Hemipteran pest damage assessment and management in white clover seed crops

Abstract

Three field experiments were conducted in white clover seed crops at Lincoln, Canterbury, to determine: (a) the incidence of pest and beneficial arthropods by suction sampling in three positions (edge, quarter and centre) into seven crops during the 1993-94 growing season (Survey Experiment). (b) the seed yield losses incurred by different intensities of potato mirid (PM, Calocoris norvegicus) released in 0.81 m² field cages (Cage Experiment). (c) the economic impact and effects on arthropod numbers from the application of two recommended insecticides (flu valin ate and dichlorvos), applied at the traditional timing for clover casebearer moth (Coleophora spissicornis and C. frischella) control (Spray Experiment). Of the insect pests collected in the survey experiment, PM, blue-green lucerne aphid (BGLA, Acyrthosiphon kondoi), and brown shield bug (BSB, Dictyotus caenosus) were the most prevalent and most likely to cause economic injury to the developing flower heads and reductions in seed yields. PM nymphs and BGLA numbers peaked in mid-November (survey experiment), while BGLA numbers peaked in mid-January (spray experiment). BSB numbers occurred later in the season coinciding with the end of flowering and seed set in mid-January through to harvest (early February). In the cage experiment the level of PM injury was highest on the stolon's second flower head and resulted in seed yield losses equivalent in value to $348/ ha at the PM (equivalent to 14 18 PM/m²). In the spray experiment fluvalinate controlled insect pest numbers for a longer period compared to dichlorvos. BGLA numbers in the fluvalinate-treated plots were significantly lower for up to 30 days after treatment. Lower numbers of the predatory Tasmanian lacewing (Micromus tasmaniae) also occurred in these plots. Seed yield gains from the application of either insecticide were financially profitable.