Issue link: http://winesandvines.uberflip.com/i/62516
CLIFF OHMART Vineyard View Perceived Risk vs. Real Risk E arly in my career, academia and years of experience as a research scientist had me convinced that pest- management decision-making should be objective and based on measurable, quantifiable, scientific observa- tions. However, once I became a practicing pest-con- trol advisor (PCA) working with California's orchard crops and observing other pest-management consultants in action, I saw that pest-management decision-making often wasn't based on objective, scientific observations. As time went on, I realized the key to good pest-management decision-making on the farm was to determine when the per- ceived risk of pest damage matched real risk. When I migrated from the orchard to the vineyard, I found the situation to be the same there, too. With respect to insects and mites, it is possible to monitor for these pests and base actions on economic thresholds. Disease management is very different in that one cannot monitor for dis- eases and hope to base control actions on economic thresholds. Once a plant part becomes infected with a pathogen, it is too late to do anything about it; therefore, disease-management decision- making is primarily based on monitoring conditions conducive to plant infection by the pathogen. When conditions are right for infection, actions are taken to try and prevent transmission. One project I was involved with a number of years ago involved 45 growers working with 16 PCAs to manage grape leafhopper in vineyards. I monitored the vineyards weekly throughout the growing season by dividing each one into four equal quadrants, counting the number of leafhopper nymphs on 10 leaves in each quadrant and calculating an average number of nymphs per leaf for the vineyard. The monitoring data was shared each week with the growers and PCAs who also monitored the vineyards for leafhoppers. Highest Seasonal Leafhopper Count in a Vineyard Sprayed Unsprayed Vineyard number Highlights • Pest-management decisions are based on the perception of risk that pest damage will occur. • That perception of risk is affected by several factors, some of which are unrelated to pest levels. • It is important to understand the difference between perceived risk and real risk. • Sound pest-management decisions are made when perceived risk of pest damage matches real risk. 108 Wines & Vines nOVeMBeR 201 1 Explaining the graph The height of a bar in the accompanying graph (above) rep- resents the highest average leafhopper nymph count per leaf recorded in each of 56 vineyards during the year. The blue bars represent the nymph counts from vineyards that were sprayed for leafhoppers, and the purple bars represent nymph counts from vineyards that were not sprayed for leafhoppers. The position, number and pairing of a vineyard on the X-axis does not mean anything other than its order from left to right. I drew the graph so that in each of the groups, sprayed and unsprayed, the bars went from lowest to highest from left to right. Twenty-nine vineyards were sprayed for leafhoppers, and 27 were not treated. When I first plotted the graph at the end of the project year I was puzzled. Taken as a whole, the leafhopper counts in unsprayed vineyards were about the same as the counts in the sprayed vineyards. The data from the two groups were almost identical. Some vineyards with very low leafhopper counts were sprayed while some were not; some vineyards with high leafhopper Nymphs per leaf