The most common soil insects attacking corn in Texas are corn rootworms, cutworms, seedcorn beetles, seedcorn maggots, sod webworms, white grubs, and wireworms. Cultural practices are vital for reducing damage by these soil pests.
Growing corn on the same land year after year increases the damage done by some soil insects. Crop rotation can reduce that damage. For example, you may reduce and sometimes eliminate losses from corn rootworms by a crop rotation scheme including soybeans or other crops that the rootworms do not feed on. In most areas of Texas, corn has been rotated successfully with sorghum without damage from the Mexican corn rootworm and western corn rootworm. However, the Mexican corn rootworm has damaged corn following sorghum in parts of South Central Texas, but this is rare.
Another cultural practice that reduces soil insect pests is to maintain weed-free fields throughout the year, because weeds serve as host plants for some soil insects.
Sample your fields for cutworms, white grubs, and wireworms before bed formation. If they need chemical treatment, use soilor seed-treatment methods. One method may be more effective for a particular soil pest than another.
Almost all hybrid seed corn comes pretreated in the bag. Clothianidin (Poncho), imidacloprid (Gaucho), and thiamethoxam (Cruiser) or thiamethoxam+abamectin (Avicta) are commonly used for this purpose. Where commercially treated seeds are unavailable, growers may have to use directseed treatment or planter-box treatments.
To treat light populations of seedcorn beetles, seed-feeding ants, seedcorn maggots, and wireworms, treat the seeds with planter-box products. The insecticide should coat each seed evenly:
- Use a concrete mixer, commercial seed applicator or homemade seed applicator to treat
- Sprinkle 1 pint of water on each 100 pounds of seed and mix them to coat the seed with moisture.
- While mixing the seed, slowly add the correct amount of insecticide, and mix it thoroughly until the insecticide is distributed evenly on all the seeds.
- Plant the seeds within 20 days of treatment, because long exposure to the chemical will affect germination in some
Do not use treated seed for human consumption or livestock feed.
Some insecticides are applied to seed in the planter box. This method is effective only against low populations of seedcorn beetles, seed-feeding ants, seedcorn maggots, and wireworms. Use as directed on the insecticide label.
For some soil pests, you must apply the insecticide before the crop is planted or at planting time. Use granular or liquid formulations, depending on the target insect and your equipment. Granular forms of insecticide are generally safer.
Three application techniques are available to treat the soil: preplant broadcast, row band or T-band, and in-furrow at planting.
Preplant insecticide application: A broadcast application generally protects against soil insects best and is the only way to control heavy infestations of white grubs. But broadcast applications are usually not recommended because they require more insecticide and are more expensive than are row band or in-furrow treatments.
When broadcast applications are necessary, apply the insecticide uniformly to the field and incorporate it to a depth of 3 to 5 inches immediately after application.
Because of label changes in recent years, fewer products are approved for preplant broadcast application.
Corn planted on a bed requires special equipment to incorporate the insecticide to 3 to 5 inches deep. This is called row treatment. For best control, treat a band of soil 7 to 10 inches wide and 3 to 5 inches deep, and place the seed in the center of the band.
Row treatments must be made during or after bed formation. Further cultivation or bed shaping will alter the position of the insecticide in the row.
Insecticide application at planting: Techniques for applying insecticides to the soil at planting time include row band, T-band, or in-furrow applications. Choose the technique according to the pest species and the insecticide label.
Some insecticides applied at planting for corn rootworm control will suppress some early-season pests such as chinch bugs, flea beetles, and fire ants on seed or seedling plants. Depending on the insecticide, the pests may be suppressed for 2 to 4 weeks.
Follow these guidelines when applying granular insecticides:
- Mount the application equipment on the planter with the spout just behind the opening plow or disc opener and in front of the covering shovels or press
- Adjust the spouts so that the treatment band is about 6 to 8 inches wide and the seed furrow and covering soil are treated.
- Incorporate the insecticide by covering shovels, short parallel chains, loop chains, press wheels, finger tines, or other suitable
- Do not apply insecticide directly on the seed unless this use is specifically listed on the label; doing so can reduce seed germination. Some insecticides are labeled only for band application behind the seed-covering devices. Poor control usually results from in-furrow application where populations of white grubs are large.
White grubs and cutworms
White grubs are the larval stage of May and June beetles. The larvae damage the plant by feeding on the roots. Small damaged plants often are killed, and large plants are stunted and may lodge before harvest.
To determine the need for white grub control before planting, examine a 1-square-foot soil sam
White grubs are the larval stage of May and June beetles. The larvae damage the plant by feeding on the roots. Small damaged
plants often are killed, and large plants are stunted and may lodge before harvest.
To determine the need for white grub control before planting, examine a 1-square-foot soil sample for each 5 to 10 acres. An average of one white grub per square foot is enough to decrease stands significantly.
