Targeted insecticide regimes perform as well as a calendar regime for control of aphids that vector viruses in seed potatoes in New Zealand

Abstract

Over two seasons (2002–03; 2003–04) at Pukekohe in the North Island and three seasons (2002–03; 2003–04; 2004–05) at Lincoln in the South Island of New Zealand, a common calendar-based insecticide regime was compared with a targeted insecticide regime for control of aphids on potatoes, and for any subsequent change in potato virus transmission by aphids. Treatments were an untreated control (1), calendar-applied fortnightly applications of methamidophos to foliage grown from untreated seed (2) and seed treated with imidacloprid (3), λ-cyhalothrin applied at an aphid threshold to foliage from untreated (4) or imidacloprid-treated seed (5) and pymetrozine at an aphid threshold to foliage from untreated seed (6). The action threshold for spraying was when apterous aphid populations exceeded 10 per 150 potato leaves. As indicated from suction traps at the trial sites, aphid flights peaked in autumn and spring. At Pukekohe in both years, aphids were not found on the potato foliage in any treatments until late January (midsummer), making 3–4 of the 5–7 methamidophos applications for the season unnecessary. Further applications of methamidophos kept apterous aphid populations at negligible levels until late summer, but populations in autumn tended to be higher than in untreated plots, due probably to the suppression of aphid predators (syrphids and lacewings) by the insecticide. No apterous aphids were found on potato foliage above the threshold in imidacloprid-treated or untreated plots until 3 weeks before desiccation of the foliage, necessitating only one application of λ-cyhalothrin or pymetrozine for each season. At Lincoln over all years, the imidacloprid seed treatment and the calendar-based 7–11 foliar applications of methamidophos kept plots free of aphids. However, the imidacloprid seed treatment alone, or one foliar application of λ-cyhalothrin alone or pymetrozine alone, were all that were required to keep aphid populations below the threshold during two of the three seasons, and were not required in the 2003–04 season. The incidence of tubers infected with potato leafroll virus (PLRV) and potato virus Y (PVY) at both sites was not significantly reduced by any insecticide treatment compared with the untreated control. The insecticide treatments also had no significant effect on total potato yields. In New Zealand, an imidacloprid seed treatment followed by a foliar application of λ-cyhalothrin or pymetrozine whenever apterous aphid populations exceed 10 per 150 potato leaves appears sufficient to maintain aphid populations below the action threshold, without compromising yields or increasing virus risk in tubers.