One insect, 300 target plants. That’s the reality when dealing with Japanese beetles (JP), especially in boom years, like the one predicted in 2017.
This half-inch beetle with a voracious appetite prefers rose plants and linden trees, but has been observed defoliating over 300 different types of plant and tree species. It’s not just the actual adult beetle which hurts. JP destroys gardens and lawns throughout its lifecycle. While in its larval stage, more commonly known as grubs, they feast on lawn roots, creating brown and yellow patches of grass which at their worst can be lifted up like pieces of carpeting.
Warming Front Range soils are waking up insects of all sorts – but none is freaking out gardeners more than JP. This year is predicted to be an up year in the cycle, due to the especially wet year we’ve experienced -which creates a more ideal breeding ground for the JP to lay eggs. Short of a long dry end to the growing season, which would destroy the drought-phobic JP in large numbers, get ready for lots of stripped leaves this year and next year.
It’s not just the actual adult beetle which hurts. JP destroys gardens and lawns throughout its lifecycle. While in its larval stage, more commonly known as grubs, they feast on lawn roots, creating brown and yellow patches of grass which at their worst can be lifted up like pieces of carpeting.
Basic Japanese beetle facts:
Japanese beetles are an invasive species that are now deeply imbedded in the ecology of Colorado. Interestingly, the beetles are not considered a pest in their place of origin, Japan. The beetles were accidentally released in New Jersey in 1916. They were first found in Colorado about 60 years ago [see japanese beetles in Highlands Ranch ].
JPs begin as grubs where they chew and damage the root structure of lawns. Beside lawn roots, they also are recorded feeding on the roots of beans, strawberries and tomatoes. The females begin to emerge in June and July and are observed feeding on vines, linden trees, roses, and over 300 other ornamental plants. Adults live about two months.
The reason Japanese beetles are such prolific breeders is that they naturally attract more beetles to a given plant as they chew the leaf structure and give off a natural pheromone that invites other adults into the fray.
Wetter years may promote the adult emergence weeks earlier than normal, and all the late-spring snow we’ve had in Denver seems to be doing just that. If you’re observing more than a few beetles on your trees and shrubs, it’s important to act as soon as possible.
Japanese Beetle Treatment Options
- Home gardeners may try knocking beetles into soapy water. Do this several times a day until the population is thinned out or eradicated completely.
- Also, introducing parasitic nematodes in August or September – by pouring them onto the soil during cool, overcast days – can keep the JP in balance. Make sure they are in the genus Heterorhabditis.
- What about over the counter insecticides? Neem based products are frequently advertised to stop JP, but often prove a mixed bag of success. Pyrethroids also help control the bug for a few days – but cannot be applied to flowering plants. (This is the agent found in many commercial household insecticides.)
- A note on backyard use of pheromone traps, sold at garden stores and online. Test after test has shown that japanese beetle pheromone traps attract more beetle to your yard than they trap, so this practice is not encouraged.
- Treatments that have not been proven effective are introducing milky spore disease as a biological control of JB grubs.
We offer professional treatment if home remedies stop working – and when JP beetles swarm, they can overwhelm even the most dedicated gardeners.
The best time to attack JP is during their egg stage and hatch in later summer and early fall. Applying a granular insecticide with a spreader to the lawn at this time offers the strongest chance of control. Control will be improved by a moderate to light watering by hose or normal sprinkler system schedule. However, significant rainfall after the application may be counterproductive and require a second round.