Common Pest Species
A wide variety of pests attack interior plants, the most common being mealybugs, scale insects and two-spotted (red) spider mites. If left unchecked, they not only make the plants unsightly, but they can severely damage the plants, which may shorten their life.
These small flies, often called fungus gnats, seldom feed on living plant tissue. Instead, they feed on algae and decaying plant matter. In some conditions, Sciarid fly larvae will feed on the root hairs of young plants. Nevertheless, any damage to plants is rare and only likely to occur when Sciarid flies are present in large numbers and the plants are already under stress.
Generally, it is the adult flies that are the biggest nuisance due to their habit of flying in an erratic manner around people’s heads. They do not settle and when waved away return almost immediately. They are slender flies, about 5 mm long, a dull grey colour with long antennae and large compound eyes. The larva is a small maggot, about 7mm long, with a small black head, translucent body and no legs.
The simplest way to reduce the risks of an infestation of these flies is to avoid over watering and to keep the surface of the compost dry. Decorative mulches may also help, although bark chippings may attract the insects if the plants are watered from above.
Mealybugs are small rounded insects related to Scale insects, aphids and whitefly. Males have wings and can fly, but the females are wingless. Their bodies are often covered in a flocculent or mealy, wax-like secretion which gives the insects their common name. Eggs are laid in a cotton-like pouch in batches of 200-300 which hatch to produce nymphs about 0.5-0.7mm long. The nymphs are mobile and spend this part of their lives searching for new feeding areas.
Mealybugs will attack a wide range of foliage plants, being typically found along the stem and in leaf and stem axils, where the white, fluffy patches become quite unsightly. As the attack progresses leaves are likely to wilt, go yellow and drop, while plant growth can become stunted and deformed. In severe infestations the plant may lose all of its leaves. Like aphids, mealybugs excrete large quantities of honeydew, encouraging sooty mould and attracting ants.
Regular cleaning of plants is the best way of preventing a build up of the pests. Colonies of mealybugs can be removed with a cotton bud or the point of a pencil.
Soft scale insects
Adult soft scales are fairly large and conspicuous. The damage they cause is a result of them feeding on the sap of the host plant. This causes physical damage and the large quantity of honeydew deposited on the leaf surface encourages sooty moulds and ants. Discolouration, leaf loss and a reduction in the vigour of the plant can result from an infestation.
Adult soft scales resemble shiny, hard lens-shaped objects between 2-10mm long and vary in colour. Black scales (Saisettia olea) are 2-3mm long, oval and convex. The upper surface has a characteristic ‘H’ marking, and the scale is black or dark brown in colour. Coccus hespiridum is much larger (3.5-5mm), has a flatter shape and is a translucent brown colour. Parthenolecanium persicae is 5-6mm long, shiny brown and has a distinct cleft at one end and a slight keel running along the middle.
Regular cleaning of the plants with water or a mild detergent solution may dislodge eggs and mobile nymphs. The removal of honeydew will discourage the growth of unsightly sooty moulds. The adult scales can be prised off the surface of the plant with a needle or pencil point.
Armoured (hard) scale insects
Armoured scales are very small rounded insects, seldom more than 2mm in diameter, that shelter under a waxy scale made from the moulted skins of its nymphal stage. Most species are dull grey or brown in colour. They tend to settle in clusters rather than individually.
Armoured scales can infest any part of a plant, although areas close to leaf veins and the underside of leaves are preferred, so it may not be immediately obvious that a plant is infested. Yellow or brown lesions on the leaves, caused by the injection of toxins, may be the first sign of damage. Unlike soft scales and mealybugs, armoured scales do not produce honeydew. When a colony of armoured scales is established, a barnacle-like encrustation will become obvious, and a waxy sheen may appear on the surface of some leaves. Left unchecked, armoured scales can kill large plants with some speed.
Regular cleaning of the plant with water or mild detergent solution should dislodge any crawling nymphs, and the physical removal of the scales will kill them. If an infestation is confined to a localized area of a plant, pruning will also be effective.
