Susceptibility of Trialeurodes vaporariorum (Hemiptera: Aleyrodidae) to insecticides in South Africa

Abstract

Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae), the greenhouse whitefly, is a highly polyphagous pest that causes severe damage to several economically important crops. Pesticide application is the most common method of control, and many cases of resistance have been reported from countries other than South Africa. No data are therefore available on the susceptibility of T. vaporariorum populations to insecticides in South Africa. The aim of this study was to assess the susceptibility of T. vaporariorum adults and second-instar nymphs of two populations (WSC-1-24 and BLF-1-25) to six insecticides. Susceptibility was assessed using two IRAC recommended bioassay methods, viz. the leaf dip method (IRAC Method No. 15) for adults, and the potted plant immersion method for nymphs (IRAC Method No. 16). Additionally, adult susceptibility to diafenthiuron (Group 12A), flupyradifurone (Group 4D) and imidacloprid (Group 4A), was also evaluated using an adapted IRAC Method No.016. Whiteflies were reared separately by population, and LC80 values for each insecticide were estimated and compared to their respective recommended field application rates to assess potential resistance. Adults from both populations were susceptible to most of the abovementioned insecticides, though variation in response was observed across compounds. Both populations exhibited low adult susceptibility to cyantraniliprole, while nymphs were susceptible. Flupyradifurone was effective across both life stages and populations, except for BLF-1-25 nymphs, whose LC80 exceeded the recommended rate. Susceptibility to pymetrozine was highly variable, with low efficacy overall. Although control failure is likely for both life stages of individuals of the WSC-1-24 population, BLF-1-25 adults were susceptible. Comparison of the two bioassay methods revealed that the plant immersion method consistently produced higher LC80 values than the leaf dip method across all three insecticides, regardless of their mode of action, or action within the plant. Despite these differences, the LC80 values estimated for diafenthiuron, flupyradifurone, and imidacloprid using both methods were still below their recommended application rates, indicating that no adult control failure is currently anticipated in South Africa with these products. These findings underscore the importance of using consistent, standardised bioassay protocols in resistance monitoring programs to ensure reliable and comparable data over time. The differences in susceptibility observed between populations likely reflect variations in local insecticide use patterns, emphasising the need for robust insecticide resistance management strategies. Integrated pest management approaches should be prioritised to reduce reliance on chemical control and promote sustainable long-term management of this economically important pest.