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While most long-time residents of the region are used to it, and expect it, no one really likes to find the inevitable worm in their sweet corn ears. Bearing a marked resemblance to Jabba the Hutt, with the personality of a shower drain, the corn earworm (Heliothis zea) is considered by most folks as the worst pest of sweet corn in the South.
Conventional management strategies call for the use of repeated sprays of synthetic insecticides. While a wide variety of materials are available, most fall into the "Restricted Use" class, which requires State "Private Applicator" licensing to apply. Interest in alternative control strategies for organic growers and others is strong.
In many books and publications on organic and small-scale growing, there are references to the use of mineral oil, applied to the silks of the ear, as a reasonably effective deterrent. Last summer, OSU researchers at the Wes Watkins Agricultural Research and Extension Center at Lane, Oklahoma, evaluated mineral oil and several similar options to see whether, and how well they worked. Good control was obtained with mineral oil, but even better results were found with vegetable oil. Control was enhanced further by mixing a small amount of pyrethrum, a botanical insecticide derived from chrysanthemums, into the oil. Future efforts will be made to improve timing of the applications, a critical factor that can make or break this strategy. Additionally, several cooperating farmers in the 1989-1990 Low Input Sustainable Agriculture (LISA) program will be testing these procedures on a small-field scale.
This is not the first time researchers have taken an active interest in this approach. In August 1942, the USDA released a circular* addressing this method of corn earworm control. Those scientists felt the mode of action of the oils were two-fold. First, if properly applied to the top of the husk where the silks emerge, the oil creates a barrier to worms trying to reach the kernels. Secondly, the oil smothers any worms already in the "neck" area. The addition of pyrethrum to the oils changed the mode of action to a "contact poison." In this case lighter oils were employed to deliver the botanical insecticide further down onto the kernels.
The optimum timing for oils, as recommended in the 1942 publication, is at wilt of the silks, approximately three days after full-brush.** The Lane researchers have suggested that the optimum period may be slightly earlier at 48 hours after full-brush...though experiments have not been conducted to confirm this. The oil-pyrethrum mix appears to have more flexible timing, according to the USDA circular. If good penetration is achieved, a few days delay is apparently no problem.
The risk in judging timing is significant. Applying oils too early will cause interference with pollination and could result in a total crop loss. Applied too late, it becomes a labor-expensive activity, which leaves the grower with wormy corn.
The oil-pyrethrum mix, while promising due to less exacting timing, is a more costly option because of the high costs and limited supplies of pyrethrum. Available formulations of pyrethrum are often less than desirable for mixing. Many are dusts, and Pyrenone, a popular field spray of pyrethrum, is difficult to mix with oil due to emulsifiers in the formulation.
The greatest barrier to use of these methods is the large amount of labor required. In the past, most organic publications have advised the use of clumsy eye droppers for oil application. This approach has made the oil option appear less feasible than it really is for the small, commercial grower. The USDA suggested that, a trained worker, using proper equipment, could treat up to one acre or 12,000 ears in an eight hour day, using about two gallons of oil (approximately 1/4 teaspoon or 0.5 to 1.0 cc per ear). Equipment used was standard oil cans designed for lubricating machinery. Large syringes are also a possibility though more clumsy.
As not all ears in a field reach silk-wilt simultaneously, two trips may be necessary. Marking treated ears is relatively easy when using an oil can, as the opposite hand remains free for marking. Researchers advised using a single paper hole punch to mark the ear-flag leaf or, suspending an open ink pad from a belt and leaving a thumb print on the treated ear.
How such controls compare economically to conventional sprays, or the marketing of wormy corn, remains to be calculated. The high labor cost of oil application may be balanced out by the high costs of insecticides, labor, and fuel associated with repeated sprayings. For organic growers, it ma, prove to be the only alternative to simply accommodating the pest.
The results of the 1990 LISA Cooperator and Lane Trials with oils will be made available through this newsletter later in the year. Should any of our readers experiment with methods like this for control of earworm, we would appreciate hearing about your experiences.--George Kuepper
*Mineral-Oil Treatment of Sweet Corn for Earworm Control, " USDA Circular No. 657. August 1942.
**Full-brush refers to the point where silks have been extended to their maximum length
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