PESTS OF DECORATIVE PLANTS

APHIDS

A great variety of aphids attack decorative plants, but the damage done is usually less spectacular than on fruit, because leaf distortion is not so common. The stems of young growth are twisted, but the aphids cause most damage by robbing the plants of large quantities of sap, causing poor growth, wilting and the formation of dwarfed, misshapen flowers. The leaves of infested plants are often made sticky by the honeydew excreted by the insects, and this sweet liquid is a medium for the growth of harmless, though disfiguring, black moulds.

Many of the fruit-infesting aphids mentioned earlier also attack flowering cherries, crab apples, currants, etc., causing similar symptoms. These can be dealt with by the methods recommended for control on fruit trees.

The more important and widespread species which attack decorative plants include the following:

The Black Bean Aphid infests dahlias, nasturtiums, poppies and spindle-trees as well as feeding on vegetables.

The Chrysanthemum Aphid, a red-dish-black species, attacks chrysanthemums.

Aphids on a rosebush.

Image via Wikipedia

The Peach-Potato Aphid, which is green, yellow or pink in colour, attacks antirrhinums, China asters, phlox, sweet peas, chrysanthemums and many other plants.

The Rose Aphid, a large green or red aphid, infests the shoots and flower buds of roses and scabious.

Control

As soon as aphids are seen on the plants, spray or dust with malathion, lindane or nicotine, repeating the application as necessary.

BEETLES AND WEEVILS

Beetle and weevil damage is usually less common on ornamental plants than on fruit and vegetables. The following may, however, be met with.

The adult Cockchafer, a large brown beetle, about l in. long, which often flies into lighted windows on summer nights, and the smaller Garden Chafer, about J in. long, with bluish-green thorax and reddish-brown wing-cases, eat the unopened buds of roses.

Leaf Weevils, green or brown insects with a metallic sheen, 1 to 4 in. long, eat holes in the leaves of trees and shrubs.

The dark brown Water-lily Beetle and its blackish larvae are common pests on water-lilies. They eat elongated holes in the leaves and they also attack the flowers, disfiguring them and causing early decay.

Control

Spray or dust damaged plants with D.D.T. But D.D.T. Cannot be used on Water-lily Beetles if fish are present in the water; therefore sink the plants for a few days with pieces of iron, or wash the insects into the water with a strong jet of water, to allow the fish to feed on them.

The brown, ¼-in. Long Clay-coloured Weevil and the black Vine Weevil are wingless and feed on the foliage of trees and shrubs at night, the latter species confining itself to notching the edges of the leaves. They also eat away the bark of the stems, and often kill the young shoots in this way. During the day the weevils hide in the debris at the base of their food-plants. The grubs, which are whitish, legless creatures with curved bodies, are also pests and feed on the roots of many plants, especially on rockeries and in pots.

Control

These weevils are very resistant to insecticides, and, in gardens, the best control is obtained by persistent trapping. Provide day-time hiding places for the weevils by laying down sacking or corrugated paper at the base of attacked plants, examine these each day and destroy the weevils. To control the grubs, dress the soil with aldrin dust, forking it well into the top 4 in.

CAPSID BUGS

The Common Green Capsid and the Tarnished Plant Bug are common pests of chrysanthemums, dahlias, hydrangeas, salvias and other plants. They attack the young leaves, which become torn and distorted as they grow larger, and they also feed on the flower buds so that they produce distorted, misshapen blooms.

Control

Spray with lindane or D.D.T. and repeat 14 days later. To prevent serious damage keep a close watch for small brown spots on the young leaves, which are signs of feeding by capsids, and, when they are seen, spray immediately.

CATERPILLARS

Caterpillars of the Angle Shades Moth, brownish-green with pale markings on the sides of the body, are common pests of such plants as chrysanthemum, dahlia, hollyhock, primula, wallflower, iris and gladiolus. Vapourer Moth caterpillars, hairy and brightly coloured, attack ornamental cherry and crab apple, hawthorn and rose, and the Winter Moth caterpillars, as well as attacking fruit trees, feed on the leaves of many ornamental trees and shrubs, eating out large holes and sometimes causing severe defoliation.

Control

Spray or dust with D.D.T.

Some caterpillars spin copious silk webbing over the leaves and young shoots of trees and shrubs, and these webs give protection to the caterpillars as they feed on the foliage and also help to protect them from sprays.

