Bruchus pisorum Linnaeus Coleoptera Bruchidae

Natural History

Distribution. Pea weevil was first observed near Philadelphia, Pennsylvania in the 1740s, and then noted in nearby states in the 1750s. It had spread across the country to Washington and Oregon in the 1890s. The origin of pea weevil is uncertain, but peas are not native to North America, and the beetles were likely introduced along with the crop. The origin of peas is believed to be the mountainous middle-eastern region from Ethiopia to Afghanistan, which may be the ultimate source of these insects, but they likely arrived via Europe. At present, pea weevil is distributed throughout the United States and southern Canada, and other temperate areas of the world including Asia, Europe, North Africa, and southern Australia.

Host Plants. Pea weevil larvae have a very restricted host range, feeding only on peas. Other legumes, including weeds, will not support growth and reproduction of this insect. Adults, particularly females, feed on pollen. If pea pollen is not available, weevils feed on pollen from Canada goldenrod, Soli-dago canadensis; yarrow, Achillea millefolium; dogfennel, Anthemis cotula; field mustard, Brassica campestris; grasses, and many other plants.

Natural Enemies. Several native parasitoids were found parasitizing eggs or larvae of pea weevil, including Uscana sp. (Hymenoptera: Trichogrammati-dae), Dinarmus laticeps (Ashmead), Microdontomerus anthonomi (Crawford), Anisopteromalus sp., and Eupter-omalus leguminis (Gahan) (all Hymenoptera: Pteroma-lidae), and Eupelmus amicus Girault (Hymenoptera: Eupelmidae). However, none were observed to be particularly effective in suppressing pea weevil abundance (Larson et al, 1938; Annis and O'Keeffe, 1987). Therefore, attempts were made to introduce parasi-toids from Europe. Despite repeated attempts at introduction in several areas of the United States and Canada, and short-term persistence of parasitoids, no significant benefit has accrued in pea weevil management (Clausen, 1978).

In addition to hymenopteran parasitoids, a few other natural enemies have been noted. A mite, Ato-mus sp. (Acari: Erythraeidae), was observed attacking pea weevil eggs in Idaho. Predators such as rove beetles (Coleoptera: Staphylinidae) and birds also feed on weevils, but the weevil's protected feeding site reduces the ability of general predators to access the larvae.

Life Cycle and Description. There is usually only a single generation annually, with weevils overwintering in the adult stage. A few beetles in Oregon and Idaho were observed producing eggs in the autumn; some eggs were fertile and resulted in a complete second generation. Beetles invade pea fields in the spring and deposit eggs on green pods. The weevil completes its life cycle in the pea, the adults emerging in the summer or autumn to seek overwintering habitat. The mean (range) duration of the egg-adult developmental period in Idaho is about 57 (46-71) days.

  1. The egg is oval, measuring about 1.5 mm long and 0.6 mm wide. The eggs, which are yellowish to orange, are deposited singly or in small clusters of about two on the surface of the pea pod. Sometimes 30 or more eggs may be found on a single pod. The egg normally hatches in 8-9 days, with a range of 5 -14 days.
  2. The young larva, on hatching, bores directly from the egg into the pod at the point of egg attachment. Occasionally, larvae construct mines in the tissue of the pea pod before entering. At this stage, the larva bears six thoracic legs and moves freely and also possesses a spiny thoracic plate. Both types of structures enable the larva to easily penetrate the pod and gain access to the pea seed. However, after burrowing into a pea seed, the whitish larva molts and loses these structures, assuming a legless form that is typical of weevil larvae. Duration of the larval stage is about 30-50 days, during which time it grows from about 1.5 mm to 6-7 mm long. There are four instars, with mean head capsule widths of 0.14, 0.35, 0.64, and

Pea weevil larva.

  1. 91 mm, respectively. Duration of each instar is about 12, 6-8, 12-14 and 10-14 days for instars 1-4, respectively. At larval maturity the insect has consumed most, or all, of the seed's contents, leaving a very thin layer of the seed coat in one area of the seed. It is through this thin area that the beetle will exit as an adult. More than one beetle larva may enter a pea seed, but only one survives to emerge.
  2. After completing its larval development, the weevil pupates within the seed. The pupa closely resembles the adult in form and size, except that it is cream-colored. Duration of the pupal stage is about 15 days (range 8-27 days).
  3. The adult normally chews through the seed coat and escapes from the pea seed immediately after completing its development. Under cool and dry conditions, however, the beetle may remain within the seed during the winter and emerge in the spring. The beetle does not attack dry seeds, so it cannot multiply in stored seed. It may persist for 18-24 months within peas, so it is readily transported with dried peas. The adult weevil is brownish, but bears patches of white, gray, and black. There is a triangular white patch on the prothorax, and white patches form an irregular band beyond the middle of the elytra. The antennae and legs are black. The beetle is oval in shape and measures about 4.5-6.0 mm long and 2.0 mm wide. Most beetles seek shelter under bark of trees,
Bruchus Pisorum

