By Eric Middleton PhD, IPM Advisor, University of California Statewide IPM Program and Cooperative Extension, San Diego, Orange, and Los Angeles Counties, for Let’s Talk Plants! March 2024.
Agave Mites - An Invisible Enemy
Agave are a common sight in Southern California and are frequently used in landscaping for homes, businesses, and in public spaces. You’ve almost certainly seen agave growing in your neighborhood and may even have some growing yourself. Many varieties of agave are produced in San Diego nurseries to keep pace with the demand for these plants across the state and country. As the climate continues to warm and California becomes increasingly dry, hardy and water-conscious plants like agave will more frequently be used in xeriscaping and as ornamentals.
However, there is an almost invisible enemy that threatens many of these agave. Greasy streaks and smudges appear on leaves, followed by lesions and plant decline. Sometimes, the entire core of the agave collapses. The plants look sickly and unattractive, dismaying homeowners and nursery growers alike.
What is causing this damage?
Identification
The answer are tiny agave mites, invisible to the naked eye. Agave mites, also known as grease mites, are a type of Eriophyoid mite. Like other Eriophyoid mites, agave mites are elongated and have a wormlike appearance, with 4 small legs positioned around their head. Adults are a translucent pale whitish color and lay oval translucent eggs. Depending on the temperature, agave mites can complete a lifecycle and develop from eggs into adults in just a few weeks. Agave mites are very small: Adults are around 1/3 mm long while their eggs are around 20 microns wide. You will not see them at all unless using high magnification. While the exact species is currently unknown, agave mites are members of the genus Oziella. Their method of dispersal is also unknown, but it is assumed they catch air currents and drift on the wind to find new hosts like other Eriophyoid mites do.
A group of agave mites (Oziella sp.).
Damage
Agave mites feed on the surface of agave leaves, living hidden at the very base of leaves or inside the core of the agave. If they are present, they are almost always on the whitish, unexposed leaf tissue and are unlikely to be found on visible sections of leaves. Feeding and damage takes place out of sight, and symptoms only appear once the damaged leaves have grown out, conservatively around 3-5 months after the agave mite infestation first began. To find the mites themselves, the agave must be cut open and leaves peeled away. The closer to the core of the agave you get, the more mites you find, and infestations are typically highest on the concave (or inside) side of leaves.
Agave mite damage is often easily recognizable. The colloquial name “grease mites” is accurate: the most characteristic sign of agave mite feeding is a greasy smudge or streak appearing on agave leaves. It will often look like someone dipped their thumb in grease and dragged it across the leaf surface. These greasy spots can be small or hidden in the event of minor infestations. Areas around greasy spots frequently appear yellowish and will lack the powdery blue-white surface color that many agaves have. When infestations are more severe, greasy spots can be seen all over the plant, and lesions or dying plant tissue are present in the greasy areas. Mites concentrated at the core of the plant can severely damage the new leaves and cause the core to collapse from their feeding.
Hosts
The complete host range of agave mites is unknown, but they appear to be able to infest most agave species. In nurseries around San Diego, we have seen them primarily on Blue Glow agave (Agave attenuata x Agave ocahui) and Agave parryi var. truncata. They have also been found on Agave guadalajarana, A. isthmensis, A. macroacantha, A. murpheyi, A. palmeri, A. parrasana, A. potatorum, A. potrerana, A. shawii, and A. titanota.
Management
Management of agave mite is both difficult and not well understood. Their small size and hidden feeding locations make agave mites extremely hard to detect, and by the time damage is found it is too late to prevent the agave from being infested with mites. Vigilance, persistence, and a willingness to make difficult choices is required to effectively manage agave mites.
First and foremost, prevention is key. For both home gardeners and commercial growers, make sure you can recognize the symptoms of an infected agave, and don’t bring in other agave that show signs of a mite infestation. Proactively monitor your agave for symptoms so you catch any mite outbreaks as early as possible. If you do find symptoms, commercial growers should err on the side of caution and proactively remove infested agave, especially if the symptoms are advanced. For home gardeners, the choice is harder. If you don’t mind some cosmetic damage, most agaves can tolerate mite damage without dying. However, this is a risk, and you may end up with some very sad-looking and damaged agave if you choose this route and don’t dispose of infested plants. When disposing of infested plants, make sure they are kept in a sealed container to prevent mites from spreading on air currents, and preferably keep it downwind of any other agave you have. If you handle an infested plant, make sure you sanitize or wash your hands and any tools used before moving on to other agave plants.