Caterpillars of the Small Ermine Moth, which are grey, attack the spindle in this way, and also attack many of the plants damaged by the caterpillars of the Lackey Moth. Lackey Moth caterpillars are blue-grey with red and white stripes, and feed on flowering cherry, crab apple, hawthorn, roses and other shrubs.

Control

Cut out the webbed shoots and burn them, then spray or dust with D.D.T.

Tortrix Moth caterpillars vary in colour between grey-green and dark green according to the species, but can be easily identified by their habit, when disturbed, of wriggling violently backward and dropping from the plant suspended by a thread of silk. They are often found on ornamental trees and shrubs such as hawthorn, flowering cherry and crab apple, holly, rhododendron and rose. They roll the leaves and tie them with silk, or tie young leaves together, feeding between them. The tips of herbaceous plants, such as helenium, phlox, rudbeckia and solidago are often tied in this way.

If possible, crush the caterpillars inside the tied-up leaves, and then spray or dust the plants with D.D.T.

EARWIGS

Earwigs are often found hiding under flat stones, in long vegetation and other damp, dark places. They feed by night on the leaves and flowers of garden plants, such as chrysanthemums, cinerarias and dahlias.

Control

Keep the garden clear of piles of rubbish and long tangled vegetation which provide shelter. Shake the blooms to dislodge any sheltering earwigs, then spray the ground and lower parts of attacked plants with D.D.T., lindane or dieldrin, avoiding any open blooms.

EELWORMS

The Chrysanthemum Eelworm is a common pest of chrysanthemum, perennial aster, pyrethrum and related plants. The lower leaves are the first to show symptoms. Brown patches appear between the main veins of the leaf, and these darken and spread until the whole leaf shrivels and falls. The infestation proceeds up-ward and the buds may be attacked and ‘blinded’, and the flowers are often dwarfed and misshapen. The worst attacks take place when conditions are wet or humid.

Control

Immerse infested dormant stools for five minutes in water kept at a constant temperature of 115° F. (-45° C.) or destroy the stock. Burn all fallen or withered leaves, as they contain eelworms and may be blown to other parts of the garden. Keep infested ground clear of all plants and weeds throughout the winter to destroy any eelworms in the soil.

The Stem and Bulb Eelworm is a common pest of daffodil, iris, hyacinth, bluebell and snowdrop, and also attacks begonia, gladiolus, onion, parsnip, strawberry and many weeds. The stems and leaves of infested plants are distorted, dwarfed and discoloured by yellow streaks. Daffodils and snowdrops may also have small yellowish swellings on the leaves. When the bulbs of attacked plants are cut across, dark rings are seen where some of the scales have begun to decay.

The final result of eelworm attack is the gradual deterioration and death of the plant.

Control

The eelworms in lightly infested bulbs can be killed by immersing the dormant bulbs for three hours in water kept at a constant temperature of 110° F. but, as special apparatus is required to do this successfully, it is easier to destroy the stock of bulbs and plant fresh ones in clean ground. Avoid growing possible host plants on contaminated ground for at least three years.

A different race of Stem and Bulb Eelworm infests phlox. The leaves of attacked plants become narrow and crinkled and the stems may be twisted and split.

Control

If possible treat dormant stools by immersing for one hour in water at a temperature of 110° F. (43° C), or destroy the stock and keep infested ground clear of aubrieta, gilia, gladiolus, gypsophila, Oenothera, primula, peas, potatoes and weeds for at least three years.

LEAF-HOPPERS

Many plants are attacked by leaf-hoppers, but usually most damage occurs on roses. The Rose Leaf-hopper is a small, pale yellow insect which resembles an aphid, but is able to jump quickly when disturbed. It sucks the sap on the underside of the leaf, causing a distinctive, coarse mottling near the mid-rib, which spreads until the yellowing is extensive and may cause premature leaf-fall in dry weather.

Control

Spray with D.D.T. or lindane as soon as damage is seen, and repeat at H-day intervals as necessary.

LEAF-MINERS

Leaf-miners are the larvae of flies and small moths, which feed on the internal tissues of leaves between the upper and lower surfaces. There are two main types of mines: the linear type, which are winding tunnels growing wider as the larva grows, and the blotch type, which are blister-like cavities eaten out by one or more larvae. The damage is whitish at first and later turns brown as the injured leaf tissue withers. A large number of garden plants may be attacked by leaf-miners, but only in a few cases are the plants severely checked and disfigured.

