In the past, I have talked about some of the wonderful aspects of winemaking. But today, I thought I would talk about something that is a real issue in the vineyards and can ultimately have an impact on the quality of the fruit in a vintage; leafhoppers.
Leaf hoppers belong to the family of Cicadellidae. They are insects, which makes them arthropods. Arthropods are organisms that have an external skeleton. There are approximately 22,000 species of Cicadellidae in the world with about 3,000 of those residing in North America. Their lifecycle, which can vary among the individual species, generally follows a basic pattern. The female will insert her eggs into living plant tissue. These eggs remain dormant for anywhere between a month to a year. When the eggs hatch, nymphs go through five molts ultimately reaching their adult stage. This process of molting can take anywhere between several weeks to months.
Within California, there are three major species of leafhopper. The grape leafhopper finds its home within the Tehachapi Mountains, as well as the San Joaquin, Sacramento and the North Coast and Central Coast valleys. The variegated leafhopper is found between southern California up through San Joaquin County and in Napa Country. The Virginia creeper leafhopper is less common, but has been spotted in the Sacramento Valley, Mendocino and Lake counties and the Sierra foothills.
Leafhoppers rely on the dead grass and falling leaves in order to survive the winter. Their lifecycle is dependent on both temperature and the length of daylight. Adults begin to awaken when the temperatures consistently reach 64oF. They immediately begin to feed on the new foliage that is developing on the vine. The females lay their eggs beneath the leaf epidermis on the bottom surface of the leaves. Each female lays between 100 and 140 eggs.
The first generation nymphs can be found along the shoot and as the vintage progresses they molt through four more stages. The first instar is pale white, with red eyes. They do not have their wing pads and measure about 1mm. Progressing through the next four instar stages, its body becomes more yellow, their eyes begin to match their body and their wing pads develop. With each molt, the leafhopper gets larger until reaching its adult size of approximately 3mm.
As the daylight hours begin to shorten, this triggers the leafhopper to enter its reproductive stage. Once under 12 hours of daylight, the females mature, mate, insert their eggs into the host plant and then begin to prepare for overwintering until the next season.
As the leafhopper feeds, they puncture the mesophyll cells on the underside of the leaf in order to suck out the contents. This leaves behind a trail of light brown/yellow or white marks known as “stippling.” This stippling, since the leaf hopper consumed the chlorophyll, causes a reduction in photosynthesis.
It is estimated that each nymph can consume 0.01in2 daily which may not sound like a lot, but multiplying that times the 140 nymphs each female can lay, it is devastating. As the chlorophyll is diminished, chlorosis can occur, causing the leaves to become dry. Within a season, the decreased photosynthesis may prevent the grapes from fully ripening, thereby affecting the quality of the wine. This may also lead to early leaf drop. If early leaf drop occurs across two consecutive vintages, the vine may be harmed for years afterwards. In fact, it may never recover resulting in the death of the vine.
Monitoring and Control
Although there are ways to monitor for leafhoppers in the egg stage, typical monitoring begins in their nymph stage as the eggs are too difficult to see. Monitoring is done by observing the top of the leaves for the white spots. We begin monitoring in mid to late May. At this point of the season, the nymphs are typically in their first or second instar stage and in their peak abundance.
Luckily, there are several natural predators that can help manage the spread of leafhoppers. Some of these include the parasitic wasp, the lady beetle, funnel weaver spider and agrarian sac spider. If the leafhoppers are persistent, removal of the basal leaves and lateral shoots prior to adults emerging is another option.
The use pyrethoids are the typical compound used in the insecticides that kill adult leafhoppers. The compound is produced from Chrysanthemums. If choosing to treat the vineyards organically, it is important to find the aphids in their young stages, as the treatment does not kill the adults. Narrow range oils, neem oil, insecticidal soaps, PyGanic, or kaolin clay may give partial control when nymphs.