Phthorimaea operculella (Zeller, 1873),
Symmetrischema tangolias (Gyen, 1913),
Tecia solanivora (Povolny, 1973) (Lepidoptera: Gelechiidae)

Distribution

The potato tuber moth (PTM), Phthorimaea operculella, originated in the tropical mountainous regions of South America. Today it has a worldwide distribution and is considered the most damaging potato pest in the developing world.

It is present in almost all tropical and subtropical regions of the world, in North, Central, and South America, Africa, Asia, Australia, and Europe. The Andean potato tuber moth (APTM), Symmetrischema tangolias, is native to South America (Peru and Bolivia) but has in the last decades spread to other regions of the world (image below). Records include North America, Australia, New Zealand, and more recently Indonesia.

Although this species is known as a pest of potato and tomato in South America and the Australian region, it does not attack any of these two crops in North America but instead feeds on black nightshade (Solanum americanum Mill.).

Adult female of Phthorimaea operculella (a) and symptoms of larvae infestation on leaves, stems, and tubers (b, c, d). (Courtesy: CIP)

Adult male and female of Symmetrischema tangolias (a), and symptoms of larvae infestation on stems and tubers (b, c). (Courtesy: CIP)

The Guatemalan potato tuber moth (GPTM), Tecia solanivora, probably originated from Guatemala and is endemic throughout Central America (image below). In 1983, the pest was unintentionally introduced into Venezuela and then invaded Colombia and Ecuador. In 2000, T. solanivora was introduced in the Canary Islands (Tenerife).
 

Adult male of Tecia solanivora (a), and symptoms of larvae infestation on tubers (b). (Courtesy: CIP)

Since then the pest has been considered as a major threat to potato throughout southern Europe and was listed as a quarantine pest by the European and Mediterranean Plant Protection Organization (EPPO 2005a; Kroschel et al. 2016a, b; Kroschel and Schaub 2013). In 2014, it was finally recorded in mainland Spain where efforts are going on to eradicate the pest (Jeger et al. 2018).

Host range

P. operculella is an oligophagous pest (i.e., an insect feeding on a restricted range of food plants) of vegetable crops that belong mainly to the family Solanaceae: potato (Solanum tuberosum L.), tomato (Lycopersicon esculentum Mill.), and tobacco (Nicotana tabacum L.). Also, wild species of the Solanaceae family, including important weeds, e.g., black night shade (Solanum nigrum L.) are hosts.

In total, the host range comprises 60 species. Further, crops of the family Chenopodiaceae are attacked including eggplant (Solanum melongena L.), bell pepper (Capsium annuum L.), Cape gooseberry (Physalis peruviana L.), aubergine (S. melongena L.), and sugar beet (Beta vulgaris L.). S. tangolias has a more restricted host range comprising of potato, tomato, sweet cucumber (Solanum muricatum Aiton), poroporo (Solanum aviculare G. Forst), kangaroo apple (Solanum laciniatum Aiton), black nightshade, and bell pepper. T. solanivora is monopahagous attacking only potato.

Symptoms of infestation

P. operculella attacks potato by mining the leaves and stems and by feeding on the tuber. Mines are the typical symptoms of leaf damage caused by the larvae eating the mesophyll without damaging the upper and lower epidermis. When the foliage dies, the larvae enter the soil through cracks where they may eventually find and feed upon tubers. Larvae enter potato tubers via the eyes and continue to bore or tunnel through the tuber just below the skin.

Larval excreta are pushed out through the holes, which can be observed immediately after larvae start their mining activity. Larvae of APTM enter the potato stem making a small hole in the plant axils (between stem and lateral petioles). From this hole, galleries made by the larvae run downward within the stem.

Excreta are pushed out through the initial hole made by the larva. When stems are severely damaged, the upper part of the stem wilts or the whole plant collapses. Young plants can suffer tip death from boring larvae. Eggs may be found in slits on the stem of a food plant. In tubers, larvae enter through potato eyes. Initially, the small hole can hardly be seen by the naked eye. As in stems, larval excreta are then pushed out through the hole, which becomes apparent after several days of mining activity.

