DEVELOPMENT OF A SAMPLING METHOD FOR FIRST INSTAR OIKETICUS-KIRBYI (LEPIDOPTERA, PSYCHIDAE) IN OIL PALM PLANTATIONS

The current study was conducted to develop a cost-effective and reliab le method to estimate population densities of Ist-instar bagworms, Oik eticus kirbyi (Guilding), in Costa Rican oil palm plantations. Total n umbers of larvae per tree were correlated with numbers on SO, 160, 240 , or all (320-360) folioles of all palm leaves. The 1-2 values of thes e correlations were used to assess the accuracy of population estimate s obtained by sampling leaves in different positions within the palm c rown or different numbers of folioles per leaf. Larvae were most abund ant on apical and subapical folioles of leaves in the upper palm crown , When a limited number of folioles per leaf(SO, 160, or 240) was samp led, leaves in the upper crown provided the most accurate estimates of larval densities per tree. Decreasing accuracy of population estimate s from upper to lower leaves resulted from increasing variability of l arval proportions from upper to lower leaves. Larval populations of 2 other oil palm defoliators, Opsiphanes cassina (Felder), and Stenoma c ecropia (Meyrick), are currently assessed by sampling leaf 17. Samplin g O. kirbyi larvae on 160 folioles of leaf 17, in the middle palm crow n, may represent an acceptable compromise among cost-efficiency, relia bility, and simultaneous assessments of several defoliating caterpilla rs. Reasonably accurate estimates of larval densities per area obtaine d in this study by sampling 1 palm per hectare should not be interpret ed as a general recommendation for sample size, because the effect of population density on optimal sample size per hectare remains yet to b e investigated.