Wines & Vines

vineyard views

vineyard views

Cliff Ohmart

July is a very revealing month in the vineyards in my part of the state, and I imagine the same is true for some other parts of the state, too. At this time of year, several things become clear: that we do not really have any good economic thresholds for either leafhoppers or mites; that treatment thresholds used by growers for these two pests are all over the map; that few growers keep quantitative records of leafhopper and mite numbers from year to year; and that some leafhopper and mite treatments are applied out of convenience rather than as a result of a real problem (i.e. I’m going through the vineyard now with an SI material so why not get the hoppers or mites now!).

I find this a very frustrating time of year, too, because it reveals that we have a long way to go in terms of developing good IPM programs for leafhoppers and mites that everyone recognizes and uses.

Are the above statements about treatment thresholds accurate? I have a little experiment for you to try that I think will lead you to the same conclusions. Take 10 growers and/or PCAs out into a vineyard that has a significant population of leafhoppers or mites and ask each one of them what they would do about this situation. I am pretty sure that some will say spray and some will say do nothing. Now both groups can’t be right. So what can we learn from this situation and how can we improve upon it?

Before I begin, I want to point out why I think it is worth discussing leafhopper and mite treatment thresholds in detail. Both of these pests do not develop into serious problems overnight. Even though some growers claim that mites “blow up” all of a sudden, mite populations don’t blow up, particularly if you are dealing with Willamette mites. (What is happening is that you either aren’t checking your vineyards often enough, or you simply missed them until they became bad!) Therefore their numbers can be watched over time to see if a real problem will develop.

Another thing is that winegrapes can tolerate a certain amount of leaf damage caused by these pests without suffering measurable losses in crop yield or quality. Furthermore, delaying a control decision is unlikely to cause a disaster, so you have some time to play with. Therefore, if there is one place in vineyard management that we can achieve pesticide use reduction as a result of monitoring and development of sound treatment thresholds, I think it is with leafhoppers and spider mites.

I want to start this discussion by admitting that developing good treatment thresholds for leafhoppers and mites is not easy. One of the main reasons for this is that both pests are what are called “indirect pests”–they don’t directly damage the fruit, but their feeding on leaves has an adverse effect on photosynthesis, which can then affect fruit quality and/or maturation time.

Quantifying the effects of leaf damage on winegrape production is difficult. The University of California Grape Pest Management manual states that Thompson seedless grapes can tolerate up to 20 leafhopper nymphs per leaf without suffering adverse affects on crop yield or maturity. As far as I am aware, this is the only published treatment threshold for leafhoppers on grapes in California. Furthermore, it was shown that vines can lose 20% of their foliage area without crop yield or maturity being affected. However, in talking with lots of growers, the situation in winegrapes is not as simple as these figures imply. There are many things that affect action thresholds for mites or leafhoppers on winegrapes. Here are some examples: Significant leaf damage due to hoppers or mites is more serious early in the season than just before harvest; * Ten nymphs per leaf with no previous leaf damage present is a more tolerable level than if there is already significant leaf damage; * Some winegrape varieties are more sus ceptible than others; * Significant leafhopper or mite numbers on basal leaves in a “California sprawl” trellis system early in the season can be tolerated if they stay on basal leaves, since these leaves are not very important for photosynthesis later in the season, or for protection of fruit (in fact when one does leaf removal, these leaves get taken off); * On the other hand, in a vertical shoot-positioned trellis, basal leaves are the only ones giving the grapes any shade at all and if these leaves suffer too much damage they may be shed exposing the fruit to direct sun.

The above discussion points out very clearly that more research is needed on leafhopper and mite numbers, and their relationship to leaf damage, yield, fruit maturity and quality. However, looking at the priority list of winegrape pest problems in California, and the number of people available to do applied research on these problems, I doubt that any work is going to be done in this area. Therefore, growers are going to have to determine their own treatment thresholds for leafhoppers and mites in their own vineyards.

The first step in determining treatment thresholds for mites and leafhoppers is realizing that you need to be willing to delay treatment beyond the threshold you have used in the past. Keep in mind that one never learns anything by spraying. That is because when spraying is done the pests are killed (usually), and you already knew that would happen. On the other hand, when you don’t spray you learn one of two things– either you didn’t need to spray and you are glad you didn’t, or you wish you had sprayed because you suffered some unacceptable crop damage. If you didn’t spray and no significant crop damage occurred, then you have just discovered that your previous treatment threshold was too low. If you suffered economic loss then the threshold to be used in the future should be lower than the one you just used. Another option, which is the more “scientific” one, is to spray one part of the vineyard and not another and compare the yield and grape quality from the two parts.

The second step in determining action thresholds for leafhoppers and mites is to keep quantitative records of their numbers on the vine and, if possible, a measure of the leaf damage caused by these numbers. By keeping quantitative records you can then correlate pest levels with effects on your crop. Although all of us pride ourselves on being able to remember pest problems from year to year in various vineyards and also our ability to judge pest problems visually, the only way we are going to make progress in refining our treatment thresholds is to collect and compare quantitative information.

Probably the most useful quantitative measure for leafhoppers is counting the number of nymphs per leaf. In cases where both nymphs and adults are causing leaf damage, an estimate of adult numbers is also necessary. This is difficult because they cannot be counted in any meaningful way. I suggest developing a simple scale of light, moderate and heavy. Although this can be very subjective, I think with a bit of practice you can develop a useful system. Mite numbers are best expressed as percent leaves with mites. Research has shown a very strong correlation with numbers of mites per leaf and percent leaves infested with mites. It is simply not practical to count numbers of mites per leaf.

The last step in the process of developing treatment thresholds for leafhoppers and mites is to decide what crop characteristics you will use as measures of damage. The simplest are yield and maturation time (i.e. time to reach acceptable sugar level). Leaf damage is undoubtedly affecting other fruit quality characteristics, but they are much more difficult to collect and analyze. I am a big believer in mastering the simple things first and then moving on to more complicated things if time, desire and money allow.

Before I close, I want to briefly discuss the topic of treating more from convenience than because leafhopper and/or mite numbers are at a problem level. If leafhopper and/or mite numbers are increasing and you are about to go through the vineyard one last time with a liquid fungicide, it seems logical to tank mix it with a miticide or leafhopper material and save the cost of another trip later. However, the whole idea behind waiting to treat until the economic threshold is reached is that pest numbers may never reach the threshold, making a spray unnecessary. If you treat before the pest reaches the threshold, you will never know if it would have been reached. The risk, of course, is that the economic threshold will be reached later and an extra trip through the vineyard will have to be made, which adds to management costs.

Developing your own treatment thresholds for mites and leafhoppers and sticking to them takes time and dedication. It really comes down to how serious you are about implementing one of the foundations of the IPM approach to pest management, which is only using pesticides when monitoring and sound economic thresholds justify their use. One final note–while spraying Out of convenience because you think things will get bad later may seem like the smart thing to do, it is definitely not practicing IPM.

(Dr. Ohmart is director of research and integrated pest management at the Lodi-Woodbridge Winegrape Commission.)

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