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Impact of Insect Feeding on Alfalfa Regrowth: A Review of Physiological Responses and Economic Consequences

DowElanco, 9002 Purdue Road, Indianapolis, IN 46268-3030
Dep. of Entomol., Georgia Exp. Stn., Griffin, GA, 30223
Dep. of Entomol., Iowa State Univ., Ames, IA 50011

^* Corresponding author.

Insect-induced injury to alfalfa, Medicugo sativa L., has been a frequent topic of research. The central focus of many investigations has been the physiological response of alfalfa to insect feeding, measured as altered root carbohydrate levels, canopy development, and forage feeding value. Leaf-mass consuming insects generally either remove leaf tissue in aerial portions of established canopies (e.g., alfalfa weevil, Hypera postica [Gyllenhal]) or consume new foliage during early regrowth (e.g., variegated cutworm, Peridroma suucia [Hubner]). In both instances, injury results in an initial reduction in leaf-to-stem biomass (L/S) ratio. Continued defoliation, however, affects canopy photosynthesis to the point that stem growth is slowed. Assimilate-removing insects (e.g., potato leafhopper, Empoasca fabae [Harris]), disproportionately reduce stem growth compared to leaf growth, resulting in a higher L/S ratio. Although specific host response varies across the range of piercing-sucking insects, herbage feeding value per plant mass is generally enhanced with the increased L/S ratio. Overall, insect-induced changes in yield and quality from leaf-mass and assimilate removal consequentially reflects a delay in maturity. From a crop management perspective, slowed phenological development may alter harvest schedules or disrupt necessary root carbohydrate regeneration. Specific knowledge of host response, combined with current economic circumstances, should serve as a basis for developing economic injury levels to manage insect pests.

Received for publication December 30, 1989.

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