Comparison of Two Different Statistical Methods for Assessing Insecticide Efficacy in Small Plot Trials Targeting Wireworms, Limonius californicus (Mannerheim) (Coleoptera: Elateridae), in Potato

  • Jeffrey A. Lojewski
  • Erik J. WenningerEmail author


The efficacy of insecticides is often assessed in small-plot field trials that compare insect damage and yield loss relative to an untreated check, typically in a randomized complete block (RCB) design. When insect damage is not uniformly distributed across a field, differences among treatments might reflect differences in local pest pressure rather than differences in treatment efficacy. One potential way to account for patchy distributions is to use a “running check” in which each plot contains an insecticide treatment and its own untreated check. Any benefits of this approach must be weighed against the additional labor involved. Wireworms (Coleoptera: Elateridae) are soil-dwelling insects that typically exhibit patchy distributions that can confound insecticide efficacy trials. Several species of wireworms feed on potato tubers, causing damage that makes the tubers unmarketable and, aside from crop rotation, insecticides remain a cornerstone of wireworm management in potato. The current investigation evaluated insecticide efficacy trials targeting wireworms in potato by comparing two different analyses for the same data sets spanning five years and seven different studies. Data collected using the running check approach were analyzed using analysis of covariance and compared with analysis of variance as if the experiment were designed as a RCB. In a majority of cases, both methods resulted in similar levels of statistical power and similar conclusions; however, including data from the running check as a covariate did sometimes eliminate or reveal differences between treatments compared to ANOVA. Overall, given the similarity of the results and the additional labor involved in using running checks, the standard RCB design would have been appropriate for the experiments assessed here. However, for wireworms and other pests that exhibit patchy distributions, care must be taken when designing experiments and interpreting results.


Tuber Feeding damage ANOVA ANCOVA Power analysis Statistical methods 


La eficiencia de los insecticidas a menudo se evalúa en ensayos de pequeños lotes de campo, que compara el daño del insecto y la pérdida de rendimiento en relación con el testigo, típicamente en un diseño de bloques completamente al azar (RCB). Cuando el daño por el insecto no está uniformemente distribuido en el campo, las diferencias entre los tratamientos pudieran reflejar diferencias en la presión local de la plaga en vez de las diferencias en la eficacia de los tratamientos. Una manera potencial a considerar para distribuciones por manchones es usar un “testigo de corrido”, en el que cada lote contiene un tratamiento de insecticida y su propio testigo sin tratar. Cualquier beneficio de este enfoque debe ser pesado contra el trabajo adicional involucrado. Los gusanos de alambre (Coleoptera: Elateridae) son insectos del suelo, que típicamente exhiben distribuciones por manchones que pueden confundir los ensayos de eficacias de insecticidas. Varias especies de insectos de alambre se alimentan de los tubérculos de la papa, causando daño que hacen que los tubérculos no sean aptos para el mercado, y, aparte de la rotación de cultivos, los insecticidas permanecen como piedra angular del manejo del gusano de alambre en papa. La investigación actual evaluó la eficacia de los ensayos de insecticidas con los gusanos de alambre en papa como objetivos, mediante la comparación de dos análisis diferentes para el mismo grupo de datos abarcando cinco años y siete estudios diferentes. Los datos colectados usando el enfoque del testigo de corrido se analizó usando un análisis de covarianza y se comparó con un análisis de varianza como si el experimento fuera designado como un RCB. En la mayoría de los casos, ambos métodos resultaron en niveles similares de poder estadístico y conclusiones similares; no obstante, al incluir los datos del testigo de corrido como una covariación, en algunas ocasiones eliminó o reveló diferencias entre los tratamientos en comparación con ANAVA. En general, considerando la similitud de los resultados y del trabajo adicional involucrado en el uso de testigos de corrido, el diseño RCB estándar pudiera haber sido apropiado para los experimentos analizados aquí. No obstante, para el gusano de alambre y otras plagas que exhiben distribuciones en “manchones”, se debería tener cuidado cuando se diseñen experimentos y en la interpretación de los resultados.



William Price provided helpful advice on statistical analyses. For technical assistance, we thank: Lucy Standley, Anastasia Stanzak, Amy Lockner, Dan Henningsen, Dave Walker, Dave Ruhter, Megan Williams, Neyle Perdomo, Jessica Vogt, Vince Adamson, and Wyatt Shewmaker. Funding for this project was generously provided in part by numerous chemical companies that funded the underlying efficacy trials.


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Copyright information

© The Potato Association of America 2019

Authors and Affiliations

  1. 1.Department of Entomology, Plant Pathology, and Nematology, Kimberly Research and Extension CenterUniversity of IdahoKimberlyUSA

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