Development of immature stages of Heteronychus arator rugifrons (Fairmaire, 1871) (Coleoptera: Dynastidae), a major crop pest in Madagascar

Abstract

Adults (black beetles), and to a lesser extent, larvae (white grubs), of Heteronychus arator rugifrons (Fairmaire) are major crop pests of cereals—particularly upland rice—in the central highlands of the island of Madagascar. Since the biology of this endemic subspecies is far less well understood than that of the African black beetle Heteronychus arator F., we undertook laboratory studies to determine the developmental durations of its immature stages at two temperatures, with measurements taken (body length and weight of all stages/instars, head capsule width of all larval instars). The mean immature stage development period was significantly shorter at 25°C (160.8 ± 11.60 d) than at 18°C (251.4 ± 11.93 d), mainly due to shorter egg incubation, L1 instar and pupal periods. Some morphological traits of immature stages confirmed that H. arator rugifrons is a distinct subspecies from H. arator. In particular, head capsule width was homogenous among larval instars, irrespective of the rearing temperature, with no overlap across instars. Head capsule width range was 1.04–1.52 for L1, 1.72–2.36 for L2, and 2.40–3.48 for L3. This information should be helpful in studies of population dynamics and economic threshold calculations with a view to management of this pest.