Diabrotica undecimpunctata Mannerheim Coleoptera Chrysomelidae

Natural History

Distribution. Spotted cucumber beetle is also known as "southern corn rootworm," which is an indication of geographic distribution as well as an alternate favored-food plant. It is found throughout the United States, though two races are recognized. In the Rocky Mountain region and westward a variant designated as Diabrotica undecimpunctata undecimpunc-tata Mannerheim is recognized, and called the western spotted cucumber beetle. The eastern form of the spotted cucumber beetle is found east of the Rocky mountains, including eastern Canada as far west as Alberta, and is designated as Diabrotica undecimpunctata howardi Barber. Spotted cucumber beetle is also found in Mexico, and consists mostly of the eastern race.

Elsey (1988) studied population dynamics of spotted cucumber beetle, and also striped cucumber beetle, Acalymma vittatum (Fabricius), and banded cucumber beetles, Diabrotica balteata LeConte, affecting cucurbits in South Carolina. His data suggested that spotted cucumber beetle was the numerically dominant species during the two years of the study, though striped cucumber beetle was also quite numerous. This appears to be consistent with most observations from the southern areas of the United States, except for southern Florida and Texas, where the banded species predominates. In the northern areas of the United States and southern Canada, striped cucumber beetle is generally dominant. Spotted cucumber beetle is highly dispersive, overwintering in southern states and dispersing northward annually.

Host Plants. This insect has a very wide host range. More than 200 plants are known to be fed upon. Beetles fly considerable distances regularly in search of suitable food. Adults are especially fond of pollen and flower petals. Arant (1929) reported that though the adult will feed on the flowers of almost any plant, they preferred cucurbits, legumes, tomato, ornamental plants, and fruit crops. He also provided a long list of plants on which he had observed leaf damage, including not only the aforementioned susceptible crops but cabbage, corn, lettuce, mustard, pea, potato, and turnip. Michelbacher et al. (1943) presented a similar list for the western race of the species, including similar relative preferences. Artichoke is also sometimes damaged by western spotted cucumber beetle in California (Lange, 1941). Elsey (1988) indicated that zucchini squash was more attractive to beetles than was cucumber. The flower candytuft, Umbellata sp., was noted as especially attractive in Alabama and wild cucumber, Micrampelis oregona, was favored in the Pacific Northwest. Adults also feed readily on sweet substances: fruits and aphid honeydew. The larvae, however, are largely restricted to feeding on the roots of cucurbits, legumes, and grains (including corn) and grasses. Winter cover crops and weeds, especially broadleaf species, often are attractive to ovipositing females (Brust and House, 1990; Buntin et al., 1994).

Natural Enemies. Several natural enemies of spotted cucumber beetles are known, the most important being Celatoria diabroticae (Shimer) (Diptera: Tachinidae). This fly deposits her larva within the abdomen of the adult beetle, which soon perishes. In Alabama, parasitism was highest during the late winter. Workers in Alabama, Arkansas, California, and Oregon have documented levels of parasitism reaching about 30%, but generally they discounted the importance of the parasitoid for regulating populations of cucumber beetles. Fronk's (1950) study in Virginia showed an average seasonal parasitism rate of only 3.7%. Rockwood and Chamberlin (1943), however, argued that the value of the parasites may be underestimated. Cucumber beetles also are fed upon by many bird species (Arant, 1929), and attacked by mermithid nematodes and the fungus Beauveria (Rockwood and Chamberlin, 1943). Fronk (1950) reported his inding several nematode species parasitizing rootworms in Virginia, with Howardula benigna (Cobb) (Nematoda: Allantonematidae) most abundant. He observed a seasonal average parasitism rate of 23.6%, and reported that Howardula-infected female beetles had poorly developed ovaries. Poinar (1979) reported experimental infection of southern corn root-worm by the mermithid nematode Filipjevimermis leip-sandra Poinar and Welch. Various steinernematid and heterorhabditid spp. undoubtedly attack larvae in some locations.

Weather. Environmental conditions, and particularly soil moisture, have been implicated as affecting the abundance of spotted cucumber beetle. Workers in Arkansas (Isely, 1929), Alabama (Arant, 1929), and California (Smith and Michelbacher, 1949) all noted that high soil moisture was a requirement for larval survival. Smith and Michelbacher (1949) further noted that in dry years it was only the clay soils or low spots that successfully produced beetles, whereas during moist years sandy soils were more suitable.

