Eighty percent of the wine made in the U.S. is from California. Grapevine red blotch disease delays grape berries from maturing and slows sugar accumulation, all important for making great wine. Grapevine red blotch virus is transmitted from vine to vine by the threecornered alfalfa hopper, a kind of treehopper. Before grapevine red blotch disease, the threecornered alfalfa hopper was only an occasional pest.

Area IPM Advisor Cindy Kron and colleagues developed a degree-day model to predict when to manage the threecornered alfalfa hopper. It is thought that adults arrive in the vineyard in late winter then lay their eggs in the cover crops and weeds found in the vineyard. The immature treehoppers that hatch from these eggs stay on the cover crops and weeds until they become adults around mid-May.

If tillage occurs too early, less nitrogen will be available in the soil. If tillage occurs too late, the threecorned alfalfa hopper adults can migrate into the vines spreading grapevine red blotch virus. Tillage needs to be timed so the maximum amount of nitrogen is incorporated but before the treehoppers become adults.

Removing cover crops and nearby host vegetation before this time would reduce the treehoppers in the vineyards and may reduce the spread of grapevine red blotch disease. Nymphs do not have wings so they cannot move very far. Grapevines are not a good food source for nymphs, so they would starve if the plant hosts that they feed on are removed by tillage. The threecornered alfalfa hopper was observed to have one to two generations per year in California so reducing the first generation could greatly reduce the local population.

However, tillage needs to be timed carefully. Growers commonly plant winter leguminous cover crops (a great host for the threecornered alfalfa hopper) in vineyards to sustainably add nitrogen to the soil. Cover crops and resident vegetation are typically tilled under in the spring. If tillage occurs too early, less nitrogen will be available in the soil. If tillage occurs too late, the threecorned alfalfa hopper adults can migrate into the vines spreading grapevine red blotch virus. Tillage needs to be timed so the maximum amount of nitrogen is incorporated but before the treehoppers become adults.

Unfortunately, small nymphs are undetectable by sweep net monitoring. Small nymphs are 1.4 to 3 mm (about 1/16 to 1/8 inch or the thickness of a grain of rice) and they blend easily into the leaf litter picked up in the sweep net due to their light or clear color. Traditional sweep net monitoring is not a technique that can be used for small nymphs of the threecornered alfalfa hopper to determine when tillage should occur.

Kron’s degree-day model predicts when treehopper nymphs are small and when tillage should occur. Degree-day models use the fact that insects are cold-blooded animals and their growth depends on the air temperature. Research can determine how much accumulated heat is needed for insects to move from one point in their life cycle to another. The degree-day model for threecornered alfalfa hopper was tested with two years of field data and successfully predicted large nymph and adult emergence timing each year. Future work includes further validation in other wine grape growing areas of California.

Kron’s article: Bick EN, Kron CR, Zalom FG. 2020. Timing the implementation of cultural practices for Spissistilus festinus (Hemiptera: Membracidae) in California vineyards using a stage-structured degree-day model. Journal of Economic Entomology 113(5): 2558–2562. doi: 10.1093/jee/toaa165.

Long-Term Impacts

Kron’s research was performed in Oakville, CA. The degree day model, if validated in other areas of California, could help time tillage of cover crops and resident vegetation that are hosts of the threecornered alfalfa hopper. It is anticipated that lowering treehopper numbers should decrease infection of grapevines with grapevine red blotch disease. If spring tillage is already practiced, costs to time tillage for threecornered alfalfa hopper should be minimal.