Phthorimaea operculella
(potato tuber moth)

P. operculella is a cosmopolitan pest, especially in warm temperate and tropical regions where host plants are grown. During recent years the species has been inadvertently introduced into Georgia (Markosyan, 1992) and the Ukraine and there is a threat of its spreading to neighbouring states (Sikura and Shendaraskaya, 1983). It has also been newly recorded from the Arabian peninsula (Povolny, 1986; Kroschel and Koch, 1994) and more widely in East Africa (Parker and Hunt, 1989). It was also observed in Germany (OP Karsholt, Zoologiste Museum, Copenhagen, Denmark, personal communication, 1996).

This is such a cosmopolitan pest that there are few countries where it is an external or potential threat. It is intercepted occasionally in Europe on imported plant material; however it is doubtful whether it will survive severe cold winters of temperate countries.

Potato is the principal host for P. operculella, but other Solanaceae are attacked, especially tomato, tobacco, chilli, aubergine and cape gooseberry. There are many related wild hosts; 60 plant species are listed by Das and Raman (1994).

Carter (1984) reviewed P. operculella with reference to Europe; Trivedi and Rajagopal (1992) gave a review of all aspects of the species, with special reference to India. A review of the knowledge of the pest over 50 years in South Africa was given by Daiber (1991).

The eggs are laid singly or in batches on the leaves of the host plant, or on exposed tubers near the eye buds. A total of 40-290 eggs are laid which take 3-15 days to hatch. In winter this stage can last up to 58 days, but eggs cannot tolerate low temperatures; eggs kept at 1-4°C for 4 months failed to hatch (Langford and Cory, 1932).

The larva at first bores into the petiole, or a young shoot or main leaf vein, and mines the leaf making a blotch. Later it bores into a tuber, making a long irregular gallery. On stored tubers feeding on the tubers begins immediately. The larval stage lasts 13-33 days.

Pupation normally takes place in the soil; the pupal period is 6-29 days.

Adults fly chiefly by night and are attracted to light. They live for up to 10 days. The moth breeds continuously where conditions permit; up to 13 generations a year have been recorded in India (Mukherjee, 1948). The complete life cycle ranges from 17 to 125 days. Development of all stages is greatly influenced by temperature: the maximum fecundity has been recorded at 28°C and temperatures tolerated are in the range 15-40°C.

P. operculella is attacked by native natural enemies wherever it is found; however, many of these are relatively ineffective. Some are partially effective and have been introduced for biological control into other countries. The area of origin of the pest was first believed to be North America, where some effective parasitoids exist. However, Lloyd (1972) convincingly argued that the pest originated in South America (where it attacks crops such as potato, tobacco, etc.) and in the warm temperate and subtropical areas east of the Andes where it is effectively suppressed by host-specific natural enemies. The List of Natural Enemies includes only species regarded as important and those that have been released as biological control agents.

Mines in the leaves, leaf stalks and stems and tunnels in the tuber are clear signs of infestation by P. operculella.

The adult P. operculella moth closely resembles some Scrobipalpa spp. but is readily separated by the spiny structure on the dorsal side of the male genitalia, which is visible without dissection.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Cultural Control

As infested seed tubers are the main cause of re-infestation, the use of healthy tubers will reduce levels of field infestation (Lakshman Lal, 1991).

In New Zealand the greatest tuber damage and economic losses occurred during the peak of the second potato tuber moth generation during February and March. Harvesting the crop as soon as possible during this period is recommended to restrict damage (Herman, 1999).

In Sudan, planting in the second week of November resulted in less insect damage and a significantly greater total yield compared with crops planted 3 weeks later. Greater depth of planting and more frequent hilling-up significantly lowered infestation; for example, insect damage ranged from 3.3% (planting 10 cm deep and hilling-up three times) to 16% (planting 5 cm deep and hilling-up once). Irrigation and mulching both significantly reduced insect damage. Light irrigation every 4 days and mulching with neem leaves during the last 4 weeks before harvest were the most effective treatments (Ali, 1993).

Field trials conducted in Meghalaya, India, during 1987-88 indicated that larval infestations of P. operculella on potatoes were consistently reduced when potatoes were grown with chillies (Capsicum), onions or peas compared to potato alone. Similarly, tuber damage was significantly lower in plots associated with Capsicum, onions and peas (11, 11 and 13%, respectively) compared to 27% in potato alone (Lal, 1993). In Egypt larval populations of P. operculella were significantly reduced by 80 and 91% on tomato intercropped with onion and garlic, respectively, in 1988-89, but not in 1989-90. In both seasons studied, tomato yield was 114-207% and 104-284% higher in plants intercropped with onion and garlic, respectively, compared to plots grown with tomato alone (Afifi et al., 1990).

