Phyllotreta armoraciae Koch Coleoptera Chrysomelidae

Natural History

Distribution. Horseradish flea beetle is an introduced species, probably from Europe. It was first found in North America in 1893 in the vicinity of Chicago, Illinois. It now is widely distributed in the northern United States and southern Canada, but principally east of the Rocky Mountains. It also has been collected from Oregon and Idaho, but in these locations it seems not to be a serious pest.

Host Plants. The adults have been collected from several cruciferous plants, but damage by adults and feeding by larvae is largely confined to horseradish. Other plants from which adults have been collected are radish, turnip, and marsh yellow cress, Rorippa islandica. The latter species occurs commonly in moist habitats, and it is considered a preferred host.

Natural Enemies. This insect is poorly studied and its natural enemies undetermined.

Life Cycle and Description. There is a single generation per year. The adult is the overwintering stage. Development from egg to the adult stage requires about 75-90 days.

Egg. The eggs are deposited from April or May until early August in Wisconsin. They are most often deposited on the petioles of young leaves, often where the petiole meets the root. Some eggs are also deposited on or in the soil. Eggs are elliptical in shape and orange. The average length is 0.57 mm (range 0.430.84 mm), and the width is 0.33 mm (range 0.260.47 mm). The eggs are usually deposited in small clusters, and individual eggs are sometimes aligned side by side. They are not securely attached to the plant. Egg production has not been exhaustively studied, but over 400 eggs have been produced by a single female. Eggs hatch in 7-14 days.

Horseradish flea beetle egg.

Horseradish flea beetle larva.

Horseradish flea beetle larva.

  1. Upon hatching, the larva is whitish with a brown head and anal plate, and measures about 1.3 mm long. The larva becomes yellowish white as it grows, eventually attaining a length of about 4.8 mm. Larvae feed in the petioles and midribs of leaves and occasionally in the roots. This mining may cause the death of the foliage. Its development requires 50-60 days.
  2. At maturity, the larva leaves the plant tissue and descends to the soil. There is a subterranean pre-pupal period of 2-7 days followed by pupation. The pupa is white and is the same size as the adult beetle. Pupation usually lasts 10-13 days.
  3. The adults are fairly distinctive, relative to other Phyllotreta spp. Newly emerged beetles begin appearing in July. The blackish beetles bear broad, straw-yellow or cream-colored stripes, one on each elytron. The stripes are considerably wider than those of other striped species, resulting in a relatively narrow dark line down the center of the back. The hind femora are enlarged. The legs are yellowish red distally, and darkened basally. The antennae lack segments that are greatly expanded, and are colored brown except for the basal 3-4 segments, which are lighter or yellowish. The beetles measure 2.5-3.4 mm long and average slightly larger than most other Phyllotreta spp. The adults seem to be long lived, being captured during nearly all months of the year. The
Horseradish flea beetle pupa.
Phyllotreta Spp
Adult horseradish flea beetle.

winter is passed by the adults in dry and sheltered locations.

The biology of horseradish flea beetle was given by Chittenden and Howard (1917).

Damage

The larvae mine the petioles and large veins of leaves, and occasionally enter roots to feed. Young leaves may wilt and die when severely mined. They feed on foliage also, sometimes making numerous small holes, but also making pits in the thicker tissue. Damage to foliage results in uneven growth and decreased root yield. Adults tend to be more damaging early in the season, and larvae later in the season.

Management

Crops should be monitored early in the year for feeding injury, because adult feeding damage often suggests additional larval damage later in the season. Crop rotation is recommended for horseradish flea beetle control, but this is impractical for home-garden production of horseradish.

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