Bioassays to compare the systemic activity of three neonicotinoids for control of Heteronychus arator Fabricius (Coleoptera: Scarabaeidae) in maize

Abstract

Two neonicotinoid seed dressing insecticides (imidacloprid and thiamethoxam) are registered in South Africa for control of the black maize beetle , Heteronychus arator, in maize. The systemically translocated antifeedant and/or repellent activity of these two neonicotinoids and an unregistered neonicotinoid, acetamiprid, were compared in laboratory bioassays. In all trials treated and untreated seed were planted in a greenhouse and stems of maize seedlings were fed to adults in glass vials. Damage to seedling stems (number of damaged leaf sheaths) was determined and beetle mortalities recorded. Results on beetle mortality indicated inadequate systemically translocated insecticidal activity of neonicotinoid insecticides at a dosage rate of 1.75 g a.i. kg −1 seed. The number of damaged maize stems, extent of damage (mean number of damaged leaf sheaths) and ability of beetles to cause severe damage, served as criteria to evaluate the efficacy of the systemically translocated antifeedant and/or repellent action. Since the majority of beetles fed on the stems before they were repelled, a high number of maize stems were damaged at the onset of the trials. The extent of damage and the ability of the beetles to cause severe damage were significantly reduced. However, in the case of acetamiprid and thiamethoxam reductions were not large enough and in some trials up to 53% of the beetles were still able to cause severe damage. This observation indicates that these two compounds may not provide adequate control during growing seasons when high beetle populations are encountered, thus increasing the probability that more than one beetle may damage a single plant.