Insecticide resistance in onion thrips (Thrips tabaci) (Thysanoptera: Thripidae)

Abstract

Onion thrips from onion (Allium cepa) crops were tested for resistance to four insecticides (deltamethrin, diazinon, dichlorvos, and endosulfan) following reports of failure of insecticides to control onion thrips in the field. Bioassays to determine baseline concentration‐mortality responses used thrips from two insecticide susceptible populations. Leaf discs (24 mm diam.) cut from leek (Allium porrum) were dipped in insecticide solutions and placed, when dry, in Petri dishes (50 mm diam.) with ventilated lids. Ten adult female thrips were added, and mortality was recorded after 24 h at 25°C. Five or six concentrations of each insecticide were tested and bioassays were usually repeated 3 times. The lowest concentration, which consistently killed over 90% thrips, was used as a diagnostic concentration to screen thrips populations collected from four onion crops and compared with both susceptible populations using the same bioassay. Each test population was also exposed to leaves dipped in water. One Canterbury and three Auckland populations were resistant to deltamethrin, whereas only the three Auckland populations were resistant to diazinon and dichlorvos. No population was resistant to endosulfan. Concentration‐mortality responses were determined for deltamethrin and diazinon. Based on the LC₅₀s, one Auckland population was 40 times more resistant to diazinon and the Canterbury population was 550 times more resistant to deltamethrin than susceptible populations. The New Zealand onion industry responded to the initial failure of insecticides by developing and implementing an insecticide resistance management strategy and research to improve onion thrips control.