Yes, the nematodes DO work. That's what we recommend at my nursery. However, they only address part of the problem - the larvae. You still need to control the adult weevils and that's pretty much a manual enterprise :-) Tanglefoot applied to the base of the rhodies can help trap some but the best plan is to go out at dusk or just before fully dark, spread a sheet under the plants and shake the patootie out of them. Collect all the fallen weevils - they can't fly and move rather slowly - and submerge them in a bucket of soapy water or diluted bleach.

Timing of the nematode application is critical, typically late summer into early fall as the adults start laying eggs. The nematodes are not cheap, need relatively warm soils and don't hang around if food is not available. And this is not a one time shot. If you live in an area where root weevils are present and active, trying to control them all is almost an exercise in futility. They hide in any wooden material - mulches, fences, tree trunks, woody debris, etc. And they hide at the neighbors place, just waiting for you to turn your head before they make their slow way over for a leaf feast. They WILL return!! But over a three year period of time, you should get noticeable improvement.

Here in the PNW - rhodie central - nematodes are a constant challenge. And they don't limit their diet to rhododendron leaves - hydrangeas, English laurel, Viburnum davidii, salal and several other mostly BLE's are favored as well. Since the damage caused by the weevils is almost entirely cosmetic - the larvae are not really an issue with the roots of established plants - the suggestion is to learn to live with the damage. I tell my clients to use the 10 foot rule - stand 10 feet away from the plant in question and evaluate the appearance. If it is hard to notice the damage from that distance, ignore it. If the plant is very visible, like along a front walkway, it may make sense to relocate it to a less visible area and replace with a variety less favored by the weevils. Apparently some rhodies taste much better to them than others - some they never bother at all. These should be the types selected for a very obvious and visually significant location.

An extraordinarily fine treatment of the topic by gg, methinks. David, if you stop by the Florida Gardening (or whatever it's called) forum here at Houzz, those folks will know quite a bit about predatory nematodes as well. They deal with them, as well as the damaging types, both of which seem to thrive down there.

"An extraordinarily fine treatment of the topic by gg, methinks"

Agreed. As someone who obviously isn't afraid to use a "chemical" here and there (as opposed to those people who, you know, live, work and breathe the pure vacuum of space LOL) I'm still tempted to do something about the adult stage that doesn't involve me schlepping around in the garden at night. Sounds like something that would upset my already precarious sleep schedule.

Neem oil, of course, turns out to contain a growth regulator which might harm bees just as neonics do, but I wonder if I do it as a one time spray application, after they are done blooming, I'd feel so guilty. Maybe it won't be as persistent as the neonics and won't be around next year.

http://www.livingwithbugs.com/root_wev.html

The thing is this just started in the past 2 years, the plants were flawless before then and one of them is in a very visible location. So I don't really see peaceful coexistent being a possibility. The worse affected variety btw is 'Vulcan' which is now almost my biggest rhodie. Besides R. auriculatum which for all I know might take another 5 years to decide to bloom.

Oh btw, interesting thing about bees. I was surprised when visiting all of the public gardens that I visited in Norcal, that there was a worrisome paucity of wild bees compared to my garden. Talking about, obviously, being near flowering plants that are known to attract them. (Right now in my garden, unless it is raining it would be a suicide mission to start deadheading my Euphorbia characiasii. I've started doing it at night. They are swarming with bees and wasps during sunny periods.)

In some cases I could suppose something about the local microclimate had an effect...you could wonder whether Fort Bragg is just too chilly for bees LOL. But not elsewhere. Whether these gardens use neonics or not, whatever is surrounding them, given how intensely agricultural, viticultural or horticultural most inhabited parts of Norcal are, I couldn't help but wonder if this hymenopteran absence was related to some such sinister environmental issue or not. I have only used neonics on a couple plants and in the one case of it being a flowering plant, I picked off all of that year's flower buds. Still I hope they aren't banned because it's nice to have them as a last resort. They've been banned in Europe.

Yes, 'Vulcan' is high on their list of favored varieties, as are most other deep red flowered forms. And the hybrids are more prone to damage than the species.

If you intend to treat for the adults rather than capture, this excerpt on the subject from the ARS might prove informative:

"For many years, one objective in landscape pest control has been to prevent
adult feeding injury to foliage, usually by applying broad-spectrum insecticides
to the leaves to kill the feeding adults. The strategy makes sense in
several ways: adult weevils are the only stage that is exposed above ground, and
the adult weevil requires about a month of intensive feeding before she is able
to lay eggs. Therefore, spraying at monthly intervals should break the
black vine weevil life cycle.

