Phthorimaea operculella Zeller Lepidoptera Gelechiidae

Natural History

Distribution. Potato tuberworm occurs widely in the United States, but normally is absent the northernmost states and Canada. It is most common in southern areas, particularly California and the southeastern states north to Maryland. Potato tuberworm appears to be native to North America, but is now spread throughout the world. Areas with warm, dry climates such as southern Europe, northern and southern Africa, India, Australia, and Central and South America all experience problems with potato tuberworm. The transport of tubers infested with insects causes extensive dissemination of this pest, and also results in occurrence records where this insect does not exist permanently.

Host Plants. This insect feeds almost entirely on members of the plant family Solanaceae. Vegetable crops supporting potato tuberworm include eggplant, pepper, potato, and tomato, though potato is the only frequent host. Tobacco is occasionally affected, and potato tuberworm is sometimes called "tobacco split-worm" when it is associated with this host. Solanac-eous weeds such as bittersweet, Solanum dulcamara; black nightshade, S. nigrum; groundcherry, Physalis spp.; henbane, Hyoscyamus sp.; horsenettle, S. carolinense; jimson weed, Datura stramonium; and matrimony vine, Lycium europaeum; also serve as hosts. A world-wide host list was provided by Das and Raman (1994).

Natural Enemies. Natural enemies affect the egg, larval, and pupal stages of potato tuberworm, though they are much more effective when the tuberworms are feeding on the aerial portions of the plant rather than within tubers. Among the parasitoids known to affect potato tuberworm are numerous species of Braconidae, Encyrtidae, Eulophidae, Ichneumonidae, Mymaridae, Pteromalidae, Scelionidae, and Tricho-grammatidae (all Hymenoptera). Reported to be the most abundant in Virginia were Bracon gelichiae Ash-mead (Braconidae) and Campoplex sp. (Ichneumonidae) (Hofmaster, 1949). In California, Apanteles dignus Muesebeck (Braconidae) was the dominant parasitoid (Oatman and Platner, 1989), though in an earlier report by the same authors (Oatman and Plat-ner, 1974), Agathis gibbosa (Say), A. scutellaris Muese-beck, and Campoplex phthorimaeae (Cushman) (all Hymenoptera: Braconidae) were most common. Many of the parasitoids of tuberworm were discussed and pictured by Graf (1917). There have been several attempted introductions of parasitoids to North America, but with few successes (Clausen, 1978).

Other natural enemies are less important. Several general predators have been noted to feed on tuber-worm, including the ants Pheidole and Lasius spp. (Hymenoptera: Formicidae), pirate bugs (Hemiptera: Anthocoridae), shield bugs (Hemiptera: Pentatomi-dae), and rove beetles (Coleoptera: Staphylinidae). Diseases have been noted (Briese and Mende, 1981; Trivedi and Rajagopal, 1992), but seem to be of little natural significance.

Weather. Weather is thought to affect the abundance of potato tuberworm. Summers that are unusually warm and dry favor increase in tuberworm populations (Langford and Cory, 1932; Hofmaster, 1949).

Life Cycle and Description. A life cycle may be completed in 15-90 days, resulting in about five generations annually in both California and Virginia; in other locations around the world the number of generations is reported to range from 2 to 13 annually. In warm climates, the generations overlap and cannot be distinguished easily. Potato tuberworm normally cannot withstand freezing, so in cold climates overwintering survival by larvae is poor except within potatoes in storage or in cull piles.

