Distribution. Crucifer flea beetle was first found in North America in 1921, in British Columbia. It moved steadily eastward, and was recovered from Canada's Prairie Provinces in the 1940s, and Ontario and Quebec by 1954. A second introduction on the east coast is also possible, as this insect was found in Pennsylvania in 1943 and was well established in Delaware by 1951 (Westdal and Romanow, 1972). It is now widely distributed in southern Canada and most of the northern United States. Crucifer flea beetle is common in prairie and other open environments, and is rare in forested areas (Burgess, 1982). Crucifer flea beetle also is found in Europe, Africa, and Asia. The expansion of acreage planted to rape seed both in Europe and North America has facilitated the spread of this insect and enhanced its status as a crop pest.
Host Plants. Crucifer flea beetle feeds principally on plants in the family Cruciferae, though other plant families containing mustard oils (Capparidaceae, Tro-paeolaceae, Limnanthaceae) have been reported to be attacked (Feeny et al., 1970). Broccoli, Brussels sprouts, cabbage, cauliflower, Chinese cabbage, horseradish, kohlrabi, radish, rutabaga, and turnip are the vegetable crops commonly damaged by this flea beetle. Preference among crucifer crops is sometimes reported, but the preferences are rarely consistent owing to changes in leaf age and leaf type (Palaniswamy and Lamb, 1992). Crucifer flea beetle also damages rape, and sweet alyssum as well as several cruciferous weeds such as tansymustard, Descurainia sp.; wild mustard, Brassica kaber; stinkweed, Thlasi arvense; pep-perweed, Lepidium densiflorum; yellow rocket, Barbarea vulgaris; and hoary cress, Cardaria draba. Reports of this insect feeding on non-cruciferous crop plants such as beets apparently stems from misidentification; these insects are easily confused with related species.
Natural Enemies. There are few effective natural enemies of crucifer flea beetle in North America. A parasitoid, Microctonus vittatae Muesebeck (Hymenop-tera: Braconidae), attacks adult P. cruciferae, placing about two-thirds of its eggs in the host's head. Although several eggs may be deposited in each beetle, only a single wasp survives, emerging 16-19 days after parasitism (Wylie and Loan, 1984). The level of parasitism by M. vittatae may be 30-50% (Wylie, 1982), but crucifer flea beetle is less preferred than striped flea beetle, Phyllophaga striolata (Fabricius), for oviposition (Wylie, 1984). Parasitized-flea beetles emerge earlier from overwintering sites than unparasi-tized beetles (Wylie, 1982). Parasitism by nematodes, particularly by the allantonematid Howardula sp., is generally low, and the nematodes are not very pathogenic. In Europe, several other nematodes attack crucifer flea beetle (Morris, 1987). General predators such as lacewings (Neuroptera: Chrysopidae), soft-wing flower beetles (Coleoptera: Melyridae), and big-eyed bugs (Hemiptera: Lygaeidae), and omnivores such as field crickets (Orthoptera: Gryllidae), are occasionally observed feeding on adults, but their impact is unde termined (Gerber and Osgood, 1975; Burgess, 1980; Burgess and Hinks, 1987).
Life Cycle and Description. There is generally only one or possibly two generations annually, although development from egg to adult can be completed in just seven weeks. Overwintering occurs as an adult in soil and leaf litter. Fence rows, windbreaks, and other forms of shelter are thought to favor overwintering sites, although this does not seem to be active dispersal to such sites. Sometimes large aggregations of overwintering beetles are found; densities as high as two million beetles per hectare have been observed overwintering in a grove of trees (Burgess and Spurr, 1984; Turnock et al., 1987). Adults diapause under short day length (8 h) conditions, and readily survive five months of refrigeration. Peak emergence of overwintering adults was reported occurring by early May in Ontario (Kinoshita et al., 1979), with summer generation adults most abundant in late June and then again in late July. The two summer generations overlap considerably, and could easily be a protracted single generation. These authors also reported only a single summer generation during a cool summer.
20 cm of the soil surface (Vincent and Stewart, 1983). They will fly throughout the season, however, and in the earlier part of the year they are more likely to be found flying at greater heights (1-2 m) (Lamb, 1983).
The biology of crucifer flea beetle was given by Westdal and Romanow (1972), Burgess (1977), and Kinoshita et al. (1979). Rearing procedures were provided by Kinoshita et al. (1979). Evidence for a male aggregation pheromone is presented by Peng and Weiss (1992).
