Effects of Girdling by the Threecornered Alfalfa Hopper on Host Plant Growth and Physiology

Abstract

The threecornered alfalfa hopper, Spissistillus festinus (Say), is a major pest to production of a wide variety of crops. Herbivory by this insect is often highly detrimental because of girdling of petioles and shoots. Although girdling by this hopper has been recorded on a variety of hosts, the physiological effects of girdling have been examined primarily on one host (Glycine max). We examined the physiological effects of girdling by four densities of hoppers on Arachis hypogaea (L.) cv. Florarunner. Densities of 0, 2, 4, and 6 hoppers per plant were maintained for a 1-week period on peanuts grown in cages in a greenhouse. Effects of hopper herbivory on growth (shoot elongation and increases in plant dry weight) and whole-plant chemistry (carbon, nitrogen, and amino acid analysis) were determined at the end of the 1-week feeding experiments, and again at 2-week intervals until maturation of plants. Differences in plant growth or chemistry were not apparent at the conclusion of the feeding experiment. However, plants subjected to the highest rates of herbivory showed pronounced deleterious effects 2 to 6 weeks after girdling had occurred. Mean shoot growth was decreased by nearly 40% and plant dry weight was reduced by roughly 20%. Foliar nitrogen concentrations were also significantly reduced; peanuts subjected to high rates of herbivory contained 30% less foliar protein that control plants. This delayed response to girdling appeared to be in part attributed to increased rates of shoot and petiole breakage well after girdling occurs. Girdles became more brittle as shoots matured and hardened with maturity. Effects of girdling may be particularly detrimental to yield, as effects are most pronounced as plants are entering the reproductive stage.