Sub-lethal effects of tefluthrin on Diabrotica undecimpunctata howardi, Barber: plant protection and larval development Paris K. Michaelides*, Ann L. Cleverly* and Denis J. Wright+ *AgrEvo Environmental Health, McIntyre House, High St, Berkhamsted, Her& HP4 ZDY, UK and +Department of Biology, imperial College, Silwood Park, Ascot, Berks SL5 7PY, UK Larval stages of the southern corn rootworm, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML Diahrotica undecimpurzctuta howardi (Barber) (Coleop- tera: Chrysomelidae), were used to assess the effects of sub-lethal concentrations of the soil insecti- cides tefluthrin and terbufos. It was shown that significant plant protection was only possible with high (LCxo) levels of terbufos, whilst protection was achieved with much lower (LC20) concentrations of tefluthrin. This protection was achieved by anorectic, poisoning effects of the pesticides on the larvae. Unlike tefluthrin, terbufos did not affect feeding activity at sub-lethal levels and a high level of pest mortality was required for comparable protection. The reduced feeding of the tefluthrin- treated larvae also led to lower larval and adult weights and lower survival to adulthood when compared with controls. 0 1997 Elsevier Science Ltd Keywords: tefluthrin; sub-lethal effects; Diabrotica; plant protection Plant protection is generally assumed to occur princi- pally by the death of the damaging stage of the pest following pesticide application (Gordon, Alcock and Jutsum, 1989; Ware, 1983). However, sub-lethal effects of insecticides can also be important compo- nents in crop protection programmes (Sutter et al., 1989). Antifeedants and repellents have been particu- larly well studied. One of the most active chemicals in this area, combining kill at high doses and antifee- dancy at low doses, is azadirachtin, the major insecti- cidal compound of extracts from the neem tree, Azadiruchtu indica (Schmutterer, 1990). Pyrethroids are also known to produce repellent and antifeedant responses by pest insects (Hirano, 1989; Kumar and Chapman, 1984; Ruscoe, 1977; Tan, 1982) and these effects are dose-related; low rates of application generally having antifeedant activity whilst higher rates can act as repellents (Herve, 1985). In the field, all recommended application rates of insecticides intended to produce high kill are degraded with time to levels that produce a changing spectrum of activity against insects. In terms of antifeedant effects at sub-lethal levels, pyrethroids are generally more potent than the organophosphates and carbamates (Bariola, 1984; Kumar and Chapman, 1984). Antifeedancy may be a specific effect, i.e. the chemical inhibits biting, piercing, chewing or associated gustatory behavioural changes before or after food intake (Hsiao, 1985; Usher et al., 1989). Alternatively, the presumed ‘antifeedant’ effect may be anorectic, whereby there is interference with non-gustatory processes that regulate appetite (Abro et al., 1993). The present paper reports on research conducted on the effects of sub-lethal concentrations of the pyrethroid, tefluthrin and the organophosphate, terbufos, against larval stages of the southern corn rootworm, Diabrotica undecimpunctata howardi (Barber). This represents the first report of the sub-lethal levels of soil insecticides tested against a soil pest to assess crop protection potential activity. Materials and methods Insects The laboratory colony of D. undecimpunctutu howardi was established from a sub-population (ex-May and Baker, Essex, UK) originating from a susceptible laboratory-reared stock in the US corn belt. Detailed laboratory culture methods were derived from a variety of sources and are described elsewhere (Jackson, 1985; Michaelides, 1995). Larvae were reared in vented plastic containers filled with sedge peat (Sedgemoor Drain, Devon, UK), fed uncontami- nated, germinating maize (Zea mays, hybrid field corn, AGIO, W.W. Johnson and Son Ltd, Boston, Lines, UK) and kept at 27&2”C with a 16-h photoperiod. Chemicals In all cases, technical material of the following insect- icides was used: terbufos [S-tert-butyl thiomethyl- Crop Protection 1997 Volume 16 Number 5 423