Evaluation of the susceptibility status of Spodoptera frugiperda (Lepidoptera : Noctuidae) larvae from different instars to insecticides

Abstract

Invasive pests such as the fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) are mainly controlled with insecticides. The labels of insecticides registered for its control in South Africa, indicate that pesticide applications should be made against small instars only. Different types of bioassays, based on the mode of action of insecticides, and the use of 3rd-instar larvae are recommended by the Insecticide Resistance Action Committee (IRAC) for susceptibility testing of S. frugiperda. Susceptibility evaluations can be influenced by pre-treatment conditions, including the source and type of the larval diet used in bioassays. The aim of the first chapter of this study was to investigate the effect of food switching from one artificial diet to another, and to maize leaf tissue, on larval development and survival. Neonate S. frugiperda larvae were placed on an artificial diet developed for the African stemborer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae), reared until the 3rd-instar, and were then divided into three groups. One group was kept on B. fusca diet, a second group was transferred to the Stonefly Heliothis diet, and the third, to maize leaf tissue. Development of the larvae were not negatively affected by the food switches, but the total development time (3rd to 6th-instar) was significantly faster on the Stonefly Heliothis diet. Sixth-instar larvae reared on maize leaf tissue were significantly heavier than those reared on the other two diets. All three diets provided male pupae with similar mass, while the female pupae from larvae reared on the Stonefly Heliothis diet were significantly heavier. The mortality of larvae reared on all three diets were similar. It is recommended that larvae are reared on the Stonefly Heliothis diet to standardise pre-treatment conditions for susceptibility testing of S. frugiperda larvae. Results indicated that larvae were not negatively affected when a food switch to maize leaf tissue was done for leaf dipping bioassays. An insecticide incorporated into the Stonefly Heliothis premix diet is recommended by IRAC (method no. 020) for susceptibility testing to diamides. No specific susceptibility test method for S. frugiperda to oxadiazines, is recommended. The aims of the second chapter, were to estimate the susceptibility of S. frugiperda larvae from different instars and to compare two bioassays, viz. insecticide incorporated into an artificial diet and leaf dipping to flubendiamide and indoxacarb. Although differently estimated by the two bioassays, larvae from later instars, showed high tolerance to both these insecticides, regardless of the bioassay used. The LC50 fold increase for flubendiamide and indoxacarb were higher for smaller compared to bigger larvae, but the insecticide incorporated diet bioassay estimated higher LC50 values for both insecticides, regardless of the larval instar tested. Leaf dipping was found to be a reliable bioassay for susceptibility testing of S. frugiperda larvae to indoxacarb. The LC80 and LC95 of S. frugiperda larvae to flubendiamide and indoxacarb were estimated well below the recommended field rate, and no control failure is foreseen in the near future. The susceptibility data estimated for flubendiamide by means of the insecticide incorporated diet bioassay, and for indoxacarb by means of leaf dipping can be used as baseline data for future insecticide resistance monitoring in South Africa.