One day your garden is lush and healthy; the next you might find clumps of metallic green beetles covering your plants; the leaves have been sucked down to their very bones. If this happens to you, don’t panic quite yet. Your yard is host to Japanese Beetles. Although there is scant possibility of eradicating this pest completely, there are a few measures you can take that will drastically reduce their numbers this season, as well as next year’s population.
A BEETLE IN THE HAND…Removing beetles from your plants by hand is the best, but most time consuming, method of population control. Use a latex glove (necessary because they bite) to pull them off, or shake individual plant branches over a plastic sealing bag. Place a paper towel soaked in rubbing alcohol or a bleach solution in the bag to sedate the beetles.
Applying a solution of water and beneficial nematodes just prior to the final larval stage of the insect can be a highly effective control measure. This is typically during early June. Check with the UW Extension Horticulture program for the exact time frame, it varies from year to year. Look for the nematodes on the ‘Gardens Alive’ online catalog.
Neem oil, a pure extract from the neem tree, is an effective spray treatment for Japanese beetles and a variety of other foliage eating pests. When ingested through treated foliage, neem oil blocks hormone activity, causing pests to neglect to eat, mate, or lay eggs. Beneficial insects, such as bees and ladybugs, do not eat plant material, so neem oil will not affect them once it has dried on the plant. However, they can die if smothered with it, so use caution. For this reason, spraying in the evening or early morning is advised, giving the treatment a chance to dry before midday, when good insects become most active. This practice also prevents leaf burn. Neem oil will also work systemically; soak the soil around the drip line of the plant with your diluted mixture, allowing absorption through the roots and into the leaves. Humans have safely used neem for centuries for a variety of ills, so it is presumably safe to use on fruits and vegetables.
OH, PATIENT USER OF NEEM! You won’t be seeing a pile of dead insects immediately, but given time their populations will lessen substantially. Many garden centers carry neem oil for around $20. Buy the 70% formula; it contains the right ratio of emulsifier and oil for the highest benefit. It will lose effectiveness quickly once mixed with water, so use it within a day and follow all directions on the label.
AVOID THE TRAP! We do not recommend the use of Japanese beetle traps. Manufacturers advise setting the traps well away from plants to lure beetles away. However, Japanese beetles don’t mind traveling for a good meal, and once one of them finds a succulent plant to feast on, he’ll send out pheromones to alert his friends. Traps could actually increase the infestation. For this reason, it is also a good idea to make controlling beetle populations a neighborhood endeavor.