If the field has an average of about one white grub per square foot, a planting time in-furrow or band treatment will provide adequate suppression (Table A1 in the Appendix).
For surface cutworms, incorporate the insecticide into the top 1 to 2 inches of soil. See Table A8 in the Appendix for cutworm control on seedling corn.
Wireworms, seedcorn maggots, and seedcorn beetles
Insecticidal seed treatments often control wireworms (Table A2), seedcorn maggots, and seedcorn beetles. Check your soil closely during land preparation to determine whether seed treatment or soil applications are needed to control these pests. If the wireworm populations are large, follow the recommendations listed on appropriate insecticide labels.
Mexican and western corn rootworm
Corn rootworm can be a significant pest anywhere in Texas, and the best defense against it is crop rotation. (The exceptions are in the Coastal Bend and South Texas, where this insect is common; crop rotation does not always control it there.)
However, many growers cannot rotate out of corn; they grow corn year after year in the same field. Laboratory studies have shown that corn rootworm can develop resistance to any of our current rootworm Bt toxins in as little as 4 years of continuous use.
In parts of the Midwest, western corn rootworm has become resistant to Cry3Bb1 and mCry3A. The first confirmed cases of resistance were in fields that were in continuous use.
In parts of the Midwest, western corn rootworm has become resistant to Cry3Bb1 and mCry3A. The first confirmed cases of resistance were in fields that were in continuous
corn planted to Cry3Bb1 for 4 or more years. Unlike the caterpillar toxins that are relatively toxic to their target pests, corn rootworm toxins are relatively less toxic overall, and the natural populations of corn rootworms have the genes to survive the current toxins, Cry3Bb1, Cry34/35, mCry3A, and eCry3.1Ab. There is some cross-resistance between Cry3Bb1 and mCry3A so that insects that become resistant to one of these toxins will have partial resistance to the other.
The keys to preventing resistance to corn rootworm toxins are to rotate crops where possible and to rotate corn rootworm toxins where crop rotation is not possible. Never use the same rootworm technology in the same field for more than 3 years, and fewer years than 3 is better.
Although you may have to buy seed from a different company, it is vital that you observe this 3-year limit to delay resistance on your farm. Corn rootworm adults tend to lay eggs in the same field where they fed on roots as larvae, and most corn rootworm resistance can be traced back to a specific field where the toxins were not rotated.
In the Coastal Bend and southern part of Texas (Fig. 2), even rotated corn may need protection from southern corn rootworm. All Bt corn comes with an insecticidal and fungicidal seed treatment. However, even the best seed treatments cannot control corn rootworms at moderate and high infestation levels.
Mexican and western corn rootworm beetles lay eggs in the soil during the summer and fall, shortly after silking time. The eggs are usually laid in a cornfield in the upper 2 to 8 inches of the soil, where they remain until they hatch the following year.
Although the hatching time depends somewhat on soil temperature, the eggs usually begin to hatch about mid-April in South Texas and about mid-May in the High Plains. They continue hatching for several weeks.
If the newly hatched corn rootworms have no corn roots to feed on, they will die. Because Mexican and western corn rootworm have only one generation per year, the best way to control these two subspecies is to rotate corn with any other crop.
In continuous corn production fields, an average of one or more beetles per plant on any sampling date during the growing season indicates the need for a soil insecticide, a transgenic Bt hybrid, or crop rotation the next spring (Tables A3, A4, and A5).
Corn rootworms usually damage crops from midApril through mid-May in South Texas and in June in the High Plains. Extensive damage to the brace roots and fibrous roots may cause the plants to lodge. A “goose necking” appearance occurs when the lodged plants continue to grow.
In parts of the Midwest, western corn rootworm is resistant to Cry3Bb1 and mCry3A, and there is crossresistance between the two toxins. As of early 2016, resistance had not been documented in either western or Mexican corn rootworm in Texas.
Southern Corn Rootworm
Because the southern corn rootworm lays eggs in the soil after the corn is in the seedling stage, rotating crops will not control this insect adequately. Unlike the Mexican and western corn rootworms, southern corn rootworm can have more than one generation per year.
In most areas of Texas, the southern corn rootworm is a minor pest if the corn is planted in fields that were not grassy or weedy during the previous year. However, in the Gulf Coast region (Fig. 2), it has been a significant pest.
Consider seed treatments or an in-furrow or band pesticide application if infestations have reduced plant stands previously (Tables A6 and A7). Seed treatments provide good control, even better as rates increase. The Bt hybrids are relatively ineffective at controlling southern corn rootworm.
For most fields, research has shown that applying in-furrow insecticides at one-half the maximum rate in Table A7 provides the most favorable economic returns for control of southern corn rootworm. Where heavy infestations occur each year, use the higher rates