Effect of Flower Thrips on Onion Seed Plants and a Study of Their Control

Abstract

Feeding injury by the flower thrips, Frankliniella occidentalis (Pergande), to onion seed plants occurs primarly in the flower filaments, causing a premature loss of pollen, and a decrease in pollination and seed set. Microscopic examinations and counts show that about 10 thrips per flower or floret are needed to cause meaningful injury. Thus, an average of 9,000 to 10,000 thrips per seed head would be necessary to cause any economic injury and seed loss, at an average of 900 to 1000 flowers per head. Field investigations of thrips damage were conducted by introducing varying numbers of adults on seed heads enclosed by fine-meshed cloth cages. The highest infestations of nymphs plus adults attained were 1900, 2400, and 3600 per head. These numbers of thrips did not cause any reduction in the seed set, yield, size or viability. Thrips increased pollination and seed set over the controls in all experiments. The addition of pollinators resulted in an increased seed production and viability over that of thrips alone. The presence of fly pollinators and 2 heads per replicate cage proved necessary to obtain adequate seed set in the thrips-damage studies. Three years of experimentation never demonstrated enough thrips per head to cause an economic loss of onion seed. Two and three applications of parathion and diazinon resulted in satisfactory thrips control. These treatments did not increase or decrease seed set or yield but the use of parathion indicated some increase in seed viability. Parathion treatments reduced numbers of bee and fly pollinators for about 3 days when applied during bloom. A single prebloom application of 1/2-1 lb per acre of dimethoate gave outstanding thrips control and no pollinators reduction. Another application during bloom reduced pollinators for up to 10 days. The single dimethoate application resulted in an increase in seed set and yield, which appeared to be in addition to any possible effect on thrips and pollinators, indicating some additional plant response to the chemical.