Fall soil sampling method for predicting spring infestation of white grubs (Coleoptera: Scarabaeidae) in corn and the benefits of clothianidin seed treatment in Virginia

Field studies were conducted in eastern Virginia from 2005 to 2008 to predict spring infestation levels of white grubs (Coleoptera: Scarabaeidae) in the soil and the subsequent benefit of seed treatment insecticides on corn, Zea mays L., in those fields. The predominant annual white grubs found in this study were Popillia japonica Newman, Maladera castanea (Arrow), and Cyclocephala spp. Forty-eight fields (i.e. blocks) were sampled in fall and again in spring in several counties over three years. Sampling involved removing a standard volume of soil from multiple locations in each field and visually inspecting it for white grubs and other soil insect pests. In the first year, a 20.3 × 20.3 cm square × 15 cm deep soil sample (compact method) was evaluated for its potential to correlate to a larger 30 × 30 cm square × 15 cm deep soil sample (standard method). No significant differences in white grub numbers were detected between the two sampling methods after correcting for differences in sampling volume using a 2.25 weighting factor. Strong correlations (r values >0.88) were observed between fall and spring sample densities of white grubs in each of the three years, indicating that fall sampling could be used to reliably predict spring infestations. Taylor's power law revealed among fall sampled fields that at least six and as many as 15 compact samples per field (95% confidence) were needed to be within 25 and 15% of the actual mean, respectively. Using the compact method, the action threshold was estimated to be ≥1.6 white grubs per soil sample in the fall and ≥1.04 white grubs per soil sample in the spring. In fields with above-threshold white grub densities, there was a significant effect of clothianidin seed treatment on stand in two of the three years, and a significant grain yield difference in 2006.

► No difference in pest density between compact and standard soil sample method.
► The compact method was accurate at predicting yield loss in fields above threshold.
► No yield differences in the control or treated plots among below-threshold fields.
► A significant yield advantage of high rate clothianidin treated seed was observed.