Life cycle, phenology and economic importance of the walnut husk fly Rhagoletis completa Cresson (Diptera: Tephritidae) in northern Italy

The walnut husk fly Rhagoletis completa Cresson, a pest originating from North-America, was detected for the first time in Europe in 1991. The life cycle and phenology of R. completa were studied, during two successive seasons, in two commercial orchards located in northern Italy. The pest develops one generation per year. Fly emergence lasted from early July to the second half of August. Oviposition was detected from late July to early September, with peaks between August 5 and 18. First instar larvae were recorded from early August and mature larvae left husks from late August onwards to pupate in the soil. The seasonal patterns of nuts infestation showed rapid growths in August following oviposition peaks. Nuts infestation levels in the untreated plots of the two orchards, ranged from 74-91% in the first year to 89-91% in the following season. Relatively dark shell surfaces were linked to a weight reduction in nuts and kernels and to an increase in darkened and mouldy kernels. Observations made in an untreated orchard for additional 3 years confirmed these trends. Baited Pherocon AM were effective in monitoring flies but catches on woody green spheres were better correlated with the first relevant oviposition phases. When pesticides were sprayed with the correct timing, i.e. against eggs or first instar larvae, infestation was kept to acceptable levels with a single application. Considering average yields, nuts prices and costs for insecticide use, 1-2 insecticide treatments are economically viable. Other Implications are discussed.