  1. The eggs are deposited singly or in poorly defined clusters, usually on the underside of leaves. If deposited on potatoes in storage, however, the egg clusters tend to be larger, up to 30 eggs. Tubers tend to be heavily infested if they are exposed, that is, not covered with soil. There also are reports of oviposition on soil adjacent to plants (Traynier, 1975). The egg is elliptical, and measures about 0.48 mm long and 0.36 mm wide. Initially white in color, they turn yellow and acquire a distinct iridescence with age. Duration of the egg stage is only about five days during the summer but may reach 30 days during cool weather.
  2. At hatching, larvae normally begin to burrow almost immediately. Larvae normally mine the leaves, but occasionally the petioles and stems, and sometimes burrow into tubers. The older or lower leaves are preferred. Larvae often plug the entrance to their burrow with excrement, but extrude the cast skins and head capsules. Sometimes considerable amounts of silk are produced by larvae, usually when they are forced to traverse the leaf surface, but also to plug larval burrows and to web together leaves. There are four larval instars. Mean head capsule widths are about 0.20, 0.36, 0.60, and 1.13 mm for instars 1-4, respectively. Body lengths are about 1.1, 2.0, 4.5, and 7.0 mm, respectively. Mean duration (range) of the instars is about 3.5 (2-6), 2.5 (2-3), 3.1 (2-4), and 7.3 (5-12) days, respectively. Initially white in color with a black head and thoracic plate, the larva acquires additional color as it grows. In the mature larva, the head, thoracic plate and thoracic legs are black. The body is principally white, with pink or greenish-pink dorsally. There are five pairs of prolegs. The anal plate is yellow. Duration of the larval period may require only 14 days during the summer, but up to 70 days during the winter months.
  3. Pupation occurs in the soil, or just beneath the epidermis of the leaf or tuber. Before pupation, the larva spins a silk cocoon that is usually covered with leaf trash, fecal material, soil particles, and other debris. Initially white or yellow, the pupa eventually becomes dark mahogany. The form of the pupa is typical of Lepidoptera, wider at the anterior end, tapering

Potato tuberworm larvae.

Potato tuberworm pupa.

Potato tuberworm pupa.

to a point at the posterior end, and with the partially developed wings twisted ventrally. The tip of the abdomen bears a hook and a circle of spines. It measures about 6 mm long. Mean duration (range) of the pupal period is 11.6 days (range 8-14 days).

Adult. The adult stage is a small grayish-brown moth with a wingspan of 12-16 mm. The wings, especially the hind wings, are fringed. The front wings are marked with dark spots, which usually coalesce to form a dark longitudinal streak or a row of dark spots. The wings, abdomen, and legs also are tinged with yellow scales. The moths are nocturnal, hiding during the day beneath debris and clods of soil. Mating occurs within 2 days of moth emergence. Oviposition is usually completed in 6-17 days, with females each producing about 150-250 eggs. Longevity rarely extends beyond 21 days. A sex pheromone has been identified and can be used for trapping under field conditions (Persoons et al., 1976).

Potato tuberworm is easily confused with eggplant leafminer, Tildenia inconspicuella (Murtfeldt). These species are similar in appearance and have overlapping host range. Potato tuberworm moths are usually larger, with yellow scaling more distinct and forming longitudinal streaks, but accurate differentiation is best accomplished by an authority. Poos and Peters (1927) presented differences in the genitalia of adults, and discussed procedures to distinguish the other stages. Eggplant leafminer does not attack potato or tobacco though it shares eggplant and horsenettle with potato tuberworm. On eggplant and horsenettle, mines of potato tuberworm begin at the midrib or one of the principal veins, whereas mines of eggplant leaf-miner begin near the leaf margin.

Excellent treatment of potato tuberworm biology was given by Graf (1917) and Poos and Peters (1927). The reports of Clarke (1901) and Hofmaster (1949) also

Phthorimaea Operculella Zeller
Adult potato tuberworm.
Phthorimaea Operculella Pupa
Adult potato tuberworm.

were useful. A brief world-wide review of this insect was published by Trivedi and Rajagopal (1992). Culture of potato tuberworm was described by Platner and Oatman (1968).