Inside the tuber, the larva tunnels just under the surface at first, but later penetrates more deeply. GPTM larvae feed exclusively on tubers during potato cultivation and during storage. Damage is caused by larvae that bore galleries into the tubers. After the larvae have left tubers, the exit hole is clearly visible. In potato fields, T. solanivora attack occurs from tuberization until harvest (Kroschel and Schaub 2013; Niño 2004).

Tubers infested by either of the three species develop a bitter taste and are unsuitable for human or livestock consumption (Keller 2003; Kroschel and Schaub 2013).

Impacts on production losses

P. operculella: Under heavy field infestation, potato foliage can be destroyed, which can result in substantial yield loss of up to 70%. High infestations early in the season can directly affect tuber yield. Strong correlation exists between leaf and consequent tuber infestation, which suggests that reducing P. operculella population density during the growing period is key to reducing potato tuber infestation at harvest.

Hence, the most devastating yield losses are largely a result of earlier tuber infestation in the field, generally where moths have laid eggs through soil cracks on the developing tubers, or when harvest is delayed. P. operculella also damages harvested potato tubers in storage. The damage to potatoes in rustic stores can be total within a few months if the tubers are left untreated.

Infested tubers are unsuitable not only for human consumption but also for use as seed. Infested tubers produce fewer yields and initiate a fast development of a new field P. operculella population (Kroschel and Schaub 2013; Kroschel et al. 2012; Keller 2003; Kroschel 1994, 1995).

S.tangolias: This species has become an economically important pest in potato fields and in storage in mid-elevation regions of the Andes (Peru, Bolivia, and Ecuador); its status in Colombia is not well confirmed. In the Andes, losses in the field may reach up to 30%, but most economically significant damage occurs when infested tubers are transferred to potato stores where reinfestation takes place.

Without adequate management, farmers can completely lose their house-stored potatoes within 3–4 months of storage. In Australia and New Zealand, where the pest was accidentally introduced from South America, it is more recognized as a local pest of tomato, poroporo, sweet cucumber and other Solanaceae crops, and is commonly referred to as 'tomato stem borer.'

However, at national level it is considered a minor pest and its economic impact on these crops is not well reported in the literature (Kroschel and Schaub 2013; Keller 2003). T. solanivora. Complete losses of harvested tubers have been observed occasionally after the invasion of the pest into new areas when farmers were not yet familiar with pest control.

Generally, tuber damage rates at harvest vary between 2 and 15% in the Andean region. When infested potato tubers are stored without application of control methods T. solanivora can destroy, depending on the storage period and temperature, of whole potato stock (Kroschel and Schaub 2013; Niño 2004).

Methods of prevention and control

Control of the potato tuber moths must take place both in the field and in storage. Implementation of integrated pest management is recommended to reduce the pest problem in field and stores. T. solanivora is the most difficult potato moth to control as the larvae feed only inside potato tubers where they are hard to reach (Kroschel and Schaub 2013; Kroschel et al. 2012; Keller 2003; Pollet et al. 2003; Kroschel 1995).
 

• Monitoring with pheromone traps.
  For all three potato tuber moth species sexual pheromones have been identified and synthesized. They are used for monitoring the flight activity of adult male populations to detect early the presence of the different moths in the field and store to take adequate control measures.


• Cultural practices.
  Some common practices for potato tuber moths are the use of pest-free seed tubers, deep planting, regular irrigation to avoid soil cracking, high hilling to protect tubers, timely harvest, not leaving the tubers after harvest exposed in the field for a long time (especially throughout the night), i.e., harvest and store immediately, and removal of leftover tubers to reduce the overwintering field population.
  
  Also, early maturing varieties can contribute to reduced risk of infestation. Biological control. Classical biological control can be an effective strategy in all those regions in which the pests has been unintentionally introduced to keep the pest population below economic threshold; for this approach, the species Copidosoma koehleri (Blanchard), Apanteles subandinus (Blanchard), and Orgilus lepidus (Muesebeck) have been widely and successfully used (Canedo et al. 2016a, b, c; Kroschel and Schaub 2013).


• Biopesticides.
  Microbial biopesticides for P. operculella field control have been tested based on Bacillus thuringiensis subsp. kurstaki (Btk) and P. operculellaspecific granulovirus (PhopGV, Baculoviridae). Btk was effective but required repeated applications because it is quickly degraded by UV light. Likewise, PhopGV has shown mixed results.
  
