Weevils

• Apple Blossom Weevil
• Clay-coloured Weevil
• Leaf Weevils
• Pea and Bean Weevil
• Strawberry Blossom Weevil
• Strawberry-root Weevils
• Vine Weevil

Weevils belong to a very successful family of beetles (Curculionidae) with more than 50,000 species worldwide. They vary in size from small seed weevils, less than 2 mm long, to the large pine weevils, 20-25 mm long. Adult weevils are fairly easy to recognise since nearly all have a characteristic rostrum or snout projecting forward from the head, with mandibles or jaws at the tip. Some species have a very long rostrum, which may exceed the length of the rest of the body, but generally the rostrum is much shorter. In addition, most species have distinctly elbowed antennae. The larval stages are relatively featureless white or yellowish grubs, usually legless, but with a well-developed head and jaws. Adults and larvae of all species feed either on living or on dead plant tissues. The larvae of many species feed enclosed inside the roots, stems or seeds of plants, and some of these types can become serious pests of agricultural crops, garden plants and stored food products. The examples illustrated here include the main species found in Europe that frequently attack garden ornamentals, fruit and vegetables.

Blossom Weevils (Anthonomus spp.)

The apple blossom weevil (Anthonomus pomorum) occurs throughout Europe and in parts of North Africa and Asia, where it attacks the flower buds of apple and pear trees. Blossom fails to develop normally in spring and flowers remain closed with dead petals attached. Careful examination of affected flowers may reveal weevil larvae, pupae or adults inside.

Adults of this weevil, with their fairly long, narrow rostrum or snout, are about 4 mm long, dark reddish-brown in colour, usually with whitish transverse marks on each wing case. They hibernate during winter under loose bark and in dead leaves or other accumulations of debris near apple and pear trees. In early spring the beetles emerge from hibernation and fly or crawl onto apple trees and sometimes pear trees, where the females lay their eggs on young flower buds. The larvae, which hatch after one to two weeks, feed inside the flower buds causing damage to the petals and other flower parts, so that the flowers fail to open. The unopened petals of attacked buds eventually wilt and turn brown, in a similar manner to buds damaged by frost. Usually a single larva develops in each affected flower and, after feeding for a few weeks, it pupates inside the flower under the dead petals. Adult beetles appear in June or July and feed on leaves for about a month before seeking hibernation sites.

Apple blossom weevils rarely cause appreciable crop losses since they tend to act as a natural thinning agent, resulting in fewer but larger fruits. However, if the beetles are known to be locally important, blossom can be protected by spraying trees with a suitable insecticide just before the flower buds open, in order to kill the female weevils before they lay eggs.

Another weevil, called the strawberry blossom weevil (Anthonomus rubi), attacks the flower buds of various plants belonging to the family Rosaceae, particularly raspberries, blackberries and strawberries. This species resembles the apple blossom weevil, but the adult beetle is slightly smaller (2-3 mm long) and entirely black or dark brown in colour (without paler marks on the wing cases).

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Leaf Weevils (Phyllobius spp.)

These are dark brown to black, short-snouted weevils, densely covered with metallic gold or greenish-bronze scales. They range in size from 4-9 and mm long, depending on species. The main pest species found in Europe are the brown leaf weevil (Phyllobius oblongus), the silver-green leaf weevil (Phyllobius argentatus) and the common leaf weevil (Phyllobius pyri), but a few other species also occur in gardens. They feed on the leaves of apples and other fruits, and on the leaves of alder, birch, lime, oak, poplars, flowering cherries, crab-apples and rhododendrons, eating small holes in the leaves and occasionally damaging blossom. These weevils are usually seen on plants in May and June. They seldom cause severe damage and can be controlled, if necessary, by spraying with a general purpose, contact-acting garden insecticide.

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Pea and Bean Weevil (Sitona lineatus)

This is one of several different species of Sitona that feed on cultivated plants. The beetles eat small, semi-circular pieces out of the edges of pea and bean leaves in spring and summer, producing a characteristic scalloped or notched effect. Small, brown, short-snouted weevils, about 4 mm long, may be seen on affected plants, but often drop off when disturbed. The pea and bean weevil is a common pest throughout Europe, North Africa and parts of Asia, and it has been spread through commerce to North America and Australia.

