Listronotus texanus Stockton Coleoptera Curculionidae

Natural History

Distribution. Carrot weevil is native to North America, and has been known as a pest since the mid-1800s. It is found throughout the United States, though it is a more serious pest in northern states. It is fairly widespread in Canada and continues to increase in geographic range and severity of damage. However, it is a serious pest principally in Ontario and Quebec.

Texas carrot weevil is also a native insect, though it likely occurs in Mexico as well. Only in recent years has it become a pest, principally in the Rio Grande Valley of south Texas, but it is also known from Louisiana.

Host Plants. These insects attack crop plants in the family Umbelliferae: carrot, celery, dill, parsley, and parsnip. Carrot weevil initially was known as a pest of parsley, and for many years as the "parsley stalk weevil." However, its apparent host range (as determined by recorded damage) was expanded to include carrot by 1925, and celery by 1936. Both umbelliferous and non-umbelliferous wild plants are suitable hosts, including wild carrot, Daucus carota, and wild parsnip, Pastinaca sativa (Umbelliferae); dock, Rumex spp. (Polygonaceae); and plantain, Plan-tago spp. (Plantaginaceae).

Natural Enemies. Several species of ground beetles, including Pterostichus melanarius Illiger, P. luca-blandus Say, Bembidion quadrimaculatum Say, Clivina fossor Linnaeus, and Anisodactylus santaecrucis Fabri-cius; (all Coleoptera: Carabidae) have been shown to feed on eggs, larvae, and pupae of carrot weevil in choice tests (Baines et al., 1990). Although some are voracious, Pterostichus spp. consuming 10-11 weevils per day, their importance in the field is uncertain. An egg parasitoid, Anaphes sordidatus (Girault) (Hyme-noptera: Mymaridae), inflicts considerable mortality, reaching nearly 50% parasitism and averaging 22% in Michigan (Collins and Grafius, 1986) and Ontario (Cormier et al., 1996), but not functioning in a density-dependent manner (Boivin, 1993). Anaphes sp. also attacks Texas carrot weevil in Texas, often achieving high levels of parasitism (60-90%) (Boivin et al., 1990). Entomopathogenic fungi, such as Beauveria bassiana and Metarhizium anisophliae, and the Listrono-tus strain of the nematode Steinernema carpocapsae (Nematoda: Steinernematidae) occasionally provide some suppression of carrot weevil.

Life Cycle and Description. The number of generations varies geographically, and with seasonal weather patterns. In New Jersey there are 2-3 carrot weevil generations per year (Pepper, 1942). There is considerable overlap of the generations, so all life stages may be found from May to October. Wright and Decker (1958) reported two generations from Illinois. In Massachusetts (Whitcomb, 1965), Ontario (Stevenson, 1976a), and Quebec (Boivin, 1988), there is usually only one carrot weevil generation per year, but some weevils complete a second generation if they first develop on weeds that are present early in the season. The period of time necessary for carrot weevil to complete development of the egg to adult stages is about 30-40 days, but another 10-17 days are required before eggs of the next generation are produced. Overwintering occurs in the adult stage, usually in grass sod or organic debris adjacent to crop fields. Reproductive diapause is regulated by both photoperiod and temperature (Stevenson and Boivin, 1990); for example, oviposition requires 14-16 h photophase at 20°C, but only 10-12 h at 28-30°C.

The biology of Texas carrot weevil is different because it inhabits a much warmer climate, where carrots are cultured during the winter months. All stages can be found throughout the year in southern Texas, though it is most abundant beginning in February when commercially grown carrots are abundant.

  1. The eggs of both species are oval, and measure about 0.8 mm long and 0.5 mm wide. Although initially light-yellow, the eggs turn brown or almost black. The eggs are deposited within cavities made by the females while feeding. She places 1-10 eggs per cavity, but the average clutch size is 3-4 eggs. The number of egg cavities per plant varies widely, with up to 100 eggs being found in some plants. The eggs are normally deposited on the interior or concave sides of leaf stalks. The female seals the opening with a black anal secretion after depositing her eggs. Eggs hatch in 5-16 days, with incubation time averaging about 11 days in the spring, but only 6-8 days in the summer months. Females deposit about 150-175 eggs during their life span.
  2. Upon hatching, larvae of both species tunnel inside the stalk, usually tunneling downwards. Some larvae exit the stalk, drop to the soil, and re-enter the plant at, or below, ground level. When plant stalks are small, larvae may move directly to the roots to feed. It may not be readily apparent that larvae are feeding in the crown or roots of plants. Four larval instars are reported by most authors, though Wright and Decker (1958) indicated five instars. Average duration of each instar is 4.1, 2.7, 3.4, and 9.2 days, respectively. Development time is affected by temperature, of course, and Simonet and Davenport (1981), and Stevenson (1986) gave temperature-related developmental data for carrot weevil. Mean head capsule width for the instars is 0.31, 0.44, 0.65, and 0.99 mm, respectively. Larvae are legless, white to pinkish brown, with yellowish brown heads. Larval development in carrot weevil requires 11-19 days, averaging 14.5 days in the spring, and about 13 days in the summer.

