Good Fruit Grower

November 2012

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Seaweeds tested for pest control S eaweed extracts are typically used by growers with the aim of improving tree growth and enhancing fruit yields and quality. Although the extracts are regulated and marketed as plant growth regulators, entomologists have been studying whether the products could also have benefits in terms of pest control. Results of a study conducted in Washington State suggest that sea- Research trials with seaweed extracts in apple weed extract applications can reduce pest mite populations. However, in experiments in Vermont, the extracts had little effect on the incidence of mites or most other pests, but did reduce fruit damage from apple maggot on some apple varieties. Dr. Holly Little, market development sci- and pear orchards have produced mixed results. by Geraldine Warner entist with Acadian Seaplants, Ltd., in Nova Scotia, Canada, which markets Stimplex (containing extracts of the seaweed Asco- phyllum nodosum), has been collaborating with Dr. Alan Knight, entomologist with the U.S. Department of Agriculture in Yakima, Washington. She presented preliminary results of their ongoing trial during an International Organic Fruit Symposium held in Washington last summer. Little said seaweed has been used for many years in agricultural production. Native Americans recognized the advantages of using sea- weed as a mulch or fertilizer and used to gather it from the shoreline and apply it to their crops. Since the 1960s, farmers have been using com- mercial products containing seaweed extracts as foliar sprays or ground applications. The idea of testing seaweed's effects on mites came after growers who were using the seaweed to enhance tree and fruit growth reported seeing fewer mites in their orchards, Little said. Washington The Washington experiment began in 2011 in two orchards at Moxee near Yakima. One was a commercial apple and pear orchard and the other a USDA research plot with just apple trees. Treated plots in both orchards received season-long applications of Stimplex. Sprays were applied at the tight cluster, pink, and petal fall stages of bloom, followed by three monthly applications. The USDA orchard was treated with pheromones for mating disrup- tion of codling moth, and no other sprays were applied. The commercial orchard was treated with six pesticides in addition to pheromones. Leaf samples collected four times during the season showed a general trend of fewer mites in the Stimplex-treated trees in both orchards. However, in the commercial orchard, statistically significant differences were seen only in an August sample. European red mite populations exceeded economic thresholds in both treated and untreated trees by the end of the season, which Little attributed to the pesticides applied to that orchard. At the USDA orchard, the Stimplex-treated trees had fewer brown mites in July and August (Table 1). There were no statistical differences between the populations of predatory mites (Typhlodromus occidentalis and Zetzellia mali) in either orchard. The trial is continuing in 2012. Ascophyllum nodosum, or knotted wrack, belongs to a group of seaweeds that often dominate rocky seashores in cooler climates. Mite species European red mite eggs European red mite motiles Two-spotted spider mite eggs Two-spotted spider mite motiles Typhlodromus occidentalis (predatory mite) Zetzellia mali (predatory mite) Stimplex (Mean number per leaf) 1.9 0.3 0.1 0.4 0.0 0.0 Effects of seaweeds on spider mites and predatory mites TABLE 1 Comparison of mite populations in treated and untreated plots in a commercial pear orchard in Washington State sampled on August 5, 2011. Treatment Control (Mean number per leaf) 3.2 1.2 0.3 1.2 0.1 0.0 TABLE 2 Comparison of mite populations in treated and untreated plots in a research apple orchard in Washington State on two sampling dates. JULY 8 SAMPLE: Treatment Stimplex Mite species Brown mite egg Brown mite motiles Typhlodromus occidentalis Zetzellia mali AUGUST 10 SAMPLE: Stimplex Mite species Brown mite eggs Brown mite motiles Typhlodromus occidentalis Zetzellia mali Source: Alan Knight, USDA GOOD FRUIT GROWER NOVEMBER 2012 41 (Mean number per leaf) 1.1 0.9 0.1 0.5 Control (Mean number per leaf) 2.5 2.2 0.1 0.5 Control (Mean number per leaf) (Mean number per leaf) 0.5 0.2 2.2 1.4 0.02 0.12 0.06 0.14 Holly Little Alan Knight According to Ashlea Barr, corporate communications officer at Acadian Seaplants, research into the mode of action of the seaweed extracts on insects, including mites, is ongoing. Previous work has indicated that reductions in mite populations may be due to improved plant health and increased resistance to a variety of environmental stresses in plants treated with the extracts. In addition, studies have shown that mites prefer to feed, settle, and reproduce on untreated plants. Vermont Terence Bradshaw, research specialist with the University of Vermont, conducted tests in 2009 and 2010 in an organic apple orchard at the research center in South Burlington to compare Stimplex and SeaCrop 16 (an A. nodosum extract from North American kelp) with an untreated control. The two products claim improved resistance to damage from

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