Sweet corn growers get updated on insect pests
HERSHEY, Pa. — At the recent Mid-Atlantic Fruit and Vegetable Convention, Dr. Galen P. Dively, University of Maryland entomologist, updated growers on the current efficacy and management of Bt sweet corn.
Dively said that in the Mid-Atlantic region corn earworm and European corn borer are the primary ear invaders, followed by sap beetles and fall armyworm. Infestation varies by year, the time of season and farm location.
Regional suppression due to high adoption of Bt field corn has lowered ECB infestations.
But CEW can damage 10 to 25 percent of ears in early plantings and often greater than 50 percent in late plantings.
Dively warned, “Timing the first spray at early silking, applying subsequent sprays on a schedule based on CEW moth pressure, and achieving adequate spray coverage of the ear zone are critical components of effective insecticide control.”
The eggs of CEW are laid directly on the silks. Once the larvae hatch, they move quickly down the silk channel and begin feeding on the ear tip. There, they are protected from the insecticidal sprays. Consequently, it is essential to target the larvae before they enter the ear — the insecticide residue must be maintained on the silk tissue.
The pyrethroids, Dively said, once the popular choice, no longer provide enough ear protection because of resistance in CEW populations.
Some of the challenges with conventional insecticides can be eliminated with Bt technology. Dively explained, “Bt sweet corn expresses insecticidal toxins from the bacterium, Bacillus thuringiensis, in all tissues of the plant.” This technology has revolutionized insect management. But now, CEW has developed resistance to several of the toxins. Because that has reduced field control efficacy, supplemental insecticides may be required for marketable ears.
Three Bt corn types are commercially available: Attribute with Cry1Abtoxin; Attribute II with Cry1Ab and Vip3A toxins; and Performance Series with Cry1A.105 and Cry2Ab2 toxins. The first two are from Syngenta, and the third one is from Seminis Seeds. Dively reported that all these varieties are highly effective against ECB, and can eliminate whorl and silk sprays where ECB is the primary problem. However, the efficacy of the first and third variety for controlling CEW has been variable since 2008.
Dively’s research has found that all Bt sweet corn varieties provide 100 percent control of ECB. Also, there has been no evidence of any change in corn borer susceptibility to the Cry or Vip toxins in the Mid-Atlantic region. Moreover, the herbicide tolerance of these varieties offers a weed control advantage over their non-Bt counterparts.
Attribute still provides good control of FAW during the vegetative stage but only moderate ear protection, no effective control of western bean cutworm, and only poor to fair control of CEW.
Performance Series provides very good control of FAW during the vegetative and ear development but no effective control of western bean cutworm and only poor to fair control of CEW.
Dively added, “Although CEW has developed moderate to high levels of resistance to the expressed toxins, the green silk tissue in the Attribute and Performance Series sweet corn is still consistently toxic to newly hatched larvae, causing intoxication and delayed growth, so those larvae that survive are exposed longer before entering the ear.” Due to this wider exposure window, Dively further explained, “The first insecticide application to both Bt types can be applied at 100 percent fresh silking, usually three to four days later than the first application in non-Bt sweet corn under the same insect pressure.”
In addition, pyrethroids and other insecticides may actually work better because larvae are weakened by the Bt intoxication. If high moth activity continues, a second spray three or four days later is usually necessary. Sometimes three or four applications may be needed depending on the desired ear quality.
Attribute II still provides excellent control of foliage feeding and ear invading insects, Dively reported, “No insecticidal sprays are required in most cases, except for secondary pests such as sap beetles.” Further, because sap beetles are mainly attracted to damaged ears, the lack of injury in the Attribute II significantly reduces their infestation.
Studies in the South also show evidence of resistance to Cry toxins in Bt field corn and cotton. Dively noted, “Clearly the high adoption of Bt field corn and cotton, both with moderate dose expression of Cry1Ab toxin being used since 1996, coupled with decreasing refuge compliance, altogether have contributed to the evolution of resistance. Unfortunately, CEW resistance to the Cry toxins will likely increase with the shift to ‘refuge in bag’ field corn that contains only 5 percent non-Bt seeds.
Furthermore, the Vip3A trait has been licensed to other seed companies, so the acreage of Vip-expressed field corn is expected to increase in the South.
Due to northward influxes of potentially resistant moths from the South, this may eventually compromise the efficacy and durability of the Attribute II sweet corn technology.