Submitted to: Archives of Insect Biochemistry and Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/13/2002
Publication Date: 12/20/2002
Citation: Schmelz, E.A., Grebenok, R.J., Ohnmeiss, T.E., Bowers, W.S. 2002. Interactions between spinacia oleracea and bradysia impatiens: a role for phytoecdysteroids. Arch. Insect Biochem. 51:204-221. Interpretive Summary: In response to insect attack, many crop plants induce chemical changes that serve to directly or indirectly reduce further damage by insect herbivores. An interesting chemical defense strategy of spinach plants is the production of insect molting hormones, termed phytoecdysteroids. Scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL, University of Arizona, and Canisius College have discovered that root herbivory by insects rapidly increases the level of phytoecdysteroids synthesized and accumulated in the roots. Using fungus gnat larvae as root herbivores, it was demonstrated that root damage by insects results in sufficient increases in phytoecdysteriods to help protect plant roots against further attack. Phytoecdysteriods are highly specific insect feeding deterrents and can result in developmental abnormalities and death when ingested by non-adapted insects. Phytoecdysteriods hold promise as control agents against selected insect pests due to their extremely low mammalian toxicity.
Technical Abstract: Plant produced insect molting hormones, termed phytoecdysteroids (PEs), are thought to function as plant defenses against insects by acting as either feeding deterrents or through developmental disruption. In spinach (Spinacia oleracea), 20-hydroxyecdysone (20E) concentrations in the roots rapidly increase following root damage, root herbivory, or methyl jasmonate (MJ)applications. In this inducible system, we consider the plant defense hypothesis by examining interactions of roots, 20E concentrations, and larvae of the dark-winged fungus gnat (Bradysia impatiens). Root herbivory by B. impatiens larvae resulted in a 4.0- to 6.6-fold increases in root 20E concentrations. In paired-choice tests, increases in dietary 20E stimulated B. impatiens feeding deterrency. B. impatiens larvae preferred control diets, low in 20E, to those constructed from induced roots and those amended with 20E (25 to 50 microgram/gram wet mass). When confined to 20E treated diets, concentrations as low as 5 microgram/gram (wet mass) resulted in significantly reduced B. impatiens survivorship compared to controls. The induction of root 20E levels with MJ resulted in a 2.1-fold increase in 20E levels and a 50% reduction in B. impatiens larvae establishment. In a paired-choice arena, untreated control roots were damaged significantly more by B. impatiens larvae than MJ-induced roots that contained 3-fold greater 20E levels. Based on dietary preference tests, the 20E concentrations present in the MJ-induced roots (28 microgram/gram wet mass) were sufficient to explain this reduction in herbivory. Interactions between spinach roots and B. impatiens larvae demonstrate that PEs can act as inducible defenses and provide protection against insect herbivory.