Damage Occurring To Joshua Trees & Mojave Yuccas. More On Yucca Weevils and Treatment.
Several folks have noticed damage on Joshua Trees and Mojave Yuccas, where the plants leaves, flowering panicles, and branches are falling off. The cause is an invasive pest known as the Yucca Weevil, 𝑆𝑐𝑦𝑝ℎ𝑜𝑠𝑝ℎ𝑜𝑟𝑢𝑠 𝑦𝑢𝑐𝑐𝑎𝑒. This pest colonize Yucca species including Chaparral Yucca, 𝐻𝑒𝑠𝑝𝑒𝑟𝑜𝑦𝑢𝑐𝑐𝑎 𝑤ℎ𝑖𝑝𝑝𝑙𝑒𝑖. Sadly treatment is difficult should be started immediately by starting to remove the infected part/s of the host plant or completely remove it and the adjacent soil because the larvae will burrow into the soil in search of another host. Pick up foliage, branches, and flowering stalks that have fallen from the host plant, put those in a garbage bag to kill the larvae and dispose of them.
If watering, allow the topsoil to dry in between watering.
I spoke to a horticulturist who has treated and written about Agave Weevils, 𝑆. 𝑎𝑐𝑢𝑝𝑢𝑛𝑐𝑡𝑎𝑡𝑢𝑠 to inquire about the success rate of treatment and if they have done treatments for 𝑌𝑢𝑐𝑐𝑎 species, she recommended removal of the infected host and treatment of the uninfected portion along with the soil around the host using in the spring and May - June. WARNING, educate yourself about the systemic insecticide you decide to work with. You will find an insecticide used called 𝗜𝗺𝗶𝗱𝗮𝗰𝗹𝗼𝗽𝗿𝗶𝗱 which is highly toxic to bees, humans, domestic pets, and wildlife. There are two forms of this systemic sold, one that lasts for 8 weeks and another for 12 months. See the link below on the toxicity of imidacloprid.
The horticulturist recommended NOT using imidacloprid and if need be then use it before flowering because it is toxic to bees. and use the product that has a duration of 8 weeks. However, this product comes in granules, and considering the wildlife that would eat the granules become poisoned and predated upon (eaten by another animal) , it would be killing wildlife as it has a domino effect. Please do not use imidacloprid. A less invasive treatment for Yucca and Agave weevils is 𝗠𝗮𝗿𝘁𝗶𝗻𝘀 𝗖𝘆𝗼𝗻𝗮𝗿𝗮 𝗟𝗮𝘄𝗻 & 𝗚𝗮𝗿𝗱𝗲𝗻 .𝟱% 𝗹𝗮𝗺𝗯𝗱𝗮-𝗰𝘆𝗵𝗮𝗹𝗼𝘁𝗵𝗿𝗶𝗻. Read directions, but the ideal time is to treat in the spring and once again in May or June. Tractor Supply and Amazon sell 𝗠𝗮𝗿𝘁𝗶𝗻𝘀 𝗖𝘆𝗼𝗻𝗮𝗿𝗮 𝗟𝗮𝘄𝗻 & 𝗚𝗮𝗿𝗱𝗲𝗻 .𝟱% 𝗹𝗮𝗺𝗯𝗱𝗮-𝗰𝘆𝗵𝗮𝗹𝗼𝘁𝗵𝗿𝗶𝗻, you may also check your local nursery, Lowes, and Home Depot.
If you live in the Morongo Basin, Cactus Mart does not sell this product but our local Tractor Supply carries this product. You can inspect for damage on the leaves by looking for small spheres. Usually, by this point, the damage is done. I am finding more Joshua Trees with damage to where flowering panicles and branches are falling off the host Joshua Tree. If you have Mojave Yuccas on your property, inspect and treat them as well, as the Yucca Weevil can colonize them as well.
It is truly heartbreaking that Joshua Trees just can't get a break. They are being attacked from all angles, from climate change, wildfire, development, big solar, and Yucca Weevils. It's heart-wrenching. If you are interested in learning more on how you can protect Joshua Trees CLICK HERE, learn more about the threats that impact this imperiled, endemic species and please write the California Department of Fish and Game (DFG) commission voicing support for CESA (California Endangered Species Act)
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This is a link to information on the toxicity of imidacloprid. http://ipm.ucanr.edu/TOOLS/PNAI/pnaishow.php?id=42.
link to the article by the nursery https://backbonevalleynursery.com/agave-snout-weevils/
Read the section under SOLUTION http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/INVERT/yuccaweevil.html