Potato tuber moth
Phthorimaea operculella (Zeller, 1873) [Lepidoptera: Gelechiidae]
Synonyms: Gnorimoschema operculella Zeller
Common names: Potato tuber moth (English), Teigne de la pomme de terre (French), Polilla de la papa (Spanish), Kartoffelmotte (German)
Host plants: Many cultivated and wild species of the family Solanaceae: Solanum tuberosum (Potato), S. melongena (Aubergine), S. incanum (Apple of Sodom), S. muricatum (Melon pear, Sweet cucumber), S. aviculare (Poroporo), S. nigrum (Black nightshade), Nicotiana tabacum (Tobacco), Lycopersicion esculentum (Tomato), Datura stramonium (Thorn apple), Physalis peruviana (Cape gooseberry), Capsicum annum (Paprika), Hyoscyamus albus(Henbane). Only few host plants are described from other families, e.g. Beta vulgaris (beet root) (Chenopodiaceae)
Leaf: Mines are the typical symptoms of leaf damage caused by the larvae eating the mesophyll without damaging the upper and lower epidermis.
Haulm: Larvae also enter into leaf axils and growing points of young plants.
Tuber: Larvae enter tubers through the potato eyes. Inside the tubers, the larvae bore irregular galleries that may run into the interior or remain directly under the skin. Larval excrement is pushed out through the holes, which can be noted immediately after larvae start mining activity. When heavily infested, tubers are often rotten making them unsuitable for human consumption. At harvest, tubers do not always show signs of damage but may harbor eggs and early instar larvae. This may result in severe post-harvest losses of stored potato in the absence of adequate control measures. The mining produces weight losses of tubers, which is exacerbated by increased transpiration through the wounds, causing them to shrink. The wounds provide entry points for microorganisms and cause secondary infestation, particularly by species of Penicillium.
Status of the pest
The potato tuber moth originated in 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. In many regions the moth is mainly considered as a storage pest of potato with losses of up to 100 %. The pest is either transferred to the stores with already infested tubers or adults enter storage facilities. The severity of tuber infestation and consequently the quality loss of harvested tubers are closely related to the population built up of the moth during the potato-growing season. Yield losses of up to 45% are therefore largely the result of early tuber infestation in the field, generally where females are able to directly deposit eggs on tubers through soil cracks. Leaf infestation becomes highest when potato is cultivated during warm dry seasons especially under furrow irrigation. Mean temperatures of more than 20°C favor a quick population development. Under these conditions highest plant damage has been observed in the Republic of Yemen with more than 30 mines per plant that directly reduces the productivity of the plant and causes yield losses. Heavy rains or regular sprinkler irrigation influences the flight activity of adults and limits infestation. In rainfed potato in the Andean highlands leaf infestation is low but delayed harvest during the dry season increases tuber infestation tremendously.
Life cycle and biology
Adults are small brownish gray colored moths (7--9 mm), with fraying on the posterior edge of the forewings and on both posterior and inner edges of the hind wings. At rest the wings are folded to form a roof-like shape. The wingspan is between 12 and 16mm. The tip of the female’s abdomen is cone-shaped, whereas the males possess two claspers at the hairy tip of their abdomen. The male’s sexual organs are situated in the middle of the ninth abdominal segment, the females in the middle of the eight. The sexual pheromones by which females attract males have been isolated and identified as trans-4, cis-7-tridecadien-1-ol acetate (PTM1) and trans-4, cis-7, cis-10-tridecatrien-1-ol acetate (PTM2). A mixture of the two is far more effective that one component used by itself.
The eggs are too small (0.5 x 0.35mm) to be invisible to the naked eye on potato leaves and tubers. The freshly laid eggs are deposited singly or in small clutches resembling strings of beads. As they develop they take on a yellowish tinge, and before hatching the black head capsule of the tiny larva can be seen through the thin eggshell. Of all the host plants, the females prefer to lay eggs on potato. The tiny eggs are laid on the undersides of leaves, on stems, and on the tubers, the eyes in particular. The females lay between 100 and 300 eggs, depending on temperature and food availability. The sex ratio is 1:1. The first instar larva (L1) is barely 2 mm long, while the fourth instar larva (L4) measures 9-13 mm in length before pupation. The color of the larva depends on their diet. In tubers they are a whitish purple, but those on the potato leaves are purple to green. A fully developed larva has six ocelli on each side of the head, mouth parts with a silk gland, a prothoracic and an anal plate, nine pairs of spiracles, and five pairs of prolegs on abdominal segments III – VI and X. The full-grown larva usually leaves the plant or tuber to pupate, which either takes place in the soil or on tubers. The pupa reaches 6-7 mm. At first it is brownish in color, turning to dark brown one day before the emergence of the moth.
The potato tuber moth can adapt to the most diverse climatic conditions. Development is possible within a temperature range of between 10°C and 35°C. The population growth rate peaks at temperatures of between 20°C and 25°C. At a constant temperature of 26°C, the development of eggs takes four to five days; the larval stage is completed within 12 days and the pupal stage within six to seven days. Daily fluctuating temperatures having the same mean temperature do not affect development times differently. In the Republic of Yemen, the moth developed eight generations, while for the Andean Highlands only three to four generations were determined. The potato tuber moth can survive low temperatures for short periods at all developmental stages. The species does not respond to unfavorable conditions by entering diapause. For development to continue, short spells of higher daily temperatures are necessary. The moth survives intervening periods on leftover potato or adults re-colonize fields after storage. Most important, infested tubers often used as seed in developing countries initiate colonization in the next growing season. Certain weeds can serve as hosts too but it has been observed that they are rarely infested.