When Weevils Attack, Tough Plants Wilt
By Peggy Katz
The desire for water-wise landscapes has prompted many gardeners to plant agaves, furcraeas and yuccas. These plants are attractive and draught-tolerant, but anyone who grows them should be aware that they can be killed by certain weevils.
The larvae of Scyphophorus acupunctatus feed on agave and furcraea species and the larvae of Scyphophorus yuccae feed on yuccas. The weevils seem to prefer some plant species more than others. They also may be more likely to attack plants that are stressed or growing poorly due to over irrigation, poor soil drainage, or a coastal location that is cool and wet.
Both weevils are rather large beetles, about 5/8 inch long (17mm), smooth, black and fully-winged with grooved wing covers. They do not fly very well, and can occasionally be seen, from March to July, walking among and feeding on the leaves of their favorite host plant. The weevils have chewing mouth parts at the end of a long down-curved snout which females use to bore into the flower stocks or stems where they deposit eggs. When the eggs hatch, the larvae, which look like white grubs, tunnel deep into the center of the plant causing the plant’s foliage to wilt and collapse. When the larvae mature, they pupate and later emerge as adults to mate and begin the cycle again.
Plants that are severely damaged should be removed and destroyed to keep the infestation from spreading to near-by host plants. If a plant is small, it may be possible to salvage it if it is not severely damaged.
To do this, dig the plant out of the ground and remove any damaged lower foliage. Then, cut through the stem retaining the top portion of the plant that still looks healthy. Check the base of the cut stem for larval tunnels and remove any larvae that are present using long forceps or a similar tool. Put the plant in a shady location for a few days until callous tissue forms on the cut stem. Then, re-root the cutting by replanting it in well-drained soil.
Identifying weevil-infested plants before they die can be difficult. Susceptible plants can be protected with a preventative soil drench containing the systemic insecticide imidacloprid. Gardeners can buy Bayer Advanced Garden Tree and Shrub insect control (with 1.47 percent imidacloprid). You can use as little as one tablespoon of the insecticide in two gallons of water. Apply it in late February or early March. This seems to provide good control of the weevil in my East San Diego County landscape.
Peggy Katz is a San Diego Master Gardener.
- 2017 - Master Gardener Association of San Diego County
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