TEMPERATURE EFFECTS ON THE ACTION OF ACYLUREA INSECTICIDES AGAINST TOBACCO HORNWORM (MANDUCA-SEXTA) LARVAE

The toxic effects of six acylurea insecticides on larvae of the tobacco hornworm were investigated at each of four environmental temperatures (20, 25, 30 and 35-degrees-C). This spans the range of temperatures which the insects can tolerate. For all the acylureas tested, mortality increased with temperature when either newly hatched or fourth-instar larvae were given insecticide in their food. Sub-lethal growth inhibition also become more pronounced at progressively higher environmental temperatures. This temperature dependence of acylurea action was not due to altered uptake of the insecticide, since there was no significant variation with temperature in the amount of [C-14]flufenoxuron taken up by fifth-instar larvae when given a single meal containing labelled insecticide. Additionally, mortality of fourth-instar larvae given a single intra-haemocoelic injection of flufenoxuron was significantly greater at higher temperatures, implying that temperature affects a process that occurs after insecticide uptake. The intrinsic ability of acylureas to inhibit chitin synthesis is temperature-sensitive, since flufenoxuron inhibited the incorporation of [C-14]N-acetylglucosamine into chitin by proleg epidermis in vitro significantly less well at 20-degrees-C than at the higher temperatures tested. However, there was no significant variation between the effectiveness of in-vitro chitin synthesis inhibition at 25, 30 and 35-degrees-C. These data show that the effectiveness of acylurea insecticides is subject to strong temperature effects in the range of temperatures likely to be experienced in the field.