Potato tuber moth (PTM)

Phthorimaea operculella

Crop: Potato

Why is it a problem? PTM is the most common insect pest of potato in Bhutan, destroying the crop both in the field and during storage. Foliar and tuber damage can result in substantial yield losses. As a storage pest, it can destroy high proportions of tubers kept as seed potato and for private consumption (21-42% in one survey).

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, so it 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 as well.

IDENTIFICATION

PTM larvae damage on tubers  
Adult PTM 

When first laid, 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 greyish-brown fringed wings with small, dark specks. They are 10 mm long with a 12 mm wingspan.

SYMPTOMS

The larva attacks all vegetative plant parts of the potato. The peculiar leaf damage symptoms are mines caused by larvae feeding in the mesophyll and  boring of stems, which causes plants to wilt (Figure 1 and 2). 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 (Figure 3)

Figure 1. Tunneling on stem (Observed at Chapcha, Chukha )

Foliar damage (mining)

Figure 2: Foliar damage (mines) (Observed at Chapcha, Chukha )

Figure 3: Tubers infested by PTM showing rot symptoms (observed at Chapcha, Chukha)

 

Confusion with other pests: PTM is the only moth pest that commonly infests potato tubers in Bhutan. It sometimes attacks tomatoes and chillies, in which case it could be confused with other pod borer species, especially the much more common and larger chilli pod-borer.

BIOLOGY

Lifecycle: Adult females lay up to 200 eggs, singly or in small batches, on the underside of potato leaves, near eye buds on exposed tubers, or on the soil. In the field and also on sprouts of stored potatoes, newly hatched larvae bore into leaves, where they make leaf mines. They gradually bore into the stems and eventually the 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, in soil, leaf litter, inside the tuber, or in the 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.

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. The 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.

MANAGEMENT PRACTICES

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.

Monitoring

  • Presence and population build up in the field and stores can be detected using sex pheromone traps. Contact NPPC for traps.
  • 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.
  • 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.

Effect of variety

little known in Bhutan. The use of resistant varieties can reduce infestation levels. Desiree is said to be moderately resistant.

Non-chemical management

Management in the field

Planting

  • Only healthy tubers free of PTM damage should be used as seed potatoes.
  • Plan seed potatoes at least 10 cm deep. However, care should be taken not to plant the potato too deep as it may hinder sprouting.
  • Sowing date should be adjusted such that it does not coincide with long dry weather as it favors PTM multiplication.

Cultivation

  • 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.

Harvesting

  • Do not leave potato tubers in  in the field overnight to be picked  and stored the following day , because the female PTM is most active laying eggs at night. Therefore, harvest and store on the same day. 
  • 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). 
  • 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).

Other

  • 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.

  • Clean storage houses and containers to remove eggs, larvae and pupae.
  • Do not keep old and new lots of potatoes in the the same store. Remove infested potato tubers.
  • 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).
  • Covering stored potato with repellent plants such as sweet flag Acorus calamus  (https://www.sciencedirect.com/science/article/pii/S2666154322001442) and Artemesia sp.  (http://www.istrc.org/images/Documents/Symposiums/Fifthteenth/s7_maharjan_fly.pdf ) may reduce the incidence of PTM, although their effects has not been properly tested yet in Bhutan.
  • Never use potato plant foliage to cover harvested tubes.

Chemical management

Management in the field

  • Use of insecticides for managing field populations of PTM is not recommended.
    • Insecticides will kill 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 re-infestation of seed potatoes.
  • Protective clothing is needed, including gloves, when planting out treated potatoes.

For more information, click on the following links: 

Version: NPPC 2022: Potato tuber moth V2.0. Bhutan Factsheet.http://pestsofbhutan.nppc.gov.bt/ . Date produced: 12 December 2022. Contact: nppcsemtokha@gmail.com
Image acknowledgement: NPPC & MyPestGuides crop

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