Life Cycle and Description. The spotted cucumber beetle normally completes its life cycle in 4-6 weeks. The number of generations per year seems unusually variable, possibly because there is no true diapause in these insects and they continue to reproduce as long as the weather allows, and because they annually re-invade northern areas. However, Elsey (1988) suggested that spotted cucumber beetle populations may have a short (30-60 days) reproductive diapause. Often there are two generations per year, but Rockwood and Chamberlain (1943) reported a single generation in Oregon, and Arant (1929) and Michelbacher et al. (1955) indicated three in Alabama and southern California, respectively. In Iowa, Sweetman (1926) reported one generation but Drake and Harris

(1926) claimed two per year. Because the generations overlap considerably, and adults can be found continuously during warm weather, field observations could lead to an underestimate of generation number. The adult is the overwintering stage. Overwintering beetles hide in plant debris and become active when the temperature reaches about 15-20°C. In the mild weather areas of the Northwest, aggregation of beetles in protected locations or "caches" may occur during the winter, but this is the exception, not the rule. Smith and Allen (1932) indicated that beetles overwinter only in areas with mild winters, principally the southern states, dispersing northward annually. Further, they noted that northward dispersants in the spring could consist of a mixture of insects that had dispersed southward during the preceding autumn, and new adults produced at southern latitudes.

  1. Normally, about 40 eggs are produced at each oviposition. Chalfant and Mitchell (1967) demonstrated that beetles prefer coarse soil or soil with cracks to fine, smooth soil; they also prefer wet over dry soil for oviposition. A female deposits, on average, six egg clusters during her life, for a total of 200-300 eggs. All values related to egg production are quite variable, with the values decreasing as the beetles progress from generation one, to two, to three. The yellow egg is oval, and measures about 0.7 mm long and 0.5 mm wide. Egg incubation time varies with temperature, from as little as a week to up to a month.
  2. The three instars generally require seven, five, and four days, respectively, or about 14-24 days for complete larval development. The larva grows from about 1.8 mm long at hatching to about 12 mm long at maturity. Head capsule widths measure 0.3, 0.4, and 0.6 mm, respectively. The larva is yellowish white, with the head, thoracic, and anal plates colored brown. The mature larva constructs a small chamber in the soil and pupates within.
  3. The pupa is white initially, but turns yellow with age. It measures about 7.5 mm long and 4.5 mm wide. The tip of the abdomen bears a pair of stout spines, and smaller spines are found dorsally on the remaining abdominal segments. Duration of pupation is about seven days (range 4-10 days). After comple-

Spotted cucumber beetle larva.

Pupa Diabrotica
Spotted cucumber beetle pupa.

tion of the pupal period the beetle digs to the soil surface.

Adult. The adult is long-lived. A life span of 60 days is common during the summer, but longevity is extended considerably, up to 200 days during the winter. The adult is about 6.0-7.5 mm long. It is generally colored yellowish green with 12 black spots on the elytra and a black head. The western form of spotted cucumber beetle has black legs and abdomen, whereas the eastern form has a pale abdomen and legs partly pale. Adults usually do not begin oviposition until two to three weeks after emergence. (See color figure 110.)

Arant (1929) provided the most complete life history study. Mendoza and Peters (1964) provided characters to distinguish larvae of D. undecimpunctata from western corn rootworm, D. virgifera LeConte, and northern corn rootworm, D. barberi Smith and Lawrence, but Krysan (1986) observed that some specimens cannot be distinguished with certainty using this key. Techniques for rearing southern corn root-worm are summarized by Jackson (1986). Biosyste-matics of Diabrotica spp. is given by Krysan and Branson (1983).

Adult spotted cucumber beetle.

Spotted cucumber beetle larva.

Adult spotted cucumber beetle.

Damage

The adults eat small holes in the leaves and flower petals of many plants. If provided a choice, vegetable crops rather than grain crops are usually attacked by adults. Fruit of smooth-skinned melons is susceptible to damage, especially before the skin hardens. Brewer et al. (1987) studied the effects of adult feeding on seedlings of zucchini squash in Louisiana. Densities of 10 beetles per plant caused significant levels of plant mortality, but their effect diminished greatly once the seedlings attained the two or three leaf stage. While beetle damage may result in complete defoliation, sometimes the feeding is limited to the pollen or fruit. Because beetles often avoid heat, the shaded portions of the fruit may be especially damaged.

Beetles also transmit cucumber mosaic virus and muskmelon necrotic spot virus (Gergerich et al., 1986), and bacterial wilt of cucurbits and of corn. Transmission of cucurbit bacterial wilt by spotted cucumber beetles is a secondary, late-season spread, so these insects are considered less important vectors than striped cucumber beetles (DaCosta and Jones, 1971).

