Some of our Denver customers have been encountering Japanese beetles devouring the leaves of the grapes and Linden trees (two of the beetles’ favorites). Japanese Beetles have the potential to cause serious damage in your gardens and landscapes- they feast on over 300 varieties of plants! The beetles are about one third to one half of an inch long and a quarter inch wide. They have a green body and head and bronze colored wing covers. The beetle form only lives 30 to 45 days, most of which are spent eating. When Japanese Beetles feast, pheromones are released by the plants which signal for more beetles to arrive. In July, the females begin taking the afternoons off to lay eggs in the lawn. They can produce around 60 eggs per season before the beetles die. The eggs hatch into white grubs that feed on grass roots through August and September. In October they go deeper into the soil and stay dormant until April when they begin feeding on grass roots again and continue through May. The grubs can cause irreversible damage to lawns during these times. In June the grubs mature to adult beetles, emerge from the ground and begin feeding voraciously on plant leaves. This goes on into July and then the cycle repeats.
In order to control Japanese Beetle and Grub populations, you may use synthetic, organic, or mechanical efforts during their early and late grub stages when they are most vulnerable (August to September and April through June). At Pine Lane Nursery, we carry both synthetic and organic product options used to control the populations.
If you choose to use a synthetic treatment, contact or systemic insecticides are both effective. Hi Yield Grub Free Zone, a systemic synthetic treatment, prevents grubs from maturing and causing serious damage. Be aware that systemic insecticides will move into all parts of the plants including the flowers, nectar and pollen, therefore they may pose hazards to pollinators. Contact sprays should be used when bees are not present on the plants- early mornings or evenings are ideal.
Orcon Grub Control, an organic control method, consists of beneficial nematodes. Nematodes are microscopic organisms that act as parasites on insect pests. Organic products such as pyrethrums of spinosads are effective contact spays with spinosads having some residual effectiveness. These are also damaging to bees and should be used according to instructions.
Mechanical controls can include traps or manually removing the beetles from your plants. Traps are best used for monitoring the arrival of Japanese beetles. The presence of Japanese beetles in your yard, alive or in traps, will continually attract more Japanese beetles due to the release of pheromones. Do not squash them or it will attract even more. The beetles can be easily handpicked off the plants- they are slow and poor flyers who don’t bite. Then drop them in a bucket of soapy water. This will kill them and mask the scent that attracts more beetles.
Japanese beetles have preferences in feeding. Plants to watch for their arrival are Linden trees; grape vines; Virginia Creeper vines; apple and crabapple trees; plum, peach, cherry and apricot trees; raspberries and roses. Keep an eye on your plants and react quickly to prevent their population from growing and causing serious damage to both your plants and your lawn!