CONTROL OF CONTAMINATION IN CORN CALLUS-CULTURES USED FOR INSECT RESISTANCE STUDIES

Bioassays using corn, Zea mays L., callus have been used successfully to differentiate among genotypes that are resistant or susceptible to leaf feeding by fall armyworm, Spodoptera frugiperda (J. E. Smith) and southwestern corn borer, Diatraea grandiosella Dyar. The use of such bioassays has, however, been limited by bacterial and fungal contamina tion of callus cultures following their infestation with insect larvae . This investigation was undertaken to determine whether transferring callus from petri plates containing a Murashige and Skoog medium amend ed with sucrose, agar, 2,4-dichlorophenoxyacetic acid-(2,4-D), and zea tin to 30-ml plastic cups containing water agar amended with gentamici n and sorbic acid prior to infestation with fall armyworm or southwest ern corn borer larvae would reduce contamination without diminishing o ur ability to differentiate among leaf feeding resistant and susceptib le corn hybrids. We found that 62% of the callus cultures on Murashige and Skoog medium were contaminated after infestation with fall armywo rm larvae while only 2% of the callus transferred to the plastic cups with water agar amended with gentamicin and sorbic acid were contamina ted. Both fall armyworm and southwestern corn borer larvae fed on call us of leaf feeding resistant corn hybrids were significantly smaller t han those fed on susceptible hybrids when the callus was placed in cup s with water, agar, gentamicin, and sorbic acid. Transferring callus f rom Murashige and Skoog medium to cups with gentamicin and sorbic acid prior to infestation with insect larvae appears to satisfactorily red uce contamination without affecting the growth of larvae fed on callus .