Cultural practices and pest management must be balanced, says Michael Costello.
Management practices for one kind of pest can affect nontargeted pests in the vineyard, warns Michael Costello, professor of entomology and acarology at California Polytechnic State University, San Luis Obispo. For example, cultivating to manage weeds can raise dust that can attract mites.
"I’ve never seen a mite that doesn’t like to hang out in dust," Costello said at the Washington Association of Wine Grape Growers annual meeting in Kennewick last winter.
The agro-system includes all living and nonliving things, and they all interact in some way, Costello said. He called vineyard vegetation management the "main tweak of the system," and listed popular practices including cultivation, the use of cover crops that can be mowed and turned under, and chemical mowing. To keep dust down, roadsides can be watered or oiled, or lignins or gravel can be used. Increasingly, growers are co-planting other kinds of vegetation, for example permanent hedgerows along waterways or vineyard edges, providing what Costello called "floral goodies for bugs."
Flowering cover crops like common vetch and alyssum are also being tried, he said. Some flowering plants can attract natural enemies of certain vineyard insects, Costello said. He cited Acerophagus sp., a parasitoid of the grape mealybug; Anagrus sp., a parasitoid of grape leafhopper; and tachinid and bethylid wasps, parasitoids of tortricid moths. However, so far, there’s little evidence that flowering plants impact insect or mite pest density, Costello said.
Floor vegetation can affect soil-water interaction and vine growth. Lower vine vigor typically lowers leafhopper density, although severe water stress can increase mite density, he said, and growers need to find a balance. Looking at season-long deficit irrigation, at 80 percent or less of the evapotranspiration rate, leafhopper density decreases, Costello said. He said his research in a Paso Robles Cabernet Sauvignon vineyard indicates that inducing water stress at the right time, generally from mid-June to the first of August, can prevent leafhoppers from laying eggs, but it can also decrease grape yields by 15 to 20 percent.
In California, growers use fungicides, especially sulfur, to control powdery mildew, Costello said. In fact, as many as 40 to 50 million pounds of sulfur are applied each year in California, mainly because it’s cheap, there’s been little or no insect resistance, and it controls eriophyid mites. But there are downsides, including bad public relations, wind drift, and blowups of other mite species. He cited research published in 2007 that found Pacific mites (Tetranychus pacificus) numbered five times the norm with the use of sulfur dust only. The research found that predatory mites were not negatively affected by the sulfur.
He urged growers to use sound IPM practices, including sampling, using cultural controls to destroy insect and mite habitats, and conserving biological controls. He also urged growers to estimate reasonable economic injury levels and use selective chemicals only when those levels are reached.