Although they can be found at any time, aphids are most active in spring and early summer. They will attack most soft-tissued plants, preferring the new growth of shoot tips and the underside of young leaves. This weakens the plant and causes distorted, curled growth in new leaves, stems and flowers. Another early sign of attack is provided by the aphids’ tendency to excrete sticky honeydew, which gives the leaves a glossy sheen and encourages the growth of black, sooty mould. If aphids persist for any length of time, they can transfer incurable viral diseases to the host plant. These can result in distorted growth which may not become apparent until the aphids are long gone.
Aphids, often called greenfly or blackfly and occasionally “plant lice”, are soft-bodies, sap-sucking insects about 2-3 mm in length. They are usually green, although black, yellow, grey and orange varieties are also found. Wingless forms are the most common although winged forms may be produced to enable an infestation to spread.
Yellow sticky traps can be effective on a small scale, but they can be unsightly, especially when covered with dead insects.
The damage caused by adult vine weevils is characterized by the notching of leaf margins where the pest has bitten into the plant. Young plants may also have their stems and shoots attacked. The damage can resemble caterpillar damage, but the notches are often deeper. Adult vine weevils are fairly large and conspicuous on plants, but less easy to see when they are walking over soil. The damage done by the larvae of the vine weevil can be considerable. They feed on plant roots causing the affected plants to lack vigour, wilt and die.
The adult Vine weevil is 8 mm - 10 mm long, black and shiny. Its wing covers are grooved and have patches of yellow hairs, although it does not have wings and cannot fly. In common with other weevils, the vine weevil has a pronounced snout and long antennae.
Physical barriers may prevent the insects crawling from one plant to another. For containerised plants, the removal of the damaged plant and complete replacement of the compost is necessary to remove any unhatched eggs. Containers need to be thoroughly cleaned before being re-used.
Although most thrips are extremely small insects, about 1–2 mm long, some tropical species may be as large as 10mm. They are characterized by their slender bodies and frilly wings, and are sometimes mistaken for two-spotted (red) spider mites.
Thrips will attack any soft foliage and may be found in large numbers on individual leaves or flowers, usually along the veins. As they remove the sap, the tissue around the feeding puncture dries out giving the foliage a silvery, flecked appearance. When the leaves expand, they may tear and form “windows”, which are another diagnostic sign of thrips. Distortion, yellowing and leaf drop may also occur. Thrips also damage flowers, which usually develop white spots and become distorted, a red-brown liquid, which gradually turns black, may also be released, speckling the flowers and the underside of leaves.
Thrips thrive under conditions of low humidity, so regular misting of plants with water will deter them. Thrips that pupate on the surface of the soil favour materials, like peat, that are rich in organic matter; sandy and loam-based composts are less attractive.
Adult whiteflies are very visible and will rise in clouds from a plant if they are disturbed. They form large populations on infested plants, and cause damage by sucking the sap. They can also infect plants with viruses. Whiteflies exude vast quantities of sticky honeydew that attracts fungal growth, such as sooty mould. Damaged plants will lack vigour, become discoloured and may have yellow spots.
The greenhouse whitefly (Trialeurodes vaporariorum) is the most common species encountered in buildings. The winged adults resemble tiny, white, wedge-shaped moths and are between 1.5 and 3 mm in length. They are mostly found on the undersides of leaves and at the top of the plant, congregating particularly around new leaves and flower buds.
Regular cleaning of the plant may dislodge the eggs, nymphs and pupae from the plant but, since the creatures are so small, they may be missed. The removal of honeydew from the plants should reduce the risk of sooty mould growth.
Mites are not insects, but are in the same class as spiders. Adult mites are small, round-bodied creatures with eight legs, no wings or antennae and bodies that are often covered in bristles. Two-spotted spider mites are about 0.5 mm long and usually either red, pink or yellow-green in colour with two dark patches on the body. They move and spread very rapidly. Broad mites are much smaller than two-spotted mites (0.14 - 0.24 mm) with a translucent body.
Two-spotted spider mites only occurs in conditions of low humidity. The characteristic symptom of attack is a speckling of the leaves, which eventually turn yellow or grey. Fine webbing is often noticeable around the leaves, especially on the underside. Severe infestations may produce stunted growth and deformed leaves.
The best way of preventing an attack of two-spotted spider mite is to maintain a high level of humidity around the plant by regular misting with water. In contrast, broad mites will only thrive under conditions of high humidity. The colonies are very slow-moving so the removal of the infested part of the plant should control the spread of the pest.