Implementing a Biointensive Strategy for Caterpillar Control in Sweet Corn

Implementing a Biointensive Strategy for Caterpillar Control in Sweet Corn

Summary

Summary
A bio-intensive strategy for the control of caterpillar pests of sweet corn is being implemented in commercial fields on eight farms. Each farm is testing the strategy in multiple plantings using direct silk applications of Bacillus thuringiensis and vegetable oil to control corn earworm, and foliar applications of Bt for European corn borer. Significantly improved control was achieved in all plantings, averaging 83% clean tips with oil compared to 59% without oil. Oil reduced both numbers and size of caterpillars. Experiments to determine optimal timing of oil for best ear quality showed silks from the top ten kernels develop late and tip fill is reduced by the oil, but ear length remains unaffected.

Objectives
To evaluate the efficacy and cost of this integrated bio-intensive strategy on eight cooperating farms throughout New England.

To conduct a qualitative assessment of the impact of using the caterpillar control strategy on production and marketing of sweet corn and other crops on the same cooperating farms.

To develop a better understanding of how direct silk applications of oil influence the developing ear kernels and larval mortality, in order to determine the optimal timing of oil treatments for highest ear quality.

Methods
Objective 1: On each of eight farms three blocks of corn were planted at least seven days apart after May 30. Each block was at least eight rows wide by 200 feet long, and contained oil and no-oil sections. The growers treated the top ear of every corn plant outside of the no-oil control area with a mixture of Bt and corn oil. When thresholds for European corn borer or fall armyworm were met, as determined by scouting, some growers also applied a foliar spray of Bt to the whole block. At harvest, 100 ears each from the control and treatment sections of each block were scored for number, size and type of caterpillars, as well as damage to the tip, side, and husk.

A pooled analysis of all of the blocks on all of the farms shows a statistically significant increase in the percentage of corn without tip damage, from 59 percent to 83 percent in the treated (oiled) verses the untreated samples. The range in improvement in the amount of tip damage on each farm varied from 8 percent to 44 percent, with the highest levels of improvement occurring on farms with the greatest pest pressure. Interestingly, a slight increase in the number of ears of corn without side damage also increased in the treated sample, from 89 percent to 93 percent, suggesting that the oil also had a beneficial effect on preventing side damage. Data also indicate that the oil treatments decrease the numbers of corn earworm, showing an increase in ears without corn earworm of 19 percent, from 66 percent in the untreated to 85 percent in the treated sample. A similar jump in worm-free corn, by 29 percent (from 47 to 76 percent), was also seen when all of the caterpillar pests monitored were pooled, indicating that the treatments have an effect on other worms as well. All differences in means were statistically significant at p =0.001 (Chi-Square test). Due to extreme weather conditions and raccoon damage, a total of five blocks failed.

Objective 2: Our qualitative assessment of the impacts of using this system on farms is on track: we have conducted pre-season interviews and will be following up with another round this winter.

Objective 3: A key question is how the application of either the oil or Bt applied directly into the husk affects the growth and development of the ear. Ears and silk growth were measured over a 12-day period (after half the plants showed silk), with the treatments applied on the third day of silk growth. The oil/Bt treatment did not affect the growth of the ear as measured by ear length, circumference, or rate of growth; however, the development of approximately the last ten kernels at the tip was stopped. At the time of oil application the silks that are connected to these tip kernels are still within the husk and receive a significant amount of oil as the oil moves down the silk channel. Next season, we will use electron microscopy to test the theory that these silks cannot receive pollen upon emergence from the husk because they are coated with oil.
To determine what day to apply corn oil to achieve the best corn earworm control, we applied the oil/Bt mixture to corn ears every day from day 3 through 11 after 50 percent of the field had silk emergence. No other insect controls were applied. The experiment was conducted twice, with two staggered plantings for evaluation under different pest pressures. The corn was harvested at maturity and evaluated for length of ear, length of cone tip, number, type and size of caterpillars, and the amount and location of damage. The data for this experiment are still being analyzed. Preliminary results indicate a significant difference among treatments with respect to the days that the oil/Bt mix was applied. Also, the later the treatment was applied to the silk channel, the shorter the length of undeveloped kernels in the tip.

Reported November 2000

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