… along comes another one. You may have heard about a black beetle which attacks plants of the Agave family. Unfortunately, this has now arrived in Algarve gardens. It is similar to the red palm weevil but with the important difference that the beetle does not fly and so all infestations are soil borne.
Agave snout weevils are beetles about 0.6 inch (15 mm) in length, brownish-black and with a dull body. Being a weevil, it has a protruding snout and chewing mouthparts. The adult female uses the proboscis to enter the base of the plant to lay eggs in the spring. Grubs hatch and consume the agave’s heart, then burrow into the soil to pupate.
The weevil was once prevalent only in Central American desert regions and Mexico but is now spreading rapidly, perhaps encouraged by the interest in growing succulents, earning it the dubious distinction of being one of the “Top 100 Worst Global Invasive Species”.
An agave’s grub-eaten core resembles a sponge and it smells really bad – hard to miss!
If you suspect an infestation, look for damaged tissue where leaves meet stem. The lowest leaves will appear wilted and slope unnaturally downward, while the centre cone remains upright. The plant, no longer anchored by roots, will rock when pushed and it is possible to push the agave over onto its side and break it at soil level, revealing a mushy, foul-smelling core infested by plump, squirming, half-inch, cream-coloured grubs/maggots with brown heads.
Nursery stock may be infested so, from now on, plant agaves bare-root. People tend to assume nursery plants are pest free, yet this is the main way snout weevil enters gardens as the beetles cannot fly.
When an agave shows signs of infestation, remove it and every plant for several feet surrounding it. If you wish, you can then sift the soil and pick out grubs and beetles. It is a lot of hard work but fortunately the beetles are slow crawlers!
Don’t let snout weevil stop you from bringing home new agaves. Before planting, remove it from its pot, set the plant (root ball and all) in a wheelbarrow, and hose the soil off the roots. Carefully examine the plant (and the pot) for beetles and puncture holes, and the soil for grubs. If a plant is infested, destroy it and inform the nursery.
You can also apply a systemic insecticide as a prophylactic (preventive) option. If you want to do this, it should be applied twice a year, in early and late spring, when the beetles are active. Systemic, as the name implies, spreads insecticide through a plant’s system, so any bug that ingests it dies. This will indeed kill snout weevils and grubs, but won’t prevent the plant from being irreparably damaged, because it is not the grubs that kill the plant by consuming the core – it is the bacteria that the weevil injects into the plant.
The weevil is a vector (carrier) of Erwinia carotovora, a micro-organism that softens the tissues for easy consumption. After chewing into an agave that has been treated with a systemic, a beetle may die, but the insecticide won’t kill the bacteria.
Insecticides kill beneficial insects too and can disrupt your garden’s natural predator-prey balance. Birds and reptiles eat snout weevils, which have coexisted with agaves for millennia. Personally, I like the idea of my garden lizards, birds and passing wildlife feasting on the grubs.
If you don’t want to use inorganic pesticides, remove an agave at the first sign of infestation and don’t plant agaves in that part of your garden again. You might replace them with aloes, which are supposed to be less attractive. There is one organic control currently under development, a pheromone trap designed to attract adult beetles in search of mates. Weevil-resistant agave varieties are also being selected and bred, but it may be years before a good supply is widely available.
For special varieties, plant agaves in containers and also look for other succulents such as larger stapelias, euphorbia species and aeoniums. Potted agaves are more difficult for weevils (which don’t fly) to access. Additional advantages to growing agaves in pots is that it elevates the plants for better viewing, enabling them to serve as garden focal points even when small. Moreover, potentially invasive agaves (such as A. americana species) stay small in containers.
The snout weevil seems to prefer Agave americana (especially older or variegated plants) but may go after aloes and other agave family species such as Dracaeana and Strelitzia. It seems to stay away from agaves with soft leaves, such as Agave attenuata; those with tough, hard-to-pierce leaves (such as A. ‘Sharkskin’ and A. victoriae-reginae); and those with slender, non-juicy leaves such as A. bracteosa and A. filifera. However, it also infests other genera in the Agavaceae family, such as Nolina, Beaucarnea, Yucca, and Furcraea.
Please let our Secretary know if you think you have this problem ([email protected]) and we will try to make a record of outbreaks. Photos and the location would be appreciated. For more info, go to https://mgaportugal.org and search ‘Agave Snout Weevil’.
Ironically, it is this grub that is often placed in the bottom of a bottle of Tequila. Cheers!
By Rosie Peddle
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www.mgaportugal.org
