Gardeners often notice notched leaves in summer and blame the wrong culprit, then lose a container plant in winter without knowing why. Vine weevil is the usual explanation. This article summarises the Royal Horticultural Society (RHS) advice pages “Vine weevil” and “Biological Control in the Garden”. It is aimed at gardeners in Britain, and the RHS describes vine weevil as very widespread and common there. This is general gardening information, not a recommendation of any product.
Two life stages, two kinds of damage
The RHS says vine weevils are broad-snouted beetles that can feed on a wide range of plants indoors and outdoors and can be especially damaging to plants in containers. The adults eat leaves in spring and summer, but it is the grubs that can cause the most damage over autumn and winter when they feed on plant roots, resulting in wilting and plant death. Most plant losses occur from September to March, when the grubs are becoming fully grown, according to the RHS biology notes.
Adults are about 9mm long, dull black with a dirty yellow mark on the wing cases, and are mainly active after dark, hiding by day in leaf litter beneath their host plants. They cannot fly but are excellent crawlers and climbers. The RHS says adults rarely cause enough damage to affect the vigour of plants; their notches around leaf margins are unsightly, especially on rhododendron, evergreen euonymus, hydrangea, epimedium, bergenia, primula and strawberry. The grubs are plump, white, C-shaped and legless, up to 10mm long, and are found among roots. They can also bore into tubers of cyclamen and begonia and the stem bases of cacti and succulents, and can kill woody plants by gnawing the outer tissues of larger roots and stem bases.
Where plants are most at risk
According to the RHS, plants in pots and containers, outdoors or under cover, are the most likely to be severely damaged by grubs, and plants in open ground are less likely to be harmed. It notes that heavy populations can occur on strawberries, primula, sedum, heuchera and young yew plants. A wilting pot in autumn or winter with no obvious pest above ground is worth tipping out to check the roots.
Why every adult is female
The RHS says all vine weevil adults are female and each can lay several hundred eggs during spring and summer, reproducing asexually. The eggs are brown, less than 1mm across and very difficult to see in soil. It adds that larger yellowish-brown spheres seen in potting compost are likely to be controlled-release fertiliser pellets. This pattern does not apply to every related species; the recently arrived Otiorhynchus armadillo, for example, is often found mating.
Non-chemical steps the RHS describes
- Tolerate adult leaf damage, as the RHS advises.
- Remove adults by hand. On mild spring or summer evenings, inspect plants and walls by torchlight and pick off adults; shake shrubs over an upturned umbrella or newspaper to collect more. In greenhouses, look under pots and on the underside of staging, where they hide by day.
- Encourage predators. Vine weevils and grubs are eaten by birds, frogs, toads, shrews, hedgehogs and predatory ground and rove beetles.
- Check susceptible plants frequently, looking for notched leaves on thick-leaved evergreens as a sign adults are present, and repot plants, removing as many larvae as possible from around roots and compost.
The RHS warns that after an apparent disappearance, numbers can build up again, so gardeners with vine weevil should keep up their guard.
Nematodes and their timing
The RHS says biological control of the larvae is available as microscopic insect-pathogenic nematodes, in several species and combinations sold by biological control suppliers and as sachets from some retail outlets. It says they are suitable for containers and open ground. For the best results they should be applied in August or early September, when the soil or potting medium is warm enough and before the grubs have grown large enough to cause serious damage. It gives temperature ranges of 5 to 20°C for Steinernema species and 12 to 20°C for Heterorhabditis species. It says the nematodes can give poor results in dry or heavy soils and work best in lighter soils and open potting composts.
The RHS guide to biological control adds general points that apply here. Nematodes are living organisms and should be used as soon as possible after purchase. Nematode biological controls are usually specific to insects or molluscs, but they have the potential to infect non-target animals, so they should be used in a targeted manner and only when there is a specific problem to treat.
The RHS position on pesticides
The RHS states that it does not support the use of pesticides and recommends non-chemical control options whenever needed. Its biological control page notes that predators and parasites are very susceptible to most pesticides, which can persist for weeks. Anyone considering a product should read the label and follow the rules that apply to it; the site’s article on pest products as biocides explains the legal position for pest control products in Great Britain, and its article on metaldehyde slug pellets covers a related garden pest product.
Common questions
Are the notched leaves a reason to treat?
The RHS says adults rarely cause enough damage to affect plant vigour, and advises tolerating leaf damage.
When are nematodes applied?
The RHS says August or early September gives the best results, when the compost is warm enough.
The bottom line
Vine weevil damage that matters happens below the compost, where grubs eat roots from autumn to spring, especially in containers. The RHS advises tolerating leaf notching, picking off adults, encouraging predators, repotting and checking roots, and using nematodes in August or early September if biological control is wanted. The RHS does not support pesticide use for this problem.
Sources
- Royal Horticultural Society, “Vine weevil” (rhs.org.uk)
- Royal Horticultural Society, “Biological Control in the Garden” (rhs.org.uk)