The developmental biology of Texas carrot weevil is similar. However, as might be expected for a species inhabiting a warmer environment, the developmental threshold is higher, 13.3°C vs. about 7°C in carrot weevil. Woodson and Edelson (1988) provided developmental data for Texas carrot weevil.

  1. The mature larva leaves the plant to construct a pupal chamber in the soil, usually at a depth of 3-15 cm. A prepupal period of 1-5 days precedes the pupal stage. The pupa is 5-8 mm long, creamy-white, and bears fairly prominent spines on each abdominal segment and elsewhere on the body. Duration of the pupa is 4-13 days, averaging 7-9 days. After transformation to the adult, the beetle remains in the pupal chamber for 1-5 days, and then digs to the soil surface.
  2. Newly emerged beetles are light brown, but gradually turn darker. The dark body is covered with tan scales. A faint-striped pattern sometimes is evident on the elytra. The adults of carrot weevil measure 3.5 — 7.0 mm long, averaging males 6.0 mm and females 6.5 mm. Texas carrot weevil averages slightly smaller, normally 4.0-4.7 mm long. The adults are quick to drop from the plant and feign death when disturbed. Although capable of flight, they rarely disperse in this manner, traveling principally by walking.

An excellent synopsis of carrot weevil biology was given by Pepper (1942), and additional useful information was provided by Whitcomb (1965) and Martel et al. (1976). Information on weevil rearing was provided by Martel et al. (1975). Texas carrot weevil was described by Stockton (1963) and its biology was given by Edelson (1985b, 1986). Boivin (1999) provided a recent summary of biology and management considerations for both species.

Damage

Both adults and larvae damage plants. Adult feeding takes two principal forms: cavities created by

Adult carrot weevil.

gnawing into the tissue (usually accompanied by ovi-position), and surface injury caused by removal of the epidermis. Females often deposit eggs on young plants, but are selective in their oviposition behavior. For example, Boivin (1988) reported that carrots were not attacked until they had attained the four-leaf stage. Larval feeding is much more injurious than adult feeding, and results from larvae burrowing through the base of the stalks or roots. On carrots, larval damage generally occurs on the upper one-third of the root, and there usually is only one weevil larva per plant. The tunneling is usually very pronounced. Fungi often invade roots that are heavily damaged by weevils. In untreated carrot ields located in areas where carrot weevils are historically damaging, crop losses of 70% may occur. In commercial ields, where insecticides are applied, losses of 10% are not uncommon. Thus, carrot weevil is considered a serious pest in some locations.

Management

  1. In Quebec, day-degree accumulations above a base threshold temperature of 7°C have been used to estimate timing of oviposition (Boivin 1988; 1999). Carrot-baited traps also can be used to monitor adult activity (Boivin 1985; Ghidiu and Van Vranken, 1995), though Edelson (1985b) reported that carrot-baited traps were not very effective for Texas carrot weevil. Perron (1971) compared several sampling methods and determined that root sampling was preferable to light traps, color-based sticky traps, and sweep nets for population estimation.
  2. Formerly, persistent liquid or granular insecticides were applied to the furrow at planting to kill adults and larvae before they could inflict serious injury. Persistent materials are no longer readily available, so multiple applications of foliar insecticides are usually used to suppress carrot-weevil adults. Insecticide must be applied before oviposition has commenced, so timing is important. In Canada, where a single generation is the rule, 1-2 well-timed applications may be adequate. Where multiple or overlapping generations occur, more sprays are applied. A technology that has not been exploited in recent years is the use of poison baits for adult suppression. However, Pepper (1942), reported good control with dried apple pomace-based baits.

In Texas, Texas carrot weevil control may require several insecticide applications for effective suppression. Late-season applications are most important, and late planting of the crop along with late-season application affords the most economic approach to chemical suppression (Woodson et al., 1989).

Biological Control. Entomopathogenic nema-todes have been evaluated for management of carrot weevil under laboratory and field plot conditions. The steinernematid nematodes Steinernema carpocapsae and S. bïbionis were more effective than the hetero-rhabditid Heterorhabditis heliothidis, and larvae were more susceptible than pupae or adults (Belair and Boivin, 1985; Boivin and Belair, 1989). In research conducted on organic soils in Quebec, application of S. carpocapsae as a soil drench or in conjunction with a bait provided moderate levels of damage suppression (Belair and Boivin, 1995).

Cultural Practices. In areas where there is only one generation per year, damage can be reduced by delaying planting until most oviposition has occurred. Organic soils tend to have more severe problems.

Sanitation and crop rotation help alleviate carrot weevil problems. Carrots left unharvested, for example, make very suitable early season oviposition sites for overwintering beetles. Crop rotation, while often recommended because beetles seem to have limited powers of flight, is often impractical because carrot and celery production tends to be concentrated in relatively small areas of organic soil, and it is difficult to achieve the necessary isolation.

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