Spatial and temporal dynamics of the threecornered alfalfa hopper (Homoptera: Membracidae) on soybeans

Within-plant distribution and within-field dispersion of Spissistilus festinus (Say) nymphs on conventionally planted and tilled soybeans, Glycine max (L.) Merrill, were quantified over a 2-yr period at two experimental sites. Within-plant distribution (vertical and horizontal location) was more closely associated with vegetative development in 1986 than in 1987. The mainstem node with the most nymphs, designated the most likely node (MLN), represented the location of 50% or mote of all nymphs through 4 July (V6-V7) and at least 30% through 10 August (Vl3-Vl8). From plant emergence until first bloom (Rl), 100% of the nymphs were found on either the mainstem, mainstem petioles, or the first lateral branch node. However, from Rl until the end of the season, significantly more nymphs were found on mainstem petioles, and twice as many nymphs were associated with lateral branches than with the mainstem. Higher nymphal densities and a greater proportion of infested plants occurred in 1986 than in 1987. Indices of dispersion calculated from individual plant samples suggested greater aggregation of nymphs than did values reported for larger sample units such as the ground cloth. Iterative nonlinear regression of proportion infested plants against mean density allowed estimation of Taylor's coefficients that were used to develop sequential sampling plans appropriate for enumerative and presence-absence methods. Results indicated that nymphs did not reach damaging levels during any of the sampling periods.