  To protect PhopGV against UV inactivation a variety of adjuvants (e.g., dyes, optical brighteners) have been tested but simple preparations of PhopGV-infected larvae macerated in water were superior. Applications of PhopGV doses sufficient to cause >95% mortality are considered not being economical, and low dose treatments are proposed for a relatively inexpensive partial suppression of the field population (Lacey and Kroschel 2009; Sporleder and Kroschel 2008; Kroschel and Sporleder 2006; Sporleder 2003; Kroschel et al. 1996).


• Attract-and-kill.
  This approach has been developed to control of P. opercullela and S. tangolias under field and storage conditions. It consists of a co-formulation of the insect pest-specific sexual pheromone, which 'attracts' males, and a contact insecticide at very low concentration which 'kills' males getting in contact with the product.
  
  The oil formulation is applied at a droplet size of 100 μL using a special handheld applicator; it is applied at 2500 droplets/ha. It effectively reduces the male population and the number of offspring, hence controlling larvae damage in the crop. It provides pest-specific control, and is harmless to natural enemies, humans, and the environment (Kroschel and Zegarra 2010, 2013).
  
  In Peru, the two products AdiosMacho-Po® and AdiosMacho-St® have been registered to be commercialized in Peru and the Andean region. Chemical control. Broad-spectrum insecticides have been commonly used to suppress potato tuber moth population and economic damage, but which has been associated with many negative effects causing resistance of the pests to various active ingredients and affecting farmers and the environment.


• Integrated Pest Management.
  Effective IPM practices for potato tuber moths have been developed, which can be applied successfully if potato tuber moths are the only economically important pests in an agroecosystem (e.g., Republic of Yemen, Kroschel 1995). However, potato is often affected by several pest species which requires a system approach to manage all economically important potato J. Kroschel et al.263 pests (Kroschel et al. 2012). This means effective IPM practices are required for all pests to fully eliminate or minimize the use of insecticides.

Storage management

Potato tuber moth infestation occurs frequently in rustic farmer-managed potato stores in developing countries, especially if temperature is suitable for rapid population build up and the storage lasts for several months. Storage facilities should be cleaned thoroughly before potato tubers are stored. Fine netting at windows should protect adult moths from entering storage facilities.

Only healthy tubers should be selected for storage. Infested potato tubers need to be destroyed. However, initial infestations cannot be easily observed and are the main reasons why potato tuber moths enter storage facilities and infest potato. Sex-pheromone-baited water traps or funnel traps (Delta) can be used for monitoring the moth presence but in known region of potato tuber moths’ occurrences, potatoes should be treated before storage.

• Biopesticides
  
  Biopesticides based on Btk and PhopGV are used in potato tuber moth storage control. The microbials are formulated in inert materials (e.g., talcum) and dusted over potatoes before storage. Since PhopGV is only effective in P. operculella and T. solanivora, in regions where all three species occur simultaneously, the use of Btk has the advantage to control all three species.
  Further, Btk is mostly available as a commercial biopesticide while PhopGV has to be multiplied in potato tuber moth larvae. The product Matapol, e.g., is a co-formulation between Btk and PhopGV, commercialized in Bolivia. It has also been shown that inert materials (e.g., calcium carbonate, kaolin, talcum, silicium rich sand) can be used effectively without the addition of active biologicals (Btk, or PhopGV) as they control first instar larvae through desiccation (Kroschel and Koch 1996; Mamani et al. 2011; Sporleder and Lacey 2013; Schaub and Kroschel 2017).


• Attract-and-kill
  
  Attract-and-kill formulations (see above) can be applied at a density of one drop (100 μL)/qm of storage area to reduce the male population and hence tuber infestation in potato stores (Kroschel and Zegarra 2013).


• Chemical control
  
  Malathion dust (WHO Class III) is often observed to be sold in developing countries to treat stored tubers. This is especially critical if precautions are not taken properly by farmers and potatoes are stored in living areas. Pyrethroids (e.g., fenvalerate) have shown to be highly effective equally to Btk treatments described above (Kroschel and Koch 1996).