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Sweet Corn > Bio-Intensive Control of Caterpillars in Fresh Market Sweet Corn: Results of On-farm Trials, 2000
Ruth Hazzard and Pam Westgate
An integrated method for the control of sweet corn caterpillar pests on organic farms has been developed here at the University of Massachusetts. It is a two-part system that combines foliar spray applications of Bacillus thuringiensis for control of European corn borer (ECB) and fall armyworm (FAW), and an application of vegetable oil mixed with Bt to the silk of each ear, to control the corn earworm (CEW). This application is made using a hand-held device called the Zea-later, which was developed at the University of Massachusetts and Hampshire College and will be commercially available this season. Whether to treat or not is determined by pest pressure, which is ascertained through currently established IPM scouting methods. This system provides growers who are following organic standards or who do not wish to use conventional insecticides with a alternative method for caterpillar control throughout the growing season. During the past growing season we conducted the second year of trials on eight farms throughout New England to test the effectiveness of the direct oil method in commercial plantings. Each farm planted 3 separate blocks of late-season corn, from the end of May through the end of June. Late plantings were used in order to test this method under high levels of corn earworm pressure. The SE variety 'Delectable' was used in all plantings. Throughout the season we monitored the insect flights of CEW, FAW and two strains of ECB (Z1 and E2) on each farm. We also scouted each block for caterpillar damage, starting in the pre-tassel stage. If threshold levels of damage or captured moths were reached, the growers were encouraged to apply a Bt foliar spray to the entire block of corn. In half of each block, the grower or one of his workers applied the oil treatment to the silk of each ear of corn, using 0.5 ml per ear of corn oil mixed with an emulsifier and Bt (Dipel DF). The timing of the oil application was determined by the age of the silk, with the target age being 5-6 days after silk has begun to grow. At this time, the corn has been pollinated and the silks are beginning to wilt. Because of the extraordinarily cold and wet season, many blocks had uneven growth, which resulted in some ears being treated earlier or later than the ideal. Corn was harvested from August 25 to October 5. We picked 100 ears of corn from the treated block, as well as 100 ears from the untreated section of the same planting. Each ear was scored for the number, size and kind of caterpillars, as well as the severity of damage and the point of entry of caterpillar that caused damage (through the side or tip of the ear). Ears were considered "clean" only if no feeding damage to any kernels was found. Blocks 1, 2 and 3 were grouped by harvest date and a total of 20 blocks were successfully harvested and included in the analysis. The results show that oil applications resulted in significantly fewer caterpillars -- both corn earworm and European corn borer -- compared to untreated controls (Fig. 1). The oil treatment also produce a significant increase in the percent clean ears, which averaged 77% clean compared to 52%, an increase of 25% (Fig. 2). Several growers achieved control at a level above 85% clean. Interestingly, the oil treatment not only reduced the damage from caterpillars that entered through the tip of the ear, but also significantly reduced the number of ears with husk damage and damage from side entry (Fig 3). Where damage was found, the size of worms and the severity of damage was lower in treated ears. Fall armyworm numbers were very low in most planting. Corn earworm pressure was higher in blocks 1 and 2, resulting in a higher proportion of tip damage (see Figures 2 and 3). European corn borer pressure was higher in the blocks that were harvested in late September, and the increase in the numbers of ECB corresponded to an increase in the amount of side damage (Fig. 3). Not all growers used the Bt foliar sprays as recommended, but those that did achieved better control of European corn borer and higher levels of clean ears. The oiling method targets caterpillars entering down the silk channel at the tip, especially corn earworm. Oil also reduces European corn borer, because some borers also enter through the tip. However, side damage, generally caused by ECB and/or FAW, is best controlled using the foliar Bt applications mentioned above. More consistent use of this part of the system could have resulted in better levels of control. The balance of corn earworm and European corn borer pressure varies greatly on different farms, and also changes with the time of year and the season. The farms in this trial include coastal locations, where the CEW pressure is extremely high every year and damage is difficult to control even using the conventional pesticides, as well as the Connecticut River Valley, where damaging levels of ECB pressure can be extremely high. Indeed we saw these extremes, with the farm in coastal Rhode Island trapping up to 45 CEW per night (when the threshold for treating to prevent CEW damage is 2 per week), and one of the Vermont farms having ECB moth captures at an equally high 48 moths per night. For this reason, it is important to utilize the IPM scouting techniques as well as both pre-silk foliar sprays and oiling of silks, as needed, to minimize damage from all of the possible caterpillar pests. Growers who participated in these trials reported that the Zea-later tool was practical to use and the time required for the single application was cost-effective (about 5 hours per half acre), given the importance of providing worm-free corn to their markets. Their markets, which include farmstands, farmers markets, CSA, and wholesale, demand anywhere from 85% to 95% clean corn. These growers plan to keep using the Zea-later and expect to see improved control as they become more familiar with this new technique. They also plan to use Bt foliar sprays more consistently when they repeat the trials in summer 2001. For an updated fact sheet on this method, contact the authors at 413-545-3696 or westgate@umext.umass.edu. The Zea-later oil applicator will be available this season from Johnny's Selected Seeds (call the commercial department, 207-437-4395). Acknowledgements: This work was made possible by suppor t from the Northeast SARE Program, the Organic Farming Research Foundation, Massachusetts Integrated Pest Management Program, and University of Massachsuetts Extension. We appreciate the donation of materials from Crookham Seeds, Valent Biosciences, and Enabling Devices, Inc and the participation of the following farms: Walker Farm, Putney, VT; Kestrel Farm, Westminster, VT; Coolwater Farm, Whately, MA; Brookfield Farm, Amherst, MA; Upper Forty Farm, Cromwell, CT; Applefield Farm, Stow, MA; Wishingstone Farm, Little Compton, RI; and Goransen Farm, Dresden ME. We appreciate the technical assistance of Eric Sideman of the Maine Organic Farming and Gardening Association, David Handley and Tori Tillson of the University of Maine, and Steve Buckley and Kathy Ready of the University of Massachusetts.
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