Dated 5/94
Placed on the Web 4/95 by the Department of Entomology, NCSU
The larvae of the potato tuberworm acts as a leaf miner on the foliage of the above crops and as a borer in potato tubers and to a lesser extent in tomato fruits.
The adult is a relatively small fragile gray colored moth with a wing expanse of about 1/2 inch. The body is about 3/8 inch long. Females are slightly larger than males. The slender forewings have a fringe of hair near their base. They are gray to grayish-brown with darker brownish spots or dots. The yellowish- brown hind wings are shorter and have a fringe of hair along the entire hind margin.
The egg is about 1/50 inch long. When first laid it is pearly white, later changing to yellow and finally brown.
The larva which hatches from the egg is waxy white or green, with a dark brown head and prothorax (area immediately behind the head). Larvae are about 1/2 inch long when full-grown and may be white, green, pink or yellow. The head and prothoracic shield are always dark brown. This is the stage that causes damage to host plants.
The pupa is about 1/3 inch long and spindle-shaped. It is usually found within a flimsy white silken cocoon. At first the pupa is white with green blotches but soon takes on a uniform brown color.
When the larva is full grown it usually leaves its host plant, leaf or tuber, to find a pupation site. In the field most larvae pupate in the soil. In storage they pupate, within their cocoons, in such situations as in and around the eyes of potato tubers, in crevices of boxes, storage bins, and on burlap sacks. The larval stage lasts 7 to 10 months under ideal conditions.
A generation of the potato tuberworm may be completed within a period of 12 days in midsummer but may require as long as 7 months during the cooler period of the year.
Infestations of tuberworms may increase in areas where cull potatoes are allowed to remain in the field following the harvesting of the spring crop. Volunteer potato plants and certain weeds also serve as sources of infestation for an outbreak of this insect, especially if a second crop of potatoes is grown on or near the same field.
Potato tuberworms gain entrance to the tubers near the surface of the ground by entering cracks in the soil. Tubers covered with at least 2 inches of soil are not subject to infestation. When tubers are exposed at harvest time they may become infested by larvae moving from the tops. Larvae move quickly from the dying potato plant and may crawl some distance to the tubers.
Potato tuberworms overwinter successfully in potatoes that are not subjected to freezing temperatures and consequently may develop in some areas in cull dumps and in storage houses and cellars. These serve as sources of early-season field infestation. They also winter successfully in seed and market potatoes held in storage and may develop through one or more generations during this period, depending upon the temperature.
Tuber-feeding larvae tunnel through the potatoes, filling tunnels with excrement and webbing on which disease organisms (fungi) grow. Such potatoes are unsightly and of little food value. A larva usually enters the tuber near an eye, covering the small entrance hole with a web and excrement. A pink coloration developing in the flesh surrounding the entrance and additional excrement make the presence of the larvae more evident within a few days. Galleries from 1 to 3 inches in length may be cut by a single larva, either just beneath the skin or deep in the flesh.
Potato tuberworm larvae mine Irish potato leaves, petioles, stems, etc. This type of damage can reduce yields. Growers who follow a rigid pesticide program in the field are seldom troubled with this pest. Always keep potato tuberworms under control, especially just prior to harvest.
Or use endosulfan (Thiodan, Phaser) 3 emulsifiable concentrate at 1 quart or 50% wettable powder at 1/2 pounds per acre. There is a one day waiting period from time of treatment to time of harvest. Two treatments 10 days apart just prior to harvest are suggested.
Any storage area should be thoroughly cleaned just prior to harvest, and 10 days before treat the area with malathion 57% emulsifiable concentrate at a rate of 1 pint to 3 gallons of water. This amount should treat 1500 to 2000 square feet of surface. This surface would include walls, floors and ceilings.
Where potatoes are used for seed, use Vapona resin strips (No-Pest Strips) either in addition to the residual spray or alone. Use according to directions for the control of flying insects.
Some growers have set up a light over a can or pail of water plus oil. This helps reduce the infestation and may be adequate for light infestations. Bacillus thuringiensis (B.t.), Pyrethrins and rotenone have registration approval for use on field growing Irish potatoes but not for use in storage.