Lurking beneath your lush garden soil is one of the most destructive and difficult to control pests that attacks garden plants. This pest is almost never seen in the light of day, and inquisitive gardeners who want to see them need a little perseverance and a flashlight. These creatures emerge at night and start munching the leaves. They leave behind an easily recognizable pattern of neatly serrated leaves that look as though a tailor had trimmed them with pinking shears. Root weevil feeding produces characteristic notches along leaf margins. Adult root weevils frequently damage euonymous and many other plants. This damage typically is mistaken for grasshopper feeding.
These pests are usually called strawberry root weevils but are more correctly referred to as the root weevil complex. Several small dark beetles cause damage by their feeding. They are all related and have similar life cycles. Control methods are also similar.Although the damage most recognizable is the all-too-familiar notching around the leaf edges, this is not the most serious problem. The larvae of these beetles live down in the root zone of the plant and feed on the tender young tissues. When these roots are destroyed, the plants start to decline rapidly. Unfortunately, the symptoms are easily confused with lack of water, since the weevils have eaten so many roots. The most common response is to dump more water on the plant. This kills the remaining roots that could absorb the water, and the plant continues to decline until it finally dies.
The adult beetles are small, dark-colored snout beetles. The largest species may reach a half-inch in length. Strawberry root weevils are shiny, brownish-black beetles about 1/4-inch long. All of the species show a common behavioral characteristic because when disturbed they drop readily to the ground.
Root weevils do not fly. Most overwinter as nearly full-grown, pale, legless larvae but a few overwinter as adults. Development and pupation are completed in the soil and most adults emerge sometime in June. Eggs are laid near the crowns or bases of the plants throughout the summer.
The diet of these creatures is extremely varied. They feed on a wide variety of trees, shrubs, vegetables and flowers. Victims include the roots of strawberry, raspberry, clover, spruce, Douglas fir, roses, peonies, euonymous, lilacs, aspens, privet and dogwood.
At times the insects create a considerable nuisance as they wander into homes. They cause no harm to humans, pets or household furnishings. Most frequently root weevils wander into homes during late June and July. Migrations greatly increase in hot, dry weather, as the insects are apparently attracted to the moisture in the building. They can be quite abundant and very unwelcome. Just why these insects are attracted to homes is unclear, but apparently the house provides shelter during the hot summer months when the insects are relatively inactive. Moisture sources in and around homes also attract the adult weevils.
Because root weevils do no harm inside homes, vacuum them up as they enter the home. Only in extreme instances are insecticidal treatments necessary to prevent infestations inside the home. The best defense is caulking around the windows and foundations and screens on doors and windows.
Because the pests are not very sociable when they feed on your garden, they are hard to control. The larvae are beneath the soil and are not easily reached by chemical applications. There are no chemicals specifically registered for larvae control. The most common strategy for most gardeners is to try to control the adults. This is also a difficult proposition because the adults are are so elusive. They spend daylight hours hidden in the soil or in litter beneath the plants. This further protects them from any attempted control measures.
Two registered controls include acephate (sold as Fertilome Acephate or as Ortho Orthene) and bendiocarb (sold as Lilly Miller Pestkill Rose and Rhododendron Dust). The first product is a liquid and is best sprayed at the base of susceptible plants in the late evening. That way the beetles emerge and are more likely to be controlled because the chemical is not dissipated. The second product is a dust and is best applied during the same time frame. Neither of these can be used on edible crops. The only synthetic product to use is one of the baits such as the Morgro Pest Meal.
The success of chemical use is highly variable and requires frequent repeated applications. It is also likely to destroy ground beetles and other natural predators, thereby giving the root weevils an even greater advantage. Organic controls are limited but effective under the right conditions. Parasitic nematodes are available from some nurseries and from mail-order sources. Keep in mind that natural controls are going to suppress the pest populations but do not eradicate them. That is because the predators would die out themselves if they were to kill all the weevils, so expect some notching to occur with this method.
The beetles can also be trapped if they are only bothering a few plants. Bury straight-sided glass or slick plastic tumblers around the plants so the top edge is even with the soil surface. Pour a half-inch of cooking oil in the bottom of the glass. As the beetles fall into the containers they will be trapped and destroyed. The traps need to be cleaned and the oil replaced regularly.
These pests are aggravating and destructive. Learn to tolerate some damage because total control is almost impossible. Concentrate controls on valuable plants that are the least likely to tolerate the damage.
- THE ART OF THE GARDENER: A walking tour of artistic and private gardens and an illustrated lecture by noted horticulturist, gardener and author Lauren Springer will be Saturday, June 21. Registration is from 8-9:30 a.m. at the Utah Museum of Fine Arts. Springer's lecture is at 9:30 a.m. Self-guided tours of 10 gardens will be from 11 a.m. to 6 p.m. The one-day event supports the art education programs of the Utah Museum of Fine Arts, University of Utah. For more information, call 581-7049.