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As hard as we try, sometimes our plants are attacked by pests. Usually, these are fairly easy to deal with, but if the environment is right, and we drop our guard, plant pests can do permanent damage. Here, I will explain the problems caused by some of the most common plant pests and give some ideas about how to prevent and control them in as sustainable a way as possible.
What is a plant pest?
A useful definition is that a creature is only a pest if it causes damage or annoyance faster than the plant can recover from that damage. That means that the best way to keep potential pests at bay is to keep your plants as healthy as possible. This means keeping the environment right and reducing stress as much as you can.
Indoor plants can be attacked by a variety of different pests, but the most common are mealybugs, scale insects and two-spotted spider mites. At this time of year (spring), you are also likely to see a large number of small gnats, called fungus gnats or sciarid flies. These aren't really plant pests (not in houses, at any rate - it’s a different matter in commercial nurseries), but they do live in pot plant soil and are very annoying. So for this article, I will consider them too.
What harm do houseplant pests do?
Most plant pests found indoors cause damage in one of two ways: feeding on plant sap, or grazing on the leaves and other green tissue.
The sap feeding pests include mealybugs, scale insects and their relatives (such as aphids and whitefly). These creatures insert sharp needle-like mouthparts directly into the sap vessels of the plant and feed off the sugars and proteins being carried around the plant’s vascular system. This can weaken the plant, as valuable resources are taken away, and it can also lead to secondary issues, such as sticky honeydew being produced.
Sap-feeding insects, such as scale insects and mealybugs, have very simple guts. As the insects insert their mouthparts, the sugary sap enters the insect at high pressure and is forced through the gut and out of the other end of the creature. Only a little of the sugar and protein is extracted, and most ends up as a sticky residue falling onto surfaces below. This can attract ants and is ideal for mould spores to grow. The insects themselves are also unattractive and their populations can grow at an alarming rate.
Spider mites and thrips damage plants in a different way. Their mouthparts are adapted to puncture the individual cells at the surface of the leaves. The cell contents are consumed and the cell is killed. The pest then moves onto the next, and the next. This means that the surface of the foliage is permanently damaged, which looks unattractive and also results in the plant being weakened as its leaves are unable to function properly.
The basics of pest management
I have been teaching professional interior landscapers about pest management on plants for 25 years. Until the early 1990s, most horticulturists and gardeners reached for a can of insecticide when they saw a pest. Fortunately, those days are long gone.
Why chemicals alone are not the answer
Chemical pesticides usually offer a rapid kill, but they don’t address underlying causes of the problem.
Over time, there is a risk of build up of resistance to pesticides in the pest population.
Pesticides are subject to legislation, which makes using them, (as well as developing and marketing them), more difficult.
There is risk of contaminating the surrounding area. Fish are especially vulnerable to chemicals.
Finally, do you really want to add more poisons to the indoor environment?
Knowing your enemy
If you understand a little about the biology of plant pests, their control becomes much easier. The principle objective is to break their life cycle at the most vulnerable point. This will vary from pest to pest, but if you can find a way of stopping a population of pests from reproducing, then you will get it under control.
Prevention is better (and easier) than cure
Always be on the lookout for signs of stress in your plants. Weak plants are more susceptible to damage, as they have fewer reserves to grow new healthy tissues. Keep your plants properly watered, fertilized and in their ideal environmental conditions.
Also, look out for the early signs of infestation. These will be described for specific pests lower down. At the same time, be aware that pests can move from plant to plant, so if you see signs of a problem on one plant, check its neighbours and move them away.
The common plant pests
Mealybugs are small, sap-sucking insects. They feed by inserting their mouthparts into the sap vessels (phloem) in the plant. The sap, which is sweet and sugary, but low in protein, enters the insect under pressure and is forced through the mealybug’s gut. Some sugars and proteins are extracted, but most passes straight through and exits the mealybugs anus and is deposited as “honeydew” – a sticky, sugary substance that can encourage moulds and ants.
