Small Group Session: Corn and Sweet Corn
Integrating Microbial Insecticides and Oils into Sweet Corn IPM
Ruth Hazzard, an extension entomologist from Massachusetts, presented results from several years of work with Bacillus thuringiensis (Bt)and vegetable oils to control damage by caterpillars in sweet corn.
The primary pest to be controlled in early season corn is the European corn borer, Ostrinia nubilalis, which can be controlled as well with Bt products as with conventional synthetic insecticides, using the same scouting methods, thresholds, and spray intervals. Replacing insecticides with Bt increased the number of beneficial insects in the plots. Weekly applications were as effective as twice-weekly applications, and the cost of Bt was equivalent to conventional insecticides.
In late season corn, there are more pest species, including fall armyworm and corn earworm as well as European corn borer. Those caterpillars entering the ear through the silk channel can be controlled using an application of a corn oil and Bacillus thuringiensis mixture (20 parts oil to 1 part Bt applied as the silk begins to wilt at a rate of 3-5 milliliters per ear). A hand-held oil applicator has been developed to make this job easier and economical for farms with less than 20 acres of sweet corn.
Corn Earworm and Other Challenges of Growing Corn Organically
Steve Mong, an organic farmer from Massachusetts, described his experience using Bacillus thuringiensis sprays and the corn oil/Bt mixture.
Biological Control of European Corn Borer in Sweet Corn with Trichogramma ostriniae
Mike Hoffmann, an entomologist from New York, reported on his work with early-season inoculative release of Trichogramma ostriniae, a wasp that parasitizes European corn borer egg masses. In his studies on four diversified vegetable farms, from 44% to 84% of the egg masses were parasitized (compared to less than 5% without the release). The cost is about $13 per acre.
Ruth Hazzard responded to questions about the practical aspects of using the hand-held oil applicator. It is not complicated or physically straining to use, and growers with 10-15 acres of sweet corn usually have succession plantings, so they can schedule hired help to apply the oil for about 8 hours per week.
Ruth Hazzard and Mike Hoffmann responded to questions about using pheromone traps to time oil applications and releases of Trichogramma ostriniae against European corn borer. Ruth said that the IPM program uses a threshold of 2 corn earworm moths per week caught by the pheromone trap. Mike said that it is tricky to use trap catches in pheromone traps for timing the release of Trichogramma ostriniae, so he recommends releasing the wasps at the mid-whorl stage of corn development.
There was also a discussion of using Bacillus thuringiensis to control European corn borers in peppers and in corn. In peppers, the current recommendation is use pheromone traps to determine when the flight of the moth begins, and then apply Bt twice per week. In corn, pheromone traps are used to determine the flight periods of the moths, combined with field scouting for caterpillars or plant damage. If 15% of the plants have damage or caterpillars present, then foliar applications of Bt begin, using the same spray intervals as with conventional insecticides.
In response to questions about the many plant hosts of European corn borer, the speakers said that European corn borer tends to be more of a problem in weedy corn, but no weeds or other plants had been successfully used as a trap crop. There is evidence that a living mulch of red clover might reduce corn borer abundance, but this must be balanced against the difficulties of establishing a living mulch and managing it so that it does not compete with the main crop.
Fall armyworm, Spodoptera frugiperda, is more difficult to control adequately with the methods described here. Steve and Ruth said that the oil/Bt combination works well against fall armyworms entering the ear from the top during silking, but they can also enter the ear before silking or from the side. Because pheromone trapping does not give a reliable early warning of fall armyworm infestations, thresholds must rely on scouting.
Sap beetles are another pest of corn, usually a secondary problem on ears damaged by caterpillars or birds. Damage may be reduced through sanitation and by growing varieties with tight husks.