Western corn rootworm (Diabrotica virgifera virgifera LeConte) resistance management for transgenic (or Bt) corn hinges on understanding the mating behavior and biology of this adaptable insect pest. During mating, the male transfers sperm and additional, previously uncharacterized material, to the female in the form of a spermatophore. We investigated the composition of rootworm spermatophores. Proteins were found to be a major component, and the stable isotope ¹⁵N was used to assess the fate of spermatophore nitrogen in mated female beetles and their eggs. We also performed longevity studies on mated and virgin females under three different diet treatments and investigated the relationships between morphometric characteristics and spermatophore volume of mating pairs of beetles. The stable isotope analysis determined that nitrogen provided to the female in the spermatophore was incorporated into the eggs. We found that virgin female beetles on a corn diet lived significantly longer than mated female beetles on the same diet. There were significant positive relationships between male size parameters (head capsule width, pronotum width, and elytral length) and spermatophore volume, and ampulla and spermatophylax volume.