Stable management of Tetranychus urticae using UVB irradiation system (UV method) together with predatory mites in strawberry greenhouse
Abstract: Based on the recent studies on lethal effects of ultraviolet-B (UVB) on spider mites,we revealed a combined use of overhead UVB lamps and light reflection sheets (LRSs) cancontrol Tetranychus urticae on greenhouse strawberries when the adequate UVB radiationreach to the lower leaf surfaces. The dominant control effect is thought to be an inhibition ofegg hatching by direct UVB irradiation. However, under practical conditions, T. urticaeincreased from April because plants grow thick so that UVB could not reach to the lower leafsurfaces. We thereby tested the additive effects of UVB irradiation system and phytoseiid mite,Neoseiulus californicus, on spider mite control in strawberry greenhouse. We artificiallyintroduced T. urticae and N. californicus on 28 November 2018 and 1 March 2019, respectively,and monitored the both mite densities under UVB+LRSs + (UV method; cumulative UVB onthe plants: 0.864kJ/m2/d) and UVB-LRSs- (control). The peak densities of T. urticae at controland UV method were 3.29 and 0.17 ♀♀/leaf on 5 May, respectively. N. californicus densitiesat control and UV method peaked 1.16 (5 May) and 0.11 (20 May) individuals/leaf,respectively. The ratios of N. californicus par one T. urticae female adult after phytoseiid mitesreleased at control and UV method were 0.20 and 0.59, respectively. High ratio of predatorymites per spider mites under UV method indicated that the UV method and phytoseiid mitespossessed additive effects or that the UV method may enhance phytoseiid mite activity,resulting stable control on T. urticae in greenhouse strawberry.