LOCALIZED INSECTICIDE TREATMENT FOR THE CONTROL OF VINE WEEVIL LARVAE(OTIORHYNCHUS-SULCATUS) ON FIELD-GROWN STRAWBERRY

Three field experiments at Horticulture Research International (HRI), Efford, in 1992, 1994 and 1995, showed that the incorporation of contr olled-release chlorpyrifos granules (78-208 g a.i. m(-3)) into the com post of the propagation modules of strawberry plants gave significant control of vine weevil larvae in the field. Better control (approximat ely 90%) was achieved when field planting was in early August so that the root system was small at the time of egg infestation, than followi ng planting in mid-May (approximately 50% control) where a much larger root system had grown. Control was as good where the treated modules had a compost volume of 80 ml as where the volume was 230 ml. Better c ontrol occurred when eggs used for artificial infestation were placed close to the crown of the plant than when placed 15 cm away. In a repl icated field experiment at Hinton Admiral, Hampshire, in 1994, pre-pla nting spot, band or whole-bed soil treatment of raised-bed, polythene mulched plants (planted as bare-root runners) with the chlorpyrifos gr anules (at 52 or 104 g a.i. m(-3)) did not reduce the numbers of larva e significantly. In a further field experiment at HRI East Malling in 1994, treatment of a 15 cm diameter by 15 cm deep cylinder of soil rou nd each plant (at 104 g a.i. m(-3)) did not significantly affect larva l numbers. Pre-planting spot treatment with imidacloprid granules (125 g a.i. ha(-1)) or a curative drench (at the same dose) was not effica cious, though good control was achieved with a standard curative drenc h of chlorpyrifos (13.1 kg a.i. ha(-1)). The survival of vine weevil e ggs and of young larvae was low (less than 9%), circumstantial evidenc e pointing to soil type and condition as being important determining f actors. Lighter soils with a structure allowing easy movement of larva e appeared to be more favourable for the survival of the pest. Where a dults were caged round strawberry plants with the surrounding surface soil replaced by sand in the field, most eggs (more than 79%) occurred in the top 0-1 cm of sand, 50% being found on or close to the surface (0-0.2 cm depth) in one experiment. Eggs were aggregated weakly round a single non-mulched plant, but there was little evidence of such agg regation round plants grown in polythene-mulched, raised beds. Surviva l to the semi-mature larval stage from eggs placed on, or 2 cm below, the soil surface 15 cm from the crown of the plant was as great as for eggs placed on, or 2 cm below, the surface adjacent to the crown. Lar vae were shown to migrate towards the crown of the plant during their development. Implications for the optimum placement of insecticide gra nules are discussed. (C) 1997 Elsevier Science Ltd.