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Bean Leaf Beetle Injury to Seedling Soybean: Consumption, Effects of Leaf Expansion, and Economic Injury Levels

Dep. of Entomology, Univ. of Nebraska, Lincoln, NE 68583-0816

^* Corresponding author (Email: entm014{at}unlvm.unl.edu).

The bean leaf beetle (BLB) [Cerotoma trifurcata (Forster)] is a major pest of seedling soybean [Glycine max (L.) Merr.], but little is known of the effects of seedling leaf expansion on final leaf tissue loss resulting from BLB injury. This field study was conducted to (i) determine a precise daily consumption rate of BLB on seedling soybean; (ii) determine the direct and indirect reduction of leaf area resulting from BLB injury and soybean leaf expansion; and (iii) develop economicin jury levels for BLB on seedling soybean. The study was conducted on a Sharpsburg silty clay loam in an ‘Elgin 87’ soybean field near Mead, NE. Experimental designs were completely random. Consumption rate treatments consisted of 2 beetles leaflet^–1, in the field and laboratory. Leaf expansion treatments consisted of an uninjured check and three levels of injury. Injury (gross removal of leaf tissue) was imposed at Vegetative Stage V3 on a lateral leaflet of the fourth trifoliate leaf. Mean BLB consumption of soybean was 0.31 cm² d^–1. An injury expansion coefficient matrixw as developedt o estimate final leaf tissue loss at late V3. Final estimated tissue loss resulting from one BLB feeding for 1 d on an unexpanded leaf was 2.74 cm². Final estimated tissue loss caused by one BLB feeding from Stage V1 through V3 (14 d) was 13.93 cm². Seedling soybean yield-loss relationships were used to calculate economic injury levels (EILs) for VC, V1, and V2 soybean. These EILs were more than twice the value of the current EIL of 1.33 beetles plants^–1.

Received for publication January 20, 1994.