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

We examined the effects of potato leafhopper (Empoasca fabae) developmental stage and alfalfa (Medicago sativa) developmental stage on the physiological response of the plant to injury. We used radioactive carbon dioxide to label the photoassimilate stream and evaluate the phloem health of alfalfa. In one experiment, six first instar, four fourth instar, and three adult leafhoppers were caged by stage on single alfalfa stems for approximately one day. Only fourth instar nymphs significantly reduced the amount of label transported to injured tissues above the source of the labeled assimilate. First instar nymphs had no effect and adults reduced assimilate transport to stem tips, but this trend was not significant possibly because of confounding variables. However, injury by both first instar nymphs and adults resulted in greater concentration of labeled assimilate in portions of the stem below the feeding site. In another experiment, the developmental stage of alfalfa stems was central to the physiological response of alfalfa to leafhopper injury. A 20 h exposure to three adult leafhoppers significantly reduced the amount of label translocated to the tip and crown tissues of early vegetative plants, and to the crown tissue only of late vegetative plants. In reproductive plants, assimilate translocation was not affected by leafhopper injury. In a final experiment, we found no evidence of an effect on the photosynthesis of leaves of similar age and position to those used as source leaves in our translocation studies. Our findings contribute to our understanding of the physiological response of plants to injury by sap-feeding insects, and suggest the need for greater refinement of economic injury levels based on leafhopper and plant developmental stage.