Weevils – Coleoptera


Identification & Descriptions
Weevils are small beetles that possess conspicuous snouts. They are often lightbulb- or pear-shaped. When immature, the legless, grub-like larvae feed on plants. After developing into adults, some weevil species are attracted to buildings. It appears they do this to seek shelter from unfavorable weather conditions, especially when it is hot and dry. These weevils enter buildings by crawling through cracks or openings around foundations, doors, and windows. They do not harm people or pets, or damage buildings or property, or infest food products. They are just a temporary nuisance.

Many weevils are important agricultural pests and several species frequently cause damage to house and garden plants. Some of the main garden pests are shown below . . . . . .

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.

Flower buds attacked
by blossom weevil

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 weevil

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).

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.

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.

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.

Strawberry root weevils
Strawberry root weevil, Otiorhynchus ovatus, is the most common home-invading weevil in Minnesota. These weevils are about 1/4 inch long, black or dark brown with rows of pits along their back. Strawberry root weevils do not fly. Sometimes people confuse strawberry root weevils for ticks; however they are easily distinguished, as weevils have six legs and ticks have eight.

Strawberry root weevil larvae feed on the roots of strawberries, evergreens–such as arborvitae, spruce, and Japanese yew–raspberries and other brambles, grapes and many other plants. Adults start to emerge in early summer. They feed on the edges of foliage, leaving a characteristic notched appearance.

Residents experience problems with these weevils from the end of June through August. They are attracted to moisture and are often found in sinks, bathtubs, water basins and similar places.

Other Weevils
Sciaphilis asperatus is 1/5 inch long with a dark brown or black body covered with brownish or tannish scales. When these scales are rubbed off, the weevil appears to be brown with irregular blackish markings. Adults have been reported from May into November. These weevils feed at night on the leaves of sugar and red maple, yellow birch, hazel and hophornbeam. The larvae feed on the roots of these plants, although they are not considered a pest.

Barypeithes pellucidus is reddish brown and between 1/8 – 3/16 inch long. Little is know about this weevil’s habits, although it appears to feed on trees. Adults are common in June and July.

The imported longhorned weevil, Calomycterus setarius, looks similar to Sciaphilus asperatus but is a little smaller at about 3/16 inch long. This weevil has a dark colored body covered with grayish brown scales. Missing scales give the appearance of irregular black patches. Adults are present in homes in July and August. Larvae feed on the roots of aster, clover, and turfgrass while adults chew on the leaves of a variety of annuals and perennials. This weevil is found primarily in southern Minnesota.

Polydrusus impressifrons is a slender 1/4 inch weevil with a dark colored body covered with lime green, sometimes irridescent-looking, scales. Weevils often appear to be green with black patches when scales are missing. This weevil is common in the northeast and north central areas of Minnesota in forested sites. Adults chew the leaves of birch, poplar, willow, and apple during June through August, while larvae feed on the roots of these trees.