title: Induced plant defenses, host-pathogen interactions, and forest insect outbreaks.
abstract: Cyclic outbreaks of defoliating insects devastate forests, but their causes are poorly understood. Outbreak cycles are often assumed to be driven by density-dependent mortality due to natural enemies, because pathogens and predators cause high mortality and because natural-enemy models reproduce fluctuations in defoliation data. The role of induced defenses is in contrast often dismissed, because toxic effects of defenses are often weak and because induced-defense models explain defoliation data no better than natural-enemy models. Natural-enemy models, however, fail to explain gypsy moth outbreaks in North America, in which outbreaks in forests with a higher percentage of oaks have alternated between severe and mild, whereas outbreaks in forests with a lower percentage of oaks have been uniformly moderate. Here we show that this pattern can be explained by an interaction between induced defenses and a natural enemy. We experimentally induced hydrolyzable-tannin defenses in red oak, to show that induction reduces variability in a gypsy moth's risk of baculovirus infection. Because this effect can modulate outbreak severity and because oaks are the only genus of gypsy moth host tree that can be induced, we extended a natural-enemy model to allow for spatial variability in inducibility. Our model shows alternating outbreaks in forests with a high frequency of oaks, and uniform outbreaks in forests with a low frequency of oaks, matching the data. The complexity of this effect suggests that detecting effects of induced defenses on defoliator cycles requires a combination of experiments and models.
authors: Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803.
authors: Bret D
authors: BD
authors: Elderd
authors: Brian J
authors: BJ
authors: Rehill
authors: Kyle J
authors: KJ
authors: Haynes
authors: Greg
authors: G
authors: Dwyer
mesh_terms: D000818:Animals; D016367:Baculoviridae / Q000472:pathogenicity; D017753:Ecosystem; D054884:Host-Pathogen Interactions / Q000276:immunology; D047348:Hydrolyzable Tannins / Q000276:immunology / Q000378:metabolism; D007313:Insecta / Q000472:pathogenicity*; D008954:Models, Biological; D009036:Moths / Q000472:pathogenicity / Q000821:virology; D009656:North America; D010935:Plant Diseases / Q000276:immunology / Q000469:parasitology*; D029963:Quercus / Q000276:immunology / Q000469:parasitology; D014197:Trees / Q000276:immunology / Q000469:parasitology*
chemical_list: D047348:Hydrolyzable Tannins
keywords: Lymantria dispar; complex population dynamics; host-pathogen model; hydrolyzable tannins; spatial model
references: J Chem Ecol. 1988 Feb;14(2):453-61
references: 24275992
references: Ecology. 2010 Jan;91(1):106-18
references: 20380201
references: Nature. 2004 Jul 15;430(6997):341-5
references: 15254536
references: Science. 1995 Aug 25;269(5227):1112-5
references: 17755536
references: Oecologia. 1993 May;94(2):195-203
references: 28314032
references: Trends Ecol Evol. 2006 May;21(5):278-86
references: 16697914
references: Am Nat. 2005 Jan;165(1):16-31
references: 15729637
references: Am Nat. 2008 Dec;172(6):829-42
references: 18976065
references: Am Nat. 2012 Mar;179(3):E70-96
references: 22322229
references: Am Nat. 2008 Nov;172(5):613-24
references: 18821838
references: Am Nat. 2000 Aug;156(2):105-120
references: 10856195
references: Proc Natl Acad Sci U S A. 1998 Jul 7;95(14):8113-8
references: 9653149
references: Science. 1982 Jul 9;217(4555):149-51
references: 17770257
references: Ecology. 2006 Feb;87(2):291-6
references: 16637353
references: J Chem Ecol. 2004 Jul;30(7):1363-79
references: 15503525
references: J Pharm Sci. 1964 Apr;53:364-8
references: 14189925
references: Am Nat. 1997 Dec;150(6):685-707
references: 18811331
references: Phytochemistry. 2011 Sep;72(13):1551-65
references: 21354580