Japanese Beetles

By: John Degroot

July 14 2012

Japanese Beetles - Garden Clippings for July 14, 2012

Consider yourself lucky if you don’t have Japanese Beetles.  There are heavy populations in pockets throughout the North end of Sarnia, and are quite abundant in the west end of Sarnia.

At first glance, Japanese Beetles look like overgrown houseflies, but larger with bronze or green shiny coats.  Take a closer look and you will find they look more like June beetles, but smaller, about 13 mm in length with a broad rounded body.  Their heads are dark green and their wings are bronze and almost metallic.

Japanese Beetles feed on almost anything.  They love Birch, Linden, Cherry and all fruit trees.  In the shrub category, Japanese Beetles are not fussy, but really favour Viburnums, Clematis and Roses.  Almost all perennials and annuals are a great food source for Japanese Beetles.

While Japanese Beetles can do a great number on plants, their damage only lasts a little more than a month.  During most of the rest of the year, Japanese Beetles are found in grub form, where they feed on grass roots. 

Japanese Beetles emerge as adults in late June till early July.  They feed and mate for a month or more, then lay eggs, usually in grass.  The eggs hatch after only two weeks, and become grubs, dirty white in colour and C shaped about 2 or 3 cm long.  Grubs are hungry so they begin eating grass roots.  Damage at this time is minimal because the grubs are small and will have little effect on grass.

By fall, damage to grass often becomes noticeable, causing irregular dead patches.  As cool weather arrives, grubs will seek protection from cold and will dig deep into the soil where they remain protected from severe cold.  In early spring the grubs emerge and begin doing more damage to grass, often causing distinct large dead patches.  Damage is often evident because sod is uprooted by skunks and racoons in search of tasty grubs.  The grubs will continue eating for a month or so, after which they will transform into adults, usually around the end of June.

Controlling adult beetles is difficult.  In Ontario, where the cosmetic pesticide ban is in effect, traditional chemical insecticides are banned.  Japanese Beetle traps containing pheromones are effective in reducing populations but care should be taken to avoid placing the traps too close to the house or host plant, for fear of attracting all the neighbours’ beetles to your garden.  Place the traps in the far corner of the yard.

Japanese Beetles are dozy early in the morning and they can easily be picked off by hand.  Another method is to place a drop sheet on the ground and shake the tree or limbs above, causing the beetles to fall into the drop sheet trap where they can be collected and placed in a bucket of soapy water.

Several gardeners have told me that neem oil is effective against adult Japanese beetles. Neem oil is not a quick fix causing the bugs to die instantly, but rather it is a slow acting organic pesticide that should be applied a few times over a period of a few weeks.

The best control for Japanese Beetles is an application of nematodes which are natural living organisms that eat the beetles during the grub stage.  For best results apply nematodes on damp soil, when the grubs are young beginning in mid August.  Follow label instructions.

Nematodes can be applied anytime from August to October, and then again in April and May.  The best window of opportunity is August to early September.  The soil should be damp when applying nemetodes and the soil temperature should be at least 10 degrees C.

The good news is that the current wallop of Japanese Beetles is heavier than normal because of last year’s summer rain and mild winter.  Grubs don’t’ like dry soil so we can expect lighter beetles populations next year.