Western Potato Flea Beetle Epitrix subcrinita LeConte Coleoptera Chrysomelidae

Natural History

Distribution. Potato flea beetle occurs widely in North America, east of the Rocky Mountains. In the United States, it is found from South Carolina to Colorado, and all northward states. It is considered as a pest mostly in northern states. In Canada, potato flea beetle is found from Nova Scotia to Alberta. It is destructive as far west as Manitoba, but is considered to be a severe pest in the Maritime Provinces.

Western potato flea beetle occurs on the Pacific coast from northern California to British Columbia, and in the intermountain area of the United States. In the intermountain area, the western portions of Montana, Wyoming, Colorado, and New Mexico support western flea beetle, as well as northern Arizona and Nevada, and all of Utah and Idaho.

Host Plants. Potato flea beetle feeds principally on plants in the family Solanaceae. Vegetable crops attacked by both larvae and adults include eggplant, potato, tomato, and pepper. Other plants reported to be fed upon occasionally by adults include alfalfa, bean, beet, cantaloupe, celery, cucumber, lettuce, pumpkin, radish, squash, sunflower, tobacco, and turnip. Solanaceous weeds such as jimsonweed, Datura stramonium; ground cherry, Physalis pubescens; horsenettle, Solanum carolinense; and black nightshade, Solanum nigrum are also suitable hosts, and non-sola-naceous plants such as redroot pigweed, Amaranthus retroflexus; kochia, Kochia scoparia; and lambsquarters, Chenopodium album may be fed upon. Western potato flea beetle apparently has the same host range.

Natural Enemies. An allantonematid nematode, probably Howardula sp., was reported from potato flea beetle in Washington, as was occasional infection by a fungus (Hanson 1933). In Ontario, Loan (1967b) recovered Microctonus epitricis (Viereck) (Hymenop-tera: Braconidae) from potato flea beetle. The importance of these biotic agents in potato flea beetle control has not been determined.

Life Cycle and Description. Potato flea beetle has only one generation per year in northern locations such as Washington and all of Canada. However, two generations occur in southern portions of the potato flea beetle's range such as Kentucky and Delaware. A generation can be completed in 30-50 days. Adults overwinter at the soil surface beneath plant litter and undergrowth, or in the soil to depths of 15-25 cm, often in or near potato ields. In the spring, after feeding on newly emerged crop plants or weeds, female beetles deposit eggs in the soil at the base of host plants.

In Colorado, the overwintering beetles are active from late May until August, with eggs produced by the overwintering individuals present from June until August, larvae from June until September, and pupae from July until September. The adults produced from this summer generation are present from late July until October. Thus, with the two overlapping broods of adults, beetles are present throughout the summer months. There is functionally only one generation per year in Colorado, but a small portion of the beetles produce eggs and initiate a second generation (Hoer-ner and Gillette, 1928).

  1. The eggs of potato flea beetle are elliptical, and pearly white in color. They measure about 0.2 mm wide and 0.5 mm long. They are usually buried by the beetles in the soil at a depth of 12-20 mm, but occasionally deeper, sometimes to a depth of 50 mm. They are laid singly, but several eggs may be placed in the vicinity of a single plant. Females produce, on average about 100 eggs during their life span, but some produce more than 200 eggs. Females may produce eggs over a period of about two months. Moisture is an important criterion for egg deposition and hatching. Females seek moist areas for egg deposition, and lack of moisture retards hatching. Temperature also influences hatching, with embryonic development being completed in 6-8 days during warm weather, but 11-12 days during cool weather.
  2. The larvae are about 1 mm long and white when they first hatch from the egg. As they mature, the head and thoracic plate and to a lesser degree the anal plate, become brown, while the remainder of the body remains pale. The larva eventually attains a length of about 4-5 mm. The terminal abdominal segment bears an anal proleg. Larvae normally complete their development in about 20-25 days, but the larval development period may range from 13-45 days, depending on weather.
  3. The larvae pupate in the soil after preparing a small pupal chamber from soil particles. Pupation occurs near the food plant, close to the area of feeding. The pupa measures about 1.5-2.0 mm long and initially it is white, but eventually darkens. Pupal development requires about 6-10 days (range 3-22 days).
  4. The adult potato flea beetle is fairly light in color after emerging. However, by the time it has dug to the soil surface it is fully darkened and cannot readily be distinguished from individuals that have long been emerged, including those that have overwintered. Adults can be found feeding until the plants have been killed by frost. Beetles measure 1.5-2.0 mm in length and are black. The antennae and legs are orange, except that a portion of the fore and middle femora are black, and the hind femora are completely black. The hind femora are enlarged. The pronotum is marked with fine punctures, the features of which can be used to distinguish this insect from the closely related tuber flea beetle, Epitrix tuberis Gentner. The fine punctures in the center of the pronotum of potato flea beetle are separated by a distance exceeding the diameter of the punctures. In contrast, the punctures at the center of the pronotum of tuber flea beetle are relatively coarse, and are separated by a distance less than the diameter of the punctures (Gentner, 1943). The pronotum of tuber flea beetle also has a deep transverse depression near the hind margin. Both the pronotum and elytra bear numerous fine hairs, but their density is less than with the similar eggplant flea beetle (see eggplant flea beetle for comparative description).

Western potato flea beetle has been poorly studied, but its biology in Washington seems similar to that of potato flea beetle (Hanson, 1933). It seems to be two-brooded in Oregon. It is similar in appearance to potato flea beetle, but is bronze rather than black, and lacks the transverse depression on the prothorax.

