Insect herbivore densities can respond to changes in host plant vigor. While. many basic studies have described the generalized patterns and-mechanisms of host plant influence on herbivore densities and biological traits, fewer studies have shown practical applications for the observed ecological patterns. We assessed the effect of different irrigation amounts on grapevine (Vitis, vinifera L., cv. Thompson Seedless) condition and the resulting changes to an insect herbivore (the variegated leafhopper, Erythroneura variabilis Beamer). Water amounts delivered to vines were manipulated with respect to amounts delivered in a weighing lysimeter: 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, and 1.4 times that used by vines in the lysimeter. We then followed plant growth measurements and leafhopper density and biological. traits throughout the growing season. By August and September,, midday leaf water potential, mean shoot length, vegetative growth, and pruning mass were a positive linear function of applied water amounts. The amount of N per unit leaf area decreased as applied water amounts increased. Similar to measured levels of vine growth, there was a seasonal response of leafhopper density to applied water amounts. By August and September, leafhopper density was a positive linear function of applied water amounts. Dry Mass of fifth instar leafhoppers, the numbers of marked and recaptured adults, and leafhopper egg deposition were also, positively correlated to irrigation amounts. Our results indicate that the greatest influence of manipulated vine growth and irrigation amounts may be due to differences in adult leafhopper movement and reproductive potential, which has implications for dispersal of pests between vineyards. We demonstrated that applied irrigation amounts to vines can be manipulated to suppress insect herbivore density without negatively influencing crop yield. We discuss our results of insect herbivore response to changes in plant water stress within the context of the plant vigor hypothesis.
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