Japanese beetles in grapes

Japanese beetle adults are seasonal pests of many ornamentals and horticultural crops, including grapes. The insect has one generation per year, with adult activity beginning in late-June or early July and ending in late August. Beetles live for up to 6 weeks, during which they apparently spend extended periods aggregating, feeding and repeatedly mating on their preferred hosts. Females lay eggs in several small batches in grassy areas between repeated bouts of feeding and mating.

Adult Japanese beetles are easily identified by their metallic green to greenish-bronze head and thorax, coppery red elytra (outer wing covers) and small white tufts on the sides and tip of their abdomen depending on the crop, aduls will feed on foliage, fruit or both. In grapes, they feed on the upper surface of foliage, chewing the tissue between the veins and creating a skeletonized or lacy appearance. Research indicates that adults are highly attracted to grape vines with existing feeding damage, resulting in aggregations on affected plants. When Japanese beetle adults appear in large numbers, extensive defoliation can occur.

Japanese beetle adults show feeding preferences for different grape cultivars, in particular those with thin, glossy and tender foliage. In general, resistance to feeding levels tend to be higher in Labrusca than in hybrids or Vinifera vines. Research in Michigan on juice grapes indicates young vines can tolerate significant (30%) leaf area loss (LAL) late in the season (veraison or later) without impacts to growth. Mature grapevines are able to tolerate some defoliation before LAL results in delayed ripening or reduced fruit yield and quality; however LAL thresholds have not been established for wine grapes. Tolerance for feeding damage on leaves will depend on the physiological state of the vine (growth stage, crop level, weather stress) and the presence of other foliar pests. In years of vigorous growth, vines will tolerate more feeding.

Monitoring traps baited with a floral lure (for females) and a sex pheromone (for male) are highly efficient for attracting beetles. These traps are not recommended as monitoring or management tools because they attract more beetles into the area than are caught (it is not uncommon to see beetles overflowing traps in heavily infested areas), resulting higher levels of localized damage. The best way to monitor for JB in grapes is to go out and look for them during routine scouting-they are rather conspicuous daytime feeders.

Grape growers have limited options for managing Japanese beetle adults. Monitoring is essential to determine the presence and extent of feeding damage. Imidan (phosmet) is the only product currently registered for control on grapes. Keep in mind that with the extended activity period and multiple sources of adults in the landscape, re-infestation may occur following treatment.