Question: We bought a small vineyard and have noticed that the whitish-yellow speckles and blotches on our grapevine leaves are looking worse and worse. Barb B., Placitas
Answer: I had the pleasure of visiting this site in Placitas a few weeks ago, so I examined the grape leaves up close—remarkably close. Using a hand lens and looking very carefully under the leaves, I took photos of tiny white insects, some of which had red spots on their backs. To me, the insects combined with the leaf damage symptoms point to a common pest in our area: grape leafhoppers, likely the Western grape leafhopper in particular.
I invited NMSU Extension Viticulture Specialist Dr. Gill Giese to help us understand what can and should be done to manage this problem so late in the season:
“The grapes have been mostly harvested by now. The days are getting shorter, and the vines are not growing as actively as they did in mid-summer. Given the wide swings in New Mexico weather lately and depending on where you are in the state, the first fall frost may come sooner rather than later. And then, the leafhoppers show up.
“Should I bother controlling, or attempting to mitigate in any way, the presence of the western grape leafhopper (Erythroneura elegantula) and the possible damage they are doing as they feed on my grapevines this late in the season?
“This is a great question. The short answer is yes, you should control the leafhoppers’ presence and reduce as much as possible the damage they inflict on your vines. Why? Because, as you are aware, the grapevine is a perennial plant that continues growing each year. So, what happens to the vine this year—insult or injury—will likely be evident and have a bearing on what the vine does next year.
“Right now, the vines are in post-harvest mode, and the carbohydrates that are being manufactured via photosynthesis will be stored in the roots, trunks, and cordons (arms). These reserved carbohydrates and mineral nutrients will support the vine’s growth next year from budburst until flowering.
“Leafhoppers, both adults and developing nymphs, puncture leaf cells with their piercing mouthparts and literally suck out the contents. This causes the yellow and white blotchy look of the infested leaves. The feeding compro-mises and reduces the amount of photosynthetic activity in the leaves, and if left untreated, the leaves will dry up and fall prematurely.
“Can the vine stand some leaf loss? Yes, it likely can, but what begins as a few lost leaves can rapidly become many more when leafhoppers are left unchecked.
“For now, especially if you have just a few vines in the backyard, control the leafhoppers. A single application of an insecticide labeled for use on grapes and for controlling grape leafhoppers should do the trick; use the lowest labeled rate. Applying the insecticide late in the evening will help minimize its impact on pollinators. As with any pesticide, always follow label directions. The label is the law.”
For more information, including a list of registered insecticides approved for both conventional and organic production, check out the NMSU Extension Guide H-332 “Managing Grape Leafhoppers on New Mexico Grape Vines” (https://aces.nmsu.edu/p ubs/_h/H332/welcome.html), which further explains these insects and how to control them for homeowners and commercial growers. From weed control and fertilizer management (like other pests, these leafhoppers tend to prefer over-fertilized plants) to routine scouting and simply removing fallen leaf litter that may be harboring insect pests over winter, this resource has a lot to offer folks who are worried about leafhoppers on their grapevines.
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For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms (http://desertblooms.nmsu.edu/) and the NMSU Horticulture Publications page at http://aces.nmsu.edu/pubs/_h/.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center.