Host-Plant Resistance

Recent research into the possibility of genetically modified varieties is making some progress as described by Li et al. (1999) and Westedt et al. (1998), these point to future research directions rather than make practical solutions. An overview of current research into the genetic manipulation of potato for insect pest resistance carried out at the New Zealand Institute for Crop and Food Research is given by Conner et al. (1996). Work is focused on transferring the genes for the insecticidal proteins of Bacillus thuringiensis into potato in order to provide resistance.

Fifteen potato cultivars were tested for their reaction to P. operculella by Berlinger et al. (1992) using artificial infestation. There was a good correlation between the rate of larval infestation in the leaves and tubers except for cv. Frisia, which had highly susceptible leaves but a low percentage of infested tubers (8.4%). Ailsa, Escort, Blanka, King Edward and Russet Burbank were fairly resistant. Cara, Maris Piper, Desiree and Nicola were moderately resistant and Frisia, Asterix, Spunta, Pentland Squire, Diamant and Alpha were susceptible.

Biological Control

Biological control of P. operculella has been attempted since 1918 when Bracon gelichiae was imported into France from USA. Lloyd (1972) showed that in northern Argentina and southern Brazil P. operculella causes little damage and is heavily parasitized. Cultures of some of these parasitoids were mass-reared by the Commonwealth Institute of Biological Control (CABI Bioscience) and widely distributed. Three of these parasitoids, Apanteles subandinus, Copidosoma koehleri and Orgilus lepidus, and also B. gelechiae have become established in a number of countries, and in some of them successful biological control has been reported (Sankara and Girling, 1980). In Victoria, Australia, Horne (1990) found that the parasitoids A. subandinus and O. lepidus were the most abundant, but C. koehleri was also recorded from several sites.

Detailed analysis of data obtained through regular monitoring revealed that, in an area free from insecticides, parasitoids were a major factor in controlling P. operculella. This result differs from previously published opinions on the effect of parasitoids, and is attributed to this study's analysis of data on a generation basis. The most successful results were claimed in Zimbabwe and Zambia after the establishment of A. subandinus and O. lepidus. In Cyprus A. subandinus and other parasitoids are credited with effective control (see Sankara and Girling, 1980). C. koehleri has also been successfully used in Peru (Raman et al., 1993). However, biological control depends on the proper selection and use of insecticides, and when indiscriminate insecticide application is practised parasitoids are unable to maintain control.

Native parasitoids can also achieve significant rates of parasitism, for example, in Sardinia the ichneumonid Diadegma turcator and the braconids Bracon nigricans, B. properhebetor and a species of Apanteles were recorded as parasitoids of the pest, the first species accounting for 65.1% of the total rate of parasitism (Ortu and Floris, 1989).

The nematodes Steinernema feltiae, S. bibionis, S. carpocapsae and Heterorhabditis heliothidis were used in an experiment in Russia (Ivanova et al., 1994). Potatoes infested with larvae of P. operculella were sprayed with aqueous suspensions. The first three species resulted in 95.5, 93.4 and 93.1% mortality, respectively. Larvae of all instars, within as well as on the surface of the potatoes, were affected, and infection on larvae of the next generation appeared in 6 days.

In Italy, a leaflet was designed to give agricultural extension workers an understanding of the biological control of P. operculella using baculovirus (CIP, 1992). Instructions for low-cost virus multiplication are included. In Israel, in tomato fields adjacent to potato fields, extensive damage was caused by P. operculella and three consecutive applications of Bacillus thuringiensis at a high volume were required for control.

In Peru, Winters and Fano (1997) found that Baculovirus phthorimaea was cost effective.

In southern areas of the Ukraine, B. thuringiensis subsp. kurstaki and B. thuringiensis subsp. thuringiensis were highly effective against the pest when applied at 10-25°C. Treatment of larvae with these preparations at 9-12°C showed that they were at least as active as Dipel, another formulation of B. thuringiensis subsp. kurstaki (Baklanova et al., 1990).

In Australia, Baggen and Gurr (1998) found in field trials to investigate the effects of habitat management, parasitism rates were greatest in P. operculella larvae recovered from potato tubers close to flowering plants of dill (Anethum graveolens), borage (Borago officinalis) or coriander (Coriandrum sativum). It is concluded that there may be value in providing non-host foods to C. koehleri by deploying flowering plants in a habitat management strategy.

Botanical Pesticides

Prevention of attack is also assisted by the application of vegetable oils. Sharaby (1988) showed that orange peel oil reduced the fecundity of P. operculella. Reproduction was significantly reduced when either males or females were exposed to the oil vapour. The effect increased with an increase of oil dose and exposure time. Egg hatch ranged from up to 30% when the moths were exposed to 160 µl of oil for 30-120 min. A further pronounced reduction in egg production and egg viability occurred when the moths were exposed to the vapours arising from paper discs treated with a drop of oil.

