Strategies and Statistics of Sampling for Rare Individuals

Abstract

Diverse subdisciplines within entomology recognize the detection of rare individuals as the precursor to effective management of these individuals. Unfortunately, detection methods have often developed on a case-by-case basis, and advances in one subdiscipline have not carried over to similarly related fields. The biology of a particular organism will certainly affect sampling methods, but the underlying principles governing the power of a sampling strategy to detect rare individuals will apply across taxa. Our review of the sampling literature demonstrates the common problem of detecting rare individuals, reviews the fundamentals of probability theory as a foundation for any monitoring program, and discusses the inferences that can be drawn from samples, especially when resources limit sampling efforts. Particular emphasis is placed on binomial-, beta-binomial-, and hypergeometric-based sampling strategies as they pertain to quarantine inspections for exotic pests, veterinary/medical entomology, and insecticide resistance monitoring.

Boll Weevil (Coleoptera: Curculionidae) Survival through Cotton Gin Trash Fans

“…The 95% upper conÞdence level (p u ) calculated for the observed zero rate of survival to 24 h of the total of 2,100 boll weevils tested at the 56.2 m/s fan-tip speed is 0.00143, or 0.143%. This represents the statistical worst case for potential boll weevil survival based on sample size alone at the lowest fan speed tested (Couey and Chew 1986, Venette et al 2002, Sappington et al 2004b). Because increasing fan-tip speed treatments provided progressively harsher conditions, the sample sizes for the lower speeds tested can be pooled for calculating the 95% upper conÞdence levels of the greater fan speeds.…”

Potential for Escape of Live Boll Weevils (Coleoptera: Curculionidae) into Cottonseed, Motes, and Cleaned Lint at the Cotton Gin

“…In some experiments, no boll weevils were recovered that survived to 24 h in any of the replicated runs. Although we have no evidence that survival is possible in those cases, we cannot conclude that it is impossible, because increasing the sample size might eventually result in a survivor (McArdle 1990;Venette et al 2002;Sappington et al 2004b, c). Instead, we calculated the upper 95% conÞdence limit (p u ) for the observed zero recovery based on the number of insects tested (n), by using the following formula:…”

Protectionism versus risk in screening for invasive species

“…Among those imports selected for a physical inspection, inspection protocols suggests a representative sample of roughly two percent, by volume (NRC 2002;Venette, Moon, and Hutchinson 2002;Work et al 2005). Inspection involves offloading the commodity, removing items from containers, and physical inspection which may consist of visual examination, dissection, and the "beat technique" (Venette, Moon, and Hutchinson 2002).…”

“…Inspection involves offloading the commodity, removing items from containers, and physical inspection which may consist of visual examination, dissection, and the "beat technique" (Venette, Moon, and Hutchinson 2002). The exporter is responsible for supplying the labor to move, open, and repackage sampled containers or boxes.…”

“…As mentioned in Chapter 2, the statistical literature on sampling for rare individuals demonstrates that the probability of detecting at least one actionable pest at a sampling rate of less than 5 percent can be approximated by an exponential distribution (Venette, Moon, and Hutchinson 2002 refer to it as a Poisson distribution). Since US agricultural inspectors sample at a rate of less than 5 percent from the total import volume selected for a physical inspection, the exponential distribution is appropriate in this empirical application.…”