About this product
A plant that was thriving wilts over the course of a week despite regular watering. You lift the pot and the root system has gone, and underneath you find fat cream-white C-shaped grubs with brown heads. Vine weevil, Otiorhynchus sulcatus, is the most destructive pest of container-grown plants in Britain, and it usually announces itself only once the damage is done.
The grubs kill plants. The adults only spoil the look of them. This treatment targets the grubs. Adults chew distinctive notched, C-shaped bites out of leaf edges, mostly on heucheras, rhododendrons, camellias, primulas and viburnums, and though it is unsightly it rarely kills. The grubs feed on roots from July through March, with the worst of it October to March. If you have found grubs, check they are the right ones: vine weevil grubs are under 1cm, C-shaped and cream with a brown head, where chafer grubs are much bigger, up to 3cm, and live in lawns.
The species here is Steinernema kraussei, a cool-temperature specialist that stays active down to 5°C, and that is the entire reason it suits this job. Watered into the soil, the nematodes seek out the grubs, enter through natural body openings and release bacteria that kill them within 24 to 48 hours, then breed inside and emerge to find the next. Nothing else is affected. Once the weevil population collapses, the nematodes die back on their own.
Autumn is by far the most important window. September to November, when the grubs are feeding hardest before winter and the soil is still warm but getting rain. March to May is a useful second pass for over-wintered grubs before they pupate. Do not treat in winter, when soil below 5°C leaves the nematodes inactive, or in high summer, when hot dry soil kills them before they find anything.
Water the soil thoroughly first, mix with cool water, apply through a can with a fine rose or a hose feeder, and water in gently afterwards. Apply late in the afternoon or in the evening, out of direct sun. Keep the soil moist for two weeks afterwards. Dry soil is the single most common cause of nematode treatment failure.
Two sizes: 12 sqm for patio pots, greenhouse benches and cold frames, and 100 sqm for mixed borders, allotments and whole-garden preventative treatment. If vine weevil is a recurring problem, treat every autumn as routine. Live product, cannot be returned.