Six on Saturday: 2.1.21-Vine weevil

Happy New Year! I should really start a new year of six on Saturday posts with something inspirational and uplifting but I’ve been meaning to write about this subject for a while, so this is what you are getting. But gardening isn’t all positive and we roll with punches. We have ups and downs and by sharing the negatives I may help someone else in future.

1. RHS pests and diseases

I have started the process of revising for my next RHS exams. In the next round of exams, I am taking two exams. The first on soil and composting, the second on plant health, pests and diseases. I purchased the RHS pests and diseases in preparation for my revision and I have found myself browsing it in odd moments for the many ways our plants can be attacked. The book is a good guide to promoting plant health and while it does list chemical solutions it largely pushes more nature-friendly methods of promoting plant health. My current garden concern is vine weevil which I will spend the rest of my six looking at in more detail.

2. Vine weevil

Vine weevil are a fairly common pest. They are more of an issue for plants in pots which I think is where my problem started. I had two pots of heuchera that I discovered a few months back were infested. I disposed of these pots but there have been signs that they are in the ground too. So far the problem seems to be just in the front garden and the back seems to be unaffected. I could dig up the plants affected but this may mean removing almost everything and even then any new additions could still be attacked again. So I am trying several approaches to tackling them.

Vine weevil are a beetle Otiorhynchus sulcatus. The larvae are plump, white, legless grubs with brown heads that can be found around the roots of plants. While I have an interest in most aspects of nature you will find few positive odes to the vine weevil grub. Most people would describe them as repulsive. Knowing what they are they are not a welcome sight. They live in the soil where they nibble through the roots of your plants. In some cases, this will just stunt growth. In more extreme infestations they will cause the death of the plant.

I knew I had vine weevil when moving a pot I moved the plant and it came away in my hand. The larvae had eaten the whole of the stem and roots detaching the crown. It is possible to clean these off and re-pot but I didn’t want to take the risk so I disposed of the plants in pots. If you would like to read how to clean off infested plants read Plantsagogo advice. They attack a wide range of plants but heuchera are a particular favourite with hostas, hydrangea and cyclamen being on the list of vine weevil diet. Pretty much the list of what I grow in the front garden.

The larvae mainly cause damage through autumn and spring. The adults emerge late spring and summer. They are about 9mm long with dull black bodies and antennae that form a right angle. They are slow-moving and eat notches out of the leaves. They are flightless so you can set up barriers around your pots. If you grow plants in pots you can keep them in a saucer of water as the vine weevil don’t swim. The adults will lay many hundreds of eggs over a couple of months.

3.Non-chemical control-nematodes and predators

There are a handful of chemical controls for vine weevil but many of these pesticides are unsuitable to infestations in the ground. The pesticides can be applied as a compost drench but all the products I have looked at involve neonicotinoids which will kill more than just the vine weevils. Instead, I have used nematodes. These are natural predators that will attack the vine weevil larvae. They are delivered as a powder that you mix into a solution that you add to a watering can to water over the area. I gave the front garden a treatment in autumn and then again late on.

I have also encouraged natural predators of vine weevil to try and control the numbers further. The front garden does get frogs visiting despite no pond. There are a number of dark damp spots such as under the water butt where they can hide. I have been intending to look at making a mini-pond for the front garden to encourage them further but this is probably a job for spring now. I have also put a fat ball feeder in the front garden to encourage a few more birds in. I feed the birds with many different feeders in the back garden but I don’t really bother in the front as there isn’t much cover around for them. Once we get to late spring I will start to check the plants in the evening to see if I can remove any of the adults myself.

4. Encouraging healthy plants

The nematodes may not prove effective enough though as they don’t work as well as the temperature gets colder. They stop working at temperatures below 5 degrees. I applied the last dose about 2 weeks before the temperature dropped to this point. So I have taken a dual approach of looking at biological controls but also looking at promoting the health of the plant. Back in autumn, I gave the front garden plants a scattering of bone meal. Bone meal is a good autumn feed as it encourages plants to toughen up for winter and root well. As vine weevil mainly cause issues with roots I want the plants to be in as good health as possible. The bone meal is sprinkled on the surface and lightly forked in.