Larvae feed on the roots of plants and also bore into the base of the stems. Root pruning may result in only a discolored and stunted plant, whereas when larvae burrow into the stem they may cause the plant's death. Latin and Reed (1985) suggested that larval feeding could increase the incidence and severity of Fusarium wilt. Some injury to the surface, or rind, of fruit may occur as larvae burrow up from the soil and feed on fruit in contact with soil, and larvae are sometimes called "rindworms." Grain seedlings, particularly corn, are often damaged by larval feeding. Older plants are less susceptible to death following feeding.

Management

  1. A sex pheromone has been identified (Guss et al., 1983), and may prove useful as a component of monitoring or suppression tactics. Traps that take advantage of the adult orientation to plant kairo-mones and the yellow color have been evaluated alone and in combination (Hoffman et al., 1996a). Although there is some evidence for improved catches, the responses are weak relative to some other diabroticite beetles.
  2. Planting time application of granular insecticides, applied in a band over the row, is often recommended for protection of plant roots if a ield is expected to be heavily infested by larvae. Foliar and blossom injury is prevented by timely application of insecticides, especially to the seedling stage, because they are more susceptible to injury. There is risk of foliar insecticides interfering with pollination in insect pollinated crops such as cucurbits, though this is not a consideration in wind pollinated crops such as corn. Insecticide applications made late in the day minimize damage to pollinators. Waiting at least one day after insecticide treatment before introducing beehives is also advisable. Control of beetles helps reduce spread of cucurbit wilt, but the concern over this disease diminishes as the plants mature.

Feeding stimulants and lures have considerable potential for increasing both the effectiveness and selectivity of insecticide applications. Cucurbitacins are arrestants and feeding stimulants for spotted cucumber beetle, though they have low volatility and therefore are not valuable as lures. Volatile components of blossoms, attractive to beetles, have been identified and evaluated as lures. Various mixtures attract beetles, but phenyl acetaldehyde was found more attractive to spotted cucumber beetle than to the other common squash blossom-infesting Diabrotica beetles. Cinnamaldehyde alone or in a mixture with trimethoxybenzene and indole also were very attractive kairomones, and have been used experimentally in the field to capture beetles. These lures are especially good for detecting beetles when they are at low densities, and otherwise difficult to locate. Combinations of volatile attractants, cucurbitacins, and low rates of insecticides have been applied to corn cob grits to make lethal baits (Metcalf et al., 1987; Lampman and Metcalf, 1987; Deheer and Tallamy, 1991; Metcalf and Metcalf, 1992).

Cultural Practices. The need for insecticides can be avoided somewhat if the timing of planting can be manipulated so that the beetles have already dispersed and deposited most of their eggs before planting. For example, Luginbill (1922a) recommended safe planting periods of April 20-May 1 for north Florida and south Georgia, May 1-May 10 for central Georgia and the southern portion of South Carolina, and May 10-May 20 for the northern regions of Georgia and South Carolina and for North Carolina.

As there is such a wide range of suitable hosts and the insects are so mobile, crop rotation is usually not considered to be a practical management approach. There are some crops, however, such as cotton, that seem to be immune to injury. Arant (1929) documented problems in corn following winter legumes, because overwintering beetles were attracted to the legumes in the spring, and deposited eggs. When corn was planted immediately after legumes, the larvae caused severe injury. Grass weeds growing in a ield immediately before planting may also predispose a crop to injury, so clean cultivation is important. Buntin et al. (1994) studied the effects of winter cover crops on subsequent injury. Broadleaf cover crops, especially hairy vetch, induced greater insect problems than grain crops such as wheat. Fallow, weed-free ields had a low incidence of problems.

Soil moisture conditions affect the insects and their damage. Crops grown in moist soil, due either to a high water table or abnormally heavy rainfall, are more likely to be damaged by larvae, probably due to enhanced larval survival. In contrast, water-stressed, wilted cucurbit plants are more attractive to beetles, and experience more foliar damage than turgid plants.

For small plantings, screens or covers provide some protection from adults.

Trap crops have some benefit for cucumber beetle control. Pair (1997) reported that small plantings of an attractive squash crop that were treated with systemic insecticide could suppress early-season populations on nearby cantaloupe, squash, and watermelon.

Host-Plant Resistance. There are some varietal differences among cucurbits with respect to susceptibility to injury. Bitter cultivars, which have a higher cucurbitacin content, were reported to be more attractive and injured (Haynes and Jones, 1975). Consequently, non-bitter cultivars have less bacterial wilt disease (DaCosta and Jones, 1971).

Biological Control. The nematodes Steinernema glaseri and S. carpocapsae (both Nematoda: Steiner-nematidae) will attack both the larval and adult stages of spotted cucumber beetle (Poinar, 1979). Their use has not been perfected, however.

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