Relationship Between Cucumber Beetle (Coleoptera: Chrysomelidae) Density and Incidence of Bacterial Wilt of Cucurbits

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Field experiments were performed in 1993 and 1994 to establish varying densities of striped Acalymma vittatum (F.) and spotted, Diabrotica undecimpunctata howardi Barber, cucumber beetles in spring-planted cucumber plots, and to determine the incidence of bacterial wilt symptoms in each treatment. These data were used to develop regression models to examine the relationship between beetle density and the severity of bacterial wilt disease. Cucumber beetle numbers peaked during the first 2 wk after colonization of plants, with a rapid decline in numbers thereafter. Bacterial wilt symptoms were first observed 12-14 d after peak numbers of beetles were recorded. Critical point linear regression analyses of both year's data demonstrated a strongly linear and positive relationship between cucumber beetle density and the severity of wilt symptoms. No significant improvement was detected by fitting quadratic models. Beetle numbers recorded two weeks after colonization of plants provided the highest r2 values when regressed on the percentage of wilted vines (final disease rating). Covariance analysis determined that regression slope values were not significantly different between years; therefore data for both years were pooled and a generalized model was developed. Substituting area under the disease progress curve values for the percentage of wilted vines per plant values resulted in a higher coefficient of determination (P = 0.0001, r2 = 0.73, slope = 70.3, intercept = 92.6), indicating that use of the progress curve, which accounts for disease development over time, provides a better regression fit than does use of a variable that reflects measurement of disease incidence at discrete intervals. The regression analyses also indicated that even low numbers (1 per plant) of beetles during the 1st few weeks after colonization of plants will result in wilt symptoms.

Document Type: Research Article

Publication date: April 1, 1996

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