Damage

Leaf mining is the most common habit of potato tuberworm, but mining of the tuber is the most damaging. Mining normally is restricted to the foliage so long as it is green and succulent. Larvae may also mine the stems, usually working downward. If the tuber is attacked, the mining may occur near the epidermis, or "skin" of the tuber, or larvae may burrow deeply. The tunnels in potato tubers normally fill with fungus (Graf, 1917). Tunneling not only destroys the food quality of the tubers, but also the sprouting potential of tubers that are used for propagation. In tomato, foliage is initially attacked, but larvae can mine through the fruit stem into the fruit (Gilboa and Podoler, 1995).

Management

  1. Pheromone traps are effective for monitoring potato tuberworm populations, and usually there is a good correlation between trap catches and damage levels (Shelton and Wyman, 1979b; Yathom et al., 1979). Pheromone-baited water pan traps are more effective than pheromone-baited sticky traps (Bacon et al., 1976), though funnel traps seem to be as effective as water traps (Raman, 1988). Sticky traps are prone to be covered with dust, thereby reducing catch (Kennedy, 1975). Larval sampling was discussed by Horne (1993), and a binomial sequential sampling plan for tuberworm in tomato was developed by Gilboa and Podoler (1995).
  2. Tuberworm often is controlled by application of insecticide to foliage (Bacon, 1960), though in some parts of the world resistance to insecticides is a problem (Collantes et al., 1986). Also, insecticides interfere with predators and parasitoids of tuberworm, which can be quite effective, so it is prudent to determine that tuberworm is present in potentially damaging numbers before implementing an insecticide-based management effort (Shelton et al., 1981). Integration of chemical insecticides with cultural practices is effective (Fuglie et al., 1993). Biological insecticides, particularly the bacterium Bacillus thuringiensis, are recommended for protection of potato tubers in storage, but not usually in the field. Suppression in the field is possible, but several applications may be required (Broza and Sneh, 1994). A granulosis virus has been used experimentally as a suppressive bioinsecticide under field conditions (Kroschel et al., 1996).

Cultural Practices. Cultural practices can greatly affect susceptibility of potato to potato tuberworm. Overwintering population tend to be low, with tuber-worm populations increasing through the year. Thus, areas where more than one potato crop are cultivated tend to experience greater loss by tuberworm, and greatest damage occurs late in the season. In some regions, potato production is limited to the spring months to eliminate the nearly year-long availability of potatoes for tuberworm breeding.

Sanitation is extremely important in potato tuber-worm management. Potatoes held in storage or in cull piles are potential sources of infestation. Similarly, potatoes left in the field, volunteer plants, and solanac-eous weeds can support tuberworms. Harvested potatoes should not be left in the field overnight as this is when oviposition occurs.

If vines are killed before senescence and tubers harvested soon thereafter, the level of tuber infestation is low. Delayed harvest increases the exposure of tubers to ovipositing moths. Infestation of tubers is especially likely if there are cracks in the soil, allowing access by tuberworm. Soil depths of 5 cm or more protect tubers from infestation. Sandy soil can also be a problem if rainfall washes away soil, exposing tubers. Irrigation practices greatly affect soil condition, with furrow irrigation producing more cracks than overhead irrigation. Frequent irrigation helps to prevent soil cracking. Hilling of the soil, wherein soil is scraped from between the rows and deposited at the base of the plants, helps to deny access by tuberworm to tubers (Langford, 1933; Shelton and Wyman, 1979a,b; Von Arx et al., 1990). Deep planting of potato seed, and culture of varieties that do not produce shallow tubers, also reduce incidence of tuberworm damage.

Some differences in tuber susceptibility or suitability exist among cultivars (Fenemore, 1980). Oviposi-tion preference, percent pupation, and moth fecundity are affected, but the significance of preference diminishes when moths are confronted with a no-choice situation.

The sex pheromone can be used to manipulate populations. Mass trapping can be used to reduce damage in the field. Trapping, and disruption of mating by saturation of the atmosphere and confusion of the moths, works best for potatoes in storage (Raman, 1988).

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