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