or within crevices of fence posts, bales of hay, and organic debris on the soil surface. Pea weevil can tolerate winter temperatures of —16° to — 19°C, if not protected by snow cover or other shelter. When overwintering in protected locations, however, temperature as low as —23°C is tolerated. Beetles become active at the time peas begin to bloom. Flights into fields occur when the temperature exceeds 21°C, and may continue for a period of two months, usually in May and June. Beetles usually alight on reaching a pea field, so the field edges are most heavily infested. Adults do not feed on the pea plant except to consume pollen. The female must ingest pollen to develop a normal complement of eggs, and usually feeds for 4-5 days on pea pollen before beginning oviposition. She may deposit up to 50 eggs per day with an average fecundity of about 460 eggs (range 235-740 eggs). Few eggs are deposited at temperature below 21 °C, and maximum production occurs above 27°C. Beetles take flight readily, and also feign death, when disturbed.

Pea weevil is easily confused with broadbean weevil, Bruchus rufimanus Boheman, as adults are very similar in appearance. In pea weevil, the terminal abdominal segment, when viewed from above, bears a pair of distinct black spots; in broadbean weevil these spots are lacking or poorly defined. In pea weevil, the posterior femora are equipped with sharp spines, and in broadbean weevil spines are absent from the hind femora or blunt.

A complete summary of pea weevil life history, based on work in the Pacific Northwest, was given by Larson et al. (1938). Brindley et al. (1946) provided a few additional observations. Information of physical ecology was given by Smith (1992).

Damage

Damage to peas is caused by larvae feeding within the pea seeds. Each larva destroys only a single pea,

Bruchus Pisorum

but contamination of peas by even a few larvae makes peas highly undesirable for human consumption. Although a lesser problem when grown for livestock food, pea weevil infestation lowers the weight and value of peas fed to livestock. Peas damaged by weevils display reduced germination potential, so they are undesirable for seed.

Management

  1. Sampling protocols for pea weevil eggs have been developed, including sequential sampling methods (Smith and Hepworth, 1992). Weevils selectively oviposit on the most exposed pods, and plants along field edges are more heavily infested. Normally, pods are picked and examined visually for eggs. Because the eggs are small and difficult to detect, the presence and absence are usually tabulated rather than attempting to count the eggs. Adults are best sampled by sweeping peas with a sweep net.
  2. The principal form of crop protection is foliar insecticides applied when the crop is in bloom. Growers sometimes treat only the perimeter (about 50 m) of the field with insecticide because beetles typically land at field margins where they encounter a lethal dose of insecticide, but this is only appropriate for large fields. Persistent insecticides are more effective (Horne and Bailey, 1991). Fumigation of infested peas destined for seed is also recommended.

Cultural Practices. Several cultural techniques help to reduce incidence of pea weevil infestation. Destruction of crop residues that may contain weevils is perhaps the most important consideration. Infested seed should not be planted unless it is fumigated to kill overwintering beetles. Careful harvesting of peas is important, because if pea pods are shattered during harvest, a great number of seeds containing weevils may remain on the soil surface and produce beetles in the spring. It is helpful to plow under the residue of crops containing pea weevils, but they must be buried to a depth of at least 20 cm to provide marked reduction in emergence. Shattered peas also may germinate and produce crops of volunteer peas. Such volunteer plants, if not destroyed, may produce large numbers of insects.

Peas are also sometimes harvested for hay, and if they contain fairly mature pods the beetles contained within may serve as a source of inoculum. Thus, pea hay should be harvested while pods are very small, or fed to livestock soon after harvest, if peas for human consumption are also grown nearby.

Early planting, especially when accompanied by early harvesting, is desirable for pea weevil suppres sion. Such crops show a lower incidence of infestation, and larvae in early-maturing seeds are more juvenile, and thus more likely to be killed by the harvesting operation.

Host-Plant Resistance. Resistance is not well developed among commercially acceptable cultivars of pea. However, several sources of genetic resistance, which function mostly in deterring oviposition, have been identified for breeding purposes (Pesho et al., 1977).

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Responses

  • jali
    What are the pea weevils natural predators?
    8 years ago
  • kifle zula
    When was the peeweevil first observed?
    5 years ago

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