No. 8 — May 13, 2019

Prepare for Their Return: Japanese Beetles

close-up of a group of Japanese beetles decimating a leaf as they feed on it

Japanese beetles are predictable and so expect them to emerge from soil between 970 to 1030 GDD (base 50). Typically this is near the calendar date of Father’s Day (June 16, 2019). Using the 30 year average of GDD, the predicted first emergence should occur between June 8 to 12. Start scouting for Japanese beetles in early June.

Be prepared and make sure to have insecticides on hand prior to when needed.

Early feeding is often on the border rows of grapevines, especially border rows that are near open grass areas.

Once Japanese beetles begin feeding the grapevines the grapevines release chemicals that attract more Japanese beetles. The beetles themselves do not have aggregation pheromones instead the grapevine wound response is what attracts more Japanese beetles.

Will treating the grass with insecticides to kill the Japanese beetle larvae reduce feeding damage to grapevines? At this time of year, most Japanese beetle larvae are at their last instar or beginning to pupate and therefore will not be susceptible to insecticides applied to grass areas. If treating grass areas the best time to treat these areas is the late summer and fall when the grubs are smaller and have yet to tunnel deeper into the soil. There is no research that I am aware of that clearly shows that treating the grass areas in a vineyard will reduce feeding damage by the adult Japanese beetles. Remember that adult Japanese beetles are very strong flyers and grapevines are a preferred host plant. Recommendation is to focus on controlling the adult Japanese beetles in vineyards.

Use indicator plant species found around your home, winery, or business as early indicators of the emergence of adult Japanese beetles. Some of favorite host plants of adult Japanese beetles are; Roses, American Linden, Fruit trees, including apple, apricot, cherry, peach and plum, and ornamental crabapple trees.

Pyrethroid insecticides are very effective in controlling adult Japanese beetles and pyrethroids also provide some repellency.

Some grape cultivars are preferred by adult Japanese beetles over other cultivars. The most susceptible are Vitis vinifera, followed by hybrids, and lastly, American Vitis labrusca.

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