Salem (1991) showed that neem seed extract was effective for control of P. operculella on potatoes in a store in Egypt. Storage loss after 6 months in potatoes treated with 100 p.p.m. neem oil was 25% (compared to 10% with carbaryl). Adults from larvae treated with neem oil were deformed.

Chemical Control

In a field study in Egypt with potatoes, abamectin was the most effective against P. operculella followed by profenofos, Bacillus thuringiensis and granulosis virus, respectively. Tuber yields with these treatments were 14.26, 14.21, 12.58 and 12.08 t/ha, respectively, compared with the control yield of 9.04 t. Under the storage conditions, abamectin was also the most effective followed by fenitrothion, B. thuringiensis and granulosis virus (Abel-Mageed et al., 1998).

Quinalphos and diflubenzuron reduced damage caused by P. operculella, and the yield was highest in plots treated with quinalphos in India (Chandramonhan and Nanjan, 1993). The efficacy of nine insecticides against P. operculella on potatoes was tested in Maharashtra, India. Treatment with phenthoate, chlorpyrifos, fenitrothion, phoxim, permethrin, cypermethrin, deltamethrin and fenvalerate at fortnightly intervals reduced pest populations. One of these insecticides should be applied on the appearance of pests with two or three repeat applications (Raj and Trivedi, 1993).

Eleven insecticides were tested in sprays against P. operculella on potatoes in Maharashtra, India, in 1983-86. Analysis of the pooled data from three field trials indicated the efficacy of three foliar applications of quinalphos at 15-day intervals, beginning 45 days after planting (Pokharkar et al., 1991). More chemical treatment details are provided by Trivedi and Rajagopal (1992).

For protection of stored potatoes in India Sharma et al. (1998) carried out tests and it was concluded that Plantmix I is a considerable improvement to Neemrich I for preventing infestations of P. operculella. Debnath et al. (1998) found that damage to stored tubers was significantly reduced following the application of dry eucalyptus leaves, and powdered sweet flag (Acorus calamus) rhizomes and eucalyptus leaves. Dusting healthy tubers with fine wood ash controlled infestations for long periods. Seed potatoes were adequately protected by 5% malathion dust and 1.5% quinalphos dust. Potatoes kept in gunny bags impregnated with solutions of malathion 50 EC and azadirachtin 1400 p.p.m. were also free of damage. Although treatment with B. thuringiensis resulted in a low level of infestation and rotting, the percentage tuber damage remained high.

In Tunisia, Das et al. (1998) found that deltamethrin, granulosis virus and B. thuringiensis were equally effective in reducing pest damage. After 3 months' storage the treatments showed no significant effect on sprouting.

Pheromonal Control

Pheromone traps are used both for monitoring populations and for control in the field and in storage (Chernii et al., 1994). The sex pheromone of P. operculella was identified as a mixture of trans-4,cis-7-tridecadienyl acetate (PTM1) and trans-4,cis-7,cis-10 tridecatrienyl acetate (PTM2) (Persoons et al., 1976). Ortu and Floris (1989) baited traps with a mixture of the compounds. The use of a large number of pheromone traps (84/ha) drastically reduced the number of captures within the field, indicating that the mating disruption techniques may be effective for controlling this pest.

In New Zealand Herman and Clearwater (1998) tested delta design DeSIRe, green plastic funnel, water and A-traps and found that DeSIRe sticky traps caught more than eight times more moths per day than the funnel traps (17 and two moths per day, respectively). This was consistent regardless of height or pheromone blend. A blend of PTM1 and PTM2 in the ratio 1:1.5 was most effective.

Tests in 1976-78 at Casablanca in Morocco, both in the field and in vegetable-packing stations, showed that the most effective traps were pans of water over which were hung rubber septa impregnated with synthetic sex pheromone of the moth (Thal, 1979). The best results were obtained with PTM1 and PTM2 at a ratio of 9:1 (Raman, 1983); all ratios tested gave better results than traps baited with virgin females. A 1:1.5 mixture remained attractive for 90 days in the field; storage at -5°C for 2 months did not reduce its efficacy. Water traps and funnel traps gave similar results to each other. Funnel traps were ideal for mass trapping: tuber damage was reduced by 23% in pheromone-treated plots. Funnel traps in stores reduced tuber and sprout damage from 60 to 8%.

Integrated Pest Management

Strategies for integrated management of P. operculella are often contained within the management of the whole complex of potato pests. The International Potato Centre (CIP) in Peru has devised a strategy to develop and implement IPM to overcome some constraints that prevent farmers from adopting such practices. Integrated pest management strategies have been developed in many areas: see also Arx et al. (1987), Das et al. (1992), Fuglie et al. (1993), Cisneros and Gregory (1994) and Berlinger et al. (1992).

Phytosanitary Measures

Because P. operculella is such a cosmopolitan species and is considered to have reached its natural limits, phytosanitary measures are unlikely to have much effect. Where it is warm enough for the moth to survive in the open, or in warm storage, regular inspections for tuber damage should be made.