Why is it a problem? PTM is the most important insect pest of potato in Bhutan. Caterpillars destroy potatoes through tunnelling, and secondary infection. As a storage pest it can destroy high proportions of tubers kept between seasons as seed potato and for private consumption (21-42% in one survey). Less commonly it can also be an in-field pest, affecting tubers in the ground, and subsequent sale prices. It can be managed through good management practices.
Where and when is it a problem? PTM causes most damage in the main potato growing areas up to about 2,500m asl. If left unmanaged substantial losses can occur each year in places where it is well established. It was still spreading through Bhutan in the early 2000s and so may not yet have reached its’ maximum impact. It has not been reported as a field pest at low altitudes, and potatoes are typically not stored for long periods there. However, it is expected to have the potential to be a very serious pest there also.
When first laid the minute, oval eggs are pearly white, changing to yellow on maturity and to black just before hatching. All larval stages have a dark brown head. Young larvae are grey or yellow-white. Mature larvae are up to 12 mm long and are tinged with pink or green. Pupae are formed in a flimsy silken cocoon covered with soil particles and debris for camouflage.
Adult moths have a narrow, silver-grey body and narrow, greyish-brown fringed wings patterned with small, dark specks. They are 10 mm long with a 12 mm wingspan.
Infestations in potato fields can be identified by leaf mines and by boring of stems which causes plants to wilt.
Detection in tubers normally requires cutting them open to reveal larvae within galleries of slender, dirty-looking tunnels. Sometimes their presence is obvious due to mounds of frass (droppings) at the tunnel entrance. Infested tubers often also become infected by fungi or bacteria.
In tomatoes caterpillars burrow into terminal stems, causing stems to die, as well as into fruit.
Confusion with other pests: PTM is the only moth pest that commonly infests potato tubers in Bhutan. It sometimes attacks tomatoes and chilli in which it could be confused with other pod borers, especially the most common, and much larger, chilli pod-borer.
Lifecycle: Adults actively fly at dusk when females lay eggs. Eggs are laid singly or in batches on the underside of potato leaves or near eye buds on exposed tubers. In the field, and also on sprouts of stored potatoes, newly hatched larvae bore into leaves in which they make leaf mines. Larvae mine the upper and lower side of leaves, leaving surfaces intact. They gradually bore into the stems and eventually tubers in which they make irregular galleries. In exposed potato tubers larvae usually enter through the “tuber eyes” from eggs laid nearby. Larvae pupate in dirt-covered cocoons inside the leaf fold or in soil, leaf-litter, inside the tuber or in cracks and crevices of potato stores.
Under warm to hot conditions the moth can pass through several generations a year, with a generation (egg to adult) taking as little as four weeks. PTM can’t establish at high altitudes (above about 2500 m asl) where temperatures are insufficient for it to pass through a generation. In cool conditions PTM overwinters as pupae.
Dispersal: Adult moths can fly easily between fields and into potato stores. Larvae can also be easily spread into clean areas through the movement of infested potatoes. Use of infested seed potatoes is an important way of infesting fields at the start of the planting season.
When can damage be expected? In areas where PTM is well established high levels of damage can be sustained every year in the absence of management. Warm potato tuber storage conditions will result in more generations being completed, and therefore more damage. In the field tuber damage is most severe in heavy soils which crack readily if they dry out late in the growing season.
Hosts: It is a pest of potato, but it can also develop on tomato, brinjal (eggplant) and chilli.
Integrated management is essential in areas where PTM is known to cause significant losses to storage potatoes, or in-field damage. Management practices will need to be carried out throughout the year, from storage, to planting to harvest. Implementing strict hygiene practices and regular monitoring is critical.
In areas where PTM is a problem stored potatoes should be inspected weekly for damage (tunnelling and frass), as well as for other issues such as excessive sprouting and rot. If damage is seen then cut tubers open to confirm identification. Action should be taken as soon as tuber damage is observed.
The presence of moths in the field can be determined by gently shaking plants by hand or with a stick to see if see adults fly up. Leaf mines can also be found as can boring damage in wilting plants.
Adult moths can also be detected in the field and stores using pheromone traps, but these are not yet readily available in Bhutan.
Effect of variety
Use of resistant varieties can reduce infestation levels. Desiree is said to be moderately resistant.
Management in the field
- Cultural practices are the most economical method of managing PTM in the field.
- Only healthy tubers free of PTM should be used as seed potatoes. Use of infested tubers will allow PTM populations to rapidly build up through the year.
- Seed potatoes need to be planted at least 10 cm deep. However, care should be taken not to plant the potato too deep as it may hinder sprouting.
- Frequent hilling-up of the soil to cover the soil cracks and exposed tubers (especially after heavy rain and on steep slopes) is needed to prevent PTM from reaching the tubers. This is especially important late in the season when the tubers are large but before they become exposed.
- The hill should be wide, with the soil thrown well up around the bases of the plant. The crop must also be free of tall weeds which will interfere with the hilling.
- Irrigation at the right times can help avoid soil surface cracking during tuber formation.
- Harvest as soon as tubers are mature (and before dying or yellowing of potato plants). Do not store potatoes in the soil in the field.
- Select healthy tubers with no signs of PTM damage for storage at the time of harvest. Discard the infested ones (consume, bury or use as animal feed).
- Put harvested potatoes intended for storage into PTM-proof storage on the same day as harvest. Moths will lay eggs on any potatoes that are left exposed overnight.
- If PTM is present in the potato fields then remove plant debris from the potato field and burn to prevent population build up in the following season.
- Never leave volunteer plants and tubers in the field between seasons (harvest thoroughly).
- Avoid crop rotation with other solanaceous crops (chilli, tomatoes and brinjal). Any volunteer potato plants growing in rotational fields should be destroyed.
- Specific parasitoids and entomopathogenic nematodes can exert significant control, but they are not known to occur in Bhutan.
Management under storage
Careful storage of seed potato is critical to ensure clean seed is available for planting.
- Storage at below about 11oC will prevent larval development. As far as is practicable potatoes should therefore be stored in cool areas, such as in cold storage or at high altitude (above about 2,600 masl).
- Use closed stores with any openings meshed to prevent access by adult moths. However, maintaining good ventilation is important for keeping conditions cool, thereby slowing moth development. Alternatively, store potatoes in nylon bags to prevent PTM laying eggs on the potatoes and place where air circulation is good (such as roof spaces).
- Thoroughly clean stores to ensure that walls, floors and storage containers are devoid of old leftover potatoes.
- Do not keep old and new lots of potatoes within the same store.
- Covering stored potato with repellent plants such as Acorus, neem, Artemesia or lemongrass may reduce the incidence of PTM, although their effects has not been properly tested yet in Bhutan. Covering potatoes with a fine net would also be effective.
- Never use potato plant foliage to cover harvested tubes.
Management in the field
- Use of insecticides for managing field populations of PTM is not recommended.
- PTM populations are rarely if ever sufficient to justify it economically.
- Insectiicides will kill beneficial natural enemies which help keep this pest in check.
- Efficacy is low as contact insecticides do not reach larvae hidden inside leaves and tubers.
- Contact NPPC for advice if outbreaks occur.
Management in storage
- Insecticides should only be used if PTM is detected within the store, and then only on seed potatoes.
- Apply Fenvalerate dust at the rate of 100 grams per 100 kg of potato.
- Never use insecticides on table potatoes.
- Store seed and table potatoes separately to prevent insecticide contamination of table potatoes and reinfestation of seed potatoes.
- Protective clothing is needed, including gloves, when planting out treated potatoes.