Abstract: The oak longhorn beetle, Massicus raddei (Blessig) (Coleoptera: Cerambycidae) is an important wood borer pest in natural oak forests (Quercus spp.) in northeastern China. This beetle takes 3 years to complete one generation and is difficult to be controlled due to a highly concealed life history in its larval and pupal stages. Determination of larval instar number and duration is the important foundation for the insect pest forecasting and the scientific management. The number of larval instars was determined using statistical method of frequency analysis through sampling periodically by dissecting infestation trees at Kuandian County, Liaoning Province during 2008-2011, during which the oak longhorn beetle larvae at different developmental stages in forests were collected and five morphological variables of the longhorn beetle larvae, including length of mandible, distance between main ocelli, width of the prothoracic plate, length of the mesothoracic spiracle, and body length, were measured under binocular microscope with an eyepiece micrometer. The results showed that the larvae of oak longhorn beetle have 6 instars without difference between male and female. The three variables including mandible length, distance between main ocelli and width of the prothoracic plate can be used for the separation of larval instars as the sclerotized structures, while length of the mesothoracic spiracle and body length were not reliable due to high variation. The average duration of 1-6 instars measured by the method of median population stadia in fields were 9.25, 266.85, 48.09, 51.29, 260.33 and 385.71 d, respectively. The total larval stage duration was over 1 021.52 d in fields. In northeastern China, the occurrence of oak longhorn beetle was very uniform and the development of population was highly synchronous in natural conditions. The larvae of the oak longhorn beetle need to undergo 3 winters to complete one generation. In the first year larvae overwinter as 2nd-3rd instar stage, the next year the 4th-5th instar larvae overwinter and the third winter all larvae enter the fullgrown larval stage. These findings further reveal the larval biology of the oak longhorn beetle and provide valuable information for the rational control of the pest in practice.

Key words: Massicus raddei, larva, morphological characters, instar number, instar duration, Dyar’s rule, population synchrony