There are several species of mealybugs, but the two most important are the Citrus Mealybug and the Long-tailed Mealybug. Both have similar lifecycles (although there is one crucial difference, which affects their control, which we will discuss later).
Citrus mealybugs (1) are about 4mm long. Their bodies are a pinkish colour and covered in a white mealy wax and protruding filaments.
Long-tailed mealybugs (2) are approximately 2.5mm long, look similar to citrus mealybugs, although the filaments are proportionally much longer.
Male mealybugs are tiny, winged creatures (much smaller than a fungus gnat) that do not feed.
Nymphs (juvenile stages) can overwinter in the soil.
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.
The best way to control mealybugs is to keep the plant clean. Physical removal of the bugs and, more importantly, the waxy egg masses, the best way of dealing with them. Often, the egg masses will be found near the growing tips of the plant, so a little light pruning will not only get rid of the pests, but might help keep your plant in shape. If your plant is strong enough, a jet of water from a hose or spray bottle will also dislodge the pests and give your plant a bit of a clean too.
Don’t be tempted to use substances such as methylated spirits or surgical alcohol. It is not only illegal (as they are not approved pesticides), but they might damage the waxy cuticles on the plant leaves.
There are some biological controls available (e.g. a type of ladybird called Cryptolaemus montrouzieri), but they work best in controlled environments, such as greenhouses. It is unlikely that they will be useful on houseplants.
These are close relatives of mealybugs and have a similar life cycle. The main difference, however, is that the adults don’t move. There are two main families of scale insects: soft scales (top), which are quite large and armoured scales (middle and bottom), which are generally much smaller. The main difference is that armoured scales produce no honeydew, whereas soft scales produce vast amounts.
Scale insects feed in the same way as mealybugs, although the damage caused can be quite different.
Soft scales are not fussy feeders – many plant species will be attractive to scale insects, and they will spread from one species to another very easily. Their rapid life cycle and feeding habits can mean that an infestation will cause severe damage very quickly. The damage they cause is a result of them feeding on the sap of the host plant. This causes physical damage, and the large quantities of honeydew deposited on the leaf surface encourages sooty moulds and ants. The presence of soft scales also detracts from the appearance of the plants. Discolouration, leaf loss and a reduction in the vigour of the plant can result from an infestation.
The damage that armoured scales do is somewhat different to soft scales – no honeydew, for example and therefore no ants or moulds. However, they can weaken a plant through the toxins they inject when they feed, the removal of plant nutrients and the possibility of spreading some plant viruses.
Armoured scales are small, and can infest anywhere on the plant, although areas close to leaf veins or phloem vessels are favoured, as are the undersides of leaves, so it may not be immediately obvious that a plant is infested. Yellow or brown chlorotic lesions on the leaves caused by the injection of toxins may be the first sign of damage
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. Plants will lack vigour and may lose leaves, and will become unattractive. The chlorotic lesions on the leaves where the insects are feeding also contribute to the sickly appearance of an infested plant.
Left unchecked, armoured scales can kill large plants with some speed.
Controlling scale insects
Scale insects are very difficult to control once established. Their hard shells make them resistant to chemicals and cleaners, and they can’t be easily removed without time and effort. The best way of dealing with them is to target their vulnerable life stage, and that is the juvenile stages where they are mobile crawling creatures before they settle under their permanent scale. This means regular and thorough cleaning and the removal of infested parts of the plant.
Two-spotted spider mite
This minuscule relative of the spider is one of the most damaging pests found on indoor plants.
Two-spotted spider mites (Tetranychus urticae) feed by puncturing leaf cells and sucking out their contents. They prefer the undersides of leaves. They favour warm, dry conditions and have an extremely short life cycle, meaning that populations can grow very rapidly.
All stages apart from the egg and larva can produce silk threads and webs which are used for shelter and for movement – mites can lower themselves by a thread from one part of a plant to another, or even to the ground.
