Bt corn has helped many growers worry less about critters like corn rootworms. But with the 20% non-Bt refuge requirement and many corn lines still without the bug-resistance gene, knowing when, what and how much to spray can prove profitable by preventing disastrous damage.

Eleven years of studying western corn rootworm (WCR) behavior are helping Texas A&M University entomologists answer some of those questions. Their data show that the root- and leaf-hungry pests nearly always reach 50% adult worm emergence when a region’s accumulated degree days reach between 1,341 and 1,364.

In the Texas Panhandle, where the majority of the studies take place, that normally occurs on July 24, says Jerry Michels, entomologist for the Texas Agricultural Experiment Station in Amarillo. It can range from July 13 to August 14. That time period can help growers determine if spraying is needed that year, or if at-plant spraying is needed the following year in a continuous corn operation.

“Our model is temperature driven, assuming that the threshold of WCR development in the soil is 50°F,” Michels says. “The model correlates closely with actual field counts of adult WCR throughout the year.”

In 2006, a dry, hot year, adult emergence began about July 1, the earliest of the 11-year study. Emergence reached 20% by July 8 and topped 50% by July 15.

Rootworm life cycle
Carl Patrick, Texas A&M Extension entomologist in Amarillo, says WCR beetles are typically ¼ in. long. They are yellow with black stripes. “There is a single generation per year, during which larvae feed on corn roots and adults feed on leaves and silk and lay eggs in the soil,” he says.

Eggs are laid in the upper 2 to 8 in. of soil in mid to late summer just after silk. Laying continues for several weeks. The eggs overwinter in the soil and begin to develop the following spring when soil temperatures reach 52°F.

“In the Texas Panhandle region, eggs begin to hatch in mid May,” Patrick says. “Larvae are about 1/8 in. long. They grow through three larval stages to almost ½ in. long.”

The cream- to yellowish-colored larvae begin feeding on outer parts of roots. Feeding lasts 30 to 40 days. Damage ranges from minor to complete loss of the root system.

“If not controlled, plants may be stunted or die,” Patrick says. “Goosenecked plants may develop and can complicate harvest and increase losses. If silks are continually pruned to within ½ in. of the shuck, pollination may be impacted.”

Stay a step ahead
To know whether to treat for rootworms, a grower needs to know whether his fields were infested the previous year in a continuous corn system.

“The beetle infestation rate must be determined during the previous season,” Patrick says. “If there were one or more beetles per plant, an at-plant insecticide, seed treatment or active transgenic corn hybrid may be warranted the following season.”

Postemergence applications may be needed if one or two small larvae per plant are counted along with obvious feeding damage. “Beetle control should be considered if silks are being eaten faster than they are emerging from husks during pollination,” Patrick says. “The generally accepted threshold is eight to 20 beetles per plant or five or more directly on the ear.”

He says beetle control also should be considered as a means to reduce preplant egg lay that will produce the next year’s larvae. “Areawide treatment of all cornfields with feeding stimulants containing low concentrations of insecticides has been successful in reducing subsequent larval populations below the economic threshold,” Patrick says, noting that this practice is more common in the Midwest.

Refuge rules
Bt corn makes up about 75% or more of the corn planted these days. However, in a Bt operation, the refuge area of 20% must include a strict insecticide regimen. The refuge, usually a hybrid similar in maturity to that of the Bt type, should be planted in an adjacent field or as strips or blocks within a Bt rootworm-protected field.

“For rootworm-resistant corn, no other field can be located between transgenic corn and the conventional corn refuge,” Patrick says. “Mixing non-Bt seed with Bt rootworm protected seed is not permitted.

“The refuge can be treated for corn rootworm larvae and other soil pests with soil-applied insecticides to control late-season pests such as corn borer or rootworm beetles,” Patrick continues. “However, if root beetles are present, the Bt protected corn also must be treated.”

Texas A&M suggests the following seed treatments for controlling WRC or Mexican corn rootworms:
• Clothianidin — Poncho 1250, the 1.25-mg rate of Poncho 600. For Poncho 600, apply 5.64 fl. oz./80,000 units of seed.
• Thiamethoxam — Cruiser 5FS. Apply 1.25 mg of active ingredient/seed. Each fluid ounce contains 17.7 g of active ingredient.

Granular insecticides
Seed treatments have been shown to provide adequate root protection under light and moderate infestations. “However, granular insecticides provide better control of high rootworm infestations,” Patrick says.

Texas A&M suggests the following at-planting granular insecticides and application rates:
• Carbofuran — Furadan 4F (western, northern and southern corn rootworm only): 2.5 oz./1,000 ft. of row, 1 qt./40-in. row spacing.
• Chlorpyrifos — Lorsban 15G: 8 oz./1,000 ft. of row, 6.5 lbs./40-in. row spacing. Nufos 15G: 8 oz./1,000 ft. of row; see label for 40-in. row spacing. Lorsban 4E: 2.4 oz./1,000 ft. of row, 2 pt./40-in. row spacing. Nufos 4E: Rates vary, so check the label.
• Tebupirimfos plus cyfluthrin — Aztec 2.1G: 6.7 oz./1,000 ft. of row, 5.5 lbs./40-in. row spacing.
• Tefluthrin — Force 3G: 4 to 5 oz./1,000 ft. of row, 3.3 to 4.1 lbs./40-in. row spacing.
• Terbufos — Counter 20CR: 6 oz./1,000 ft. of row, 4.9 lbs./40-in. row spacing. Counter 15G: 8 oz./1,000 ft. of row, 6.5 lbs./40-in. row spacing.

Suggested insecticides for controlling adult corn rootworm beetles include:
• Carbaryl — Sevin 80WSP: 1.25 to 2.5 lbs./acre (days from last application to harvest, 48, to grazing, 14). Sevin XLR Plus 4 lbs.: 1 to 2 qt./acre (days from last application to harvest, 48, to grazing, 14).
• Malathion — Atrapo ULV 9.9 lbs.: 4 oz./acre (days from last application to harvest, 5, to grazing, see label). Fyfanon ULV 9.9 lbs.: 4 oz./acre (days from last application to harvest, 5, to grazing, 5).
• Methyl parathion — Penncap-M 2F: 1 to 2 qt./acre (days from last application to harvest, 12, to grazing, 12). Methyl parathion: 0.5 pt./acre (days from last application to harvest, 12, to grazing, 12).

Treatments and labeling will vary for different regions. Consult your regional extension entomologist or your agrichemical dealer for the recommended rootworm control measures for your area.