Temporal decay in the efficacy of insecticide treatments against Andean potato weevils (<i>Premnotrypes</i> spp.), as applied by farmers.

<p>The x-axis shows the parameter estimate ± SEM associated with the effect of a single insecticide application on the proportion of tubers infested with weevils (sqrt-transformed). The y-axis shows the month of the insecticide treatment.</p

Forward stepwise development of the global (least parsimonious) statistical model explaining Andean potato weevil infestations.

<p>The x-axis shows the progressive addition of explanatory variables in order of their contributions to lowering AIC. The y-axis shows cumulative reductions in AIC from the AIC associated with using only the mean to estimate infestations. The first dashed line shows the point where the addition of variables started to penalize the AIC, whereas the second dashed line shows the point where this penalty started to exceed two AIC values. The global model included all variables before the second dashed line.</p

Standardized predicted impacts of explanatory variables on Andean potato infestations.

<p>The model is initially set to predict infestations for a field with no pesticide applications and with mean (for continuous variables) or most common (for ordinal and categorical variables) values for all other explanatory variables. For continuous explanatory variables, bars reflect predicted changes in infestations in response to a one standard deviation increase in the explanatory variable. For ordinal explanatory variables, the bars reflect predicted changes in infestations in response to a single unit increase in the explanatory variable; except for the number of hillings, for which only a decrease could maintain predictions within observed bounds.To obtain multi-model predictions, parameter estimates were multiplied by their corresponding parameter weights. Hence, predicted effects are “attenuated” for explanatory variables with parameter weights smaller than 1.</p

Parameter estimates ± SEM for the best models predicting Andean weevil infestations based on local factors only (i.e. Best local), landscape factors only (i.e. Best landscape) or both local and landscape factors together (i.e. Best combined).

<p><i>Notes</i>: Lower AIC values suggest better model performance. The Akaike weight, <i>w</i>_i, is interpreted as the relative probability that a given model is the best in the set. The models included control variables for observer effects (not presented in the table).</p>*<p><i>P</i>≤0.05.</p>**<p><i>P</i>≤0.01.</p

Community knowledge worker assisting farmers with record-keeping activities associated with their potato harvest.

<p>The individual pictured in this manuscript has given written informed consent (as outlined in the PLoS consent form) to appear in the published photo. Photo credit: Soroush Parsa.</p

Local and landscape features of hypothesized to exert important influences on Andean potato weevil infestations (<i>Premnotrypes</i> spp.).

a<p>Farmers apply a layer of ash directly below the potato seed at the time of planting; this practice is intended to kill potato weevils.</p>b<p>Fertilization can influence crop defenses against herbivores <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036533#pone.0036533-Throop1" target="_blank">[17]</a>.</p>c<p>Many agronomists recommend hilling the plants (piling dirt up around the stem of the plant) higher to lengthen the distance weevil larvae must travel to find tubers.</p>d<p>Early harvest shortens the exposure of tubers to neonate larvae <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036533#pone.0036533-Khne1" target="_blank">[18]</a>.</p>e<p>Larger fields have lower perimeter to area ratios and have been suggested to have lower infestations <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036533#pone.0036533-Rios1" target="_blank">[19]</a>.</p>f<p>Early emerging plants may experience greater infestations <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036533#pone.0036533-Calderon1" target="_blank">[20]</a>.</p>g<p>Planting density may influence the abundance of many insect pests <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036533#pone.0036533-Speight1" target="_blank">[21]</a>.</p>h<p>Planting potatoes following a potato planting should lead to very high infestations <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036533#pone.0036533-Rios1" target="_blank">[19]</a>, but implementing a single host free period should eliminate this risk. Rotation 2007 indicates if potatoes were sown in the field the previous season while Rotation 2006 indicates if potatoes were sown there two seasons before the study.</p>i<p>The study hypothesized that weeds may serve as refuges for adult weevils before potato plants emerge.</p>j<p>Weevils are poorly adapted to elevations above 3,700 meters <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036533#pone.0036533-Khne1" target="_blank">[18]</a>.</p>k<p>The study hypothesized that greater soil erosion in steeper slopes may increase tuber exposure to weevils.</p>l<p>Modern cultivars like Yungay may be more susceptible to insect pests <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036533#pone.0036533-Harris1" target="_blank">[22]</a>.</p>m<p>The study was interested in exploring any soil influences on weevil infestations without any strong <i>a priori</i> expectations.</p>n<p>A measure of potato fields sown the within 100 m of the focal potato field; these current fields dilute the effect of immigrating weevils <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036533#pone.0036533-Parsa2" target="_blank">[15]</a>.</p>o<p>A measure of potato fields harvested the previous season that lie within 100 m of the focal potato field; these previous fields may be sources of overwintering weevils that immigrate into focal fields <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036533#pone.0036533-Ortiz1" target="_blank">[14]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036533#pone.0036533-Parsa2" target="_blank">[15]</a>.</p>p<p>Potato storage units are facilities adjacent to farmer houses and are known to concentrate high densities of overwintering weevils that may immigrate into focal fields <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036533#pone.0036533-Ortiz1" target="_blank">[14]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036533#pone.0036533-Parsa2" target="_blank">[15]</a>.</p