Since I moved to East Kent, and closer to the coast, it seems my problems with vine weevil have increased. I wonder if it is the milder climate, the salty air, or just the higher amount of tough evergreens that can cope with the salt spray that makes these pests more prolific in this area. The amount of vine weevil damage is certainly bigger than it was when I was further inland.
Vine weevil tends to eat evergreen plants, and often you can see damage on Rhododendron, Bergenia and Laurus. I have also seen Aspidistra with vine weevil damage on the leaves. The leaves will appear serrated. The weevil lays eggs, and they seem to have a preference for potted plants. Especially with Heuchera for instance you may not realise the grubs are in the ground until one day your plant seems to dry out despite your watering, and then you find the plant is actually loose on the ground, completely detached from all its roots.
Last week I went through my Ophiopogon japonicus collection. I put them in the greenhouse a few weeks ago to protect them from the worst weather. Although they are pretty hardy, I wanted to protect them a bit as I will use some of the plants for a display when I open my private garden for the National Garden Scheme. My collection is a modest one, with about 10-15 different cultivars. So far I haven’t applied for it to become a National Plant Collection yet, but I have thought for a while it may be a nice addition to my Hakonechloa as it is also a Japanese plant.
When I went through my plants and wanted to re-pot one, I noticed quite a few of the leaves came off, and those bits had no roots at all. When I examined the compost a bit better, and broke it up, I discovered the small white grubs that are the young vine weevil grubs. One of my plants was in a 9 cm pot, and it had 15 vine weevil grubs in it. A true infestation.
When I got to one of my rarest cultivars, one called ‘Ofuna Punctata’, a lot of leaves came off, and I ended up with the centre of the plant, having lost about a third. As I went through all the plants, I shook all the compost off, leaving only the roots. I killed all the vine weevil by hand, and then re-potted the plants, making sure that after I had done one plant, I would dispose of the old compost before I started with the next plant, to prevent cross contamination. Some of my plants were in multi purpose compost, others in a soil that looked more like a clay based compost. I didn’t notice any difference in numbers between the different composts. I went through my entire collection.
When I did this job, I noticed that some of the plants did look more yellow than others. As it is still quite cold, I previously just dismissed this as winter damage, and them being old leaves. Now I had the plants next to each other though, I could clearly see which had had more damage from the weevil (see below).
Prevention of vine weevil is quite difficult, but there are ways. A lot of pesticide products that we could use in the past, have now been banned, but there are natural ways to fend these grubs off.
First of all I would recommend putting a layer of grit over the soil if you have plants in pots. This could prevent the vine weevil from laying its eggs in the soil. You can also order nematodes from online suppliers. These nematodes attack the grubs and will kill them. There is a minimum soil temperature for the nematodes, although some suppliers now have different species for different times of the year, to overcome this problem. Generally you would still not use nematodes in the coldest months, but greenhouse use is possible during this time. The greenhouse needs to have a minimum temperature of 12-15C then. Applying nematodes is quite simple as they can be watered in with a watering can. Make sure the compost doesn’t dry out, as otherwise your nematodes may die. It is also wise to water the plants before applying nematodes. The cost of nematodes is quite low, and certainly worth it. usually the minimum amount is £15-£20 pounds for a pack that would easily be enough to treat your greenhouse, or patio pots. Personally I use Dragonfli for my predators, but there are of course other companies who supply biological pest control. Although I have replaced all the soil, and killed all the vine weevil I could find by hand, I will still use nematodes to be sure I get them all. I will also treat all my other greenhouse plants.
Another method I have heard of, but never tried, is the dunking method. Vine weevil grubs need oxygen, so I have heard of people who dunk their plants in a bucket full of water for 24 hours, killing the grubs by drowning them. I wouldn’t do this with plants that like very dry soil as the compost can stay wet for quite some time afterwards, but I can see how this would work with perennials and shrubs.
Monitoring vine weevil in your pots can be difficult, and often you don’t see anything until it is too late. Losing plants from vine weevil damage does give you an indication of plants they particularly like, so you are learning which ones to keep an eye on.