Appl. Entomol. Zool. 39 (2): 203–208 (2004)

To clarify the reason for the recent dramatic outbreak of the leafminer, Chromatomyia horticola (Goureau), on the green pea in Japan, the insecticide susceptibility of two leafminer populations collected from grower’s pea fields in Shizuoka and Kagoshima Prefectures was measured in a laboratory assay. Of the five insecticides registered for leafminer control on the pea, cartap was moderately effective, but phenthoate, malathion, permethrin and tralomethrin were less effective on larvae of the leafminer populations at their recommended dilutions. The leafminer population from Shizuoka was slightly less susceptible than the population from Kagoshima to all insecticides tested by LC50 assay. Emamectin-benzoate, fipronil, spinosad, chlorfenapyr, cyromazine and isoxathion were very effective in controlling leafminer populations. The impact of insecticide applications on the leafminer and associated parasitoids in pea fields in Shizuoka Prefecture was also discussed. Of the four samples of pea shoots collected from three fields, two samples collected one month after insecticide application had very few leafminers and many parasitoids. However the sample taken immediately after insecticide application had a very large number of emerged adult leafminers, but few parasitoids. The results suggest that the recent large outbreak of C. horticola might have resulted from a combination of low susceptibility to insecticides in the species and the destruction of the parasitoid complex due to frequent insecticide applications.