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

Abstract

Insect-induced injury to alfalfa, Medicago 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 saucia (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, Empoascafabae (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.

Alfalfa physiological response to potato leafhopper injury depends on leafhopper and alfalfa developmental stage

“…The entire stem was exposed to leafhoppers parts (e.g., tips) probably cannot be ascribed to differences in carbon fixation by leaves from healthy versus injured plants, but most likely are attributable to translocation disruption by potato leafhopper. The differences others have found in photosynthesis may be due to injury that is more severe and longer in duration (Womack, 1984;Hutchins et al, 1990;Flinn et al, 1990). Moreover, these studies examined whole plant rates of photosynthesis, whereas we measured photosynthesis in individual leaves and, in particular, only leaves at this distance below the apex.…”

“…The ability of vascular-feeding insects to disrupt nutrient movement and partitioning varies with the insect's growth and development. For alfalfa, older potato leafhopper stadia (third instar through adult stage) are more injurious to herbage biomass, nutrient content, photosynthesis, and transpiration rate and root non-structural carbohydrate levels than younger instars (Womack, 1984;Hower, 1989, Hutchins et al, 1990Flinn et al, 1990). As leafhopper development proceeds, their energy and nutrient demands increase as does their nutrient procurement abilities.…”

Alfalfa Consumption by Adult Clover Leaf Weevil (Coleoptera: Curculionidae) and Development of Injury Equivalents for Stubble Defoliators

“…Alfalfa stubble defoliators, such as alfalfa weevil adults and cutworms, consume dry matter, delay regrowth initiation and subsequent plant maturity, and reduce growth rates after defoliation (Hutchins et al 1990). The reduction in alfalfa vigor also encourages weed encroachment which may reduce forage quality and stand lonj!…”

Interactive effects of insects and ungulates on root growth in a native grassland

“…Although behavioral and physiological explanations are both plausible, we recognize they are not necessarily independent. Aside from changes in chemical defenses in response to herbivory, insect feeding can also alter the nutritive value of remaining (and regrowing) vegetation (Hutchins et al 1990). Forage quality is a common cue influencing ungulate forage selection (Bailey et al 1996), and thus plant chemical responses to insects could alter subsequent ungulate behavior.…”