Vegetable production: Controlling potato pests

Learning how to identify insect potato pests helps us to decide on a course of treatment.

Taking a close look at the most important of the more than 60 insect pests that attack potatoes will help potato farmers to protect their crops and income.

POTATO TUBER MOTH

The moths are nearly 1 cm long, and the wings are greyish, with dark grey spots.
They are active at night and hide between the plants during the day.
Moths live for about a week and lay up to 250 eggs.
Potato tuber moth larvae damage foliage and tubers beneath the ground, as well as tubers in storage.
The moth does not feed on the potato plants, but lays her eggs on or near them.
The resulting larvae tunnel or mine into all plant parts, causing severe yield losses when the tubers are reached.
When the foliage dies down, usually at the end of the season, the larvae move down cracks in the soil to reach the tubers.
They tunnel into the tuber just beneath the skin. In storage the eyes (where the sprouts form) are usually attacked first, and there is no sprout development.
This can lead to severe losses in stored seed potatoes.
Various insecticides have been registered to control this pest.

POTATO LEAF MINER

The leaf miner is the larva of a small fly that also attacks a wide range of other vegetables.
The female fly damages plants by piercing leaves with her ovipositor, causing “stippling”.
These “punctures” may also stress plants.
The female lays her eggs in some of the puncture marks.
A small cream-coloured maggot hatches from an egg and starts to mine between the leaf surfaces.
Severe infestation (leaf mining by the larvae) may give leaves and haulms a burnt appearance and usually results in foliage dying off prematurely.
This “removal” of green material before the plant is ready to die off results in a yield reduction of up to 70%.
The larvae don’t attack tubers. Several registered insecticides help to control this pest.

APHIDS

These are small insects with sucking mouthparts.
They don’t usually damage plants directly, but are virus vectors that possibly transmit viral diseases when they arrive in a potato field after feeding on infected plants elsewhere.
After multiplying in such a field, offspring that have fed on an infected plant may spread the virus to other plants in the same field.
The 2 most important potato viruses are potato leafroll virus (PLRV) and potato virus Y (PVY).
Tubers infected with such viruses may not be sold as certified seed.

CUTWORMS

The cutworm moth lays eggs on the soil or lower parts of the plant.
The young larvae feed on plants during the day, but as they get older they are usually found below the surface of the soil.
From here they eat through the stems, causing the plant to die. If you dig into the soil next to a dead plant you will probably see the larvae.
Sometimes cutworms move down much deeper and attack tubers when they find them.

MILLIPEDES

Millipedes eat mainly dead or dying plant material.
They don’t tunnel into the soil but may damage tubers if they reach them through cracks or when they come across exposed tubers.
You can reduce millipede problems if you ridge often to ensure that all cracks and exposed tubers are covered.
Although you can buy millipede poison baits, control is rarely needed.

BOLLWORMS AND LOOPERS

These are the larvae of night-flying moths.
The African bollworm attacks many crops and is very destructive.
Moths lay many single eggs on plants and they hatch in a few days.
The larvae have a characteristic white to beige stripe along both sides of the body.
Larvae can grow up to 3 cm in length.
They eat leaves and sometimes flowers, but this is usually only a problem when there are many larvae feeding in a field or when the plants are still small.
Loopers walk by arching their backs and moving their back “legs” towards their front legs.
Young larvae eat leaf surfaces between the veins; older larvae eat holes in leaves.
They are usually only a problem when many caterpillars attack small plants at the same time.

NEMATODES

These microscopic, slender roundworms live in the soil.
Nematodes attack roots and tubers, and negatively influence the transporting of water and nutrients through the plant.
In South Africa, the 3 most important nematodes that attack potatoes are: root knot nematodes, lesion nematodes and potato cyst nematodes.
Root knot nematodes are the most common.
They prefer a hot climate and sandy soil and survive in the soil for many years in the form of small eggs.
The eggs hatch when a host plant grows close to them, and they move through moist soils to reach the roots.
Infected tubers make galls which house the female worms and egg sacs.
Planting infected tubers introduces nematodes into clean soils.
Fallow cultivation, rotation with cereals or grasses and applying nematicides are some of the control measures.
Lesion nematodes are less common but can cause economic losses.
Infected tubers look unhealthy: they could have purple-brown pimples, pustules or wart-like protuberances.
Potato cyst nematode infections must be reported to the SA Department of Agriculture as it is a quarantine pest.

MITES

Red spider mite is the best known mite on potatoes.
Females lay their eggs on the underside of leaves and the infestation is spread to other plants by wind, humans and moving machinery.
They prefer warm, dry conditions, and their feeding causes white, yellow, silver or brown lesions.
Mites attack leaves as well as stored potatoes.
If not controlled, they can destroy plants in fields and stored tubers in a matter of weeks.

This article was written by Diedrich Visser and first appeared in Farming SA.