Phyllotreta ramosa Crotch Coleoptera Chrysomelidae

Natural history

Distribution. Striped flea beetle, Phyllotreta striolata (Fabricius), is common in Europe and Asia, and gained access to North America before 1700 (Bain and LeSage, 1998). It is now widely distributed in Canada and the United States; however, it is not abundant in the Rocky Mountain region and west coast. It is well known from Canada's Prairie Provinces, where crucifer oil-seed crops are grown extensively, but is not as abundant, or damaging, as crucifer flea beetle, Phyllotreta cruciferae (Goeze). In the United States, striped flea beetle is most common in the northeast and midwest.

Western striped flea beetle, Phyllotreta ramosa (Crotch), apparently is native to North America. Its distribution is restricted to the western United States, principally Washington, Oregon, and California. A report of this insect from British Columbia is suspect.

Host Plants. Striped flea beetle is a crucifer-feed-ing species, and is often called the striped cabbage flea beetle. As is the case with other Cruciferae-feeding insects, it will also feed on mustard oil-containing plants from the families Capparidaceae and Tropaeola-ceae. It commonly attacks such vegetables as Brussels sprouts, cabbage, cauliflower, collards, kale, mustard, radish, rutabaga, turnip, and watercress. It is reported to have attacked non-cruciferous garden vegetables, but these are erroneous, with the insects actually feeding on cruciferous weeds found amongst the vegetables. Weeds serving as suitable hosts include black mustard, Brassica nigra; flixweed, Descurainia sophia; pepperweed, Lepidium densiflorum; shepherdspurse, Capsella bursa-pastoris; yellow rocket, Barbarea vulgaris; and other common cruciferous weeds. Information on host preference was provided by Feeny et al. (1970) and Tahvanainen (1983). Western striped flea beetle has similar host preferences.

Natural Enemies. A parasitoid, Microtonus vittatae Muesebeck (Hymenoptera: Braconidae) attacks the adults of several crucifer-feeding flea beetles, including striped flea beetle. The biology of this parasite was given by Wylie (1982, 1984) and Wylie and Loan (1984). General predators such as the shield bug, Podisus maculiventris (Hemiptera: Pentatomidae) also sometimes feed on beetles (Culliney, 1986).

Life Cycle and Description. In Canada, striped flea beetle has only one generation per year, though a complete life cycle requires only about one month. Overwintering occurs in the adult stage in the soil and leaf litter (Burgess and Spurr, 1984; Turnock et al., 1987). Adults become active in early spring, which is March in the southern states, April in the midwest and southern Canada, and as late as June in northern Canada. In Saskatchewan, abundance of overwintering adults is high through mid- to late-June, with summer generation beetles becoming abundant in late July. Similar population trends have been observed in New York. Beetles are more active fliers early in the season, and fly at greater heights (1-2 m) than later in the season (Lamb, 1983). They become active about two weeks earlier than crucifer flea beetle, with which they commonly coexist. A second generation is reported from New York and North Carolina, but this is uncertain. The life cycle of western striped flea beetle is unknown, but likely is quite similar to that of striped flea beetle.

  1. Soon after their emergence, beetles begin to deposit eggs in the soil adjacent to host plants. In Manitoba, egg-laying females were collected from April until early August. Eggs hatch in about five days.
  2. There are three instars. The larvae measure about 1.25 mm long at hatching, but eventually attain a length of about 4.9 mm. They are white or yellowish white, with a brownish head and anal plate. The larvae bear an anal proleg.
  3. Pupation occurs in the soil within a small cell formed from soil particles. Pupae resemble the adults in form but lack fully developed wings.
  4. Adults are about 2.0-2.4 mm long. The hind femora are enlarged. The head and pronotum are black, and the elytra are brownish black, each with a slight metallic luster. The elytra usually bear an irregular yellow band running nearly the length of each elytron. However, the band is sometimes discontinuous, resulting in two yellow spots on each elytron. The proportion of the population with the aforementioned discontinuous banding pattern is higher in southern portions of the beetle's range. The adults emerge and are very numerous in July and August in northern states.

Striped flea beetle and western striped flea beetle are very similar in appearance. Males can be easily differentiated, however, by the size of the fifth antennal segment. In western striped flea beetle, antennal segments four to six are nearly the same size. In striped flea beetle, the fifth segment is wider and longer than adjacent segments, nearly twice the length of the sixth segment. The other common striped flea beetle that may get confused with these species is Zimmermann's flea beetle. However, male P. zimmermanni have their ifth antennal segment even more enlarged, about three times the length of the sixth segment.

Information on striped flea beetle biology was given by Dugas (1938), Burgess (1977), and Wylie (1979). Rearing techniques were provided by Burgess and Wiens (1976). Information on western striped flea beetle can be found in Chittenden (1927) and Smith (1985).

Damage

Striped flea beetle damages both the above-ground and below-ground portions of crucifers. Adults are damaging to seedlings in the spring, but even mature plants can be severely damaged when beetles are particularly abundant. Dugas (1938), for example, reported that the foliage from an entire ield of mature turnips in Louisiana was so damaged that it appeared

Adult striped flea beetle.
Phyllotreta Striolata
Antennae of some Phyllotreta flea beetles: (left to right) western striped, striped, Zimmermann's.

burned. Although the beetles produce only small holes in the foliage, when excessive feeding occurs the adjacent tissue dies, leading to a bronzed or burned appearance. Even modest levels of feeding reduce the value of foliage crops such as mustard if the plants are nearing maturity. Beetles will sometimes feed on the young florets of broccoli, greatly reducing yield.

Larvae feed on both rootlets and the principal portions of the plant root. Larvae often feed along the surface of the main root, leaving shallow trenches on the root surface. On plants where the root is harvested, such as turnip, larval feeding destroys the crop's marketability. Even in crops where the root is not harvested, such as cabbage, flea beetle larvae can be very damaging, because their root pruning activities stunts the plants and may cause seedling death.

Management

These insects are very similar to crucifer flea beetle, and often coexist simultaneously. The management considerations discussed under crucifer flea beetle are also applicable to the striped and western striped flea beetles.

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