Seasonal occurrence and impact of natural enemies of Hypera postica (Coleoptera: Curculionidae) larvae in Iowa
Giles, K.L. | Obrycki, J.J. | Degooyer, T.A. | Orr, C.J.
The entomopathogenic fungus Zoophthora phytonomi (Arthur), (Zygomycetes: Entomophthorales), infected populations of Hypera postica (Gyllenhal), (Coleoptera: Curculionidae), larvae at four sites in central and south-central Iowa in 1990, 1991, and 1992. Six epizootics (> 50% host infection) were observed during this 3-yr study; five were associated with above average rainfall. First occurrence of disease after 1 January 1991, occurred following the accumulation of 168-207 degree days above 9 degrees C. From 30 March to 28 May (1991), rainfall averaged between 3.6-7.0 mm per day, promoting epizootics of disease that reduced total larval populations from 15-61% at four locations in Iowa. In 1992, enzootics (< 50% infection) of Z. phytonomi occurred during late May and early June on declining populations of H. postica larvae at three sites. An epizootic occurred in central Iowa despite below-average rainfall. Parasitization of H. postica larvae by Bathyplectes curculionis Thomson (Hymenoptera: Ichneumonidae) and B. anurus Thomson ranged from 2.2 to 27.6% in 1991 and 1992. B. curculionis parasitization rates at two southern Iowa locations declined during epizootics of Z. phytonomi each year, suggesting a negative effect of Z. phytonomi on this larval parasitoid. B. anurus became the most abundant larval parasitoid at the two locations in southern Iowa after epizootics of Z. phytonomi.
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