This adulticide (adult-killing) strategy is unfortunately flawed in two
important ways. First, black vine weevil populations appear to be
surprisingly adaptable, considering that they reproduce asexually. Various
populations of black vine weevil adults that I've tested in my laboratory
survived applications of acephate (Orthene), chlorpyrifos (Dursban), endosulfan
(Thiodan) and carbaryl (Sevin). More importantly, I have observed
populations of adult weevils in nursery and strawberry fields that survived any
of the insecticides already mentioned, along with azinphos-methyl (Guthion),
bendiocarb (Turcam), fenpropathrin (Tame), and bifenthrin (Talstar).
Putting it simply, some black vine weevils have apparently become resistant to
any of the organophosphate, carbamate, or pyrethroid insecticides used for their
control. Therefore, it is possible that the insecticides sprayed to kill
the adult weevils in the landscape won't work. "

Ok, that is useful information. But I wonder whether that selection for resistance was more likely in the PNW where as you know, they are a more common landscaping material than here. In fact, all BLEs are...which as you say they will attack. Part of my reason for wanting to be aggressive is to keep the problem from spreading. Maybe I should order more of the nematodes and apply them to any reds I have, just in case a weevil grub is lurking. I do loves me my red rhodies LOL.

Maybe I should be glad that so many dunderheads around here just landscape their yards as though they are in Minnesota. (sorry, Minnesotans! nothing personal. Maybe I should have said North Dakota LOL) In fact right in my own 'hood I've twice seen nice big BLEs given the chop for no reason.

Damage is worse under building overhangs and low-branching trees, particularly evergreen ones - as though the insects like the shelter. I've moved a rhododendron out from under a redwood before and had the damage level fall right off starting with the next growth flush.

tom as I think you know, I agree with everything you say. I didn't know about the lack of colony collapse in AU, very interesting that. The pseudo-scientific hysteria over glyphosate is both breathtaking and terrifying at the same time. As I've pointed out, it was far more extensively studied than many of the GRAS chemicals that end up in flavorings of children's candy, coffee mixers and whatnot. But I see my garden as an ecological "system" and want to minimum the impact I have on the non-plant life forms LOL. Never been stung by a native bee or wasp here, so I figure I should look out for them. (I have been stung by a GD European hornet, and relished the chance to kill off the local swarm by vacuuming them up at night. They are the only stinging insect attracted to light so they are easy to get rid of this way. Since then I only see a "scout" a couple times a year. They might have been attracted to my yard due to a large planting of hideous, highly diseased European lilac bushes. Those are long gone.)

Embothrium, interesting that you say that because the 'Vulcan' is almost under an American holly, but moving it certainly isn't an option at this point. It's the perfect spot to get morning sun, afternoon shade, and I don't have many such spots considering my overly ambitious rhododendron collection program.

Have you tried the Tanglefoot, David? I have the same issues on some plants, no fun.

Yeah I've read about that too. Issue is there are several shrubs contacting the 'Vulcan' (and I admit, some weed tree seedlings in there. If I could file a lawsuit against my neighbor for growing rose of sharon, I would) so they would find other ways to climb.

Weevils were not a problem on 'Vulcan' growing in the open for decades here. However I eventually and gradually lost its attractiveness to what I took to be foliage mildew (lace-bug damage is similar).

Yes, I had a reputable landscape company doing my grass for a few years. I showed them this weird little beetle, I kept seeing and they saw them too, sent me to the nursery for nematodes. I had lost one huge woody shrub and had many sick. It worked so well, and things recovered overnight. I have done it every couple years now.

Laurie can you be specific on which nematodes?

Well, I am afraid they come in a clear plastic bag. I assume they have them in bulk in the back of the nursery somewhere. I know they were for Root Weevil, and soil larvae. I will sure call my nursery for you and see if I can provide better information. I won't be home until 6:00 but I will post with any news.

Beneficial nematodes are sold by most good nurseries and garden centers here in the PNW simply because root weevils are so dang prevalent in the landscape!! Nematodes are microscopic roundworms and are usually sold in the form of a sponge or other fabric inoculated with the organisms that are then "reconstituted" by soaking in a bucket of water. They are not inexpensive (the package sold at my local nursery is around $35) and need to be stored before use under controlled conditions, usually refrigeration. And the timing of the application needs to be considered carefully - nematodes are most active under certain soil temps and you must be able to coordinate both this and the proper timing in the life cycle of the larvae attempting to be controlled to be successful.

And a single application is seldom adequate. Generally, it is recommended that this is at least a three season project, with trapping or control of the adults as an essential part of the program. If you don't halt the adults, then there is really no point on wasting time and money attempting to kill off the larvae.....they will be back!!

Since the damage is almost always limited to the cosmetic - and some plants are more prone to weevil predation than others - I would consider replacement or relocation of heavily affected plants before embarking on a nearly endless (and expensive) mission to control these pests.