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The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte and northern corn rootworm (NCR) Diabrotica barberi Smith are key pests of corn in the north-central United States (Chiang, 1973; Levine and Oloumi-Sadeghi, 1991). Both species are commonly found as larvae in the same cornfields in areas where geographic distributions overlap. Attempts to distinguish larvae of each species using external characters have proven to be difficult. Mendoza and Peters (1964) reported that differences in anal plate (pygidial shield) morphology could be used to separate larvae of each species. The WCR generally has a notched, dark brown anal plate whereas the anal plate of the NCR is oval and pale. However, Piedrahita et al., (1985) found that these characteristics were not consistent in second-third instar larvae reducing the diagnostic utility of the characters (i.e., third instar NCR were misidentified as WCR 52% of the time). Electrophoretic and molecular techniques have proven to be more reliable diagnostic tools. Piedrahita et al., (1985) surveyed 20 enzyme systems and identified differences among NCR and WCR larvae using horizontal starch electrophoresis. Clark et al., (2001) and Roehrdanz (2003) utilized PCR-RFLP and multiplex PCR respectively, to identify mitochondrial differences among each species. This paper reports an alternative and reliable way to differentiate WCR and NCR larvae using morphological characters on the sclerotized head capsule of each species.