Vine Weevil Control

Answered by Harrod Horticultural Posted in Category Pest Control & Nature

Dear Martin

Last year I got my order of plants to fill the victorian tub planter and had some plants over. Some have thrived but others have wilted and appear to have moulded away - is this indicative of this vine weevil? If so, what's the treatment?
I have heard a lot about nematodes - what are they and would it be beneficial to have them in the soil as a preventative rather than cure? If so where do I get them from and what type? Loads of questions but being a rank amateur need all the help I can get.   Many thanks for your help in advance.

Yours truly
J.Parsons

Dear John

Many thanks for your enquiry concerning the possible presence of vine weevil in your strawberries. It’s actually the soil-dwelling larvae or grubs of the weevil which do the most damage by feeding on plant roots. They can be very destructive and on occasions kill plants, but usually the damage is limited to wilting and general ill-health! 

First of all, I suggest you check the soil around the roots of the strawberry plants to see if the larvae are present. It’s about this time of year that they begin feeding again after a period of relative inactivity throughout the winter. The larvae are creamy coloured, around 8mm long and curved into a ‘C’ shape with a brown head. You’ll find them in and around the roots of plants in pots, beds and borders and it’s not unusual to introduce them to the garden in the soil of a potted garden centre-purchased plant.  

Fortunately, there are quite a few techniques available to the gardener which put paid to vine weevil activity, one of them being the nematodes you mention in your message. Nematodes are microscopic worms which occur naturally in the soil but by applying a concentrated dose in the form of the vine weevil larvae-killing Nemasys H, their results are more tangible. Nature has a gruesome way of keeping pest numbers down and the nematodes’ mode of action is no different; the nematodes (Steinernema kraussei) seek out the vine weevil larvae and attack the pest by entering natural body openings. Once inside, they release bacteria that stops the larvae from feeding, quickly killing it. They do not stop there; they reproduce inside the dead pest and release a new generation of hungry infective nematodes, which disperse and search for further prey.  

Each application gives around four weeks of treatment before the nematodes slowly die back to their previous background numbers but because the nematodes are generally used in an outdoor environment, we are unable to supply the treatment until soil conditions are favourable – a temperature of 5 degrees C must be maintained before successful applications can be made.   The good news is that we’re right on top of suitable soil conditions and we’ll be despatching nematodes very soon – but if you’d like to get out in the garden and on the case almost immediately, there are s a few cultural methods you can fall back on in the meantime. Of course there are some chemical controls available as a last resort, but we would not advocate this approach due to our firm organic beliefs. Removing any adult weevils is the first step, and their presence is far easier to spot than that of the elusive larvae. Small, neat notches are cut out of the leaf edges by the adult weevils, and these nocturnal shenanigans can be severely disrupted by placing a sheet of paper beneath any nibbled plant and shaking. You should find a number of adults come tumbling out of the leaf cover onto the paper, where they can be safely transported away and dealt with. This practice must be carried out under the cover of darkness to be successful. 

You can also target the destructive larvae and potentially destructive eggs with a programme of re-potting or re-planting. The eggs and larvae live exclusively amongst the roots of plants and if any are discovered during re-potting, it’s advisable to sterilise or dispose of the infected soil. It’s also good practice to check the roots of any garden centre-purchased plants in future for evidence of infestations as mentioned above.   But back to the nematodes! It’s the March and autumn nematode treatments which cause maximum disruption to the vine weevil’s lifecycle. The larvae, which have over wintered in the soil and would have developed into egg-laying adults, are killed by the nematodes and an entire generation can be almost wiped out – good news indeed for your plant’s roots! You can still apply Nemasys H every 6 weeks throughout the growing season if necessary but unfortunately, Nemasys H is not really suited to preventative control as the nematodes need the pest to be present to feed on and use as a host for reproduction.

If you decide to try out a course of Nemasys H, applying the nematodes is relatively simple.
When you receive your packet of Nemasys H Vine Weevil Killer, it should have a use-by date stamped on or inside the outer packaging – this date is normally within 2-4 weeks of the pack arriving on your doorstep. You can safely refrigerate the nematodes (but don’t freeze them!) until you are ready to apply them or the use-by date is about to expire.   You’ll need to dilute the nematodes in water and apply with a watering can to the soil around your potted plants. You can also use on the garden borders and yes, it’s good practice to apply any left-over solution to healthy looking plants as once you’ve mixed the nematodes with water they cannot be stored. It’s worth investing in, or making, a rose for the watering can you intend to use with large holes as a fine rose can easily clog up. 

Further treatments can be made at six-weekly intervals if required, and other crucial application window is in the autumn – this puts paid to any larvae before they over winter, which should mean less pests at the start of the next year.   I hope this mini-biology lesson has given you some background as to how this fascinating method of control works, and given you an insight into this common pest and how best to control it! As always, please don’t hesitate to contact me with any other queries you may have and good luck!

Martin 

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