STUDIES ON INSECTICIDE RESISTANCE IN TUTA ABSOLUTA (MEYRICK), WITH SPECIAL EMPHASIS ON CHARACTERISATION OF TWO TARGET SITE MECHANISMS

Tuta absoluta Meyrick (Lepidoptera: Gelechiidae) is a primary pest of tomato plants and is native to South America. Since the first documented European case in 2006, it has spread throughout the Mediterranean basin and North Africa. Larval stages cause direct feeding damage and reductions to both yield and fruit quality. Chemical insecticides have been the main control tools used against T. absoluta, but decreasing efficacy has been attributed to the development of insecticide resistance. During this study, leaf-dip bioassays were used to quantify responses of five field strains of T. absoluta to insecticides belonging to different chemical classes. The results showed significant variation in susceptibilities to organophosphates and pyrethroids, which are a major class of neurotoxic insecticides and acaricides used extensively over the last decades. One important mechanism of resistance to pyrethroids, termed knockdown resistance (kdr), has been shown to arise through alterations (point mutations) in the para-type sodium channel protein leading to reduced sensitivity of the insect nervous system to the pyrethroids. Mutations in the Ace gene have also been reported to cause insensitivity to organophosphates. A combination of PCR-based molecular methods and biochemical assays was used to investigate possible existence of any mutations in these two insecticides targets in several laboratory strains. Cloning and sequencing of domains II, III, and IV of the T. absoluta sodium channel gene revealed the presence of several kdr mutations previously reported to confer reduced sensitivity in other arthropod species. These included L1014F, M918T, T929I and L925M mutations. Characterisation and sequencing of Ace gene revealed the existence of a single mutation A201S previously related to organophosphate insensitivity in several insect species. Diagnostic tools that allow detection in individual larvae and adults were developed and used to screen field samples of diverse geographical origin and assess their distribution in global T. absoluta populations.