extended leaf height of 17 inches. At this point, corn borer larvae will be able to survive on the plant. Inspect plants for the presence of egg masses, whorl feeding, and active larvae. Observing moth activity around field margins or within the field may alert you to developing infestations. Recent corn borer infestations in ND developed in mid to late July and August as a result of the late emergence of the numerous single generation type borers. In other years, the two generation borers emerging first may contribute more to significant infestations.

Field scouting for corn borers:

Whorl stage corn . . . . Pull the whorls from 10 plants at 5 locations across the field. Select whorls at random, avoiding damaged plants. Unwrap the whorl leaves; count and record the number of live larvae found.

Tassel stage or older corn . . . . Examine the underside of the middle 7 leaves (3 leaves above and 3 leaves below the ear leaf) on 20 plants from 5 locations in the field. Multiply the number of egg masses found by 1.1 (correction factor for eggs on other leaves). Complete worksheet to determine the need for treatment.

Economic Threshold (Corn borer/plant) when factoring Crop Value and Control Costs

¹ Crop value = expected yield (bu/acre) X projected price ($/bu)
² Control costs = insecticide price ($/acre) + application costs ($/acre)

RUP - Restricted use pesticide

GRASSHOPPERS

In the northern plains, grasshopper egg hatch normally begins in late April to early May. Peak hatch occurs about mid-June. Heavy infestations typically occur in areas of low rainfall or during drought years. Outbreaks are usually preceded by several years of hot, dry summers and warm autumns. Cool, wet weather increases disease occurrence and delays development of grasshoppers, reducing the overall population.

For more information on grasshopper management and treatment thresholds, refer to discussion under Small Grain Insects.

RUP - Restricted use pesticide

Page 1, CORN INSECTS

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