by David Haviland, Entomology advisor, UCCE Kern Co.
Effective management of glassy-winged sharpshooter (GWSS) requires coordinated efforts on a regional scale between citrus and grape growers. Since late 2015 citrus growers in the General Beale and Edison regions of Kern County have made two insecticide applications to combat the overwintering GWSS populations. Now it is grape growers’ turn to take action.
Late April, 2016 marked the transitionary period when GWSS from overwintering hosts started laying eggs in grapes. Last week (May 4th, 2016) I made a visit to a table grape vineyard in the General Beale area where three of us found more than 40 egg masses in approximately 10 minutes. We have heard similar reports from CDFA surveyors that are keeping tabs on the situation. Many of the egg masses had hatched recently, many were still viable, and a few had been parasitized. Evaluations of the GWSS being found on sticky cards also revealed that a high percentage of the females being captured had a white patch of brochosomes on the wings, indicating that they are gravid and laying eggs.
Small nymphs that hatch from sharpshooter egg masses are not an immediate threat for spreading PD because they don’t move very far. However, once these nymphs become adults, they can be responsible for significant amounts of vine to vine movement of the disease. In order to prevent this spread, grape growers right now should be in the process of treatments for sharpshooters.
The standard spring treatment for GWSS in grapes is soil-applied imidacloprid in late April to early May. At this time a new flush of roots quickly takes up the product where it is available in the plant to kill adult GWSS that are moving in from overwintering hosts like citrus to lay eggs, as well as any nymphs that hatch from egg masses. When treating, ensure maximum efficacy by using a top of the label rate and sufficient pre-irrigation and post-irrigation to move the active ingredient into the root zone. By making this treatment, growers are ensuring that any nymphs that hatch in grapes will die before spreading the disease.
History has shown that the combined effects of removing PD-positive vines, treating overwintering hosts, and making a spring insecticide treatment to grapes is a very effective trifecta for mitigating the May 2016 2 threat of this disease. As of early May, the first two steps are complete, and it is now up to grape growers to make sure that GWSS that enter their vineyards will suffer the consequences of a terminal mistake.