Diel Patterns of Threecornered Alfalfa Hopper (Homoptera: Membracidae) Nymphs on Soybean

Diel patterns of intra- and interplant movement, vertical distribution, molting, tending by ants, and attrition of Spissistilus festinus (Say) nymphs on soybean (Glycine max (L.) Merrill) were investigated by observing nymphs at 4-h intervals in 24-h observation sessions. Both intraplant (defined in relation to main-stem nodes) and interplant movements occurred most often between 0800 and 1200 h and least often between 2400 and 0400 h (CDT). Most (80.9%) intraplant movements covered short distances (<4 nodes). The frequency of intra- and interplant movements usually increased with nymphal age (instar). Periodicity of nymphal movement did not result in periodicity in vertical distribution. Most molts occurred between 0400 and 0800 h (85%) when daily high leaf temperatures were ≥ 31.8° C or between 0800 and 1600 h (86%) when high temperatures were ≤ 30.1° C. The frequency of tending by ants was reduced or absent at 1200 and 1600 h at two sites, but periodicity at other sites was not detected. Differences in tending among S. festinus instars were observed but trends were different among sites. Attrition (found dead, killed by predators, or not located) of nymphs was greatest between 0800 and 2000 h, and least between 2400 and 0400 h. Observed predation accounted for only 13% of the total attrition but underestimated actual predation.