Holly Krock's organic approach to treating agave mites

Everybody knows what aphids and mealybugs are and what a pest (literally) they can be. But many gardeners, including succulent collectors, have never heard of agave mites. Ignorance is bliss, until something like this shows up on an agave or mangave:

Photo © 2023 Ben Faber | https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=57252

The smudges that look like someone dragged a thumb dipped in grease across the leaf surface, and the scar-like lesions, are classic signs of an agave mite infestation. Agave mites, also known as grease mites (appropriate!), are tiny critters in the family Eriophyidae. Adults are around 1/3 mm long, and their eggs are around 20 microns wide (that's 0.0008 inches). For comparison, a human hair has a thickness of approximately 70 microns, so the eggs of agave mites are 3× smaller.

If the infestation is mild, the symptoms may seem purely cosmetic. Why worry, you might wonder, when agaves planted in the ground get all kinds of nicks and dings as a matter of course? Because if left untreated an infestation is likely to get worse, stunt the growth of the affected agave, interference with the production of offsets (if it's a clumping species), and ultimately lead to the death of the plant. Even worse, the mites might migrate from the infested agave to other agaves or mangaves nearby. How agave mites spread is not entirely known, but it's assumed they're dispersed by air currents.

When the first signs of a mite infestation become visible, rapid action is called for because the infestation is well under way (it takes 3-5 months for symptoms to appear). Since mites aren't insects, common insecticides, like those containing imidacloprid, won't work. Typically, agave mite infestations are treated with translaminar miticides such as Forbid, Judo, Avid, or Pylon.​ These are specialty chemicals (you won't find them at The Home Depot or Lowe's), and they're pricey. Even Avid, the least expensive option, costs close to $100 for a small bottle. What's more, experts recommend rotating between several different insecticides, which makes effective treatment unaffordable for many agave owners. If you try to avoid the use of heavy-duty chemicals in your garden, you're out of luck anyway because all of these miticides are potentially hazardous to the environment and to you.

Now, however, there's an alternative: Horticulturist Holly Krock, co-owner of Krock Nursery in Goleta, California, has developed an organic alternative. In collaboration with Ron Whitehurst, pest control advisor and co-owner of Rincon-Vitova Insectaries, she has created a comprehensive program based on biological and organic controls. It was tested for two years at Krock Nursery, a specialty grower of agaves for the collectors market, and has proven to be highly effective. (Holly makes a brief appearance in my video of her husband Tony demonstrating how to core and cut an agave for propagation.)

Holly Krock in the Krock Nursery greenhouse

Holly will publish a more technical account of her management protocol elsewhere, but she has graciously provided a summary for Succulents and More. The biological and organic controls needed are available online, and the steps are easy to implement. Even so, it may be too much effort for people who discover a mite infestation on one or two agaves; if they're common species, it's easier to simply remove and destroy the plants. But if you have a collection of special agaves – whether they're rare or simply special to you – it's a welcome alternative and well worth considering.

Holly's management protocol hasn't been tried on aloe mites, but there's every hope that it might work, considering both agave and aloe mites are in the same family.

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Management of Agave Mite – Working with Nature

By Holly Krock, Horticulturist & Co-Owner Krock Nursery

© 2024 Holly Krock. All rights reserved.
No distribution of reproduction without prior written permission. 

In collaboration with Ron Whitehurst,  co-owner of Rincon-Vitova Insectaries and a Pest Control Advisor, we have developed an effective and ecologically balanced approach to treat agave mite:

Part 1 – Optimize plant health and natural immunity by converting to an organic biologically active grow system

–      Use only organic fertilizers and pest management products.

–      Add mycorrhizae and compost to the soil mix.

 Part 2 – Implement a multi-tiered approach of biological & organic controls

–      Scheduled applications of Chromobacterium subtsugae (sold as Grandevo CG), which is a microbial insecticide. It works as a toxin when ingested, it repels, and it reduces reproduction. The three modes of action make agave mites less likely to develop resistance.

–      Timed release of the following three predatory mites:

▪       Stratiolaelaps scimitus – 1x

▪       Neoseiulus fallacis – 1x

▪       Neoseiulus californicus – repeat seasonally

The release of these predatory mites is recommended even outside the greenhouse because it helps build up a population in the area. They won't all survive or stay, but some do and it moves you closer to a balanced system. They can use pollen as an alternative food source when eriophyid mites are not present; to create a more inviting habitat, you can plant flowering annuals like Alyssum.

Part 3 – Ongoing evaluation of plant & soil health

–      Continually monitor insect activity, soil health, and plant vigor. Adjust as needed.

 Summary of Results

Grandevo CG has been the most effective component in this program for mite management. However, a comprehensive approach including all three parts will create a more stable and sustainable system.

Grandevo CG was applied together with polysorbate 20 as a non-ionic surfactant. The product was not effective without this surfactant. In contrast to other surfactants, polysorbate 20 doesn’t remove the waxy coating of blue agaves.

The product rate recommended and followed is:

–      Grandevo CG: 0.5 oz dry product per 1 gal H₂O

–      Polysorbate 20: 0.4 tsp per 1 gal H₂O

For small applications (a dozen plants or so), use a typical household spray bottle with the following rates:

–      1 cup water

–      1/8 tsp. Grandevo CG (granules)

–      2 drops polysorbate 20 (liquid)

Recommended application schedule:

–      Grandevo CG and polysorbate 20 about every 2-3 months (this includes three applications 1 week apart, so a single treatment is a 3-week process in total)

Follow the label precisely with regard to rate and best-use practices. It is recommended to maintain a pH of 7 in the tank or sprayer while using this product. If you have tap water with a high pH, try using reverse osmosis water. Products used were sourced through Hydrobuilder.com and Amazon.com. There are many suppliers available online and prices will vary. You can purchase the recommended predatory mites from Rincon-Vitova or your local insectary.

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16 oz. of Grandevo CG is $46 on Amazon, 12 oz. of polysorbate 20 is $12 on Amazon. In a non-commercial setting, this will last a long time. I suggest you split them with fellow agave aficionados.

If you have a ½ gallon sprayer (like this), use these amounts:

• ½ gallon (8 cups) of water
• 1 tsp. Grandevo CG
• 16 drops (0.8 ml) polysorbate 20 

I find it easier to convert drops to ml and use a small 1-ml syringe for dosing. Here is a handy drop-to-ml converter.

The solution is only good for about 24 hours, so don't make more than you can use in one day.

Grandevo CG and polysorbate 20, the two main ingredients

I have a few agaves in my garden that have suspicious-looking grease spots. I don't know for certain whether these are symptoms of a mite infestation, but I've decided to treat them and all the agaves near them anyway. 

Grease spot caused by agave mites – or not?

I made the first of three applications yesterday (I used the entire content of my ½ gallon sprayer) and will repeat it after one week and then again after two weeks. As Holly says, a single treatment is a 3-week process that includes three applications one week apart. 

MORE INFORMATION ON AGAVE MITES

• Agave Mite: A Quick and Dirty Primer (article by Ron Parker)
• Operating Agave Might! (article by Matt Maggio)
• Agave mite (comprehensive post on Agaveville)

© Gerhard Bock, 2024. All rights reserved. To receive all new posts by email, please subscribe here.