Hints & Tips

PESTS - BLACK VINE WEEVIL - PESTS
Black Vine Weevil, (Otiorhynchus salcatus), is a British native beetle, which, until the middle of this century, only very occasionally cropped up as a pest of conservatory plants. Now, however Black vine weevil is considered to be the most serious garden pest in the UK. Why? Mainly due to the change in the way plants are produced. Most plants today are grown in containers, which bring benefits such as better plant growth and improved shelf life. Unfortunately container plant production also brings benefits to the vine weevil in terms of an ideal medium for the development of the larvae and freedom from natural predators such as ground beetles. For many years powerful pesticides such as Aldrin dust were used to control the vine weevil, but has since been withdrawn by the Ministry of Agriculture, thus removing the only effective control available at the time.
So what do they look like?
The vine weevil starts to lay between 500 and 1600 eggs over a two to three month period one to two months after emergence. The eggs are very small, less than 1 mm in diameter, and are typically laid on the compost or soil surface, very close to the stem of a choice plant. Within 10 to 20 days the eggs hatch into small larvae, which burrow down and commence feeding on fine roots and dead material. The larvae are creamy white, legless with brown heads and typically adopt a 'C' shape. They grow to some 10 to 12 mm in length in six stages, or instars. The first three instars are root feeders, with stage four also tackling stems and storage organs.
The final two stages are immobile. Black vine weevil over winter as larvae, pupating for three to four weeks as temperatures rise in the spring. They pupate about 20 cm below the surface of the soil in an earthen cell formed by the larvae. The adult Black Vine Weevil is 8 to 12 mm long with roughened, dull black wing cases on which there are patches of small yellow bristles.
The adults are all female, reproducing parthenogenically, and are incapable of fight. Outdoors, the adult females emerge from the soil or compost in late spring as temperatures rise, feeding only at night and resting by day under containers, plant debris, crevices etc. After emergence the adults typically only migrate a few metres although occasionally much greater distances have been observed. The insects are, however, excellent walkers, with modified legs, which enable them to grip sheer surfaces and climb to feed.
Spotting the symptoms:
Diagnosis of vine weevil in the garden or house tends to start with seemingly inexplicable yellowing and wilting of plant foliage, despite plants being well watered and tended. Closer examination can reveal characteristic notching around the outer edges of foliage caused by adults feeding at night. Knock the container out or uproot a suffering plant and examine the root ball. The larvae of the vine weevil are unmistakable - white, 'C' shaped, legless maggots about 10 mm long with a brown head present in the soil or the lower stems of fleshy plants. It is the feeding activity of these grubs, which kills the plants by removing all the roots. In situations of heavy infestation the adult can be spotted.
Preventative measures:
With containers, don't keep the compost from year to year. Complete replacement every year can significantly reduce the level of vine weevil problems in the following season. Collect adults at night by torchlight when they are active. Place newspaper under potted plants and knock the plant to dislodge the adults. In the garden place a tray under plants. Squash and dispose of adults regularly. Providing a moist dark hiding place will also trap the adults. Place a damp piece of sacking on a path to provide a daytime resting place. Lift the sacking during the day and dispose of adults.
Treatments against vine weevil based on biological control agents such as nematodes can be effective. Typically the main drawback with this technique is that the vine weevil larvae are damagingly active at temperatures much lower than those required for effective control by the nematode worms. Nevertheless, this technique can be useful under protection, where temperatures are higher.

For more information www.vine.weevil.org.uk

back | 1 | 2 | 3 | 4 | 5 | 6 | forward