The adult weevils overwinter in plant debris and coarse vegetation and move onto peas, beans and other leguminous plants in early spring. Females lay eggs in the soil during warm weather and the larvae, which hatch about two weeks later, feed for about a month on the nodules found on the roots of pea and bean plants, before pupating in the soil. Adults appear in June or July and feed on various plants until the autumn, when they seek hibernation sites. Some virus diseases of broad beans are transmitted by these and other weevils.

If necessary, young plants can be protected by dusting or spraying the leaves with a suitable insecticide. Older plants are not greatly affected by this pest and rarely need treatment.

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Vine & Root Weevils (Otiorhynchus spp.)

The vine weevil (Otiorhynchus sulcatus) is the most important species of Otiorhynchus because it attacks many kinds of house and garden plants both indoors and outside, but several related species, such as the clay-coloured weevil (Otiorhynchus singularis) and the strawberry-root weevils (Otiorhynchus ovatus and Otiorhynchus rugifrons), also attacked some garden plants outdoors. The clay-coloured weevil is particularly associated with damage to apples, currants, gooseberries, raspberries, roses, rhododendrons, polyanthus and clematis, whereas the strawberry-root weevils are mainly pests of strawberries. The biology and treatment of the clay-coloured and strawberry-root weevils is much the same as for the vine weevil. All these species are distributed worldwide.

Vine weevils are especially troublesome on container-grown plants in houses, conservatories and glasshouses, but they also attacked plants growing outdoors in borders, rock gardens and similar situations. Potted cyclamen, primulas and begonias are most susceptible to attack, but this pest also affects camellias, crassulas, ferns, fuchsias, gloxinias, geraniums, grapevines, orchids, pansies, saxifrages, sanseverias, stawberries and many other plants. Destruction of roots by a the larval stages checks growth and may cause sudden wilting and collapse of shoots and leaves. Adult vine weevils are seldom seen since they are mainly nocturnal and hide during the day. Their presence is generally indicated by irregular notches and holes eaten out of leaves and on some woody plants by the death of young shoots due to ring-barking. Camellias and rhododendrons are very susceptible to this kind of damage.

The adult weevils (7-10 mm long ) are entirely dull black, usually with small patches of yellowish scales on the wing cases, and the rostrum or snout is relatively short and broad. The legless, white larvae (up to 10 mm long) live in the soil and look like miniature chafer grubs, but can be easily distinguished from these by the lack of thoracic legs.

The biology of the vine weevil is unusual. Nearly all vine weevils are female and they can lay viable eggs without being fertilised by a male. This ability to produce viable, unfertilised eggs is known as a parthenogenesis. Male vine weevils have been found occasionally but are very rare. Each female weevil can lay several hundred eggs over a period of three to four months during spring and summer and, although many of these eggs fail to hatch, a single female has the potential to start a serious infestation. Eggs are laid in the soil or potting compost near suitable host plants. The larvae, which hatch after about two weeks, feed on the underground parts of plants (i.e., roots, bulbs, corms or tubers) for several months before pupating in the soil. Adults sometimes emerge in autumn, particularly on indoor plants, but generally not until the following spring outdoors. There is one generation each year but, because of the staggered emergence of adults, there is often some overlap of generations in late winter and early spring when eggs, larvae, pupae and adults may all be present at the same time. Adult weevils are unable to fly but crawl into glasshouses through doors and ventilators or may be introduced on newly acquired plants. They hide at soil level during the day, in leaf litter, cracks and crevices of loose brickwork or woodwork and similar situations, and crawl up onto plants after dark.

Vine weevil damage can be reduced by regular inspection of plants, good hygiene, and by the use of chemical and/or biological control methods if available. In small gardens and greenhouses, frequent torchlight weevil hunts after dark on warm spring and summer nights can help reduce adult populations and so limit egg-laying. During re-potting of container-grown plants destroy any larvae, pupae or adults seen. Remove all accumulations of plant debris that could provide shelter for the adult beetles. It is difficult to keep up to date with the range of chemical and biological pesticides available to the general public for controlling household and garden pests. Seek advice from a good pesticide stockist or garden centre for currently approved insecticide products.

REMEMBER TO USE INSECTICIDES SAFELY AND FOLLOW ALL THE INSTRUCTIONS ON THE PRODUCT LABEL