Populations of Sitobion avenae and Aphidius ervi on spring wheat in the northwestern United States Spatial distribution and sequential sampling plans based on numerical and binomial counts

Abstract

During a three-year field survey of the English grain aphid, Sitobion avenae (F.) (Homoptera: Aphididae) and the parasitoid, Aphidius ervi Haliday (Hymenoptera: Aphidiidae; represented by aphid mummies) on spring wheat, data sets were generated which consisted of 47 estimates of mean density (m aphids per tiller), variance (s2), and the proportion of empty tillers (Po) for S. avenae and 22 estimates of the same statistics for A. ervi, respectively. Each estimate of the aphid and parasitoid populations was based on counts per individual tiller on each sampling occasion. Taylor's power law was found to fit the data sets better than Iwao's m*-m regression model. Taylor's slope for S. avenae (1.3076) was significantly higher than that for A. ervi (1.1519), indicating that S. avenae was more aggregated than A. ervi. Based on temporal changes in spatial aggregation represented by an index, 1/k [where k was estimated as m2/(s 2-m)], S. avenae was most aggregated at low densities during the early infestation period and tended to be less aggregated as density increased, whereas A. ervi was more likely to be randomly distributed. A common k was detected for neither S. avenae nor A. ervi because the slopes from the linear regression of k on m significantly exceeded 0. Sequential sampling plans, based on numerical (complete) and binomial (presence or absence) counts, for the aphid and parasitoid species were developed using Taylor's parameters and those estimated from the linear regression ofln(m) on In[ -ln[Po) ], respectively. Suggestions are made concerning the use of the sampling plans and the levels of precision that may be attained.

A General Method for Estimating Cereal Aphid Populations in Small Grain Fields Based on Frequency of Occurrence

“…where m is the mean number of aphids per tiller and p is the proportion of infested tillers, and ci and p are parameters that are estimated from this equation. Species for which parameter estimates are available are Rhopalosiphum maidis (Fitch), R. padi (L.), Sitobion avenae (Fitch), Schizaphis graminum (Rondani), Metopolophium dirhodum (Walker), and Diuraphis noxia (Mordvilko) on three crops (spring wheat, winter wheat, and spring barley) (Ekbom 1987;Elliott et al 1990;Feng and Nowierski 1992;Feng et al 1993;Legg et al 1992;Schaalje and Butts 1992;Ward et al 1986). We were intrigued by the similarity in parameter estimates among aphid species sampled from various small grain crops, A d decided to investigate the potential of developing a single 'generic' set of parameters for Eq.…”