A good account of potato flea beetle was given by Johannsen (1913), based on the work conducted in Maine. However, a more comprehensive treatment of biology, including rearing methods, was given by Hoerner and Gillette (1928) from Colorado. This and most other life-history research from the western United States was conducted before a systematic revision of the Epitrix flea beetles resulted in recognition of tuber flea beetle as a separate species (Gentner, 1944). Thus, some studies may contain mixed populations, and despite Gentner's attempt to identify the true species involved in previous research on "potato flea beetle,'' the biology of potato flea beetle remains somewhat uncertain. The work of Hill and Tate (1942) in Nebraska is particularly suspect, because tuber flea beetle appears to be present in this area. Beirne (1971) and Campbell et al. (1989) provided a useful perspective on potato flea beetle in Canada.

Epitrix TuberisEpitrix Subcrinita
Adult western potato flea beetle.

Damage

The damage by adult potato flea beetles is typical flea beetle injury. Small pits are eaten in either the upper or lower surface of the foliage. Although the beetles may not eat completely through the foliage, the remaining epidermis soon dries and dies. Dead leaf tissue eventually falls away, leaving a hole. Leaves on heavily infested plants may be riddled with holes, although the veins usually remain. Small plants may be killed. Once potato plants become established, however, they can withstand considerable flea beetle feeding. Senanayake et al. (1993) estimated that densities of 100 beetles per plant would not depress potato yield of healthy plants. If potatoes were previously stressed by Colorado potato beetle feeding, then densities as low as 5-25 flea beetles per plant would depress yield. In eastern Canada, flea beetle populations regularly reduce potato yield by 15% unless corrective actions are taken. Damage by adults is greater under hot and dry conditions.

The larvae are root feeders, but occasionally cause considerable damage by feeding on potato tubers. By feeding on the tuber surface, larvae produce tracks or scars that materially damage the commercial value of the potato tubers. Occasionally they burrow directly into the tuber to a depth of up to 3 cm. The pits eventually are filled with corky material that blackens. Although tuber feeding is noted wherever potato flea beetle occurs, it is not as serious a problem as it is with tuber flea beetle. In the case of potato flea beetle, adult damage is the predominant form of damage.

Potato flea beetle has been implicated in transmission of several plant diseases. Tuber feeding also enhanced infection of tubers by Rhizoctonia and potato scab fungi (Hanson, 1933). Potato flea beetle also has been shown to be capable of transmitting ring-

rot bacteria (Christie et al., 1993), and probably many other diseases.

Damage to plants by western potato flea beetle is similar to potato flea beetle damage, although much less frequent. Also, western potato flea beetle larvae are less prone to feed on the tubers of potatoes, feeding instead primarily on the fibrous roots.

Management

Sampling. Senanayake and Holliday (1988) compared various sampling procedures for potato flea beetle monitoring. Use of a sweep net was efficient early in the season, but declined as plants matured. Visual samples were not highly satisfactory because beetles were often feeding on the underside of foliage, and difficult to observe. Whole plant bag sampling was determined unwieldy for routine sampling despite its high degree of precision. Stewart and Thompson (1989) determined the spatial distribution of potato flea beetles on potato and recommended that 10-20 plants be sampled when beetle densities were four per plant or higher, but that sample size be increased to 50-60 plants when beetle densities were only one per plant.

Because the beetles are small, mobile, and often feed on the underside of foliage, plant damage may be a better indicator of insect density than beetle counts. The relationship of feeding puncture abundance to yield has been studied, but owing to variability in plant response, the damage threshold remains uncertain (Howard et al., 1994).

Insecticides. Foliar applications are made for suppression of adults, usually 1-2 weeks after adults first appear. Commonly, several applications are necessary to protect young plants from overwintering beetles, and the first or spring generation adults, due to protracted emergence. Granular formulations are applied at planting or after, if there is reason to expect damage by larvae. Systemic insecticides have been applied at planting, using various formulations; although some systemic treatments provide excellent seedling protection against both adults and larvae, phyotoxicity is a potential hazard (Chalfant et al., 1979b). Insecticide resistance has been detectable since the 1950s (Kring, 1958), but populations remain manageable (Ritcey et al., 1982). In many areas, excellent control of flea beetles occurs as part of Colorado potato beetle chemical suppression. As more selective management techniques are developed for Colorado potato beetle, flea beetle problems may be increased.

Cultural Practices. Glandular trichomes deter feeding by potato flea beetle (Tingey and Sinden, 1982), but this character is not yet incorporated into horticulturally acceptable varieties.

Row covers can prevent attack of plants by adult flea beetles, and subsequent damage by larvae, if the crop is grown on land not previously infested by potato flea beetle. If the beetles were abundant in the previous season, however, the adults may be overwintering in the soil and can emerge under the row cover. Thus, there is considerable benefit from crop rotation with a non-solanaceous crop.

Destruction of solanaceous weeds is beneficial. Because adults do not fly readily, planting of crops at some distance from previous crops or weed hosts is considered beneficial. Wolfenbarger (1940), working in New York, studied the distribution of potato flea beetle injury in potato fields with respect to weedy, uncultivated areas. Damage to tubers was greatest within 25-30 m of uncultivated areas containing sola-naceous weeds. Further, the number of beetles overwintering in soil of uncultivated areas was estimated about 470,000 per hectare, whereas it was only 55,000 per hectare in potato fields.

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