LIMITATIONS OF USING REGRESSION AND MEAN SEPARATION ANALYSES FOR DESCRIBING THE RESPONSE OF CROP YIELD TO DEFOLIATION - A CASE-STUDY OF THECOLORADO POTATO BEETLE (COLEOPTERA, CHRYSOMELIDAE) ON POTATO

The relationship between Irish potato tuber yield. Solanum tuberosum L ., and defoliation by the Colorado potato beetle, Leptinotarsa decemli neata (Say),was investigated and the use of regression and mean separa tion analyses as a basis for developing economic injury levels were co mpared. The type of statistical analysis used to describe this relatio nship greatly affected the interpretation of data. Mean separation (= multiple-comparison) procedures were inadequate to detect accurately d efoliation levels tolerated by potato, because these procedures did no t have significant statistical power to detect small reductions in yie ld. Similarly, regression analyses failed to identify a level of defol iation tolerable to potato because a consistent biological relationshi p between yield and defoliation could not be determined. For example, a quadratic term (nonlinear) in the analysis of variance was significa nt in only 2 of 8 experiments, and the relationship between 'Atlantic' and 'Superior' yield and defoliation in these experiments was concave , indicating that yield losses were greatest at low levels of defoliat ion. A plateau model (nonlinear) suggested that Atlantic potato could tolerate low levels of defoliation (1.8-13% in 4 of 6 data sets), but it did not fit data for the variety Superior. Although a negative line ar model provided a statistically significant fit to 7 of 8 data sets (average slopes [+/- SEM] for Atlantic and Superior were -0.12 +/- 0.0 1 and -0.15 +/- 0.03 T/ha per the percentage of defoliation. respectiv ely), it is likely that a linear model underestimates the actual level of defoliation that potato can tolerate, especially at low levels. Th e implications of these findings for the development of economic injur y levels and economic thresholds for the Colorado potato beetle on pot ato are discussed.