Conoderus amplicollis Gyllenhal Coleoptera Elateridae

Natural History

Distribution. Several species of Conoderus damage vegetables, especially in southeastern states and in California. Their origin is uncertain, but southern potato wireworm and Gulf wireworm also occur in South America and Asia, so they likely were introduced. Southern potato wireworm and Gulf wireworm were first detected damaging crops in Georgia and Alabama, respectively, in 1927, and by 1955 they had spread along the South Atlantic and Gulf Coast from North Carolina to eastern Texas. Southern potato wireworm was first detected in California in 1963, and Gulf wireworm in 1938. Tobacco wireworm is known from many eastern states and provinces, but in the south it occurs as far west as Arizona. Among other Conoderus spp., C. exsul (Sharp) is sometimes damaging in California and other southwestern states, and C. rudis (Brown) and C. scissus (Schaeffer) are occasional pests in the southeast (Seal et al., 1992c; Seal and Chalfant, 1994).

Host Plants. Vegetable crops reported to be damaged under field conditions include the below-ground portions of cantaloupe, beet, cabbage, carrot, celery, corn, cowpea, mustard, potato, sweet potato, tomato, and turnip. Sweet potato is the most severely injured crop. Other crops injured are gladiolus, peanut, strawberry, and tobacco. In studies conducted in South Carolina, grain seeds such as oat, sorghum, and wheat readily supported larval growth, and seeds from non-grasses such as cowpea, lima bean, and Amaranthus supported fair survival and growth. Potato, sweet potato, and carrot also supported moderate survival and growth (Day et al., 1971).

Natural Enemies. Day et al. (1971) reported that the fungus Metarhizium anisopliae was commonly found in dying southern potato wireworm larvae, and that an unspecified nematode was a common parasitoid. Also, birds frequently follow tractors as fields are plowed, and feed upon exposed wireworms. There is no data, however, to suggest that any of these mortality factors are significant. Larvae of southern potato wireworm, and undoubtedly other species, are susceptible to infection by Steinernema carpocapsae (Nematoda: Steinernematidae) under experimental conditions (Poinar, 1979).

Life Cycle and Description. Generally there is one generation of southern potato, tobacco, and Gulf wireworm annually in the southeastern states. Most insects overwinter as partly grown larvae and pupate in March-May. This results in adults that oviposit throughout the summer and produce overwintering larvae. A small proportion of the southern wireworm larvae complete their development in the summer and overwinter as adults, and the adults also may produce eggs early enough that larvae do not enter diapause but go on to complete a second generation. In southern Florida, a third generation is possible. The following is a description of southern potato wireworm, but the other Conoderus spp. are similar.

  1. The eggs are spherical in shape, smooth in texture, and white in color. They measure 0.30.4 mm in diameter. When first deposited the eggs are coated with a sticky secretion, causing soil particles to adhere as the material dries. Eggs are deposited near the surface, usually in the upper one centimeter. Day et al. (1971) reported that beetles deposit an average of about 36 eggs, with a range of 22-63, but this level of fecundity seems low. Mean duration of the egg stage is about 8-20 days at 23-25°C and 7-9 days at 27-30°C.
  2. The larvae, also called wireworms, initially are whitish, but as they mature they become cream or yellowish, with the head, pronotum, and terminal abdominal segment becoming reddish orange. The number of instars is thought to be 3-4. Mature larvae measure about 17 mm long and 2 mm wide, and are smooth, shiny, and cylindrical. The caudal plate at the tip of the abdomen bears a small notch. This plate is also marked by a pair of setae near the notch, and another pair near the anterior margin of the plate. The characteristics of the caudal plate are useful diagnostic characters. Larvae are found in various grassy and weedy fields but not in heavily wooded areas. They also are infrequent in fields that previously lacked grass and broadleaf weed cover. In addition to providing larval food, plants prevent soil temperatures from becoming lethally high, and provide hiding places for adults. Larvae are principally located near the soil surface; about three-fourths are found in the upper 5 cm, and most of the rest in the 5-10 cm range. Development time of larvae in spring and early summer is typically 40-70 days, with a range of about 3090 days. However, most larvae developing in August, and all larvae developing in September and October, overwinter in the larval stage and do not pupate until the following spring. Thus, larval development time in the overwintering larvae is 200 to over 300 days.

The shape of the notch in the caudal plate of the larvae can be used to distinguish among the common species. In C. falli, the notch is closed or nearly so, whereas in C. amplicollis and C. vespertinus the notch is open and V-shaped. In C. amplicollis the notch is relatively shallow, the depth being equal to approximately one-third the distance from the notch to the pair of setae on the dorsal plate. In contrast, the notch in C. vespertinus is relatively deep, being approximately equal to the distance from the notch to the pair of setae on the dorsal plate. Rabb (1963) gave a key to several Conoderus larvae.

  1. When larvae have completed their development they twist and roll in the soil to form a pupal cell. The cell is usually within the upper 10 cm of soil. Pupae are white to yellowish, with the head and thorax becoming tan just before the emergence of the adult. In form, the pupae greatly resemble the adults except that the elytra are reduced in size and twisted ventrally. The pupae tend to be slightly larger than the adults. Duration of the pupal stage is about six days at 29°C and 18 days at 19°C.
  2. The adults, called click beetles, are elongate oval, and measure 6-9 mm long and 2-3 mm wide. They are uniformly dark, ranging from light to dark-brown, except that the legs are tan. The elytra are marked with numerous parallel channels, and are moderately shiny. The adults are nocturnal, and are found hiding under organic material during the daylight. The pre-oviposition period of adults is reported to be 12-30 days, and are capable of oviposition from March through September; beetles lay few eggs in the autumn or winter.

