By PR Extension Office
The primary cause of dying yucca plants has recently been detected, according to MSU Powder River Extension Agent Mary Rumph.
“For a few years, I have noticed dying yucca plants throughout rangeland and have received reports from others throughout the county of noticing the same,” said Rumph. Last week, a local resident who was digging up dead yucca plants in the yard discovered a black beetle on the plants and a larva in the roots. Further investigation led to the conclusive identification of the “yucca weevil (Scyphophorus yuccae)”.
Most of the information regarding the weevil comes from Southern California, where yucca is more common. The adult black weevils are most commonly observed on hosts in the summer. They are up to 3/4 inch long and have long, narrow, snoutlike mouthparts and a small, elongated head. Adults have a smooth, finely punctured thorax and lengthwise grooves on the wing covers.
The mature larvae are up to 1 inch long (ugly grub-like looking!). They are cylindrical, legless, and white or yellowish with a brown head. Larvae feed hidden in the apical meristem (central growing point) of hosts, the base of green flower stalks, and in roots.
Weevils develop through four life stages: egg, larva, pupa, and adult. After mating, adult females chew an egg-laying tunnel in the base of hosts. After hatching from an egg, larvae develop through several increasingly larger instars as they feed and tunnel in plants. Mature larvae generally exit hosts to pupate in topsoil; pupae can also occur in the base of plants.
One generation egg to adult can be completed in less than 2 months when temperatures are warm. There can be 4 or 5 generations per year (in California). “I would anticipate 2 or 3 generations in our area,” commented Rumph.
Adults feed on host sap and this does not threaten plant health. Adult feeding causes discolored, punctured spots or small holes in the foliage. The larvae cause the damage by boring in the base of hosts, causing plants to decline and facilitating entry of plant pathogens. Larval feeding in combination with decay microorganisms that colonize wounded tissue commonly cause infested plants to collapse and die.
“Recognizing control methods are highly unlikely on rangeland; however, there are control recommendations for a yard situation,” said Rumph.
Provide the plants with well-drained soil and limited irrigation. If other yucca plants are still growing in the area and aren’t infested, then preventive insecticides can be applied (those containing the active ingredient imidacloprid, products such as Bioadvanced Tree and Shrub Insect Control).
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