5. Mycorrhizal fungi

Mycorrhizal fungi are beneficial fungal that form associations between roots and plants to give the plants better access to nutrients. The RHS profile page partly dismisses them as the use of lots of fungicide and other fertilisers can disrupt the process. However, as we learn more about the effect these fungi the more gardeners are seeing the benefit of using this funghi. We still know relatively little about how these relationships work but they are truly astounding. I am reading Merlin Sheldrake’s entangled life and it is fascinating reading all the things fungi are capable of doing. Empathy sells many different mycorrhizal products for the gardener aimed at many different functions. They have products aimed at acid lovers, roses, evergreens but I went with the general rootgrow product. This is ideally spread on the roots when planted but as I have already got the front garden planted up the plants received a sprinkling around and it was forked in around some. So this will, hopefully, allow the plants attacked by vine weevil manage to grow back strong enough to survive any future attacks.

6. Mulching

Mulching may not seem to have much to do with vine weevil but let me explain how this may potentially help. The front garden is heavy clay. So heavy you can sculpt with it. Bricks were made locally from it. I have discussed my efforts to improve the soil previously here. Nematodes do not work as effectively on heavy soil in part because the temperature drops colder. So in order to make sure I get the most from the nematodes I need to improve the soil. I am using a no-dig approach of applying top dressings. The soil organisms will gradually take it into the soil improving the structure of the soil. I have used Dalefoot clay buster which I can get delivered free locally. This is a lovely product for enriching the soil. Not cheap, but it feels and looks beautiful. Most gardeners will appreciate this while any of the non-gardeners reading this will think I’m crazy. As well as improving the conditions for nematodes, the mulch can also help improve the drainage while also helping water retention which may seem contradictory. It also provides the plants with some additional nutrition. Clay soil is rich in nutrients but it is not always accessible to plants. All of this should improve the health of the plants to make them more resilient to vine weevil and other problems.

I hope you have found some of this of interest. We’ll see next month if vine weevil come up on my exam. If they do I feel pretty prepared for answering a question on them. The one time I’ll actually be wanting vine weevil to appear.

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28 thoughts on “Six on Saturday: 2.1.21-Vine weevil”

  1. As I started reading your post, I wondered to myself if mycorrhizal fungi would help, as I got Sheldrake’s book for Christmas! I was amazed to learn that fungi can actually hunt for creatures like worms under the soil, amazing. There’s so much going on in the soil, isn’t there?

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    1. The Mycorrhizal may not make much difference to the already infected plants since it’s the roots getting eaten. But it may slow or help the surrounding plants survive. But it seemed worth doing an application over the front garden as it’s all still pretty young having only set up the front garden a year ago. The Sheldrake book is fascinating. I don’t know how accurate some of it is. But considering how much we know about plants we know so little about fungus in comparison.

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      1. Yes I agree re Sheldrake, I get the impression he gets carried away with his own rather weird notions, perhaps he took too much LSD, as he likes to tell us he did for that experiment. He’s a bit of a show-off, I think, but despite that, I did find the content so far interesting and informative.

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  2. Very interesting post. I bought some plants from our local garden centre recently – I mentioned the cyclamen and pansies a couple of weeks ago – each plant had at least 2 of those white vine weevil eggs. I haven’t managed to contact the garden centre to warn them about this, but I will if they are open this week. The plants look fine at the moment but should I plant the cyclamen around the garden once the troughs are needed for summer plants? I was hoping to. Happy New Year to you and your family.

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    1. The advice is to dispose of infected plants but this just isn’t practical a lot of the time. If you think you’ve removed the eggs I’d risk it and use nematodes in spring, then keep an eye out for the adults later in the year and signs of nibbling. I’ve taken to pulling plants out of pots at the garden centre. Found so many last trip heavily slug egg infested that I gave them a miss.

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    2. I have never seen a vine weevil egg. They are 0.8mm diameter spheres, white when laid but quickly turning brown so almost impossible to see against soil or compost. Are you sure that what you saw wasn’t controlled release fertiliser granules?
      We had a lecturer at our garden club who told us how she cleaned all the compost off her fuchsias and removed the vine weevil eggs, showing us a fertiliser pellet as the offending egg. I kept schtum, but it wasn’t easy.

