Black Vine Weevil on Indoor Plants

A black Vine Weevil attack on your plants can show itself in at least two ways. The most serious sign is a sudden wilting of growth at the top of the plant but without any signs of damage to the leaves. This means that the larvae of the Weevils are attacking the roots of your plant. Another danger sign is holes or chewed edges to the leaves of the plant. This means that the adult Weevils— 1 cm (1/2in) black beetles— are living on the leaves of your plant.

The first sign may not be all that easy to distinguish from several other plant problems. The second is so certain that you must go into action at once, or the Weevils will not only destroy the plant but may move on to other nearby plants.

Is It Weevils?

If your plant is only infested with the larvae and not the beetles then the signs of damage it shows can range from slight limpness to the plant dying very quickly. It is not easy to confuse a slight infestation of Weevil larvae with problems such as dryness, draughts or overwatering.

The way to be certain that the Weevil larvae are responsible is to look in the top layers of the compost where you might see lcm (1/2in) white grub-like insects actively attacking the roots. If you don’t see them, do not assume that it is not Vine Weevil larvae— they can live at depths of up to 25cm (10in) down in the compost. To confirm your suspicions probe deeper or gently take the plant out of its pot to inspect more thoroughly. Shake the compost off the roots over a tray and look to see if any of these fleshy larvae drop out.

The signs of the adult beetles are easy to detect. The beetles themselves are black, 6-legged and have prominent snouts and they will have taken chunks of the plant’s leaves for food. If you do not spot them during the daytime, have a look in the dark (use a torch) as the beetles are more active at night.

Which Plants Are Prone?

Both the larvae and the beetles attack a wide range of plants but particularly Cyclamen and Primulas. They also like Azaleas, Begonias, and succulents.

Pests on the Move

Weevils are often spread in infested compost so be very careful with any plants you bring in to your home. A large number of nurseries use fast-draining potting soil — well suited to the Weevil’s lifestyle. The beetles do not travel too far as they can’t fly but they can move from plant to plant if the plants are close to each other, so move any infected plants away from their neighbours.

Combating Vine Weevils

When you first spot the symptoms of attack, pick off any beetles by hand and kill them. As soon as you have done this, spray, then drench the plant with an insecticide containing malathion. The best time to apply the insecticide is at night when the insects are more active.

Next take the plant out of its pot and gently wash the roots to get rid of the larvae. Be thorough, as there are not only larvae likely to be present; the adult beetles lay eggs — which are very small — and these also must be removed as soon as they are found.

Know Your Enemy

The Black Vine Weevil has a complicated lifecycle. Knowing something about it could help you to deal with it properly. The adult beetles, which are large, black and six legged, lay minute eggs in April in the compost of chosen plants, often quite deep down.

Larvae on the Loose

About two weeks or so later the larvae hatch out of the eggs and begin their attack on the roots of the plant. Unless checked, they will eat all the roots until the plant succumbs. However, the larvae are only active if the temperature is above 7°C (45°F). Below this temperature the larvae go into hibernation until the weather becomes warmer again, when they will re-awaken.

The Beetle Emerges

After over a year, the larvae, which eat by day and night, will have eaten enough to be able to go into the pupal stage in the following May. During this stage the larva covers itself with a protective coating and slowly transforms into the adult beetle. When the beetle hatches it finds its way out of the compost to the leaves and starts to eat. When it too has eaten enough, it lays eggs and the life cycle starts all over again, unless the Weevil can be halted at critical growth stages.

Fighting Back

Research has shown that the best time to counter-attack Black Vine Weevils is at the point in their lifecycle just after the adult beetles have hatched in mid spring. Then is the time to spray with insecticide. The beetles are nocturnal, so apply sprays or drenches at night when they are most active.

Checklist

Do

• Be on guard. Check all new plants for signs of infestation.
• Spray or drench the plant and its compost as soon as you see signs of Weevil attack.
• Apply BHC dust at night when the insects are most active.
• Wash the plant’s roots gently to remove all the larvae and any of the eggs that may be there, and mix an HCH Gamma dust into the compost when repotting.

Don’t

• Let the infestation spread from plant to plant. Make sure that you throw any contaminated compost away.
• Use old compost. Use only sterilized commercial potting compost.
• Leave fallen leaves or loose litter around which the beetles can hide in.
• Think that one application of insecticide is enough. Dust or spray with BHC Gamma dust or liquid until they have gone.