ESTIMATING APHIDOPHAGOUS COCCINELLID POPULATIONS IN ALFALFA

Precise yet time-efficient sampling methods for coccinellids would be useful in integrated pest management (IPM) research and decision-makin g in alfalfa. We compared quadrat sampling, removal sampling, counting coccinellids for a constant amount of time while walking slowly throu gh a field (timed counts), and sweepnet sampling for estimating adult and larval coccinellid density in alfalfa. Removal sampling gave biase d estimates of density of adult Coleomegilla maculata Degeer, but gave acceptable estimates of density of adults of other species. Regressio n models were developed to convert relative population estimates of ad ult and larval coccinellids obtained by sweepnet and timed count sampl ing to absolute population estimates (number per m(2)). For adults, pl ant height was included in the best regression models for converting s weepnet catch and timed counts to population density, For larvae, plan t growth stage was included in the best regression model for convertin g sweepnet catch to density. The number of aphids per 10 sweeps was in cluded in the model for converting timed counts of larvae to density; however this model was of little value because it explained only 13% o f the variation in the relationship between timed counts and populatio n density. Coefficients of determination of regression models for esti mating adult density from timed count and sweepnet sampling were 0.90 and 0.93, respectively. The model for converting sweepnet catch of lar vae had a coefficient of determination of 0.71. In terms of statistica l precision achieved per unit time spent sampling, a 25-sweep sample w as the most efficient sample unit for estimating adult density. A doub le la-min removal sample was most efficient for sampling larvae, but t his method gave biased estimates of larval density, and overall, sweep net sampling was better.