Just about anyone who has raised a houseplant or two has come across the mealy bug. These pests, if left to their own devices, will eventually create a mealy mess of monstrous proportions. This not only causes the plant to look unsightly, but also severely weakens and even kills plants. Often a limited outbreak of mealy bug can be cleaned off a small houseplant with use of alcohol or meths, but if an infestation has taken up residence in your hydroponic system then immediate action is called for.
Identification of mealy bugs
To the uninitiated who have not battled the mealy monster before, the first signs of an infestation can sometimes be mistaken for a fungal disease attacking indoor or greenhouse plants. The first sign of these pests is usually a white waxy, or floury looking deposit, often on the undersides of the foliage or hidden in the leaf axils at the point where the leaf joins the stem. Sometimes, the whitish powdery clumps are found inside flowers or around the base of the plant and on top of the media surface. While this white cottony growth does look similar to a fungal growth, it is in fact the waxy covering of the mealy bug.
The adult female mealy bug is about three millimetres long, oval and wingless, with short antennae and a fringe of filaments around the body. One of the most common mealy bug species found in hydroponic systems is the ‘long tailed mealy bug’, which has two very long filaments at the tail end of the insect. Underneath the white waxy covering, the insect itself is usually yellow or grey and may have a darker strip running down the middle of the back. Other types of mealy bug such as the Citrophilus mealy bug has two dark stripes on its back and when crushed, the body becomes red. The Obscure mealy bug has pinkish grey body contents and no long tail filaments. There are also the grape mealy bug, the Mexican mealy bug and the Solanum mealy bug which can infect the foliage of plants.
Mealy bugs, however, do not just infest the top portion of the plant. There are species which attack the root system and live in the soil or media: These species often termed ‘ground mealy bugs’ are small, lack filaments and are uniformly covered in a white waxy powder. Root mealy bugs feed on terminal or outer roots of plants and can rapidly infest a container of growing media or soil. Control of these types of mealy bugs is very difficult as often they are not even discovered until a plant is repotted and it involves removal of the soil or growing media and drenching in potent chemical insecticides.
Adult male mealy bugs are not often seen. These are winged and very small, only about one millimetre in length. They can be distinguished from other flying insects by the two filaments of white wax that are attached to the stomach area and by the single pair of transparent wings.
Mealy bug life cycle
Most mealy bugs lay eggs, up to 600 at a time, in a loose cottony waxy deposit on the undersides of the leaves. These eggs hatch into a first instar nymph or crawler which is about 0.3mm in length and similar to the adult, though not at first covered in the waxy layers. These hatchlings crawl about for a few days and may be carried from plant to plant on air currents, or other methods. The juvenile stages go through a second and third instar nymph stage, growing larger with each moult. As the young mealy bugs settle down to feed they begin to exude the white waxy material that soon forms a covering over the whole body.
Some of the most common types of mealy bugs infesting hydroponic systems, such as the long tailed mealy bug, can produce live young and hence populations can build up very rapidly under good breeding conditions. Each adult female, long tailed mealy bug can give birth to as many as 200 crawlers in a cottony mass of wax secretion, with there being three to four generations per year depending on climate. Under cooler winter conditions, the insects tend to retreat under bark, into crevices and other places to hide until temperatures warm up sufficiently for further breeding.
Damage caused by Mealy bugs
Under all the white mealy mass, these insects have piercing and sucking mouth parts which injure the plants by sucking the sap. A heavy infestation of these pests can in fact ‘suck a plant dry’ and weaken it to the extent that growth stops, foliage may yellow and drop off and gradually the plant will die. Mealy bugs will often cause twisting and distortion of the foliage and disfiguration of the leaves and fruit with their feeding. As they feed, the insects extract large amounts of sap in order to obtain sufficient proteins. The excess sap is excreted as honeydew which forms a sticky deposit on the plant leaves. Sooty mould may eventually grow on this honeydew, resulting in a black tacky mess. This build-up prevents the plant from photosynthesizing and producing assimilates for growth.
Root mealy bugs carry out a similar process; sucking the sap from the plant’s roots, weakening the whole plant and breeding in the root zone. High populations of this pest feeding on plant roots will weaken the plant, often prevent further growth and development and will eventually cause root death and foliage drop.
