Your guide to the most important pests that attack plants in hydroponic systems

By Dr Diedrich Visser

Growing conditions in a hydroponic system differ from those in open fields, but most pests attack plants in both systems with equal vigour because the plants’ chemical make-up stays the same, irrespective of the growing conditions.

But the ecosystem does affect certain insects. The most important component of a hydroponic system that differentiates the ecosystem from that of an open field vegetable crop is the growing medium.

Insects that depend on soil for the completion of their lifecycle will be significantly hampered by, or won’t survive in, a hydroponic system. These include soil-dependent insects such as certain cutworms, white grubs, snout beetle larvae, black maize beetle, mole crickets and the nematodes.

The larvae of some insects need soil to pupate in, but this won’t stop the adults from flying in and laying their eggs on plants. Other larvae, such as those of certain moths, will pupate in any suitable place if soil isn’t available. Therefore only soil insects/nematodes known to attack underground parts of plants may be adversely affected – to the advantage of the hydroponic farmer.

The majority of vegetable pests aren’t soil-dependent, however, and effortlessly locate and attack crops in greenhouses. The protective environment inside them is favourable for most insects and may result in more severe attacks.

Hydroponic farmers, for this reason, should familiarise themselves with the most important vegetable pests so that they can put corrective measures in place before there’s economic damage.

These are some of the important pests:

Leaf miners

Leaf miners are larvae (or maggots) of yellow and black flies and are about 2 mm long.
The flies have a distinct yellow dot on the upper thorax, between the wings.
The cream to orange larvae don’t have legs and are never found outside their tunnels in the leaves.
Pupae are formed on leaves, but are easily dislodged, and are nearly always found on the soil, underneath plants.
Many flies enter greenhouses from surrounding areas and lay their eggs in small punctures they make on the upper sides of leaves.
The eggs hatch within a few days.
The larvae make long, thin tunnels while feeding between the upper and lower leaf surfaces, and this results in the leaves dying off.
Affected tissue looks poor and may resemble diseased plants.
The condition of plants eventually deteriorates to such an extent that early dying off occurs. Yield losses in crops such as tomatoes may be as high as 80%.
Fruits aren’t usually attacked and the larvae don’t move down to the roots.
Various leaf miner insecticides have been registered – remember always to use them as instructed on the label.

Aphids

The most common aphids in greenhouses include the black bean aphid, green peach aphid, potato aphid, cotton aphid and the cabbage aphids.
There may be both winged and wingless aphids on crops.
The initial infestation nearly always originates from winged females that fly in from somewhere else.
Female aphids don’t need a male to reproduce and they bear live young.
Nymphs reach adulthood in as little as 6 days.
A colony usually consists of aphids of different sizes, all originating from a single female. While feeding, they may transmit a virus to the plant.
Visible symptoms on infected plants include leaf necrosis, mosaics, yellowing, smaller plants and early dying.
There aren’t any pesticides that can control viral diseases.
Although aphids mainly damage crops by sucking their sap and transferring viral diseases, a secondary negative affect is the formation of sooty mould, which is secreted by aphids, on honeydew melons.
Various aphid insecticides have been registered.
Always use them as instructed on the label.
It’s essential to destroy old plant material after harvests because aphids could spread from it to new plantings.

Caterpillars

Several caterpillars damage vegetable crops, and all of them are the larvae of night-flying moths.
These include the African bollworm, semi-loopers, tomato moth, the lesser armyworm and the diamond back moth.
Various insecticides have been registered for use against some of them – always follow the instructions on the labels.
Collecting larvae by hand is another way to keep their numbers reaching the economic threshold.
Insecticides are only recommended if their numbers reach very high proportions or when very small plants are attacked.
Regular scouting is, therefore, an important control strategy.

Mites

Mites are small arachnids (not insects) and have 4 pairs of legs and no wings (most insects have only 3 pairs of legs in the adult stage).
Nearly all mites resemble microscopic spiders, are usually smaller than 1 mm and only visible to the naked eye as tiny, moving dots on leaf surfaces.
Various mite species have been reported to feed on vegetables, the best known being red spider mites.
They are red (or orange) and are easy to spot on green leaves.
Mites are especially destructive on tomatoes.
Under certain conditions – for example, in the absence of natural enemies and acaricides, and under warm, dry weather conditions – mite numbers may increase to abnormally high levels. When such conditions prevail, crops could be completely destroyed in a period of 2 weeks.
Various insecticides have been registered for use against mites – remember to follow the instructions on the label.
Mites are one of the most difficult pests to control if insecticides aren’t an option.
They reproduce very quickly and hide between plant material and webbing, so they aren’t easy to see in the early stages of infestations.
When they are noted, it is usually too late to use sanitation measures such as removing infested leaves.

Whiteflies

They’re not flies, but tiny insects belonging to the same order as aphids (Hemiptera).
They have piercing/sucking mouth parts and, like aphids, suck sap from plants.
Whiteflies hide on the underside of leaves and are usually only noticed when leaves are turned upside down.
They’re 1 mm to 2 mm long, white, and swarm upwards if disturbed.
Whitefly nymphs (larvae) are very different from the adults.
They’re wingless and translucent and become immobile after the first moult.
Their translucency means they take on the colour of the leaf and aren’t often noticed.
They may cause yield losses as a result of their feeding and by transmitting viral diseases to healthy plants.
Various insecticides have been registered for use against whiteflies; remember to use them as instructed on the label.
Because whiteflies are excellent at hiding, you should always check the underside of leaves for them.
Whiteflies are extraordinarily difficult to control using insecticides – in fact, very few have even a limited effect.
So it’s very important to use a registered product to control them.