The Q&A Archives: Japanese Beatles are invading my garden. How can I kill them all?

Question: Please suggest short and long term fix.Problem is growing every year. Location is in Plymouth, New Hampshire.

I am very frustrated so suggest harsh and mild fix for this problem. I heard heard positive and negative aspects of bacterial treatment and pheromones.


Answer: There are chemical and physical control measures for Japanese beetles. Remember though, that these methods are intended to manage beetle populations, not eliminate them altogether, which would be nearly impossible. Following the philosophy of integrated pest management (IPM) practices, it is possible to have an insect pest present, but in such low numbers it does not do significant damage. Aiming to significantly lower the numbers of Japanese Beetles on your property is the most realistic scenario.

Chemically, there are two major chemicals recommended for controlling Japanese beetles. The first is carbaryl, commonly sold under the brand name Sevin, which is usually a spray or fine dust used to coat the leaves and/or flowers on affected plants. Control can last for up to two weeks, but the beetles must actually contact the carbaryl, and it must be reapplied if washed off by rain.

The second is imidacloprid, commonly sold as Merit, which is a systemic treatment for non-food crops such as shrubs, trees and flowers. Formulations differ, but imidacloprid is generally applied early in the growing season, in mid to late spring, to allow sufficient time for the plant to absorb the chemical and develop some protection from the beetles. Control can last for months to years depending on the formulation. The beetles must ingest the chemical to be affected, so some plant damage will occur. Also, imidacloprid does not accumulate well in petals, so the beetles may still devour your roses. But the foliage should be protected, making their bounce-back that much quicker once the adult feeding ends.

Remember that with either chemical method, potentially beneficial insects will be affected, including valuable pollinators such as bees. Remember too that the crop or plant you wish to treat must be listed on the label of the insecticide. Some formulations are not appropriate for some plants. Read the label!

There are some natural bacterial controls available such as milky spore, sold under the name Doom or Grub Attack intended to target only the Japanese beetle grub, or larval stage of the beetle. Other grub treatments contain synthetic insecticides like Merit that targets grubs in general, not just Japanese beetle grubs. Considering that adults can travel ten to fifteen miles from where they were larvae, controlling Japanese beetle grubs is not likely to have any effect on numbers of adults feasting on your garden. But if you have significant damage from grubs, you may want to consider their use.

A device mistakenly used by many for Japanese beetle control that will not work, and may in fact make your problem worse is traps. These exploit the beetles clumsy flying ability and amorous intentions with a cardboard or plastic wall they slam into while seeking the source of the irresistible pheromone in the bottom of the trap. The only real use for them is for monitoring beetle populations, not controlling them. If using them for monitoring, they should be placed at the edge of your property, away from valued landscape plants, as they will attract beetles from miles around. If the trap fills in a short time, treatment is probably in order. A few beetles over the course of the day would not indicate any need to use controls.

Practically speaking, we can just as easily observe Japanese beetles feeding in our landscape and note the numbers. Also, considering the proximity of other homes in most neighborhoods, your neighbors will likely not appreciate your use of a trap. One person I know never had a huge Japanese beetle problem until this year, when her neighbor installed a trap near their property line. Now the beetles are devouring everything in sight in her yard!

The best physical control method for adult beetles continues to be hand picking or shaking them from plants and dropping them into soapy water. They are very clumsy, so if you place a pan of soapy water below an affected plant and shake the branches, the beetles should easily drop to their death using this soapy weapon. This does get tedious, and sometimes the beetle population will be too large for hand picking. The two basic options at that point are: try the chemical sprays, or just let nature take its course. Most woody plants will survive near to total defoliation by the beetles, despite their ragged appearance post-feeding.

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