In the San Joaquin Valley of California, leafhoppers (Erythroneura elegantula and Erythroneura variabilis) can severely damage the foliage of grapevines resulting in economic loss. Most Thompson Seedless raisin vineyards, however, don't require treatment for leafhoppers every year. To help make the correct treatment decision, monitoring guidelines and action levels are important. This study provides information on monitoring techniques and action levels for this leafhopper complex. A sustained population of 20 nymphs per leaf during summer broods results in 20% to 30% visible damage to the canopy by harvest in early September. Populations higher than this may require chemical intervention to prevent an economic loss. The photosynthetic activity was reduced in proportion to visible leaf damage. Methods of estimating damage to the canopy from leafhopper activity are presented.
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