Webs and threads are also easily moved by air currents, allowing the mites to move from one plant to another over quite large distances – the plants don’t need to be touching to allow the mites to spread.
Larvae, nymphs and adults all feed and damage the plant. By sucking out leaf cell contents, they remove chloroplasts, preventing photosynthesis. The individual cells die and turn yellow and damage is permanent.
The spider mite life cycle is very straightforward. An egg hatches and produces a 6-legged larva (above). After a while the larva settles, withdraws its legs and grows into an 8-legged nymph (protonymph), thence a deutonymph and finally an adult. Between each stage, the mites have a short resting phase.
Adults mate and the females lay eggs – unmated females can also lay eggs, all of which hatch into males.
Controlling spider mites
In commercial situations, such as glasshouses and interior landscapes, spider mites can be very effectively controlled with biological control agents, such as predatory mites. However, in domestic situations, that can be more difficult to manage and can get expensive.
Physical cleaning is effective. The removal of any webbing as soon as it is seen will be helpful. Also, keeping the foliage of plants misted will certainly deter the pests from establishing. If your plants like humid conditions, then bathrooms and kitchens are going to be very spider mite unfriendly locations.
Fungus gnats are a group of small flies that all have similar feeding and lifecycles. They are all in the Sciaridae (pronounced see-yah-rid-ee-ay) family and are also known as sciarid flies. Fungus gnats are so common and abundant, most people know them when they see them flying around. However, there are other pests that look similar, including fruit flies and shore flies.
Fungus gnats (sciarid flies) are slender insects, about 5 mm long, a dull grey/brown colour with long antennae and large compound eyes. As with all true flies, it only has one pair of wings. One of the most distinctive features of sciarid flies is the “Y”-shaped vein in each wing. You will need a hand lens to see it, though.
The larva is a small maggot, reaching a maximum length of about 7mm. It has a small black head, translucent body and no legs.
Adult shore flies have a stockier body, shorter legs, tend to be darker in colour and lack the distinctive wing venation. Fruit flies are usually paler in colour and frequently have bright red eyes.
The trouble with gnats
These small flies seldom feed on living plant tissue. Instead, they feed on algae and decaying plant matter. In some conditions, fungus gnat larvae will feed on the root hairs of young plants. Nevertheless, any damage to plants is rare and only likely to occur when fungus gnats are present in huge numbers, and the plants are already under stress.
Fungus gnats are usually a symptom of another problem, not a cause. Their presence often indicates poor watering practices, which damage plants.
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 are attracted to carbon dioxide in your breath). They do not settle and when waved away return almost immediately. Because they fly around the face, they often seem more numerous than they really are.
The precise time of the fungus gnat flushes varies according to location, but is usually in the spring and autumn.
The flies are often confused with midges and mosquitoes and falsely accused by people of causing insect bites. In fact, their mouth parts are not able to penetrate human skin - they are just about able to take a drink of water.
Fungus gnats lay their eggs in the top 2 – 3 cm of the soil. In order for the eggs to hatch and the larvae emerge, the soil must be moist. Larvae feed on moist algae in the soil, but don’t burrow very deep. After the larvae have fed, grown and shed their skins a few times, they pupate. The pupae remain in the soil. The adults emerge from the pupae, climb out of the soil, fly away and seek a mate with which to breed, and then die after a few days.
Controlling fungus gnats
The easiest way of preventing fungus gnats is to water your plants from the bottom up (see our post about watering plants). This keeps the surface layer of the soil dry, which means that most eggs that are laid in the soil won’t hatch.
However, if you have a persistent gnat problem, then there are some very effective biological controls that you might consider. One of the most effective, and readily available, is a microscopic, parasitic eelworm called Steinernema feltiae. It is invisible to naked eye, especially in soil. It works by entering the larvae of the fungus gnat where it releases pathogenic bacteria that reproduce and digest the larva from the inside.
Yellow sticky cards are also good at trapping newly emerging adults from the soil, but can be unsightly.
by Kenneth Freeman
15 March 2021