The adult injury is in the form of small holes in the foliage. They do not eat completely through the leaf, but leave the lower epidermis intact. However, the remaining tissue soon dies, dries, and falls from the plant, producing a hole. When beetles are abundant, all leaf tissue may be riddled with holes, resulting in drying of adjacent tissue and death of emerging seedlings. Severe injury typically occurs in spring when the weather is hot and dry. Defoliation of young seedlings and small broccoli transplants in Manitoba at levels of less than 10% caused stand reductions of 5% (Soroka and Pritchard, 1987). Older plants, while rarely killed, may suffer reduced growth. Crucifer species differ in susceptibility to attack and ability to tolerate defoliation (Bodnaryk and Lamb, 1991a). Beetles also may feed on the young florets of broccoli, greatly reducing yield.
The larvae feed on roots. Although root hairs are eaten, roots are sometimes scarred or contain burrows, reducing the marketability of root crops such as radish and rutabaga (Kinoshita et al., 1978).
The ability of crucifer flea beetle to transmit Xantho-monas campestris pv. campestris, the causal agent of black rot of crucifers, was investigated by Shelton and Hunter (1985). Although the potential to transmit this bacterium mechanically was demonstrated, there was little evidence that the beetle was an efficient or important vector of this disease.
Biological Control. Although some Phyllotreta spp. are susceptible to the steinernematid nematode Steinernema carpocapsae under laboratory conditions, this does not appear as a viable option for crucifer flea beetle management. Morris (1987) studied susceptibility in the field, and concluded that the beetle larvae may be too small for ready entry by infective juvenile nematodes.
Cultural Practices. Various cultural manipulations have been investigated for avoidance of crucifer flea beetles or their damage. In the studies conducted in Virginia, intercropping collards with nonhost plants resulted in lower beetle numbers and damage levels relative to collards monoculture, but collards yield was not increased owing to plant competition in polycultures (Latheef et al., 1984). Similar results were observed in North Dakota with canola-field pea intercrops (Weiss et al., 1994). However, in California, slight benefit was noted from similar polycultural systems (Gliessman and Altieri, 1982). Larger plot size tends to result in higher beetle densities owing to random dispersal from smaller plots (Kareiva, 1985). Dispersal rates are higher from dicultures than monocultures (Elmstrom et al., 1988).
Mustard, a highly preferred food plant, can be inter-planted or planted as a border to reduce beetle densities on other susceptible crops (Kloen and Altieri, 1990). Companion herbs, often suggested to confer benefit to adjacent crops, differ in their effect on cruci-fer flea beetle. Catnip, southernwood, tansy, and wormwood reduced beetle densities, whereas hyssop and santolina did not affect beetle numbers on collards (Latheef and Ortiz, 1984).
The role of weeds in flea beetle management is variable. Traditionally, weeds have been viewed as alternate hosts that favor survival of beetles when crops are not available. If the crop is not yet available as a food source, destruction of weeds can deprive flea beetles of food, leading to their demise. However, weeds also confer a benefit by attracting beetles from the crop plants, thereby reducing beetle density and damage to crops (Altieri and Gliessman, 1983), especially if the weeds are more preferred than crops. Reduced tillage practices, which leave more weeds, thereby may result in reduced flea beetle damage (Reed and Byers, 1981; Milbrath et al, 1995).
Early season plantings tend to have more severe flea beetle problems, so delayed planting is recommended if constraints such as season length and market conditions will allow this practice (Milbrath et al., 1995). Direct seeding of broccoli, rather than the use of transplants, necessitates the use of granular insecticides to protect the young seedlings (Soroka and Pritchard, 1987). Plants growing from small seeds are less tolerant to flea beetle defoliation than those growing from large seeds (Bodnaryk and Lamb, 1991b).
Crucifer cultivars differ in the level of waxy bloom present on the leaf surface. Low-wax or glossy lines offer promise for reduction in caterpillar numbers, owing both to less preferred oviposition by adults and lower survival of young larvae on glossy host plants. However, such glossy varieties tend to support higher numbers of flea beetles (Stoner, 1990, 1992). Wild crucifers, with high densities of leaf trichomes, are resistant to feeding by crucifer flea beetles, but this character has not yet been adequately incorporated into commercial cultivars (Palaniswamy and Bod-naryk, 1994).
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