Larvae of Liogenys suturalis (Blanchard, 1851) (Coleoptera: Scarabaeidae: Melolonthinae) are often found feeding on the roots of plants in the central-western region of Brazil where it is major crop pest. Herein we describe the third instar and investigate the mating behavior of L. suturalis. Adult insects were collected with light traps from September to December during 2011–2014 at the State University of Mato Grosso do Sul, in Aquidauana and Cassilândia, Mato Grosso do Sul, Brazil. In the laboratory, eggs were gathered from ovipositing females and used to initiate a rearing system to obtain third instars. The larvae were preserved in alcohol prior to preparing the description. Mating pairs were formed to observe mating behavior. Adults mated from 18:00 to 23:00 h, and a higher number of matings was observed between 18:00 and 20:00 h. Adults were found in the field feeding on leaves of Schinus terebinthifolia Raddi (Anacardiaceae).