Experiments were undertaken to determine the potential for using rubidium chloride (RbCl) or cesium chloride (CsCl) to mark southwestern corn borer, Diatraea grandiosella Dyar, for use in applied ecological studies. Maize, Zea mays L., plants were sprayed with aqueous solutions of RbCl or CsCl at rates of 100, 1000, or 10,000 μg/g and inoculated with D. grandiosella neonates. Rubidium and cesium were successfully absorbed and translocated in maize plants. There were only a few minor effects of the treatment on maize or on southwestern corn borers. Rb and Cs were detected in plants, but not in insects, by using flame atomic absorption spectrophotometry. Graphite furnace-atomic absorption spectrophotometry (GF-AAS) and neutron activation analysis (NAA) allowed identification of Rb and Cs in adults. Rb and Cs were detected by GF-AAS in feral unmarked adults, and they contained higher levels of Rb than Cs. Males and females contained similar amounts of Rb, but Cs levels were higher in males than in females. Adults recovered from field maize treated with 1000 μg/g Cs had higher levels of Cs than did those from untreated plants. Using NAA, neither Rb nor Cs was detected in adults recovered from greenhouse-grown untreated maize. Males and females recovered from maize treated with 1000 μg/g RbCl and CsCl contained similar amounts of Rb, but females contained more Rb than Cs. We conclude that application of 1000 μg/g RbCl or CsCl on plants is effective in marking adults of D. grandiosella with Rb or Cs and would be useful for mark–recapture dispersal studies.