Methods of Artificially Infesting Corn with the Corn Earworm and Factors Influencing Resistance1

Abstract

Corn cars artificially infested with laboratory-reared 1-day-old corn earworm larvae, Heliothis zea (Boddie), produced nearly 100% infestation, and damage ratings were higher than under conditions of natural infestation. Although the application of 6 and 10 larvae per ear decreased the percentage of uninfested ears and increased damage grades, 3 larvae per ear appear adequate for differentiating resistance. Application of eggs and second and third-instar larvae did not appear adequate for artificial infestation of ears. Clipping the husk and silk of susceptible strains generally did not increase damage grades, indicating that larvae did not prefer the silks in these strains and moved rapidly to the grain. Under some conditions resistance of TIIS×T15S is probably due to silk balling in the silk channel. The very long husk of P.I. 186225 is the main reason for its high resistance under natural conditions. The resistance of P.I. 217413 (Zapalote Chico) is due to a tight, tough husk, and perhaps some form of silk resistance; a resistance factor in the grain was also indicated.