Issue link: http://winesandvines.uberflip.com/i/62516
GRAPE GRO WING ing costs money and has environmental side effects. Growers make spray decisions because they think that if spray is not applied, unacceptable damage will occur. A grower decides not to spray if he thinks the number of leafhoppers present will not cause significant damage. For the leafhopper situation in each vineyard represented on the graph, the grower and PCA felt their decision was cor- rect—after all, who would purposefully do something they thought was wrong? Counting vs. not counting The missing piece that explains the pattern is the fact that I was making quantitative estimates of leafhopper counts while growers and PCAs were not. At the time of the project, very few growers and PCAs were making quantitative counts of leafhopper nymphs on leaves. Most were making visual estimates and classifying pop- ulations as light, moderate, heavy and so forth. If a decision was made to spray, the perception was that the leafhopper population was at an unacceptable level. Without numerical counts of leafhopper nymphs, the only way to summarize the results would be that leafhoppers were perceived to be at treatable levels in the sprayed vineyards and not in the untreated. It is only when leafhopper counts are superimposed on these same vineyards, however, that one sees the real variation in economic thresholds among growers and PCAs. The key, in my opinion, is that without the numbers we would have no idea that different economic thresholds were being used. The second thing I learned from the graph is that one of the biggest challenges of pest-management decision-making is Statement of the Ownership, Management and Circula- tion as required by the Act of Congress of October 23, 1962, Section 4369, Title 39, United States Code of Wines & Vines, published monthly in San Rafael, CA. 1. The date of this filing is October 1, 2011. 2. The location of the office of the publisher is 65 Mitchell Blvd., Ste. A, San Rafael, CA 94903. 3. The name and address of the publisher is: Chet Klingensmith, 65 Mitchell Blvd., Ste. A, San Rafael, CA 94903. The name and address of the editor is: Jim Gordon, 65 Mitchell Blvd., Ste. A, San Rafael, CA 94903. 4. Wines & Vines is owned by Wine Communications Group, a California corporation. 5. For the year preceding this filing, the average number of copies printed per issue was 5,409. Paid circu- lation averaged 3,449. Free distribution averaged 1,560, and total distribution averaged 5,009. 6. For the single issue published nearest the filing date, 5,432 copies were printed, Paid circulation was 3,275. Free distribution was 2,004, and the total number of copies distributed was 5,279. I certify that the above statements are correct and complete. —Chet Klingensmith, Publisher, Wines & Vines 110 Wines & Vines nOVeMBeR 201 1 determining if perceived risk of pest damage is in fact real risk. Experience and research has shown that leafhopper counts of less than five nymphs per leaf clearly do not present a real risk of damage. So, for vineyards on the left side of the graph that were sprayed for leafhopper, the perceived risk was high but real risk was low. On the other hand, research and experience has shown that 20 or more nymphs per leaf present a real risk of damage. So, for vineyards on the right side of the graph that were not sprayed, real risk is much higher than perceived risk. For vineyards with low leafhopper counts that were not sprayed, perceived risk was low and it matched real risk. For vineyards with high leafhop- per counts that were sprayed, perceived risk was high and it also matched real risk. What about perceived versus real risk of pest damage in vine- yards where the leafhopper counts are in the middle part of the If no damage occurred, I know that my economic threshold for leafhoppers has not been exceeded. graph? In my opinion, this is where pest-management decision- making is the most difficult. To be blunt, it is a no-brainer to forego spraying when there are almost no leafhoppers, just as it is to spray when there are a lot of leafhoppers. It is that middle ground where the separation of perceived from real risk is the most difficult. The only opportunity to learn in this situation is to not spray. If I choose to spray I learn nothing, because if an efficacious mate- rial is used and it is applied properly I know the outcome. There will be no damage because the leafhoppers will be controlled. If I choose not to spray, however, I learn one of two things: Either I am glad I did not spray because no damage occurred, or I wish I had sprayed because damage did occur. If leafhopper nymph counts were used in the decision-making, then one can learn quite a bit from this exercise. If no damage occurred I know that my economic threshold for leafhoppers has not been exceeded and I can tolerate more than what was there at the time of the monitoring. On the other hand, if damage did occur, then I know my economic threshold is lower than the count in the vineyard at the time of monitoring, and next time I need to spray at a lower number. In this way, a grower and/or PCA can develop an accurate, numerically based economic threshold for leafhoppers in any vineyard. This ap- proach can also be used for other insect and mite pests that can be monitored quantitatively. In a sense it is trial and error, but the alternative is to be spraying when it is not necessary. Since there are no scientifically validated economic thresholds for most of our vineyard insect and mite pests, and given the num- ber of varieties and strategies for growing winegrapes, I know of no other way to minimize insecticide sprays (either organic or conventional) than by developing one's own economic threshold in this manner for each situation. Other factors in perceived risk Perceived risk can be affected by a number of things besides the lack of quantitative data—for example, how often a vineyard is monitored. If I see a mite problem developing in a vineyard, and