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Development of sweet corn hybrids having resistance to ear feeding by the European corn borer (ECB) (Ostrinia nubilalis, Hübner) would save growers millions of dollars annually. Sweet corn breeding lines have been developed which show resistance to ear feeding. To determine whether resistance in these lines was heritable, testcrosses were made between 6 resistant lines and 2 susceptible inbreds. The testcrosses, resistant lines, susceptible inbreds, and Jubilee, a standard susceptible hybrid, were planted in randomized complete blocks replicated 8 times at each of 2 locations. Primary ears were infested, on the ear tip, at mid-silk (R3) with approximately 50 neonate larvae and were evaluated for damage 25 - 30 days later using a 9 point scale. The resistant parents and their testcrosses had 15% to 64% more resistant ears (less than 1% kernel damage at the tip only) than Jubilee. The inbreds and their commercial hybrid did not differ in resistance from Jubilee. Resistance to ECB appears to be polygenic and it has been suggested that some level of non-additive gene action is important. While the specific traits responsible for ear resistance are not known, the resistance of the breeding lines was transmitted to the testcrosses.
Concern over insecticide usage for control of European corn borer (Ostrinia nubilalis Hübner) in sweet corn (Zea mays L.) in recent years has increased the need for genetic control. Our objectives were to determine the degree of ear feeding resistance transmitted by resistant breeding lines to testcrosses and to investigate the relationship between resistance and both ear silk channel length, and infestation level. Testcrosses averaged 35% acceptable ears compared to 7%. for two commercial control hybrids and 45% for the lines per se when artificially infested at the ear tip at mid-silk at two locations. Generally, resistance in the testcrosses was closer to the resistance level of the resistant parent, indicating good combining ability for resistance. Heterosis above the resistant parent was found in 8 of 12 testcrosses. Across entries, ears having relatively longer silk channel length tended to have less damage but the relationship was not strong (r 2 = 0.24, P < 0.01). When silk channels were artificially shortened at infestation, resistance levels were lower, but five of seven lines had a higher proportion of acceptable ears than did the control hybrid. Across increasing levels of infestation from 50 to 200 neonate larvae per ear eight of nine lines had higher resistance (P < 0.05) than the control.
Insect resistance in vegetable crops carries a new urgency as insecticide availability and usage become more restricted. The European corn borer (ECB) long has been the most costly sweet corn pest in the northern states, with corn earworm (CEW) and southwestern stalk borer most serious farther south. Resistance to ECB and CEW, as developed by classical methods, will be discussed. Newer methods (such as RFLPs) should speed transfer of this resistance to commercial stocks, and also facilitate transfer of resistance from other species. Gene modification and endophyte exploitation are longer-term possibilities. Resistance must be free from association with undesirable traits, such as long silk channel length (r = 0.3 to 0.4), unless consumer and processor specifications change.