Spatial distribution and optimum sample size for monitoring of water lily aphid Rhopalosiphum nymphaeae (L.) in Makhana Euryale ferox Salisb

Water lily aphid Rhopalosiphum nymphaeae L. (Hemiptera: Aphididae) is the major pest of Makhana/Gorgon nut Euryale ferox Salisb, which is an important aquatic crop of tropical and subtropical regions of South-East and East Asia. The basic information such as distribution pattern and optimum sample size for estimating population density in the field is unknown. Therefore, we studied spatial distribution patterns for two stages, i.e., nymphs and adults of R. nymphaeae , using five different distribution indices and two regression methods viz., Taylor’s power law and Iwao’s mean crowding index for two years. All the indices showed aggregated distribution by nymphs and adults of R. nymphaeae in both years individually and with the pooled data of two years. The values of aggregation parameter (b) of Taylor’s power law and density contagiousness coefficient (β) of Iwao’s mean crowding index were found to be more than one for both nymphs and adults for each of the years, and pooled data of two years indicated an aggregated distribution of the R. nymphaeae . The optimal sample size (n) was estimated at three fixed precision levels of 0.05, 0.10, and 0.25 with Iwao’s regression coefficients. The results showed that the optimum sample size decreased with increased levels of precision and mean aphid density. The optimum sample size (n) for estimating the lowest (3.3/leaf) and highest density (217.6/leaf) of both nymphs and adults ranged from 305.2 to 26.3, 76.3 to 6.6, and 12.2 to 1.0 with 5, 10 and 25% levels of precision. This basic information on spatial distribution pattern and optimum sample size helps monitor and formulate effective management strategies against R. nymphaeae .