Understanding the Threat of European Grapevine Moth and Its Remedies

European Grapevine Moth

Author:

Terrraview

Imagine being quarantined over an insect. That’s what California winegrowers experienced in 2009 when the larvae stage of a grapevine moth chewed its way through the flower and grape clusters of a single growing season and invited Botritys cinerea fungus to set in and destroy the entire crop.

Michigan State University Extension Pest Control Office,
UC Agriculture and Natural Resources Statewide Integrated Pest Management Program

Soon nine counties in California dealt with the infestation in their vineyards, which is why quarantine procedures were put in place to contain its spread. The quarantine order described guidelines for the proper transportation and disposal of grapes and other favored plant species in Fresno, Lake, Mendocino, Merced, Monterey, Napa, San Joaquin, Santa Clara, Solano, and Sonoma counties. Fortunately, the quarantine worked and, according to the U.S. Department of Agriculture, the state of California eradicated the grapevine moth in 2016 with help from Chilean and Italian winegrowers. Since then, the California winegrowers believe the pest arrived earlier than 2009, and possibly as early as 2005, when they started noticing Botritys infections.

Lobesia botrana, also known as the European grapevine moth (EGVM), was first spotted and identified in Southern Italy. While California is safe for now, vigilance continues as vineyards in Southern Michigan reported spotting EGVM egg-laying at high-pressure sites in July. Western Europe, North and West Africa, the Middle East, and eastern Russia are all experiencing EGVM, too. Chile spotted the pest in their vineyards in 2008, prior to the discovery in California. Chile’s response to EGVM helped shape California’s response to it in subsequent years. More recently, Japan dealt with the infestation of the EGVM. While the grapevine moth enjoys several types of fruit berries and flowers, Vitis Vinifera is a preferred host.

Different Generations of Larvae Cause Different Types of Damage

The European grapevine moth has an annual life cycle of two generations in northern Europe, three generations in southern Europe, and even a partial fourth generation in warmer regions of Spain, Greece, Jordan, and Egypt. The adult moth, or tortricid, is approximately 6-8 mm long, with a half-inch (11-13 mm) wingspan. The wings are tan-cream, with bluish-gray blotches and brown and black markings. The adult moth’s lifespan is 1-3 weeks depending on climatic conditions.

The majority of females mate only once, although they are capable of mating multiple times. When the adult female tortricid lays eggs, these flat, lentil-shaped eggs are visible to the naked eye. The eggs are laid singly (alone) on or near smooth surfaces of developing flower clusters in spring by the first generation, and on berries by the second and third generations. Initially, the eggs are iridescent creamy white, turning yellow as the embryo develops, and later black when the head of the developing larva appears. The larva emerges from the edge of the egg and leaves the translucent, iridescent chorion (outer shell of the fetal membrane) attached to the flower cluster or berry. Hatching of eggs occurs anywhere from 3-10 days after they were laid, depending on ambient temperatures. A female can lay 35 eggs per day for six days, with a mean of 80-140 eggs laid per female, depending on the generation.

In May and June, the first-generation larvae will web flower parts together and feed on individual flowers and pedicels. They may enter the peduncle—the main stalk—and cause the bunch to dry up. Damage from redbanded leafroller, which causes rolling of the leaves as the name implies, can be mistaken for EGVM at this time, so it is important to identify the larvae to determine the appropriate management strategy. Larval development is completed in three to four weeks, depending on temperature. The first-generation population tends to be the largest, although it is not the most damaging. The first generation is shorter than the summer generations. Larvae feeding on flower clusters develop faster than those feeding on grape berries later in the season, and this influences generation time. First-generation adults emerge from pupation when air temperatures exceed a threshold of 50°F (10°C) for 10-12 days, usually in the middle of spring. Adult males emerge about a week before the females.

In July and August, the second-generation larvae feed on green berries, hollowing them out and leaving the skin and seeds. In countries where L. botrana is established, control measures are targeted at the second generation. This is due in part to the prolonged emergence of the first generation and because of possible re-infestation from untreated neighboring vineyards. However, treatment of the first generation is recommended if populations are high or if treatments are conducted on an area-wide basis. Insecticides are less effective after bunch closure.

Third-generation larvae appear August-September. Both second and third-generation EGVM cause the greatest damage by feeding on entire bunches during and after veraison and leaving the clusters contaminated with frass (excrement), which exposes the clusters to Botrytis and other secondary fungi infections, as well as other pests that are attracted to damaged berries.

With 50°F (10°C) being the lower developmental threshold and 86°F (30°C) being the upper developmental threshold, eggs hatch in about 118 degree-days Fahrenheit (DDF) or 66 degree-days Celsius (DDC). But the degree days may vary. In Turkey, for example, the base temperature for EGVM is set to 12 °C (54°F). The first generation emerges at about 120 DDC, the second generation at 520 DDC, and the third generation at 1047 DDC. Some researchers report that larvae die when the temperature exceeds 93°F (34°C).

Monitoring for Insecticide Application

Currently, strategies for eradicating EGVM include chemical control, mating disruption, and fruit removal. Monitoring egg-laying and determining egg hatch are also essential to the management of this pest.

Insecticide applications should be timed for larval emergence by laying traps. Several reduced-risk insecticides are registered for use in grapes to control tortricid larvae. These include insect growth regulators, both natural and synthetic spinosyns obtained by the fermentation of Saccharopolyspora spinosa bacteria, and Bacillus thuringiensis, a microbe naturally found in soil that makes proteins toxic to larvae. For the first generation, egg-laying should be monitored—from the peak until the end of the adult moth flight cycle. Search for eggs on the peduncle of 100 clusters, selecting one cluster per vine. Begin monitoring for second- and third-generation eggs on berries one week after the moths of the respective flight are caught in the traps. Continue monitoring bunches for eggs weekly until one week after peak flight, and for feeding damage (holes or hollow berries), webbing, and presence of larvae. Infestation is often greater on the border than the interior of vineyards, particularly near woods or hedgerows. Regular cluster sampling in the vineyard interior and at the borders (particularly next to woods) can help to assess berry moth infestation levels and determine management needs.

Fruit removal from infested and surrounding grapevines has been prescribed as a first-step control measure for commercial growers in affected areas. However, research in this area is ongoing as scientists study how parasitoids that attack the egg stage and the pupal stage are reportedly effective at suppressing this pest. Eggs parasitized by wasp parasites turn black and become easier to spot with a hand lens.

A mating disruption product, Biocontrol Isomate-EGVM, which works by mimicking female European grapevine moth pheromones, is proving useful in adult males becoming disoriented and thereby delaying or preventing their mating with receptive females. Mating disruption has been studied in Europe for several years. It has proven most effective when grapevine moth populations are low and when applied to large areas over 10 acres.

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