Effect of application rate and beetle age on efficacy of imidacloprid (Gaucho®) against black maize beetle, Heteronychus arator Fabricius (Coleoptera: Scarabaeidae)

Abstract

Laboratory trials were conducted to study the effect of application rate and beetle age on damage inflicted by black maize beetle, Heteronychus arator, to maize seedlings treated with imidacloprid. Application rate was evaluated in two trials using young adults during autumn and older adults during spring after hibernation. Three trials evaluating the effect of beetle age were conducted at fortnightly intervals during spring when adults were in a progressively changing reproductive phase. Only females after hibernation were used in the latter trials. In all trials treated and untreated seeds were planted in a greenhouse and maize seedling stems fed to adults in glass vials. Damage to maize seedlings (number of damaged sheaths) was determined and beetle mortalities recorded. Results indicated that with older adults a rate of half the registered rate might be as effective as the registered rate. However, it would be safer to use the higher registered rate until all factors that might affect the efficacy of imidacloprid are known. Females occurring during mid-spring were less affected by the systemic activity of imidacloprid than those occurring later during the season, which might possibly be related to their ovipositional cycle. In the majority of fields in the area where H. arator pest outbreaks traditionally occur, maize seedlings are in a vulnerable growing stage during mid-spring. The systemic component of imidacloprid might therefore not be able to control H. arator effectively in growing seasons when beetle population levels are high, especially when it is used at half the registered rate.