Potato moth

Scientific name

Phthorimaea operculella

Description of adult

Both male and female adult moths are about 12mm across the outspread wings, and have brownish grey forewings with tiny dark scattered markings. A fringe of fine hairs borders the pale-cream hind-wings.

Immature stages

The eggs are white and very small. The larva fully fed is about 12mm long and has a dark head. The body is greyish pink if the larva is in a tuber and dark green if it feeds on foliage. The pupa is dark brown and about 8mm long.

Life history

The moths remain quietly amongst the plants during the day, but run out of sight if disturbed becoming more active towards dusk. They then fly readily, and deposit minute white eggs on the plants or tubers. On leaves, the eggs are usually laid singly on the undersurfaces; on tubers, they are laid in groups around the eyes or surface scars. Each moth can lay up to 100 eggs in 2 weeks. Incubation requires from a week in summer to several weeks in winter.

The larva usually crawls on the leaf for a short time and then mines into it. The eaten-out area becomes brown and brittle, with particles of frass enclosed in the blister. As the larva grows it eats its way into the leaf stalk and on into the stem where its feeding usually kills the terminal section. In the tuber, the larva tunnels just under the surface at first, but later penetrates more deeply. The larval stage lasts for about 2 weeks in warm weather and much longer in winter.

The mature larva leaves the plant or tuber and shelters amongst plant refuse on the ground, or between tubers that are in contact, or in folds of bags. There it spins a flimsy cocoon, often building in particles of soil or debris. Inside the cocoon the dark brown pupa develops into a moth in a week or 2. The life cycle is completed in about a month in summer, and there are several generations in a year. In winter the time required from egg to adult is about 10 weeks and the adults live for about 3 weeks. The moth breeds freely in cull potatoes lying on the ground.

Distribution

Throughout crop growing areas of Queensland. Potato moth is a serious pest of tomatoes in south-east Queensland, especially where potatoes, as the major host are grown.

Host range

Potato moth moth infests pepino, potato, tomato and tobacco, and several solanaceous weeds including thornapple, false cape gooseberry and the nightshades.

Damage

Major and sporadic.

The larvae mine the leaves, stems and fruit. Young plants can suffer tip death from boring larvae. Larvae can enter through the calyx end of the fruit, or where 2 fruit or a leaf and fruit touch, obscuring damage beneath. Damage is often significant and tends to be more serious if other susceptible crops are grown nearby.

Control

Examine the leaves and stems of five consecutive plants in the row at six widely spaced locations throughout the crop. Sample the crop weekly from emergence. Spray pepino crops if there is more than an average of 2 mines per plant when the plants are less than 200mm tall (i.e. 60 mines over 30 plants). When the plants are more than 200mm tall, spray if there is more than an average of 10 mines per plant (i.e. 300 mines over 30 plants). Plants can tolerate considerable foliage damage.

In tomato crops more than 1 larvae per 2 plants is a high level of infestation.

Cultural

Avoid planting adjacent to crops such as potatoes, tobacco, pepinos and tomatoes that are susceptible to potato moth.

Biological

This pest is usually suppressed by parasitoids. Pheromones are available to trap male moths and indicate changes in numbers.

Chemical

Spray when necessary.

Check the Australian Pesticides and Veterinary Medicines Authority database for chemicals registered or approved under permit to treat this pest on the target crop in your location. Always read the label and observe withholding periods.