Experimental methods for evaluating natural enemies of Russian wheat aphid, Diuraphis noxia (Homoptera : Aphididae)

Abstract

Methods for evaluating natural enemies of Diuraphis noxia (Kurdjumov) in multiple habitats were examined during 1991- 1993 at Bushland, Texas. Methods examined included cage exclusion, insecticidal exclusion, and host exposure. Cage exclusion was tested using walk-in and bucket-type cages. Experiments demonstrated significant impact of enemies on D. noxia. Bucket-cages generated data with greater variability and were more labor intensive. Bucket-cage effects on pest dispersal were minimal. Walk-in cages assessed the collective impact of D. noxia enemies; bucket-cages assessed only predation. Laboratory bioassays identified insecticidal exclusion agents toxic to D. noxia and with al toxicity to natural enemies. Malathion was highly toxic to parasites and predators and showed low toxicity to D. noxia. Insecticidal exclusion experiments assessed parasites and predators separately. Data showed less parasitism on malathion-treated versus untreated plants. Numbers of coccinellid predators were similar on malathion-treated and untreated plants. Mean D. noxia numbers were greater on treated plants in closed versus open cages. This indicates that malathion reduced activity of D. noxia parasites, but not predators, which limits the value of insecticidal exclusion in evaluating D. noxia enemies. Three series of host exposure experiments were conducted: general phenology, parasite recruitment, and predator recruitment. Experiments exposed potted wheat infested with aphids in different fields. General phenology experiments showed several species of parasites and predators, mostly coccinellids, attacking D. noxia. Parasite activity was very low during the spring, and increased in late summer; predators occurred earlier. D. noxia was less heavily parasitized than Schizaphis graminum (Rondani), but numbers of coccinellids collected were similar on both aphids. Parasite recruitment experiments showed both parasite numbers and percent parasitism similar among fields. Predator recruitment experiments showed less coccinellids collected from CRP field than either sorghum or wheat fields. However, less coccinellid recruitment in CRP did not result in lesser impact. By measuring impact of recruited individuals, recruitment experiments provided a better assessment of enemy activity than general phenology experiments.