Chrysanthemums, cinerarias and other related plants are often attacked by the Chrysanthemum Leaf-miner, particularly when they are raised under glass. The linear mines of this pest usually start on the edge of the leaf where they may not be noticed at first, and then quickly invade the rest of the leaf, so that the whole leaf is destroyed in severe cases.

The Holly Leaf-miner forms yellowish-brown blisters on the leaves of holly.

Mines of the blotch type are made on lilac and privet by the Lilac Leaf-miner and, if the attack is severe, the shrubs look very unsightly when the damaged leaves shrivel.

A similar type of mine to that made by the chrysanthemum leaf-miner is made on rose leaves by the Rose Leaf-miner.

Control

If possible, remove the mined leaves by hand and burn them. Otherwise, spray with lindane or malathion two to three times at 14-day intervals, starting as soon as the damage is seen.

RHODODENDRON BUG

The Rhododendron Bug which feeds on rhododendron leaves, is cream-coloured when young, while the adults are shiny black with broad, lace-like wings. The leaves become finely mottled with yellow and sickly in appearance, and the undersides, where the small, flat bugs can be seen, are stained with rusty-brown marks.

Control

Spray the underside of the leaves with D.D.T., lindane or nicotine in mid-June, or when the damage is first seen. Give two or three applications at three-weekly intervals.

SAWFLIES

Sawfly larvae resemble moth caterpillars but have more legs. Like caterpillars, many species eat pieces out of the foliage of the plant and in severe cases may strip it completely.

The green larvae of the Banded Rose Sawfly and the Large Rose Sawlly feed on the rose leaves, and the former species also burrow in the stems.

The Rose Slug Sawfly also attacks roses, but the damage caused is distinct from that of other species. The yellowish, semi-transparent larvae of this species feeds only on one surface of the leaf, leaving the other to turn brown and wither, which results in the appearance of brown blotches on the foliage.

In the south of England spiraea is commonly attacked by the yellowish-green larvae of the Spiraea Sawfly which eat away the soft tissues of the leaves, leaving only the main veins intact.

Control

Spray the infested plants with D.D.T. or derris. In the case of the Rose Slug Sawfly treat both surfaces of the leaves. Another sawfly which attacks roses is the Rose Leaf-rolling Sawfly. The female lays its eggs on the edges of the young leaves, which then roll inward towards the mid-rib, so that when the greenish-grey larvae hatch out they are protected within the rolled-up leaves.

Control

Hand pick and burn the distorted leaves when they appear. Chemical control is difficult, but nicotine dust can be applied when the temperature is 65° F. (18° C.) or over, and the vapour from the chemical will penetrate the rolled leaves.

SCALE INSECTS

Out-of-doors they are usually found on woody plants, and those species found on fruit trees are also common on some ornamental shrubs. The stems of ceanothus, cotoneaster, hawthorn, flowering currants, mountain ash and other trees and shrubs may become infested by the Brown Scale or Mussel Scale.

The Rose Scurfy Scale is a flat, white scale found encrusting the stems of roses, especially in sheltered places.

The Yew Scale is similar to the Brown Scale, but is found only on yew on which it is a common pest.

Control

On deciduous trees and bushes and on yew, spray with tar oil in December or January. On other plants and during the growing season, apply malathion two or three times at 14-day intervals.

THRIPS

Three species of thrips damage outdoor ornamental plants. The Gladiolus Thrips is found on gladiolus, and sometimes on iris, lily and freesia. The Rose Thrips is found on roses, morning glory, larkspur, lupin and other plants. These minute insects suck the sap of the leaves and flower petals, leaving white or yellow spots on the feeding areas and, if the attack is severe, the flowers may be misshapen or may wither before opening.

The Privet Thrips attacks the foliage of privet and lilac, causing a silvery mottling on the surface.

Control

Spray with D.D.T., lindane or malathion.

Repeat at ten-day intervals as necessary.

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06. September 2013 by admin
Categories: Plant Biology, Weed & Pest Control | Tags: , , , , , , , , , , , , | Comments Off on PESTS OF DECORATIVE PLANTS

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