Potato tuber moth

Potato tuber moth (Phthorimaea operculellais the most widely distributed potato insect in the world.  It is also named as potato moth, potato tuber worm, stem end grub, tobacco leaf miner, tobacco split worm and split worm. It is usually found in warm climates for overwinter survival and considered a subtropical pest. The moth or worm is considered the most serious pest of potato in tropical and subtropical regions.

Potato tuber moth (PTM) or Potato tuber worm (PTW) has appeared and spread in the US in the past century. The moth consists of several species. Phthorinaea operculella, the most common, is widely distributed in the world, found in North Africa, and parts of Asia, Europe, the Americas, and Oceania. Recently PTM/PTW has been found in traps in northern latitudes. Whether this northern migration is due to global warming, mutation, etc. is not known. Since it is not a good flier, its migration is attributed primarily due to movement of tubers carrying the pest into storage facilities further north.

Life Cycle:

POtato Tuber Moth has four stages: adult, egg, larva (damaging), pupa.

Female moth lay about 100-150 eggs at dusk on the surface of the potato near the eye or in the cracks on the exposed skin of tuber. In field, the female laid eggs singly on the undersurface of the potato leaves. Eggs are oval, smooth and yellow, laid alone or in clusters on leaves or near eyes on infested tubers. 

Within 3-6 days larvae hatches out from the eggs. The larva bores deep into the potato tuber and feed upon its content. The larval period lasts for 5-16 days. The fully grown larva comes out of the tuber and pupates inside a silken cocoon. In field the cocoons are formed among the dried leaves or thrash on the ground or in the soil.  Larvae, caterpillar-like (PTW, worm), are gray, cream or pale green with a dark brown head about half to three-quarter inch long in the final instar.

In storage the cocoons are formed in the cracks on the floor and wall of the store house or on the seams of storage bags. Pupal period lasts for about a week. The pest remain active throughout the year or till the food is available. Pupae are yellow or rust colored; pupation occurs among dead leaves or debris, in soil, or on stored tubers.

Adults have a narrow, silver-gray body with grayish-brown wings patterned with small dark spots. The body length is around a third of an inch and a wing span of an inch (2.54 cm). It is mostly nocturnal and attracted to light. They are poor fliers.

Low rainfall and moderately high temperature are favorable conditions for the growth and multiplication of this pest. The life cycle is 17 to 125 days depending on temperature, commonly one month. Adult = up to 10 days; egg = 2 to 6 days; larva = 13 to 33 days; pupa = 6 to 29 days. Several generation may form per year. There are about eight or nine generations in a year. In colder parts, the larvae & pupae are reported to hibernate during winter. 

Host Range

PoPotato tuberworms are mainly associated with potatoes; however, they have been observed feeding on other plants such as tomatoes, eggplants (Solanum melongena L.), peppers (Capsicum spp.), tobacco, and wild solanaceous plants like Jimson weed or datura (Datura stramonium L.) (Alvarez et al. 2005). In the Pacific northwest, potato tuberworms have only been reported infesting potatoes (Rondon et al. 2007).

Damage

Larvae feed on potato leaves, stems, petioles, and more importantly potato tubers in the field and in storage. The newly hatched larvae create mines on leaves by feeding on leaf tissue while leaving the upper and lower epidermis of the leaf intact. They prefer feeding on young foliage (Trivedi and Rajagopal 1992). Typical damage results from larvae boring tunnels in tubers. Larvae depositing their excreta make tubers unfit for consumption. Potato tuber eyes become pink due to deposition of silk and excrement by potato tuberworm infestation. Severe infestations result in yield and quality losses during storage where previously infested tubers are stored with healthy potato tubers (Malakar and Tingey 2006, Rondon 2010). This generally destroys the entire crop of stored potatoes.

Most economic damage occurs to potato tubers in storage conditions in developing countries and is caused by larval feeding. Presence of even one larva is sufficient to spoil and destroy a tuber. Rapidly moving caterpillars penetrate the tubers, form galleries coated with silken threads and eject frass outside the tuber. On leaves, caterpillars form galleries and then penetrate other plant parts. After two to three weeks, caterpillars leave the plant (caterpillars can move through cracks in soil) and pupate on walls of potato bags lying in potato fields. Fungi, bacteria, and mites can develop inside the tunnels made by the larvae, which causes the tubers to rot and emit an unpleasant smell.

Stored crop losses in potatoes ranging from 50% in Yemen and Peru, 86% in Tunisia, Algeria, and Turkey, 90% in Kenya, and 100% in India and the Philippines have been reported (Alvarez et al. 2005). In Egypt, potato tuber moth has caused up to 100% losses to potato plants in fields as well as in storage (Ahmed et al. 2013). Potato tuber moth is also a pest of tomatoes where larvae damage the leaves, stem and the unripe fruits.