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Red bean pod injury by adult release of the legume pod borer, Maruca vitrata, in fields
The injury of red bean (Vigna angularis) by the legume pod borer, Maruca vitrata (Lepidoptera: Pyralidae), was evaluated through an artificial inoculation experiment in cages randomly arranged in a field. Virgin adult pairs (0, 1, 2, 4 and 8) were released into screen-nets at the flowering stage of red bean, and the damage was investigated at harvest. Any insecticides were not sprayed through all plant-growing season. The absolute number of uninjured pod in a net showed a decreasing tendency according to the increase of the number of released adult pairs, but did not show a statically significant difference among treatments. The total production (number of pod and seed) of red bean was highly variable even in a treatment. The variation seemed to be caused by pre-infestation by M. vitrata and other insect species before netting. Instead, therefore, relative damage was compared among treatments. The total ratio of injured pod in a net increased according to the released adult number. The result clearly indicated that the mating ratio and fecundity within the screen-net also increased according to the number of released pairs and that the hatched larvae affected the damage. The corrected rate for pod injury at 8 pair release was about 46%. On the other hand, the ratio of injured seed per injured pod was about 80% regardless of treatments. The result suggested that one pod continuously be attacked till almost disappearance of itself.
