Keep plants healthy to thwart weevil attacks

LATE August is a great time to enjoy the garden and it's also an opportune time to control weevils in the garden.

There are several species of weevils native to B.C. including the obscure root weevil (Sciopithes obscurus), woods weevil (Nemocestes incomptus), Horn's Woods weevil, (Nemocestes horni) and the Woodburn weevil (Dyslobus granicollis).

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But the black vine weevil (Otiorhynchus sulcatus) and the strawberry root weevil (Otiorhynchus ovatus) both introduced to B.C. around 1895 from Europe are the most prolific weevil pests of plants in gardens, nurseries and berry fields.

Weevils look similar to some beetles, having a three segmented body that is about eight millimetres long, with a head with black antennae, a long snout, a small mid section called the thorax and a large abdomen. The body colour is black or brown and sometimes spotted with yellow or tan spots.

Black vine weevils are flightless females and parthenogenetic, meaning reproduction happens without fertilization from a male. Female weevils lay on average 200 eggs. Adult weevils lay eggs on the soil surface in July and the eggs hatch in two to three weeks. The newly emerged larvae descend into the soil to feed on roots and root bark.

Weevil young (called larvae) are white in colour, with brown heads, no legs and often found curled in the soil lying in a "C" shape. Larvae are usually smaller than the adult or slightly larger depending on their stage of development and how much food is available to eat.

During their development, the larvae molt five or six times within earthen cells in the soil.

Adult weevils eat the leaves of many different plants including rhododendron, yew, strawberry, lily, camellia, salal, rose, viburnum, many perennials, trees and many other plants. Damage is indicated by the telltale crescent-shaped notches on the edge of the leaf with severe damage leaving the leaf looking jagged. Adults are nighttime feeders and they feed on new or current year's leaf growth. A few or several weevil notches on the leaf is not a plant health concern, it is merely a cosmetic issue.

Weevils do not eat holes in leaves but they can eat notches into leaf holes caused by other pests. Weevil larvae eat plant roots and they can eat the soft bark where the trunk/stem touches the ground, causing stem girdling and plant death.

Effective weevil control can be obtained by using integrated control methods, not just one method but several methods applied simultaneously. Keeping plants healthy is always the best line of defence and it can be obtained by properly watering, applying mulch, avoiding the use of chemical fertilizers which induce soft leaf growth that is prone to pest attack and choosing the right plant for the right place. Encouraging natural weevil enemies like birds, frogs, toads, shrews, hedgehogs and predatory ground beetles is very effective. Applying sticky barriers like Tanglefoot or StickEm to the trunk will prevent adult weevils from travelling up the trunk to eat the leaves. Apply a plastic wrap or masking tape to the stem before applying Tanglefoot to avoid stem tissue burn.

Chemical control of adult weevils in a residential setting is generally ineffective due to the lack of effective personnel training, poor quality spray equipment, insect resistance and the fact that most people spray after the damage is seen, which is too late to control the adult and the larval stage can't be controlled with foliar sprays.

Weevil traps can be made from burlap, small pieces of wood, small pots or corrugated cardboard that is placed under the target plants. The weevils will hide in the traps during the day where they can be collected before their nighttime feeding begins.

Physical barriers placed on top of the soil work but barriers can be time consuming to install and they need monitoring for effectiveness. A layer of sharp

sand placed around target plants may prevent egg laying. Coffee grounds are also said to be an effective deterrent but too much coffee is not good for the soil pH.

The use of beneficial and parasitic nematodes can be effective in controlling weevil larvae that are found in the soil in late August and September. Commercial nematode products include BioSafe, Biovector and Nemesis, among others. Soil temperatures must be at least 13 C, preferably higher, and lots of irrigation must be applied to the soil before and after nematode application to allow the nematodes to swim through the soil to find the larvae. These nematodes do not like direct sunlight, so apply them during the evening.

One of the most effective weevil controls can be obtained by hand-picking them off the plant during their nighttime feeding hours.

This is best done during the "summer weevil hunt party" that includes the company of friends and lots of wine.

Todd Major is a journeyman horticulturist, garden designer, writer, consultant and organic horticulture teacher. For advice contact him at stmajor@shaw.ca

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