Agromyza oryzae (Munakata, 1910)
Closely related to the European species A. alunulata Hendel, 1931 (= distorta Griffiths, 1955). Larva in Hering, 1956. Arrangement of anterior spiracles and mouth hooks nearly identical and also male genitalia very similar. The larval characters are diagnostic and provide clear differences to identify the two species which have no overlap in distribution.
IMPORTANT CHARACTERS
Male terminalia
Wing length: 2.5 - 3 mm. Aedeagus and male postabdomen are shown in (Ag oryzae postabd.pct).
Immature stages
Larvae and puparia were carefully described and illustrated by Sasakawa, 1961 (Ag oryzae larva.pct): Each mandible with two large and three smaller mouth hooks (Ag oryzae larva.pct). According to the author the right mandible is somewhat larger than the left one. A small patch of hairs is situated at base of mandibles. Cephalopharyngeal skeleton similar to the one in albipennis with long ventrally convex intermediate sclerite and a rather narrow dorsal arm of the basal part (Ag oryzae larva ceph.pct).
Posterior spiracles with three opening slits, without spiracular hairs. Both spiracles are situated at the end of a segment-like articulated shaft. The anterior spiracles are of unusual shape among Agromyza species: On the large nearly hemispherical surface up to 150 tiny openings can be found (fig. from Sasakawa). The development of many small spiracular openings could be an adaptation to the frequently inundated ground where pupariation takes place (Spencer, 1973)
Puparium short and broad, from lateral view the ventral side appears straight and the dorsal side strongly convex. There are some distinct differences between hibernating and non-hibernating specimens. The former, hibernant, type is more convex and shows peculiar transverse keels on some segments (Ag oryzae puparium.pct).
BIONOMICS
The lifestyle of oryzae was studied by Kuwayama, 1950: In northern Japan the adults emerge at the end of May or early June strongly depending on the weather conditions. Three generations occur per year. Only a part of the puparia of the second generation emerge in the same season to produce the third generation. The remaining puparia hibernate and emerge in the subsequent year. The last generation lives almost exclusively on the second host plant Zizania, whereas the first ones on both plants (Klimanova, 1971).
The eggs are laid at the tip of the leaves, as a consequence the hatching larva has to mine downwards. When fully grown the larva produce a mine extending over almost the entire width of the leaf. The larval stage lasts from 11-44 days.
Puparia of the first, non-hibernant, generation can be found mainly on the host plant but not within the mine. So do the hibernant puparia of the subsequent generations but Kuwayama, 1950 reported that these puparia adhere not as tightly as the non-hibernant puparia. So they can easily fall down where they can hibernate on the ground, frequently covered by dead plant matter.
The pupation on the plant and not in the soil must be interpreted as an adaptation to the temporary inundation of paddy fields (Spencer, 1973).
HOST PLANTS
Oryza sativa L. (rice, paddy), Zizania latifolia Turez.
DISTRIBUTION
Northern Japan, eastern Siberia (Spencer, 1973), China: District Zhejiang (Li et al., 1994), Central and South Korea (Suh and Kwon, 1998).
ECONOMIC IMPORTANCE
Agromyza oryzae is known as one of the most important pests on rice plants. Mining can destroy the affected leaves especially if there are several larvae in one leaf. Young plants can be seriously weakened. Beside the application of insecticides the lifestyle of the grass miner suggest two further control measures (Kuwayama, 1950).
1: Removing the second host plant, Zizania, from the vicinity of the paddy fields where it often occurs. 2: Removing the leaf litter where the overwintering puparia can be found.