Natural enemy ravine revisited: the importance of sample size for determining population growth

Abstract

Abstract. 1. The population growth of three aphid species, Metopolophium dirhodum (Walker), Rhopalosiphum padi (L.), and Sitobion avenae (F.), on winter wheat, was analysed by regression. The calculations were based on censuses of aphids made in 268 plots at 3-or 7-day intervals for 10 years on leaves and 6 years on ears. The calculations were made separately for each plot each year, then repeated on the pooled data from all plots monitored in a year.2. At the level of individual plots, no population growth was detected at very low densities. At high densities, the populations grew exponentially and the growth rates did not decrease with increasing aphid density.3. Significant growth was always detected in the pooled data. These growth rates decreased significantly at the highest densities. Field estimates of the intrinsic rate of increase derived from these data ranged from 0.010 to 0.026 for M. dirhodum, 0.0071±0.011 for R. padi, and 0.00078±0.0061 and 0.0015±0.13 for S. avenae, on leaves and ears respectively.4. The apparent lack of growth in the individual plots at low densities is attributable to small sample size. It is concluded that the natural enemy ravine in the population dynamics of cereal aphids, identified by Southwood and Comins (1976), is a consequence of low population densities at which population increase is undetectable unless very large samples are taken.

Seasonal Phenology ofAmphorophora agathonica(Hemiptera: Aphididae) and Spread of Viruses in Red Raspberry in Washington

“…Lastly, aphid counts may not have been a signiÞcant factor in the constructed models because of the relatively small sample size at each of the 50 locations (10 leaves per wk). In a study on cereal aphids, population growth in individual plots was unable to be tracked when aphid densities were low, whereas data pooled over all plots were more accurate (Jarosik et al 2002). Increasing the sample size in each location will give a better estimate of true aphid population densities and possibly shed more light on the relationships with virus spread.…”

Forecasting migration of cereal aphids (Hemiptera: Aphididae) in autumn and spring

“…1990). Appropriate assumptions on spatial distributions are required for the determination of sample size for aphid density estimations in the field (Jarosik et al. 2003; Sutherland 2006; Klueken et al.…”

“…Moreover, suction trap catches can indicate reliably changes in abundances of aphid species between case studies. In contrast, direct visual field counts during the early time of aphid immigration are too laborious due to large sample sizes needed for obtaining reliable data (Jarosik et al. 2003; Sutherland 2006).…”

“…This is because cereal aphids may differ significantly not only from the Poisson or quasi-Poisson distribution, but also from the negative binomial distribution (Ekbom 1985(Ekbom , 1987Krebs 1989;Elliott et al 1990). Appropriate assumptions on spatial distributions are required for the determination of sample size for aphid density estimations in the field (Jarosik et al 2003;Sutherland 2006;Klueken et al 2008). This confusion about the appropriate assumption of distribution was the reason for the combined analyses in table 2.…”

Within-field distribution of the damson-hop aphid Phorodon humuli (Schrank) (Hemiptera: Aphididae) and natural enemies on hops in Spain

“…A knowledge of spatial distribution of prey and predator is important in evaluating the system's persistence and the potential of natural enemies to reduce prey density (Stavrinides & Skirvin, 2003). The spatial distribution of an insect can be employed in investigating population dispersal behavior, establishing a precise sampling scheme and for sequential sampling (Margolis et al, 1984), binomial sampling (Binns & Bostanian, 1990), the study of population dynamics (Jarosik et al, 2003), detecting pest levels that justify control measures (Arnaldo & Torres, 2005) and assessing crop loss (Hughes, 1996).…”