Japanese beetles

Asked April 5, 2017, 4:02 PM EDT

Is there a natural concoction we can spray on our grass that will kill the grubs before they hatch? If so, when is the best time to spray? Also, is picking them off our plants and then drowning them in soapy water the best thing to do? Our Washington Park area is under siege from these monsters! Help!

Denver County Colorado

1 Response

Here is an excerpt from the CSU Extension fact sheet that discusses biological controls for Japanese Beetles (the link to the full fact sheet follows below):

"[A] biological control alternative for control of Japanese beetle larvae [in the lawn] is the use of soil applications of certain insect parasitic nematodes. (These organisms are discussed in more detail in CSU Extension fact sheet 5.573, Insect Parasitic Nematodes.) Specifically effective are certain nematodes in the genus Heterorhabditis (e.g., Heterorhabditis bacteriophora, H. megadis) and several biological control suppliers will provide these organisms (Figure 7). (A list of biological control suppliers can be found on the Insect Information website at: http://bspm.agsci.colostate.edu/outreach-button/insect-information/.) Applications of Heterorhabditis nematodes are made as a soil drench, preferably during cool, overcast periods, and must be immediately watered into the turfgrass. They should be applied when Japanese beetle larvae are present and active.

Another biological control that has received considerable past attention for Japanese beetle control is milky spore (Bacillus popilliae), a bacterium that produces ‘milky disease’ in Japanese beetle grubs. (The currently available formulation is sold under the trade name St. Gabriels’ Organics Milky Spore Powder.) Milky spore powder is applied to turfgrass areas where Japanese beetle grubs are active and may infect some of the grubs, producing a chronic infection that reduces survival and reproduction. Applications of milky spore powder will not produce immediate reductions in number of Japanese beetles; successful establishment of milky spore at a site, and its subsequent natural spread, may somewhat reduce overall numbers of Japanese beetles in future years."

And while it seems never-ending, hand-picking is still the recommended control method for adults.