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Sugar beet cyst nematode (SBCN) is a serious pest of beet and Brassicaceae crops and was first detected in Japan in 2017. Two varieties of SBCN-resistant radish (‘Kobuberi-daikon’ and ‘KGM1804’) were selected for trap cropping based on pot experiments. However, their effect on the SBCN population was not confirmed under field conditions. In this study, field experiments were conducted to investigate the efficacy of these radishes on the SBCN population along with chemical treatment (1,3-dichloropropene and dazomet) and fallowing. Trap cropping using ‘Kobuberi-daikon’ and ‘KGM1804’ reduced the population density of SBCN to the same level (55–60% reduction in unhatched eggs) and was superior to fallowing, whereas the anti-nematode effect of both nematicides were higher than that of trap cropping (1,3-dichloropropene: 82% and dazomet: 66%). Considering the other advantages of trap cropping such as lower cost, environmentally friendliness and mitigating soil erosion, trap cropping using radish varieties tested in this study is a useful option for nematode management.

The diamondback moth Plutella xylostella(L.) (Lepidoptera: Plutellidae) is one of the most destructive insect pests worldwide which has developed resistance to many insecticides. Previously, a point mutation (I4790K) in a ryanodine receptor gene, a target gene of diamides, was identified as a major factor of resistance of the diamondback moth to cyantraniliprole in Japan. Although the diamondback moths have not yet widely developed resistance to cyantraniliprole in Japan, increasing resistant diamondback moth populations with the I4790K mutation in the future is a concern. To simply and quickly monitor the frequency of the I4790K mutations in field populations, we developed a multiplex polymerase chain reaction (PCR)-based molecular diagnostic method that can identify a genotype of the I4790K mutation site. We evaluated the sensitivity and specificity of the diagnostic method by comparing its cyantraniliprole-resistant and cyantraniliprole-susceptible allele detection results with those by Illumina MiSeq sequencing data. The results show sufficiently high sensitivity and specificity. Moreover, cyantraniliprole-resistant allele frequencies calculated by the molecular diagnostic method were almost comparable with those by MiSeq sequencing data. The molecular diagnostic method would help in performing continuous monitoring of the cyantraniliprole resistance level of diamondback moth populations in the field.