After slugs and unenthusiastic spouses the most common problem suffered by those that grow hostas, and for that matter many other plants, is the accursed vine weevil. The help desks organised by the RHS at many of their shows are regularly blessed with the query 'What can I do about vine weevils?' and related questions, as is their Wisley post bag.
Often the first sign that vine weevils are about are the tell-tale notches on the edges of leaves. Sometimes these notches are also visible on the petioles. This damage is distinct and is very unlike damage caused by caterpillars, slugs, snails and other beasts. The appearance of weevil notching on your hosta leaves is a sign that steps should be taken to protect your plants from the grubs beneath the surface, because although a common garden pest, adult vine weevils are seldom seen and its the grub that does the real damage.
Otiorhynchus sulcatus, for that is their name, are about one centimetre long when adult. They have pear shaped bodies, are dull black and their wing cases (although they cannot fly) have small, yellow-brown markings when the adult has newly emerged. From mid spring to mid autumn they hide in leaf litter, at the base of petioles and in other dark, untidy places during the day and emerge at dusk to spend the nocturnal hours eating the leaves of a variety of plants including Hydrangea, Camellia, Rhododendron, Primula and strawberry, and laying eggs.
All vine weevils are female and do not need to waste time looking for a mate. Each one can lay hundreds of eggs over a period of several months. Although adults begin to emerge in mid spring and commence to feed on your plants they do not start to lay eggs until the summer. The eggs, brown and spherical, are less than 1 mm in diameter and are therefore much smaller than Osmocote granules and very difficult to find in the compost.
Although vine weevil notching on the leaves of your best plants is unsightly it is not normally sufficient to cause terminal damage. It is the grub that is much more harmful to the plant. The larva are plump, creamy white and legless. They are not too dissimilar to a small maggot, but they have distinct light brown heads and are only about 10mm in length. These grubs live in the soil where they fed on the roots. A small plant in a pot can be killed by this action and small plants that die off or larger plants that look sad or begin to die crown by crown in summer for no apparent reason should be carefully examined. The container should be tipped out on newspaper and a very careful search made for larva and pupa. Vine weevils pupate in the soil as soft white pupae of a similar size and shape to the adult that eventually-emerges.
So what to do? Insecticides available to the amateur do not give good control of adult or larval vine weevils. (By implication, there are products available to commercial growers but it seems that we cannot be trusted with them!) The best means of protecting plants against the grubs is to use one of the available pathogenic nematodes products. These are sold in nurseries and garden centres as Bio Safe and Miracle Natures Friends for Vine Weevil. They can also be obtained mail order as Nemasys from Defenders (Tel: 01233 813121). Such pathogens are most effective on containerised plants. They can be used on garden soil but are not as effective, especially on heavy clay soils. The time for treatment is late summer when the compost is warm and before the grubs have grown large enough to cause damage. Chemicals such as pirimiphos-methyl (Miracle Sybol) or lindane dust (Murphy Gamma BHC or Doff Weevil Killer) can be used at dusk just prior to adult activity.
But if the vine weevil really is a problem there is only one solution. Summer and autumn evenings must be spent among the hostas with a torch. The adults must be captured and destroyed (Finger or heel pressure of less than 20lbs per square weevil is sufficient) before they have a chance to lay eggs. Hopefully the winter will be severe and if you keep your plants in pots, every pot will be frozen solid for a few days. Then in spring, every plant has to be re-potted, the compost carefully shaken away from the roots and brand new compost used throughout. The old compost must be sealed in old plastic bags and taken to the village dump. With it should go all the surviving eggs, grubs and pupae.
It could be worse. At least they can't fly.
The above article originally appeared in the 1997 Bulletin.