Mealy bug control in hydroponic systems
It’s sometimes said that mealy bugs only infest succulent plants which have been fed on high levels of nitrates, water stressed plants, or that infestations only occur with relative low humidity and dry conditions. However, in a protected indoor or greenhouse situation, mealy bugs seem to not be that fussy about the type of plant or conditions for infestation and will attack whatever plants are available at the time. The host range for infestation is limitless and infection can be rapid in an indoor or greenhouse crop. One of the best ways of controlling mealy bugs is with prevention. However, this is often difficult as mealy bug eggs and nymphs are not visible and also tend to shelter in leaf axils and other places where detection is not possible. Checking any seedlings or new plants that are being brought into the cropping area for the telltale white cottony signs is useful. Often, these eggs and nymphs and root mealy bugs escape unnoticed on new plants.
The best method of control is regular checking of plants and taking action as soon as mealy bugs are detected on any plant. As soon as signs are seen on one plant, others are likely to be infected with eggs and nymphs as well.
Foliar mealy bug control
Mealy bugs are very difficult to control with conventional sprays since a water-repellent waxy covering protects the adults. Mealy bugs, particularly the young, also tend to hide down in leaf axils, buds and other places where sprays can’t penetrate well enough to have any effect. For an individual plant, it’s often recommended that the insects be brushed with a solution of alcohol or meths that dehydrates the pests. If treating more than one plant, or even for just a heavily infested plant, this is extremely time consuming and often not effective as many of the pests will not be detected or reachable in their hiding places.
The most effective control options for foliar mealy bug infestation are the use of oil sprays or natural predator or parasite insects. Oil sprays, combined with a good wetting agent will adhere to and smother the insects. These are most effective if combined with a insecticide such as Maldison. For those who prefer not to use synthetic pesticides, neem oil combined with a good wetting agent, or (pre-formulated into an insecticide product) is also effective provided it is applied on a very regular basis (every three days). Usually the most common cause of a lack of control with botanical agents such as neem oil, is due to insufficient coverage or infrequent applications. Neem oil breaks down in sunlight and being a natural product, needs to be applied every few days to break the life cycle of the insect. The mode of action of neem oil is as an insect growth regulator, which means, although it does have the smothering effect of an oil, it mostly works by disrupting the moulting and feeding of the insect. This attack breaks the life cycle and controls the infestation. Sprays should keep being applied until at least one month after the last of the insects are seen, to control any eggs that may have hatched in this time.
Predators and parasites
Where an established population of mealy bugs exists on a number of plants in a confined growing area such as a grow room or greenhouse, the use of natural predators often proves very effective. The most commonly used biological controls of mealy bugs are the mealy bug predator ladybug Cryptolaemus, the Hypoaspis mite which is particularly fond of mealy bug and the Lacewing Larvae. All of these are available from suppliers of beneficial insect predators.
The Cryptolaemus ladybug predator are introduced to the plants as adults, placed close to the mealy bugs. This predator likes warmth and heat (at least 16° C) and will hibernate if conditions become too cool for feeding. All stages of this predator will eat mealy bugs and their eggs, and unlike the mealy bug they can fly and are very efficient at seeking out their prey.
The Hypoaspis mite is a tiny insect which feeds on a range of insects, although it prefers mealy bug adults and eggs. This mite is often used in conjunction with the Cryptolaemus ladybug. The Hypoaspis will control mealy bugs on the smaller plants while the Cryptolaemus prefers to fly to the top of larger plants and prey on the pests in that region. Hypoaspis mites are usually delivered in vermiculite media which should be spread over the media surface.
Lacewing larvae prey on a wide range of insect pests and will also eat mealybug immediately on release and quickly reduce these pests. Lacewings are used as predators for many pests and are also best combined with either the Cryptolaemus or Hypoaspis predators for good long term control.
Root mealy bug control
Often, the only control for a heavy infestation of root mealy bug is to throw the whole plant and media away and start again. However, for some plants, removal of all of the infected media followed by dunking the root system into a strong solution of chemical insecticide such as Carbaryl may control the problem. Repotting into fresh, sterile media is then required after disinfection of the planting container and surrounding areas (mealy bugs can hide underneath the growing area, in plant residues, pots, trays and media).
Mealy bugs may be a monstrous pest, and one that sometimes appears invincible. However, the way to deal with these cottony invaders is with patience, persistence and predators. The spraying option, either using insecticides or natural products such as neem oil requires a dedicated spray program with treatment every few days.
Using predators relies on the grower monitoring both the pests and predators to make sure a balance is achieved and new predators are brought in as required. Control of mealy bug is a long term process: They have a habit of popping back up months after the population was supposedly eradicated, so watching for the first powdery signs of insect feeding is an ongoing process. However, with a little effort, mealy bugs can be beaten and banished from your hydroponic system forever.