The navel orangeworm, Amyelois transitella, is not a worm.
It's a caterpillar, or more specifically, the larval form of a phycitid moth.
It is considered an agricultural pest, which, despite its name, is not known for its damage to navel oranges (although it has been known to lay its eggs in the navel end of these). NOW (as entomologists and farmers like to abbreviate it) is a serious pest damaging almonds, walnuts, and pistachio crops.
NOW will also attack figs, pomegranates, and citrus fruits, but where I live, in California, it does not appear in citrus groves, preferring instead to go after the bountiful (1 million acres) of nut trees in the Sacramento and San Joaquin Valleys.
Navel orangeworms invade the nut, damage the meat, and if left undetected long enough, consume the entire nut, leaving a shell filled with frass and webbing. Though a typical infestation may typically damage only a small percentage of a crop (0.5%, or with a severe population explosion, up to 3%), an infestation may invite more serious consequences: nuts opened by the orangeworms allow bacillus to move in, where it can produce aflatoxin. NOW itself carries an enzyme that breaks down the chemical, so it remains unaffected, while the nuts can become toxic.
NOW is controlled by insecticides and careful harvesting (removing "mummies" where moths, eggs, and larvae can overwinter). Recent studies of the moth's sex hormones, biochemistry, and mating cycles have led to deployment of traps using pheromones, leading to mating disruption.
Almond Board of California, http://www.almondboard.com/Growers/orchardmanagement/PestManagement/InsectsMites/NavelOrangeworm/Pages/Default.aspx (November 6, 2012)
Bob Curtis, "Focusing on NOW management," Western Farm Press. October 21, 2009. http://westernfarmpress.com/tree-nuts/focusing-now-management (November 6, 2012)
Kathy Garvey, "Navel orangeworms prefer pistachios." January 21, 2009. University of California Newsroom. http://www.universityofcalifornia.edu/news/article/19272 (November 6, 2012)
W.S. Leal, A.L. Parra-Pedrazzoli, K.-E.Kaissling, T. I. Morgan, F. G. Zalom, D. J. Pesak, E. A. Dundulis, C. S. Burks and B. S. Higbee. Unusual pheromone chemistry in the navel orangeworm: novel sex attractants and a behavioral antagonist. Naturwissenschaften Vol. 92, Number 3 (2005), 139-146, DOI:10.1007/s00114-004-0598-5 http://dx.doi.org/10.1007/s00114-004-0598-5
University of California Integrated Pest Management Online,
http://www.ipm.ucdavis.edu/PMG/r3300311.html (November 6, 2012)
http://www.ipm.ucdavis.edu/PMG/r605300111.html (November 6, 2012)