Seasonal Abundance and Parasitism of the Sunflower Beetle (Coleoptera: Chrysomelidae) on Cultivated Sunflower in the Northern Great Plains

Abstract

Postdiapause adults of the sunflower beetle, Zygogramma exclamationis (F.), appeared in plots of sunflower, Helianthus annuus L., between 28 May and 9 June and were present until mid-July 1985-1987. Peak density of adults occurred between 13 and 23 june when plants had 6-15 leaves. Eggs were present on plants for approximately 1 mo (June) with maximum egg densities occurring between 13 and 26 June. Larvae were initially observed between 10 and 19 June when plants had 5-12 leaves and were present an average of 53 d before entering the soil to pupate. There were four larval instars. New-generation (prediapause) adults emerged from the soil at the end of july or the beginning of August. By late September, all prediapause adults had left the plants and entered the soil to overwinter. Although population densities varied, life history patterns were similar among the 3 yr. There was one generation per year. The parasitoid Erixestus winnemana Crawford (Hymenoptera: Pteromalidae) was reared from sunflower beetle eggs, but egg parasitism was extremely low (1985, 0.1%; 1986, 1.9%; 1987, 0.4%). Doryphorophaga macella Reinhard (Diptera: Tachinidae) was the only larval parasitoid recovered. Larval parasitization varied between 0 and 14.8% in 1985, 35.7 and 66.7% in 1986, and 0 and 100% in 1987. The life history of D. macella showed good synchrony with its host; sunflower beetle larvae were parasitized during the entire period they were present. The tachinid parasitoid Myiopharus sp. was recovered from prediapause adult sunflower beetles in 1987, but its rate of parasitization was only 1.7%. Parasitism by D. macella may have contributed to the reduction in sunflower field infestations by the sunflower beetle during recent years.