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      1. Yes, the grubs were there and while there are other similar the worst of the heuchera lifted off the ground as stem had been eaten through with few roots left. So even without seeing the grubs I’d suspect vine weevil.

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      2. Yes, I am sure they are the eggs. I squeeze them and they squelch! (Sorry, I hope you have finished your lunch.) A couple of years ago I wondered why a pot plant suddenly died and just lifted off the soil. If I remember correctly, the grubs were there doing the damage to the roots. I did check on the internet what the eggs looked like and thought that mine could have been the models for their pictures!

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  3. Oh, I definitely do know more about vine weevils now! Thank you. I don’t have them yet but if I do get them, I am prepared. Many thanks and Happy New Year to you too.

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  4. Oh vine weevils! How I hate them! Can I give you a couple of links to info about them that may be helpful, if a little OTT.https://projectblue.blob.core.windows.net/media/Default/Imported%20Publication%20Docs/Vine%20weevil%20control%20in%20hardy%20nursery%20stock.pdfhttp://www.dovebugs.co.uk/doveassociates.htm (click on information sheets, a wealth of useful stuff)
    Also, I was astounded at what you wrote about bone meal. The head of horticulture at Askham Bryan told me and my fellow students how useless it was nearly 50 years ago. One of my lecturers from the time is still blogging and his blog is a treasure trove of good information, like this: http://www.nodiggardener.co.uk/2014/07/garden-myths-discussed-does-bonemeal.html

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    1. Cheers for links. I’ve read a few different accounts on the effectiveness of bone meal depending on the soil. Either way I have to know what it supposedly helps for my RHS exam next month. I got the box for Ā£1.50 at the end of summer when shop was doing clear out. So not too bothered if it has little effect. It’s secured in my head what it is meant to do for passing exam. The RHS syllabus is getting updated and I’ll be interested to see what gets taken off as there are quite a few practises that few people do still covered.

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      1. The no dig gardener article interesting. The issue that most bonemeal contains excess calcium may actually be of some use for my thick clay soil. With the calcium helping hold the particles together though I imagine there are still better ways of going about that.

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      2. Horticulture and Agriculture turned their backs on many old, essentially organic practices when pesticides and artificial fertilizer became widely and cheaply available and are now living with the consequences and often turning the clock back. It’s an immensely complex subject (I’m just re-reading Jim Al-Khalili on quantum biology – quantum weirdness underpinning photosynthesis for example) and drawing up a syllabus for any sort of course of study is a massive exercise in oversimplification. Even so, if the RHS are recommending bone meal it’s time I stopped my subscription (after 53 years).

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      3. It’s mentioned as a source of phosphorus. I imagine there will be quite a bit that changes as peat compost is mentioned a lot. It does talk about the environmental impact briefly. But regardless of opinions on peat it is getting phased out and people taking the course need to be prepared for that.

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      1. Blood fish and bone has around the same content of available nutrients as Growmore, so is probably a fairly expensive way of applying nutrients but is organic. All of Roger Brooks posts on gardening myths are worth reading but you get the sense that bone meal exercises him the most.

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  5. Thanks for this – my garden is infested with vine weevil. Things in the ground are mostly ok but I can’t grow anything perennial in a pot so stick to annuals only – bulbs, summer flowers etc. I’ve used nematodes which might be having a small effect but it is hard to tell.

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  6. I appreciated your discussion of the mutiple approached you have used to counteract vine weevils. In particular, I think that strengthening your plants by improving the soil and encouraging natural predators are wise. Thanks for showing the nematode powder and explaining how they are applied. I was curious!

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  7. Great post! I also had all kinds of vine weevils, beetles and borers eating my squash. I tried nematodes which helped. The thing that worked best was to ‘rotate’ my crop. I moved the squash to the backyard. Eventually I’ll return them to the front yard. One thing I didn’t try, and wish I had, was diatomaceous earth which is organic and non-toxic. I also have red clay and found, by accident, that adding a thick layer of bark mulch works wonders for plants.

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