Efficacy and environmental fate of alphacypermethrin applied to rice fields for the control of chironomid midge larvae (Diptera : Chironomidae)

Chironomid midge larvae are the most important pests affecting the establis hment of aerially sown rice crops in New South Wales (NSW). The pyrethroid insecticide alphacypermethrin was evaluated for the control of chironomid m idge larvae in small plot field trials at Yanco Agricultural Institute duri ng the 1997-1998 and 1998-1999 rice seasons. Treatments were applied at sow ing (6 days after flooding) in both trials. In the 1997-1998 trial, alphacy permethrin applied as an emulsifiable concentrate formulation at 10, 20, an d 30 g active ha(-1) provided between 54 and 73% control of target Chironom inae in the first 19 days after application (DAA), whilst a chlorpyrifos st andard treatment (75 g active ha(-1)) also provided 73% control. Population s of Chironominae were very low during the 1997-1998season (estimated contr ol density 14 DAA approximately 485 larvae m(-2)), and control was not refl ected in improved plant establishment. In the 1998-1999 season target Chiro nominae, predominantly Chironomus tepperi Skuse, were far more abundant (es timated control density 14 DAA approximately 13 100 larvae m(-2)). Alphacyp ermethrin provided >99% control of Chironominae for 19 DAA at all rates eva luated (6, 10 and 20 g active ha(-1)), whilst the chlorpyrifos standard (75 g active ha(-1)) gave 97% control over the same period. Plant establishmen t 30 DAA averaged 23 plants m(-2) in the control bays, from 169 to 188 plan ts m(-2) in the alphacypermethrin bays, and 204 plants m(-2) in the bays co ntaining the chlorpyrifos standard. Differences in plant establishment betw een the alphacypermethrin and chlorpyrifos treatments were not significant (P>0.05). Results indicate that alphacypermethrin applied at between 6 and 10 g active ha(-1) will be equal or superior to chlorpyrifos (75 g active h a(-1)) for the control of chironomid larvae during rice crop establishment. Concentrations of alphacypermethrin in the water column and sediment were monitored during each trial. Concentrations in water for the 20 g active ha (-1) rate ranged from 0.23 to 0.41 mu g l(-1) 1 DAA down to 0.008 mu g l(-1 ) at 18 DAA, whilst at the 6 g active ha(-1) rate alphacypermethrin concent rations declined to below the detection limit (0.001 mu g l(-1)) at 18 DAA. Surface sediments (top 2 cm) still contained measurable concentrations of alphacypermethrin at 25 DAA (e.g. 29 mu g kg(-1) for 20 g active ha(-1) rat e, 1997-1998 trial). No alphacypermethrin residues were detected in plant s amples taken 42 days after treatment, or in grain or forage samples at harv est (1997-1998 trial, detection limit 20 mu g kg(-1)). (C) 2000 Elsevier Sc ience B.V. All rights reserved.