Saving already infested plants is difficult. While some predators like Phytoseiid mites and predatory thrips do feed on Eriophyoid mites, it appears unlikely they can provide adequate control of agave mites. The predatory mite Neosiulus californicus may prevent agave mite infestations if applied before an infestation starts. However, most commercially available predator mites appear unable to clean up existing infestations. So while some predators may provide a measure of control and could potentially prevent new agave mite infestations from starting, they will probably not eliminate already-present agave mites.
Miticides labeled for use against Eriophyoid mites may be effective against agave mites. The biggest challenge is finding a miticide that can affect agave mites protected in the core and at the base of leaves. For this reason, systemic insecticides like Spiromesifen and Spirotetramat appear promising, although contact insecticides like Fenpyroximate could also work if very thorough coverage is achieved. Similar products have been effective against other Eriophyoid mites but have not been directly tested against agave mites. Anecdotally, agave plants can grow out of the damage caused by agave mites if treatments eliminate infestations. However, this takes time, and effective treatments have not yet been established for agave mite.
If all else fails, an extreme option involves coring an infested plant and waiting for new pups to emerge. To do this, remove most of the inner leaves with scissors or a knife, and then destroy the agave core with a drill fitted with a shovel bit. Be sure to collect and promptly dispose of the macerated tissue and remove all the leaves on one side of the plant to ensure water does not pool in the now damaged and removed center of the agave. Removing the core and some of the inner leaves should remove most of the mites. If this is successful, the agave should still survive, and will produce pups even though it will no longer be able to grow itself. This technique is best used in combination with chemical control to increase the chance that mites are eliminated. For commercial growers, it is unlikely to be feasible due to the time it takes to implement. Again, this is an extreme option that is not guaranteed to work, and will result in serious damage to your plant, so only use it as a last resort for particularly prized agave.
Conclusions
In short, there are currently few options to treat infested agave plants. While some miticide treatments exist, little is known about their efficacy against agave mites. There are many unknowns and preventing agave mite infestations from occurring in the first place seems to be the best option for home gardeners and commercial growers alike.
As a summary, consider the following if you have infestations of agave mites or are worried that you may in the future.
Monitoring
Check if any of your agave have symptoms and strongly consider getting rid of agave with advanced symptoms. Cover and dispose of these plants downwind of the rest of your agave. Continue to monitor your plants for symptoms, especially plants close to previously infested agave.
Prevention
After removing already infested agave, you can deploy predatory Neosiulus californicus mites or use miticides prophylactically to help prevent infestations. Sachets of N. californicus are available to purchase and may provide preventative control for several weeks. If using miticides, products containing fenpyroximate, spiromesifen, or spirotetramat are likely good choices.
Curing Infestations
If agave mites are established in your plants, applications of miticides labeled for use against eriophyids are the most likely to be effective. However, the damage the mites caused will persist and may take months to grow out. Unfortunately, predatory mites will not be able to clean up existing agave mite infestations.
Monitor Again
Check your plants multiple times after treatment to evaluate what has worked and what hasn’t. Remember that just because symptoms appear later does not necessarily mean plants are still infested with agave mite. If possible, cut up a plant or two and check for mites under high magnification (30x or greater at minimum) to see if agave mites are still present.
Agave mites are difficult to manage, but hopefully this gives you a decent starting point for identifying and controlling this damaging pest!
Dr. Eric Middleton is the UCCE Integrated Pest Management advisor for San Diego, Orange, and Los Angeles Counties. Eric is passionate about sustainable agriculture and investigating practical pest management solutions. His goals are to conduct research that is directly applicable to the needs of growers, to promote practices that have less of an impact on the environment, and to share his findings in a way that is accessible to everyone.
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