Integrating Microbial Insecticides and Oils into Sweet Corn IPM in Massachusetts

Objective 1, Experiment I. On-Farm trials with Bt products, 1994-1996

The purpose of these trials was to determine whether products containing Bt provide control of European corn borer (ECB) and fall armyworm (FAW) as effectively as conventional broad-spectrum insecticides. On-farm trials were conducted from 1994-1996; SARE funds supported the second and third year of trials.

Project description: Seventeen farms participated over the three years, with 10-13 farms each year. Early plantings of corn were split, with one half treated with Bt products and the other half with conventional materials. Some farms compared Bt to unsprayed blocks. The same monitoring methods, action thresholds and timing of applications were used in both treatments, with 1-4 applications per block. Bt products contained B.t. kurstaki; conventional products included methomyl, permethrin, esfenvalerate, and thiodicarb. Scouting was conducted by UMass field technicians. All crop management activities and harvest samples were done by farmers. In 1996, beneficial insect populations were assessed at six farms.

Results: Thirty-four trials were successfully completed. ECB populations exceeded the action threshold in all but two trials. No fall armyworm was present, so the efficacy of treatments against this pest could not be evaluated. The mean ear damage at harvest was equivalent in Bt and conventional treatments (see Table 1). Two categories of beneficial insects (ladybeetles and insidious flower bugs) were consistently higher in Bt plots on the six farms where sampling was done. Higher numbers of dead beneficial insects were found in all conventional plots.

In six trials, corn earworm flights began during silking and the Bt applications were followed by a broad-spectrum insecticide for corn earworm control. There was no difference in the ECB control obtained in these trials compared to those where no CEW flight occurred and only Bt was used. This pest situation is common in mid-season corn. The combination of tassel treatments for ECB with silk treatments for CEW can be readily integrated into existing pest management practices during mid-season. See Pest Management Systems for Sweet Corn, Appendix 2.

Six trials in three years used unsprayed blocks to compare to Bt treated blocks. The average damage levels were 3.4% Bt blocks and 19.9% damage in unsprayed blocks.

Grower evaluations: Of the 17 growers who participated in these trials, 14 (82%) were satisfied with the control they achieved with Bt and plan to use Bt products in the future for ECB control. Three were not completely satisfied and wanted to test Bt further before using it widely on their sweet corn crop. Growers reported that their reasons for choosing Bt over conventional products were applicator safety, easier relations with neighbors, shorter re-entry and preharvest intervals, and conservation of their beneficial insects.

Attachments: For further details, see Using Bacillus thuringiensis (Bt) Products for European Corn Borer Control in Sweet Corn. 1994-1996 On-Farm Trials: Final Report

Objective 1, Experiment II. Replicated trials with Bt products, 1996

The purpose of this experiment was to evaluate several factors in the use of foliar applications of Bt products against European corn borer (ECB).

Project description: Two Bt formulations, Dipel EStm (Abbot Laboratories) and MVPtm (Mycogen Corp.) were evaluated with and without spreader-sticker, and in weekly vs. twice-weekly applications, in field trials using a randomized complete block design at the UMass Research Farm. In one treatment tested, oil was applied to silks (Dipel ES/corn oil, 1:20) in addition to foliar Bt applications. Ears were harvested and rated for damage and presence of pests.

Results: This trial was designed primarily to evaluate control of ECB; however, corn earworm (CEW) was numerous and caused more feeding damage than ECB. All treatments resulted in more clean ears (61-91%) than the control (51%). Although foliar sprays of Bt are generally regarded as ineffective against CEW, in this trial five of six foliar Bt treatments suppressed CEW. The best control (91% clean) was achieved with an oil direct silk application in addition to foliar sprays. The benefit gained by using a spreader-sticker or spraying twice weekly was not significant in most instances. However, one formulation (Dipel ES) resulted in more damage when used weekly without a sticker. In other respects, there were no differences between the two Bt products tested.

Attachment: Foliar Sprays of Bacillus thuringiensis in early corn, 1996. Ruth Hazzard and Mark Mazzola. Arthropod Management Tests: 1997, pp. 122-123.

Objective 2. Develop an alternative control for corn earworm and other late-season pests.

Introduction: Since 1992, we have been investigating oils as an alternative control for corn earworm and other late-season pests of corn. We began this work as a result of an earlier SARE-funded project, New England Farmer to Farmer Information Exchange, which brought together farmers to discuss biointensive methods for sweet corn. One grower was using the oil method successfully on his farm, and others were interested. The method, which was widely used in corn before the introduction of broad-spectrum insecticides, is very simple: small amounts of oil are applied directly to the silk of each corn ear shortly after the corn silk develops. Studies conducted from 1992-1995 demonstrated that either mineral oil and corn oil, mixed with a Bt product and applied once at 0.5 ml/ear, significantly reduces damage from both corn earworm and European corn borer larvae that enter the ear through the silk channel. The oil is applied by hand, by an individual walking down each row. Growers with a total of 10-15 acres of sweet corn would typically be treating a one-quarter to one-acre block of corn at one time.

We have worked closely on this project with the Lemelson National Program for Invention, Innovation and Creativity at Hampshire College. The major barrier to adoption of the oil method is the lack of an efficient applicator tool that will minimize the time required for oil applications. In 1996, with Lemelson funding, we designed and built a hand-held syringe pump oil applicator. Subsequently, the University of Massachusetts and Hampshire College submitted a patent application for this design to the US patent office. Timing trials by researchers and growers indicate that the labor for oil applications to an acre of corn ranges from 6 to 10 hours. Cost estimates for the oil method are based on these figures.

All experiments described below were conducted with four replicates in a randomized complete block design.

