Geoff Zehnder, Eric Simonne, Terry Briggs, Jim Bannon, and Marvin Ruff

Vegetables in Alabama are often plagued by insects, particularly caterpillars that can cause severe feeding damage to the marketable parts of the plant. While synthetic insecticides are available, recent AAES studies have shown that garlic, red pepper, and other organic materials also provide effective control of these pests on lettuce and cabbage.

Some of the more damaging of the lepidopterous species include the diamondback moth, the cabbage looper, and the imported cabbage worm. These pests primarily feed on cruciferous crops (cabbage, collards, broccoli, kale, etc.). Several armyworm species also may occasionally attack crucifers and other Alabama vegetable crops, including lettuce.

Adult female moths lay eggs on the cabbage and lettuce plants, and the hatching caterpillars, or “worms,” feed on the outer leaves and heads causing defoliation and reducing the grade or marketability of the crop. Application of synthetic insecticides is one method that growers can use to protect vegetable crops against insect attack. However, organic growers or growers interested in adopting integrated pest management (IPM) practices are interested in alternative methods of insect management.

Organic insecticides and insect repellents, usually derived from plants or microbes, have a long history in agriculture and reports (many are anecdotal) vary as to their effectiveness. Unfortunately, few formal studies have been done to compare these materials, particularly the organic insect repellents, with standard, synthetic insecticides.

A two-year AAES study was conducted to compare commonly used, synthetic insecticides with some readily available organic materials, including garlic juice and red pepper powder, for control of worm pests in cabbage and lettuce. The study demonstrated that application of garlic and other organic materials provided levels of worm control sufficient to protect cabbage and lettuce heads from insect attack.

Separate experiments were conducted with Atlantis variety cabbage transplanted on March 25, 1997, at the E. V. Smith Horticulture Substation in Shorter and Salinas 88 Supreme variety head lettuce transplanted on June 6, 1997, at the Sand Mountain Substation in Crossville. Cabbage treatment plots consisted of a single, 25-foot-long treatment row bordered on each side by a nontreated or buffer row, and lettuce treatment plots consisted of two-row plots with 40 plants per plot. Cabbage treatments were sprayed on a weekly schedule beginning on April 25 until harvest using a spray volume of 55 gallons per acre. Ivory Snow™ liquid laundry soap (six drops per two-liter bottle) was added to the spray mixture in all spray treatments to ensure the cabbage leaves were evenly covered.

Karate™, applied at the rate of 0.75 ml per two-liter bottle, was used as the standard synthetic pyrethroid insecticide in the cabbage trial. This amount was equivalent to the recommended field rate of 2.6 fluid ounces of Karate per acre applied in 55 gallons of water. (Note: a gallon is 3.8 liters and a fluid ounce is 29 ml.) Commonly used by commercial growers to control caterpillars in vegetables and other crops, this product contains 13.1% lambdacyhalothrin, the active ingredient. The organic materials under study in the cabbage trial included the following: Garlic Barrier™, Align™, Javelin WG™, and McCormick™ ground red pepper.

The garlic mixture, which is 100% garlic juice mixed with water, was applied as a foliar spray; 1% garlic, 1% fish oil, 98% water. This product is advertised as an insect repellent that enters the plants through stomata and moves systemically through the plant.

The McCormick ground red pepper obtained at a local super market, has also been reported as an insect repellent. Align is a botanical insecticide containing 3.0% azadirachtin, a natural insecticide obtained from the tropical neem tree. Azadirachtin is reported to have activity against a wide variety of insect species, including lepidopterous pests. The active ingredient in Javelin WG is a toxin obtained from a bacterium, Bacillus thuringiensis variety kurstaki. The toxin is active only against caterpillar pests, and acts as a stomach poison that must be consumed by the worms to be effective.

Lettuce spray treatments, applied approximately weekly beginning on June 12 through July 18, were made using 36 gallons per acre spray volume. In the lettuce experiments Garlic Barrier was evaluated as a foliar spray using the same formulation of garlic, fish oil, and water, and as a foliar spray plus a transplant drench application. A combination of Sevin XLR™ insecticide (at a one-pint-per-acre rate) plus Kocide DF™ bactericide (at a two-pounds-per-acre rate) was used as the chemical standard treatment in the lettuce trial.

In the cabbage test, worm counts were recorded weekly on five cabbage plants per plot, and a visual damage rating was assigned to five plants per plot at harvest on May 28. Plants in the lettuce trial were evaluated at harvest using a 0 (no insect or disease damage) to 5 (severe insect feeding damage and disease symptoms) visual damage rating scale to assess insect damage and foliar disease incidence. Caterpillar larvae were not identified by species in the lettuce trial.

Worm counts taken in the different cabbage treatment plots indicated that the Javelin biological insecticide, Garlic Barrier, and red pepper treatments resulted in equivalent or better control of cabbage worms than the Karate synthetic insecticide treatment (Figure 1). All these treatments resulted in significantly lower numbers of worms than in the nontreated control. Align provided the least effective worm control of all the spray treatments. Insect damage ratings taken at harvest indicated that cabbage plants in the Javelin treatment exhibited the least worm damage, followed by red pepper, Karate, Garlic Barrier, Align, and the nontreated control, in order of increasing damage (Figure 2). The average worm damage ratings in all organic treatments except for Align were below 4, indicating that most of the damage occurred on the outer wrapper leaves and did not affect marketability of the cabbage heads.

Fig. 1. Average number of cabbage worms per plant in the different cabbage spray treatments. DMB=diamondback moth larvae; CL=cabbage looper; ICW=imported cabbage worm.

Fig. 2. Insect feeding damage ratings in the different cabbage spray treatments: 1=no apparent insect feeding; 2=minor feeding on wrapper leaves; 3=moderate feeding on wrapper leaves with no head damage; 4=moderate feeding on wrapper leaves with minor feeding on head. A rating of 4 and above is considered unmarketable because even slight damage to the head is not acceptable.

Fig. 3. Insect feeding and disease damage ratings in the different lettuce spray treatments (0=no damage; 5=most severe damage).