Spider Mites and Dry and Hot weather
Greg Loeb, Cornell Entomology
August 9, 2016

August is often the time we see the emergence of spider mite problems in vineyards and this summer it is particularly important to scout for them because we often see more mite problems under dry conditions. There are several contributing factors but temperatures are usually above average when its dry and these higher temperatures lead to more rapid mite development and more generations and potentially higher populations. Also, beneficial mites often are not able to keep up with the population growth of spider mites with hot temperatures. Perhaps an even more important factor is that with drought conditions the vines will shut their stomates during part of the day to help conserve water and this has the result of increasing leaf temperatures due to lack of transpiration and evaporative cooling. Also, the vine stops adding new leaves earlier in the season in drought conditions and this has the effect of concentrating mites on less leaf material. 
There are good reasons to scout your vineyard for spider mites and/or spider mite damage. I reviewed mites in my spring update, but briefly, there are two species of spider mites that attack grapes in the Eastern US, two-spotted spider mite (TSSM) and European red mite (ERM), but ERM typically is the more common. It is important to know the difference between the two species since some miticides are more effective against one than the other. As the name indicates, ERM is reddish in color and lays red eggs. Adult female TSSM tend to have large black spots on the top of the abdomen but this is a pretty variable. TSSM eggs are clear to opaque. TSSM tends to stay on the bottom side of leaves and produces obvious webbing while ERM can be found on either side of the leaf and does not produce much webbing. Both species have the capacity to go through a number of generations during the season. Because of their small size, it is often difficult to know if you have mites. Foliar symptoms (bronzing of leaves) are one clue, although if you have wide spread, obvious symptoms then economic damage may already be occurring. The working threshold for spider mites (TSSM and ERM combined) in our area is 7 to 10 mites per leaf, although this will vary depending on health of the vineyard, crop load, value of the grape, etc. The impact of mite feeding on grapes includes reduced photosynthesis, delayed accumulation of sugars thereby delaying harvest date, and the potential of reduced yield the following season. I suggest sampling at least 50 mid-shoot leaves from both the edge and the interior (25 leaves each) of a vineyard block, examining both sides of the leaf. A hand lens will be necessary to see the mites for most people. Even with a hand lens, it is challenging to count the mites. Thus, we recommend estimating the proportion of leaves infested with one or more mites and use something like 50% infested as a treatment threshold. A leaf is considered infested if it has one or more spider mites. Remember to keep rough track of which species is most common.
There are several chemical options available for mite control in New York and Pennsylvania: Vendex [fenbutatin-oxide], Agri-Mek and several generics [abamectin], Nexter [pyridaben] (not on Long Island), Acramite [bifenazate], JMS Stylet Oil [aliphatic petroleum distillate], Zeal Miticide1 [etoxazole], Onager or Savey [hexythiazox], Danitol [fenpropathrin], Portal [fenproximate] and the newly labeled miticide called Nealta [cyflumetofen]. Read labels carefully. JMS Stylet Oil is not compatible with a number of other products including Captan, Vendex, and sulfur. Also, although Stylet Oil can help with mite problems, it is not likely to provide complete control in problem vineyards. Nexter is very effective against ERM but higher rates should be used for TSSM. Agri-Mek currently has TSSM on the label but not ERM, although in apples both species are on the label. Acramite includes both TSSM and ERM, although it calls for higher rates for ERM. The new label for Zeal miticide 1 includes both ERM and TSSM in NY whereas the old label only had TSSM. You need a 2(ee) recommendation, which is readily available, for use against ERM with older material. Since Zeal miticide 1 affects eggs and immatures, it is advised to apply before populations reach damaging levels to give the material time to work. Similar advice can be applied to Onager, Savey and Portal. Danitol and Brigade (two-spotted only) are broad-spectrum insecticides that also have fairly good miticidal activity. Pyrethroids are hard on beneficial mites, however.

Spider mites are often thought of as a secondary pest. In other words, something must happen in the vineyard that disrupts their natural control by predators, particularly predatory mites, before their populations can increase to damaging levels. Since Danitol and Brigade have miticidal activity they would not be expected to flare spider mites in the short term. However, in the past, spider mites have been quick to develop resistance to frequent use of pyrethoids. This may or may not happen but it is worth keeping in mind. One of the first things to watch out for is initial good suppression of mites followed by a resurgence indicating the spider mites recovered more quickly than the predatory mites. The other miticides (Vendex, Onager, Savey, Zeal, Acramite, Nealta, and Nexter) are generally pretty easy on natural enemies, although at high rates Nexter can negatively affect predatory mites. Overall, paying attention to conserving predatory mites can pay economic dividends since miticides are quite expensive.

In summary, given how dry things are its worth getting out in the vineyard and scouting for mites and mite damage. While you are out there, also scout for grape berry moth and leafhoppers. As of today (August 9, 2016) we are at about 1600 DD using the grape berry moth phenology model (in Geneva, NY) at the NEWA web site (http://newa.cornell.edu/), so its about the right timing for the third flight of grape berry moth. As we get closer to harvest, you also will want to be cognizant of multicolored Asian beetle in clusters and Drosophila fruit flies (see my spring review for more information). 
For questions, contact Greg Loeb, Cornell Entomology at gme1@cornell.edu 315-787-2345.






European Red Mite (jpg; 15KB)

• Adult European red mite on bottom side of leaf

Two Spotted Spider Mite (jpg; 11KB)

• Two spotted spider mite adult. Photo: Jack Clark, UC Davis