Corn earworm is the most destructive pest of sweet corn (Del Valle and Miller, 1963; Pimentel et al., 1997; Snyder, 1967; Yadav, 1980) and one of the costliest of all crop pests in North America (Huang, 2015). The migratory nature of corn earworm makes it unpredictable and challenging to manage on a farm scale, and climate change is likely to expand its overwintering range, increase the number of generations per year, and cause earlier infestations. Corn earworm is ubiquitous; it feeds on more than 200 host plants, including important crops as well as weedy and uncultivated plants (Hardwick, 1965; Huang, 2015; Kennedy and Storer, 2000; Neunzig, 1963; Olmstead et al., 2016; Sudbrink and Grant, 1995). This ubiquity limits the ability to use cultural controls such as crop rotation and increases the pest’s adaptability and probability of successful development (Kennedy and Storer, 2000; Olmstead et al., 2016). The oviposition and feeding habits also limit the efficacy of chemical controls (Barber, 1941; Cook et al., 2003, 2004). Plant breeding remains one of the few promising tools for reducing the economic damage caused by corn earworm.
Corn earworm is an especially challenging pest for organic sweet corn growers, because few effective management strategies are permissible under the National Organic Program (Cook et al., 2003, 2004; Ni et al., 2011). Organic producers are in particular need of breeding for resistance to corn earworm since the advent of transgenic Bacillus thuringiensis cultivars has supplanted classical breeding for earworm resistance. However, novel forms of corn earworm resistance are of interest to conventional farmers as well, since increasing reports of Bacillus thuringiensis resistance indicate a limited lifespan for one of their main management tools (Reisig and Reay-Jones, 2015; Tabashnik and Carriere, 2015; Tabashnik et al., 2009).
As early as the 1910s, plant breeders have examined husk characteristics as potential mechanisms of resistance to the corn earworm, including husk thickness, husk texture, and presence or absence of flag leaves (Collins and Kempton, 1917). Longer husk extension past the ear tip has been proposed as a resistance mechanism either by increasing the distance the larva must travel before it can feed on the ear and/or due to availability of non-kernel food for the larva to consume (Barber, 1944; Collins and Kempton, 1917; Kyle, 1918). Husk tightness has been considered primarily as a physical barrier: a tight husk leads to tightly bunched silks, which may slow or halt larval penetration to the ear tip. The combination of long and tight husks may improve this physical resistance (Barber, 1941, 1944; Phillips and King, 1923). Husk extension and tightness also have been thought to interact with the cannibalistic behavior or corn earworm larvae to improve resistance. A tighter husk with a “tube-like” silk channel increases the likelihood the larvae will meet and reduce the population through cannibalism (Barber, 1936).
Over the decades, many studies have evaluated husk extension and tightness as corn earworm resistance factors, but results have been mixed. Some studies have shown strong negative relationships between corn earworm damage and husk extension, husk tightness, or both (Collins and Kempton, 1917; Del Valle and Miller, 1963; Douglas, 1947; Kyle, 1918; Ni et al., 2007, 2008). However, a number of studies have shown an inconsistent or absent relationship between corn earworm damage and these husk characteristics (Ni et al., 2012; Painter and Brunson, 1940; Snyder, 1967; Yadav, 1980), or even a positive relationship with corn earworm damage (Widstrom et al., 1970).
One major challenge in assessing husk traits as a corn earworm resistance mechanism is the presence of significant epistatic and environmental interactions. Several of the aforementioned authors (Del Valle and Miller, 1963; Painter and Brunson, 1940; Widstrom et al., 1970) explained the contradictory evidence on husk traits and earworm damage as effects of genetic background and interactions with other traits that confer resistance or susceptibility. In addition to these epistatic interactions, several studies indicate interactions between genotype and both environmental and management factors (Barber, 1944; Del Valle and Miller, 1963; Dicke and Barber, 1944; Douglas, 1947; Wiseman et al., 1970).
In addition to the inconsistent relationship between husk traits and corn earworm resistance, one challenge in breeding sweet corn for longer husks in particular has been an inverse relationship between husk length and yield. Butron et al. (2002) noted that longer husk extension past the ear tip has frequently been achieved by shortening the ear rather than actually lengthening the husk.
The goals of this research were to determine the feasibility of selecting a sweet corn population for longer husks without shortening the ears, to determine whether direct selection for longer husks indirectly confers improved resistance to the corn earworm, and finally to compare husk extension, ear length, earworm infestation rate, and extent of damage of each cycle of selection to commercial cultivars used on organic farms in Wisconsin.
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