Ways to Fight Root Weevils
If the edges of the leaves of your rhododendrons, azaleas, camellias, viburnums or other broadleaf evergreens are chewed and appear lacy, the culprit is probably root weevils. If you look for the insect, chances are you will not find it on the plant, because it does its damage at night.
You can control this pest by treating the plant with an insecticide, treating the soil with a beneficial nematode, and treating the trunk of the plant with a sticky substance. Some organic gardeners recommend the use of diatomaceous earth as a barrier.
Nematodes - This is a natural, biological method to control root weevils. One product on the market is called BioSafe, and a 20gram package contains 10 million insect-killing organisms of the species Steinemema carpocapsae. This size container covers 225 square feet. The recommended application period is every three to four weeks during the period when root weevils are active. This type of insect control is applied to the soil.
Sticky substances - Since the adult weevils emerge from the soil at night and climb the plant to eat the edges of the leaves, one can apply a sticky substance to the trunk of the plant to trap the insects. For years, the product Tanglefoot has been used for this purpose. It is generally recommended that the trunk of the plant be wrapped with a 4-inch band of masking tape or duct tape. Then apply Tanglefoot right over the tape surface. Make sure the tape is tight enough that the insects cannot make a path under it. This product comes in a sticky paste, or it can be applied by aerosol spray. Check the surface occasionally, removing some of the dead insects so they do not form a bridge over the treated area.
Insecticide treatment - If soil and foliage are sprayed in May or June as root weevils become active, they can be controlled. The product Orthene is generally used to control root weevils. The manufacturer recommends that the. product be applied in May or June (when the pest becomes active) and again two more times at three week intervals.
Diatomaceous earth - Over the years many organic gardeners have used this method of discouraging root weevils. Simply dust the soil at the base of infested plants with diatomaceous earth. In bad infestations, the lower stems and undersides of leaves can also be dusted with D.E. Watch the insects' movement, as they are apt to move onto other plants.
Whichever way you choose to control root weevils, be certain to read and follow label instructions on the product you use.