Vine Weevil

Vine weevil is a pest of numerous plant species, both outside and under cover. Fairly indiscriminate, the vine weevil will attack almost anything.

In summertime damage is mainly caused by the adult, a nocturnal beetle which feeds on the foliage, cutting notches around the edge of the leaves. This type of damage, though unsightly, is not normally fatal to the plant. The larval stage of the beetle lives in the soil, and feeds on the roots of plants during spring and autumn. If enough root feeding takes place, and it usually does, the plant withers and dies. Peat based compost is the ideal environment for vine weevil larvae, allowing it far more freedom of movement than in ordinary garden soil. Improving the soil structure of ones garden by digging in compost and manure therefore actually encourages the vine weevil. Unless bought in plants are from a known and reliable source, one can bring the pest in also.

Under normal conditions the life-cycle of the pest is quite predictable. In the spring the warmer soil temperatures “wake up” the overwintering grubs, which start feeding and growing. When fully grown, the grub burrows deeper into the soil and enters a pupal stage, which lasts about 20 days. When this is completed an adult beetle emerges and works its way to the surface and is active throughout the summer until October. At night adults feed on foliage and lay eggs near the base of the plant. During the day they hide under pots, in crevices or beneath the soil. The adult beetle is about 10mm long, greyish black, with dull yellow spots. It cannot fly, but can walk and climb very efficiently. The eggs take 2-3 weeks to hatch depending on the temperature. The grubs are very small at this stage, resembling very small maggots. When fully grown they can be 10mm long. They are a creamy white colour with a brown head, and do not have legs. At rest they form a curved position like the letter ‘C’. The grubs burrow into the soil and start feeding on the roots of the plant. Very young grubs feed on the fibrous root hairs, but larger grubs attack the larger roots and even the stem below ground. As autumn progresses and temperatures drop, the grubs stop feeding and remain inactive during the winter. As spring temperatures warm the soil, the life-cycle continues.

Outside, usually only one generation occurs each year, though there is evidence to suggest that a warmer than normal winter can produce two generations. If the adult beetle manages to survive the winter, egg laying can begin as early as March. This will produce grubs active in the soil during April, May, June and July that will become adults that same year. Grubs that have overwintered will have all pupated by the end of June, but egg laying from overwintered adults can result in active soil borne grubs throughout the summer.

In heated conservatories and greenhouses, the grubs may not enter the inactive winter stage, and the life-cycle can be continuous.

The Natural Enemy.
Parasitic nematodes, which occur in minute numbers naturally in the soil, are watered into the growing medium in very large numbers. The nematodes carry a bacteria with which they infect the vine weevil grub to kill it. This bacteria is very specific, and has no effect on mammals, reptiles, or earthworms. The nematodes then invade the body to feed on the contents and breed. The next generation then leave the host and seek out more vine weevil larvae. If there are none to be found the nematodes die after about 4 weeks. For the nematodes to be able to infect the vine weevil with this bacteria, the soil temperature must be above 10°C.

Nematodes swim in the water contained between the soil particles, but only in their third stage of development can they live outside of the host. During this stage they do not need to feed, and can search for a live host to infect. They can only live in this stage for a few weeks, so if no host is found within 4 weeks they will die.

How much to use.
First one has to calculate how many packs of nematodes will be required. If the existence of vine weevil is confirmed for the first time, one must assume the whole garden is infested. Once the problem has been successfully treated, subsequent years should only require a routine autumn treatment on a limited scale.

Treating the total area of a garden would be prohibitively expensive, and largely futile, as the following summer adults will walk in from hedgerows, or even next door.

In Autumn, treat all pots and containers containing plants that will be kept. Dispose of all compost from plants that are discarded at the end of season. Do not put this on the compost heap. In beds and borders, treat plants on an individual basis. If you have a greenhouse or conservatory, treat everything in it, even bare soil if it was planted during summer but has now been cleared.

Autumn treatments.
Even if nematodes were applied in the spring, a further treatment will be required in the autumn. Adults from surrounding areas will have laid eggs during the summer. Any incidence of adult feeding during the summer certainly confirms the need for autumn treatment.

Apply the nematodes from the middle of August to the end of September. Soil temperature can be guaranteed to be warm enough, as can the presence of the grubs. Earlier applications can falter due to several reasons, mainly weather and the possibility of a second generation of adults. If the nematodes are applied too early, it is possible for them to work too well, killing all present grubs. If no further grubs can be found they will die out. The adult beetle can still be laying eggs during September, and even October in warm years. The grubs from these late eggs may not cause significant damage at the time, but will overwinter and cause problems in the spring.

Later applications are more sure of total success as they will be present during these months. If damage from grubs is causing unacceptable plant damage in July, treatment will cure it, but another application in September may be needed.

The nematodes are supplied in a clay carrier medium. Nematodes are microscopic, so without a microscope one is unlikely to be able to see them. As long as the carrier is moist there should be no problem. If there is any mould or the product has a strong smell, similar to rotten eggs, contact your supplier. A faint smell upon opening the package is acceptable, as nematodes give off methane which can collect in the envelope.

The product should be used straight away. If this is not possible, store the unopened packet in the bottom of the fridge (not freezer) until ready to use. Nematodes will not escape from the packet into the contents of the fridge, as they can only swim in water and will not leave the carrier.

The instructions supplied with the nematodes will give details on how to prepare and apply them. They are mixed with a measured amount of water and this is watered onto the soil. If your pots are all the same size, just divide the mixture between them. This is unlikely to be the case, so some more calculation is necessary.

The water is only a method of applying a certain amount of nematodes to a corresponding volume of compost. You may use less or more water, as long as you know how much water you are using for each pack of nematodes. If your pack of nematodes is sufficient to treat 600 litres of compost, and you decide to use 12 litres of water, then each litre of compost must receive 20mls of solution.

This amount of solution needed per. pot is quite a small, so to ensure it does actually enter the compost, water the plants well for a few days before, and immediately prior to the application.

One can apply to borders with a watering can and fine rose. If doing so, a good seal is necessary between the spout and rose, as dribbles here will upset your calculations. Use extra water in the can to dilute the nematode solution and keep watering until it is all gone.

Keep the rose as close to the soil as possible. Repeat with more water to wash any nematodes off the foliage into the soil. Apply the nematodes in the evening, or on a dull day, so that the water can drain into the soil without much evaporation. Once the nematodes have been applied, the soil or compost must not be allowed to dry out for 6-8 weeks. Soil is less important, as even if the top looks very dry, it can still be moist underneath. With pots and containers, especially using peat based compost, try to water often and regularly, but do not overwater. If the water drains through the compost and runs out of the bottom, nematodes can be flushed through. For this reason pots should always be standing in trays and watered from the bottom. Watering from underneath is far better for root development also. In especially hot weather, mulches or bark on the surface help to retain moisture by inhibiting evaporation.

Nematop Adult Vine Weevil Trap and Nemasys Vine Weevil Killer see the
Green Gardener web site