If you have been noticing some fresh holes in your roses, crab apples and raspberries the likely culprit are the Japanese Beetles. The adult beetles can be very destructive and not very selective in what they eat. The beetles start as a white grub and the adult beetles typically come out in late June and hang around through August. During that time they eat, mate, and lay eggs to overwinter in your lawn.
When in feeding stage the grubs eat the roots of lawn grass and can reduce the root system enough that grass can die in the summer heat and dry winds. The adult beetles chew on leaves, leaving them with a lacy or skeletonized look. Their favorite plants include grape vines, crabapples, hollyhock, roses, plum, apples, lindens, and birch to name a few. Bring a sample of the insect and damage they are causing to one of our Events and our Master Gardeners can help you identify the issue and suggest solutions.
There are several ways you can help reduce the damage to your plants however it can be difficult to eliminate the beetles from your yard. Adults beetles can fly for miles! For turf, make sure you have a large infestation before applying an insecticide for grubs. Look for areas of brown grass and search in adjacent green areas for grubs and pupae. Insecticides are needed to control grubs and adults if the damage is extensive.
Removing beetles by hand may provide protection for backyards if beetle numbers are low. Beetles on a plants attract more beetles so by not allowing beetles to accumulate, plants will be less attractive to other beetles. Shake beetles from small plants into jars filled with soapy water.
If you choose to use an insecticide, foliage and flowers should be thoroughly treated. Follow label directions and avoid spraying under windy conditions. Never spray when bees are foraging. Be sure the insecticide is registered for use on the plants you want to spray. If it is a food crop, note the minimum number of days that must be observed between the date of the last application and the date of harvest.
Research has demonstrated that more beetles fly toward traps then are caught; resulting in surplus beetles that feed on your plants. Think twice before purchasing and installing a pheromone trap. If you are really frustrated with the number of beetles you see, try this method and see if it helps.
For more information on beetle identification and lifecyle visit the U of M Extension – Japanese Beetle Management.
Source: University of Minnesota Extension