The adults of tobacco wireworm measure 7-10 mm long. The pronotum and elytra are yellowish brown, with a pair of longitudinal dark bands on the prono-tum and an irregular dark band dorsally on the elytra. Adults of Gulf wireworm measure 8-9 mm long. They are dark brown with yellowish legs and antennae.

The biology of southern potato wireworm was given by Day et al. (1971), tobacco wireworm by Rabb (1963), and Gulf wireworm by Cockerham and Deen (1936) and Stone and Wilcox (1979). The ecology and management of wireworms were reviewed by Thomas (1940) and Glen et al. (1943). Chalfant and Seal (1991) provided an excellent review of wireworm problems on sweet potato. A key to distinguish Conoderus from the other common vegetable-attacking genera of wireworms can be found in Appendix A. Keys for the identification of the genera of adult Elateridae can be found in Arnett (1968), and of larvae in Becker and Dogger (1991).

Tobacco wireworm larva.

Adult tobacco wireworm.

Elateridae Larva
Terminal abdominal segments (dorsal view) of some Conoderus wire-worm larvae: (left to right) southern potato, Gulf, tobacco.

Damage

The larvae, called wireworms, feed principally on seeds, roots, stems, and tubers of several plants. They seem to prefer plant parts that are high in starch content, and sometimes attack fruit where it comes into contact with soil. Larvae often scar the surface of potato and carrot, which causes slight loss in quality, but may also tunnel into these vegetables, causing serious loss. However, they can be cannibalistic and predatory on other soil-dwelling species such as root-worms (Coleoptera: Chrysomelidae). The adult stage of wireworms causes little injury.

Wireworms sometimes cause considerable damage to irrigation systems. Polyethylene and some other plastic materials may have holes completely eaten through the tubing, causing loss of pressure and uneven watering. Thick-walled tubing and harder, high-density tubing are more resistant to damage (Ota, 1973).

Management

Sampling. The adults are readily attracted to blacklight traps (Day and Reid, 1969; Stone and Wilcox, 1983). This attraction is useful for population assessment, but it is not adequate for effective suppression (Day et al., 1973). Adults also can be captured on sticky traps, but the population trends ascertained by light and sticky traps are not completely equivalent (Genung, 1972). Larvae can be separated from the soil by sifting; a screen with 6-7 meshes per centimeter is desirable. Soil sampling followed by screening is accurate but labor intensive means to predict wireworm problems before planting. Spatial distribution of larvae is variable, but sometimes aggregated, requiring extensive sampling (Seal et al., 1992c).

Considerable work has been done to develop baits for wireworm sampling. Growers can assess the population densities of wireworm larvae before planting in the spring (Jansson and Lecrone, 1989; Seal et al., 1992a). Baiting is accomplished by burying whole wheat, corn, potato, sweet potato, carrot, or other attractive food sources in the soil at a depth of 1015 cm and counting the number of wireworm larvae attracted. Soaking the bait for one day increases its attractiveness. Baiting does not work well if the soil is cold or dry; when soil is cold a sheet of transparent plastic is sometimes placed over the section of soil-containing bait as this warms the soil and increases the mobility of wireworm larvae, allowing more accurate counts (Bynum and Archer, 1987). Wire-worm counts of one or more per bait station suggest that wireworms perhaps will be a problem and insecticides may be used. Simmons et al. (1998) compared several methods of sampling and reported that corn/wheat baits were the most accurate, precise, and cost effective.

Insecticides. Insecticide is often applied to prevent susceptible crops from being injured by wireworms, principally because unless preplant sampling is done carefully, considerable damage may occur before the infestation is detected. The most common approach to wireworm suppression is to apply persistent liquid or granular formulations at planting, and to incorporate the insecticide into the soil near the seeds or seedlings. Preplant applications generally, but not always, are more effective than postplant treatments (Linduska, 1979; Chalfant et al., 1992b). Sometimes insecticides are applied broadcast over the entire field, but more commonly they are applied in bands over or along the rows. For long-duration crops, the plantingtime application may be followed by a mid-season application. Insecticide also may be incorporated into the irrigation water (Chalfant et al., 1992a). Effectiveness varies considerably among insecticides (Day and Crosby, 1972), and some insecticide resistance has been reported.

Cultural Practices. The type and temporal occurrence of weeds and cover crops can affect subsequent damage by wireworms. Grass cover crops such as sorghum-sudan are highly attractive to Conoderus spp., and it is important to have the fields free from such grasses during peak flights (Jansson and Lecrone, 1991). Similarly, the rotational sequence of vegetable, or vegetable and field crops, affects wireworm abundance and species composition (Seal et al., 1992b). Native grass sod is often heavily infested with wire-worm larvae, and damage to vegetables might be high in years immediately following conversion of sod to crop land. On the other hand, legumes are generally poor hosts for wireworms, and little damage follows these crops.

Field flooding causes destruction of wireworms in Florida (Genung, 1970). A six-week period of flooding is needed; alternatively, two four-week periods separated by a two-week period of drying are effective.

Host-Plant Resistance. Resistance to wireworms in commercially available crop cultivars varies among crops. Cuthbert and Davis (1970) reported high levels of resistance among sweet potato cultivars, and Wiseman et al. (1976) demonstrated some resistance among field and sweet corn varieties. However, most crops remain susceptible to injury. Interesting work conducted in South Carolina by Schalk et al. (1992) demonstrated that interplanting a resistant variety of sweet potato within a susceptible variety would reduce injury by wireworms and other insects to the susceptible cultivar.

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