Multistate validation of node-based sample units to estimate soybean aphid whole-plant density (Hemiptera: Aphididae)

Soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an important pest of soybean, Glycine max (L.) Merrill in the North Central United States and southern Canada. Sampling to estimate whole-plant densities of A. glycines is time intensive. To decrease this cost, methods were previously developed to estimate whole-plant A. glycines densities based on counts from subsets of nodes rather than whole-plant counts. Here, we validate the accuracy of fourteen different node-based sample units from data collected over multiple states and years (six state-years). A. glycines densities were recorded separately by node in all 95 fields sampled. Node-based sample units were used to calculate aphid counts on the remaining plant parts (i.e., those not included in the sample unit). Linear regression was performed on the predicted and observed counts of aphids on the remaining plant parts. Preliminary analyses indicated that use of the single node with the highest aphid count (N_MAX) was the best sample unit for estimation of A. glycines counts on remaining plant parts (r²>0.92). For sample units comprised of more than one node, prediction of aphid counts on remaining plant parts was improved by using a weighted formula compared to a node-averaged formula for calculation of number of aphids on remaining parts. Accurate estimates of whole-plant aphid densities can be obtained using node-based sample units. Future work will include development and validation of a sampling plan for estimating soybean aphid density at the field or plot level using node-based sample units.