Trichobaris trinotata Say Coleoptera Curculionidae

Natural History

Distribution. Potato stalk borer is found widely in eastern North America, west to the Rocky Mountains. It is considered a native insect, though it also occurs in Mexico.

Host Plants. This insect feeds only on plants in the family Solanaceae. As suggested by the common name, it feeds principally on potato. However, it can also affect eggplant. The favored weed host is horse-nettle, Solanum carolinense; but it also feeds on buffalo bur, Solanum rostratum; black nightshade, S. nigrum; and several species of groundcherry, Physalis spp. Other early records of feeding on crops and weeds are suspect, and likely due to misidentification of the insect.

Natural Enemies. The natural enemies of this poorly studied insect seem to be few in number. The most important natural enemy is Nealiolus curculionis (Fitch) (Hymenoptera: Braconidae), but Eurytoma tylo-dermatis Ashmead (Hymenoptera: Euytomidae) is also recorded from potato stalk borer.

Life Cycle and Description. There is only one generation per year throughout the range of this insect. Adults overwinter in reproductive diapause. In the spring, usually during May, adults become active and mate, with oviposition commencing in May or June. Larvae are present in early summer, and following a brief pupal period, new adults appear, usually in July or August.

  1. The adults usually deposit eggs within cavities in the stems and petioles, though some eggs are deposited in axils of terminal leaves. Usually only a single egg is found in each cavity, but sometimes two eggs occur at the same site. The egg is white, oval in form, and measures about 0.65 mm long and 0.39 mm wide. Mean duration of the egg stage is 7.4 days at 24°C (range 6-11 days).
  2. The larvae are whitish to yellowish, with a pale brown head. The larva is quite elongate in form, and attains a length of 9-11 mm. There are 5-6 instars. Mean duration is about 4.1, 4.4, 6.8, 8.4,18.4, and 16.4
Trichobaris Trinotata
Potato stalk borer weevil larva.

days for instars 1-6, respectively. Total duration of the larval stage is 20-84 days for larvae with five instars, and 33-106 days for larvae with six instars. The larval stage lacks legs, but bears deep thoracic ridges which aid in mobility. Larvae feed within the plant, primarily within stem tissue. Prior to pupation, the larva burrows outward to the epidermis of the plant, but leaves a thin covering; this forms the point of adult escape from the plant. A quiescent period of 2-6 days precedes the molt to the pupal stage.

  1. Pupation occurs in the plant, within a pupal case consisting of plant particles. The pupa is oval in form, greatly resembling the adult except for poorly developed wings. It is white or yellow initially, but become dark within the final few days of this stage. Mean duration of the pupal stage is 12.1 days at 24°C (range 8-18 days).
  2. The adult is a small ash-gray weevil, with males measuring about 4.2 mm long and 1.6 mm wide, and females 4.5 mm long and 1.7 mm wide. Close examination reveals that the true color of the weevil body is black, but its surface is covered with minute gray scales. This is most apparent at the head, which lacks scales, and therefore is black. There also are three black spots at the base of the elytra—one dorsally at the juncture of the elytra and two laterally. These three spots are the basis for the species name, trinotata. The

Potato stalk borer weevil pupa.

Potato stalk borer weevil pupa.

Potato Stalk Borer Scientific Name
Adult potato stalk borer weevil.

rostrum (beak) curves strongly at the base, resulting in a downward orientation in the beak. Unlike the body form of some weevils, the body of the beetle is not strongly curved. Adults mate in the spring, and ovi-position commences just one day after mating. The female chews a hole to the depth of the rostrum and then oviposits within the cavity. The hole is sealed with a clear fluid secreted by the female.

This insect and its biology were described by Faville and Parrott (1899), Chittenden (1902), and Cuda and Burke (1985, 1986). A key to the species of Trichobaris was published by Barber (1935).

Damage

Once considered a serious pest, current agronomic practices have relegated this insect to insignificant status over most of its range. The principal form of damage is larval burrowing. Larvae gradually move from the point of hatching downward through the stem. As the larva grows it produces a correspondingly larger tunnel, causing greater injury. At some point the larva reverses direction, feeding upward again, and enlarging the tunnel. Length of the burrow may reach 30 cm. Larvae may also burrow into the roots. Adults also feed in the spring before oviposition, but apparently not in the autumn. The damage by adults consists of irregular holes eaten in the leaves.

Management

It is difficult to detect these insect, because they dwell within the stems of potato and weeds. Also, the beetles are quite small, and drop readily if the plants are disturbed. Sanitation is effective at eliminating this insect as a pest. The most important practice is to destroy the potato vines, as this is the overwintering site of weevils. Secondly, because the weevils develop on, and overwinter within, solanaceous weeds, it is useful to destroy these plants. Late-planted crops are less injured by weevils than early plantings due to the early activities of adults in the spring. If insecticides are used, they should be applied to the foliage as the plants emerge.

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