Objective 2, Experiment I. Applefield Farm, 1996. The objective of this trial was to determine the effect of different ratios of Bt to oil in the direct-silk oil barrier method.

Project Description: A commercial block of corn was divided into replicated plots with five treatments: Bt (Dipel ES) mixed with corn oil at three ratios (1:20, 1:10 and 1:5), oil alone, and no treatment. The corn was treated on the fourth day after silk initiation.

Results: Untreated ears were heavily infested with corn earworm, with only 18.5% of ears free of damage. ECB and FAW were negligible. Oil alone gave 46.5% clean ears, and the addition of Bt at any ratio significantly improved control (55-65% clean ears). There was no difference in the control achieved by the three oil:Bt ratios.

Objective 2, Experiment II. Applefield Farm, 1996. The objective of this trial was to compare two Bt products and two types of oil for efficacy against the insect complex. Mineral oil has given better control in previous experiments but would not be acceptable on certified organic farms because it is a petroleum product.

Project Description: A commercial sweet corn planting was divided into replicated plots. Two Bt products, Mattchm (Mycogen Corp.) and Xentaritm (Abbott Labs) were applied with either corn oil or mineral oil; three additional treatments with each oil alone or no treatment were used as controls. Xentari, a granular product, was premixed with water but because of a mixing error it was applied an excessively low rates. Applications were made three days after silk initiation.

Results. Corn earworm was the dominant pest. Ear damage was high, with 48% clean ears in untreated plots. There was no significant difference in the control achieved with the two types of oil, whether they were mixed with Bt or used alone. The addition of Xentariâ„¢ to either oil did not give improve control; this is very likely due to the low rates used. The addition of Mattch did significantly improve control, providing 81% clean ears when mixed with corn oil.

Objective 2, Experiment III. Food Bank Farm, 1996. The timing of oil applications relative to pollination of the corn silk affects the length of the filled portion of the ear. Some oil-treated ears have a short, distinct section of unfilled kernels at the tip, which we call a “cone tip�. Untreated ears usually show a gradation of unfilled kernels out to the tip. The timing of application also influences the degree of control, because too late an application will allow insects to enter the ear before the oil is present. The objective of this experiment was to evaluate the effect of the timing of oil application on these factors.

Project Description. A commercial corn planting was subdivided into replicated plots. An oil/Bt mixture (20:1) was applied at 0.5 ml per ear at 2, 4, 6, 8, 10 and 12 days after silk initiation.

Results. Both corn earworm and ECB were present, but infestations were light. Untreated plots were 87.5% clean. The best control (98% clean) was achieved with treatments on day 2 and 4. Subsequent applications provided levels of control that were not significantly different from untreated ears. However, there was a significant trend over all treatments toward better control with earlier applications. Day 2 treatments had a much higher proportion of “cone� tips (26%) compared to day 4 (8%). All later dates had 1% or less, the same percentage as the untreated control.

Objective 2 and 3, Experiment IV. UMass Research Farm, 1996. The major objective of this experiment was to determine whether the rate of the oil/BT mixture applied to silk would effect the degree of tip fill and would change its efficacy against late-season pests.

One limitation of oil applied to the silks is that it does not control insects that burrow through the side of the husk. Many ECB and FAW larvae enter ears through the side, after moving down from the tassel where they fed as young larvae. This “side damage� is a major reason that oil treatment experiments have not consistently achieved the 90-95% control that is needed in most markets. In this experiment, we tested an integrated system in which two foliar applications of a BT product were made at the tassel stage to reduce the number of ECB that would move into ears through the side of the husk.

Project Description. In replicated plots of late sweet corn, a 1:20 mix of Dipel ES: corn oil was applied four days after silk initiation at the following rates: 0.3 ml, 0.6 ml, 0.9 ml or 1.2 ml per ear. An additional treatment used 0.6 ml per ear but was preceded by two applications of MVP II (Mycogen Corp.) five days apart, at the pretassel and tassel stage. At harvest, ears were measured for total length and the length of the filled portion of the ear in addition to damage and insect ratings.

Results: ECB was the major cause of ear damage in this experiment, although corn earworm was also present. There was no fall armyworm. Only 24% of the corn in untreated plots was free of insect damage, and both side and tip damage were present. Oil treatments yielded 64-77% damage-free corn, a three-fold improvement. The addition of two foliar Bt sprays gave 100% clean ears, a significant improvement over all the oil-alone treatments, and certainly a level of control that is acceptable in all markets.

There was no difference in the level of control or the length and proportion of filled kernels with different oil rates. Oil applications resulted in a mean reduction in total ear length of 7% (1.4 cm), a 9% reduction in the length of the filled kernels (1.5 cm.), and a higher proportion of “cone� tips, compared to untreated ears. However, the lowest oil rate (0.3 ml/ear) resulted in a significantly lower percentage of “cone� tips (61% compared to 13% in untreated and 80-85% in other oil treatments).

Objective 3, Experiment I. UMass Research Farm, 1997.

The objective of this experiment was to evaluate the efficacy of elements of the biointensive system for late-season sweet corn.

Project Description: Two foliar applications of Bt at the tassel stage and application of the oil barrier directly to silks were used singly or in combination in late corn that was harvested in mid-September.

Results: Both CEW and ECB caused feeding damage, but CEW larvae were most numerous. Foliar Bt sprays by themselves were ineffective in this study. This may be due to the fact that ECB infestations were low during tasseling and occurred primarily during ear development, with small larvae entering directly through the silk channel. The control achieved by oil treatments was 32.9% better than the control. There was no statistical difference between the control achieved with oil alone and that gained with oil plus Bt sprays.

Attachments: See 1997 Annual Report of the Vegetable and Small Fruit Integrated Crop Management Program, p.30-31, for further details.