Weevils
Weevils (family Curculionidae) are also called snout beetles because the front of the head and mouthparts of adults are elongated. There may be more than 1,000 weevil species in California. Some species are common pests, but most are innocuous.
Identification
The larvae of many weevil species feed hidden within plant parts or soil. Adults of many species hide during the day and feed at night. Therefore, symptoms of their presence as described below in Damage may be the first obvious indication that these pests are present.
In addition to their snout, characteristic of adult weevils is the club-tipped antennae that are held bent (elbowed) and attach to the sides of the snout near its tip. Adults of some species have fused wing covers and are flightless. Many species are dark and dull colored, mostly blackish, brown, or gray. Others are brightly colored or a mix of bright and dull coloration. Adults of many species are about 1/8 to 2/5 inch long, but others are larger such as giant palm weevils that are about 1-1/2 inches long.
The pale eggs are oblong and, in most species, occur hidden where the adult female laid them inside plant tissue or soil at the base of host plants. Larvae are distinctly segmented, commonly legless, and when mature are about the length of adults of the species excluding the snout. In many species larvae are pale yellow or whitish with a brown head. When exposed they commonly assume a C-shaped posture. Life cycle
Weevils develop through four life stages: egg, larva, pupa, and adult. Many species have only one generation per year and overwinter as larvae in plant parts or soil. The peak abundance of adults commonly is during spring and summer. Adult females generally feed on host plants for several weeks before they begin to lay eggs. Many species consist only of females that reproduce without mating (parthenogenesis).
Eggs laid by the adult female commonly occur in soil near the base of hosts or in a hole chewed in aboveground plant parts. The larvae generally develop through 4 increasingly larger instars while feeding for a period of about 2 to 8 months. Pupation generally occurs in host plants or soil where they fed as larvae. Damage
Damage varies greatly according to the species of weevil. Adults of many species chew foliage of the same plant species where the larvae feed. Adult chewing commonly occurs along the leaf edges and appears as irregular notches.
Adult agave and yucca weevils make small punctures in hosts and feed on the exuding sap. Rose curculio weevils chew holes in flower buds, which can cause the rose buds to drop or, if the flower buds mature, cause the petals to be ragged.
In most species, the damage that threatens plant health and survival is caused by the larvae, which entirely consume small roots and chew the bark and cortex of larger roots. With other species, the larvae tunnel into aboveground parts such as the flower buds of roses or the growing point of palms. With root-feeding species, the first indication of this damage can be that plants appear water stressed, foliage wilts, fades in color, and dies. Most anything that makes roots unhealthy can cause similar symptoms, including drought stress and root decay pathogens. To determine the cause, you may need to dig in soil and examine roots. In some species such as Fuller rose beetle the larval feeding is of minor importance and it is the adults chewing foliage that causes most of the plant injury. Solutions
If chewing is observed on plant foliage, but no cause of this (such as caterpillars) is observed, there are several ways to try and determine whether weevils are infesting the plant. On trees and single-trunk shrubs, the trunk can be wrapped with corrugated cardboard then inspected for weevils during the day. First peel the off the outer layer from one side of the cardboard and place the exposed corrugated side (that with the exposed alternating ridges and grooves) against the bark overnight. In the morning unwrap the cardboard and examine it for any weevils, which can be disposed of in a bucket of soapy water or crushed.
Alternatively, several pitfall traps created as in this illustration can be buried in the ground around the base of plants, then examined in the mornings for captured weevils. Shaking shrub branches over a collecting surface such as a white cloth placed on the ground beneath shrubs may dislodge some weevils that remain sheltered in the plant during the day. Small plants can be inspected for feeding weevils with a flashlight or head lamp during the night.
Weevil management varies with the species and management is commonly difficult. For some weevils and host plants there are alternative cultivars or species that are less extensively or not at all fed upon in comparison with others; for example, some rhododendrons are resistant to root-feeding weevils.
Provide valued plants with appropriate cultural care and a good growing environment so they are better able to tolerate pest damage. When buying new potted plants, if possible remove the plant from the container to examine whether roots are healthy looking and free of chewing damage that indicates weevil larvae or certain other pests are present.
On trees and single-trunk shrubs, trim branches that provide a bridge to other plants or the ground and apply a 6-inch band of sticky material (e.g., Tanglefoot) encircling the trunk to exclude ants, flightless weevils, snails, and certain other pests. First wrap the trunk with landscape fabric or other flexible material and place the sticky material on top the wrap to avoid direct contact with bark and possible bark damage. Trapping adults as described above for monitoring and destroying them may also help to reduce the abundance and damage of weevils.
For root-feeding weevils, entomopathogenic nematodes such as Heterorhabditis bacteriophora can be drenched onto soil under the plant canopy whenever the weevil larvae or pupae are expected to be present, commonly midsummer through fall, varying by the species. The nematodes are commercially available from online venders. To be effective soil must be warm and kept moist (not soggy) for at least two weeks after the application.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Integrated Pest Management Program (UC IPM).
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