Sweep-net Sampling for Western Spotted Cucumber Beetle (Coleoptera: Chrysomelidae) in Snap Beans: Daily Fluctuations in Beetle Counts and Correlation Between Relative and Absolute Density Estimates

Abstract

Sweep-net sampling was evaluated as a method of sampling populations of adult western spotted cucumber beetles (WSCB), Dtabrotica undecimpunctata undecimpunctata Mannerheim, in snap beans, Phaseolus vulgaris L. Each of 18 fields was sampled initially between 0800 and 1000 hours; 1–6 subsequent sets of samples were collected later on the same date. Field means of beetle counts in sweep-net samples collected after the initial sampling episode (subsequent means) were significantly lower than the field mean of initial samples collected in the same field (initial mean) in 16 of 42 comparisons. Subsequent mean/initial mean ratios were negatively correlated ( R 2 = 0.72) with the amount of increase in solar radiation intensity between initial and subsequent sampling episodes. When fluctuations in beetle counts in sweep samples were described in relation to an average daily radiation curve, estimated counts were lowest for sampling conducted at 1300 hours. Time based correction factors used as multipliers to stabilize relative density estimates according to a standard (0900 hours) sampling time ranged from 0.9 for 1800 hours to 2.5 for 1300 hours. The linear relationship between corrected counts of WSCB in sweep-net samples and measures of WSCB absolute density (beetles per meter of row) was described by the equation: y = 0.44 + 2.55 x ( R 2 = 0.69), where x = number of WSCB per meter of row and y = corrected number of WSCB per 10 sweeps.