Distribution. Imported cabbageworm is found principally in temperate regions of Europe, Africa, Asia, and North America, but is now firmly established in Australia and New Zealand, southern Mexico, and Hawaii. Imported cabbageworm was first observed in North America in 1860 at Quebec City, Canada. It spread to Massachusetts and New Jersey by 1869, reached Ohio in 1875, and by 1886 it was found in the Gulf Coast and Rocky Mountain states. P. rapae attained British Columbia in 1898 and the Pacific Coast by 1901. It is now widespread in North America. It is a strong flier and also is spread long distances annually by strong winds. Few cabbageworms reportedly survive the winter in most of Canada, but much of the country is invaded annually by dispersants from the United States or from southern Canada (Beirne, 1971).
Pieris rapae is easily confused with other common cabbage white butterflies: Pontia protodice, southern cabbageworm; Pieris napi, mustard white; band Ascia monuste (Linnaeus), southern white. Before introduction of imported cabbageworm, P. napi (Linnaeus) was the dominant cabbage butterfly in the north, and Pontia protodice (Boisduval and LeConte) was the principal cabbage-feeding butterfly in the south. Both have been largely replaced by P. rapae, though they sometimes co-occur on cultivated crucifers or on weeds. A key for the differentiation of these species is included in Appendix A.
Host Plants. Larvae of this insect feed widely on plants in the family Cruciferae, but occasionally on a few other plant families that contain mustard oils. Vegetable crops attacked include broccoli, Brussels sprouts, cabbage, cauliflower, collards, horseradish, kale, kohlrabi, mustard, radish, turnip, and watercress. Also attacked are flowers such as nasturtium and sweet alyssum, and weeds such as field pennycress, Thlaspi arvense; pepperweed, Lepidium spp.; wild mustard, Brassica kaber; horsehair mustard, Conringia orientalis; and yellow rocket, Barbarea vulgaris. Adults sip nectar from flowers, and are commonly seen at mustards, Brassica spp.; dandelion, Taraxacum officinale; aster, Aster spp.; purple heliotrope, Heliotropum spp.; thistle, Cirsium spp.; and both weedy and cultivated crucifers (Harcourt, 1963a). Sea kale is reported to be attractive for oviposition, but larvae fail to complete their development on this plant (Richards, 1940).
Natural Enemies. Imported cabbageworm is subject to numerous predators, parasitoids, and diseases. General predators such as shield bugs (Hemiptera: Pentatomidae), ambush bugs (Hemiptera: Phymati-dae), and vespid wasps (Hymenoptera: Vespidae) attack them, as do many insectivorous birds. Chittenden (1916a), for example, noted 90% predation of overwintering pupae by birds. However, parasitoids are considered to be much more important mortality factors. Harcourt (1963a) identified three important species in Ontario. Cotesia glomeratus (L.) (Hymenoptera: Braconidae) attacks the early instars, and emerges from the mature larva as it prepares to pupate. Phryxe vulgare (Fallon) (Diptera: Tachinidae) attacks mature larvae and emerges from the host pupa. Pteromalus puparum (L.) (Hymenoptera: Pteromalidae) attacks and kills cabbageworm pupae. Cotesia glomeratus has long been considered to be the most important parasi-toid in Canada and in the northern United States. Cote-sia glomeratus is readily observed in the field, searching diligently on foliage for larvae. Dead cabbageworm larvae are often found with clusters of 20-30, C. glomer-atus cocoons attached. However, in Europe, Cotesia rubecula (Marsall) (Hymenoptera: Braconidae) is the most important parasitoid, and in recent years it has become established in North America where it is now assuming a dominant role (Godin and Boivin, 1998b,c).
In contrast, tachinids were more important in California, particularly Madremyia saundersii (Williston) (Diptera: Tachinidae) (Oatman, 1966b). As observed in Canada, however, C. glomeratus was also a significant larval mortality factor in California. A low level of egg parasitism by Trichogramma pretiosum Riley (Hymenoptera: Trichogrammatidae) occurred, but failure of eggs to hatch, which was attributed to infertility caused by cool weather, resulted in more deaths than did egg parasitoids. The pupal parasitoid P. puparum was also effective in California, but this is not always the case. In some locations, relatively low levels of parasitism have been observed (Oatman and Platner, 1969; Ru and Workman, 1979; Lasota and Kok, 1986); high rates tend to occur late in the season.
Virus and fungal diseases of imported cabbage-worm have been reported, but the predominant natural disease in a granulosis virus (GV). Pieris rapae GV occurs most commonly under high density conditions, and often among late instar larvae after they have consumed the exterior foliage of plants and are forced into close contact. Harcourt (1963a) observed over 90% mortality of larvae due to natural occurrence of this disease. In the early stages of infection, larvae are inactive and paler in color. As the disease progresses, the caterpillar body turns yellow, and tends to appear bloated. After death, the body blackens, the integument ruptures, and the liquefied body contents ooze on the plant foliage. Rainfall has a major roll in assisting the spread of the virus on the plant, and from the soil to the plant. Beirne (1971) suggested that outbreak of granulosis virus in the second generation of cabbageworm often prevented the third annual generation from causing extensive damage.
Life Cycle and Description. The complete life cycle of this insect requires 3-6 weeks, depending on weather. Godin and Boivin (1998a) reported that about 320 degree-days above a threshold of 10°C was required to complete a generation in Quebec. The number of generations reported annually is two-to-three in Canada, three in the New England states, three-to-five in California, and six-to-eight in the south. Imported cabbageworm can be found throughout the year in the south.
Imported cabbageworm larva.
(2-5), 4.1 (3-6.5), and 7.8 (5-18) days, respectively. All larval stages except the first instar bear a narrow yellow line running along the center of the back; this stripe is sometimes incomplete on the early instars. A broken yellow line, or series of yellow spots, also occurs on each side. The mature lava typically wanders before pupation, spending an average of 1.8 days (range 1-4 days) without feeding, and finally spins a silk pad or platform, where pupation occurs. (See color figure 93.)
Adult male imported cabbageworm.
Adult male imported cabbageworm.
the crop to flowering weeds to feed. This movement tends to result in a preponderance of the eggs being deposited on the edges of crucifer fields unless there are flowering weeds contained within the crop. (See color figure 200.)
The biology of imported cabbageworm was given by Chittenden (1916a), Wilson et al. (1919), Richards (1940), and Harcourt (1962, 1963a). Culture was described by Webb and Shelton (1988).
The larvae defoliate crucifer crops, sometimes killing young plants. Severe damage to young plants often prevents head formation even when the caterpillars are later removed, so early season protection is important (Wilson et al., 1919). If left unchecked, cab-bageworm often can reduce mature plants to stems and large veins. Although they prefer leafy foliage, larvae may burrow into the heads of broccoli and cabbage, especially as they mature. Larvae are often immobile and difficult to dislodge, and they may be overlooked when cleaning produce. Larvae produce copious quantities of fecal material which also contaminate and stain produce.
Imported cabbageworm larvae are present, and potentially damaging, throughout most of the period that crucifer crops are under culture. In northern latitudes such as Wisconsin (Radcliffe and Chapman, 1966) and Ontario (Harcourt, 1963a) damage typically occurs during summer months, in southern California they are most abundant during the autumn (Oatman and Platner, 1969), and in southern latitudes such as South Carolina (Reid and Bare, 1952) and Louisiana (Smith and Brubaker, 1938), damage occurs during the autumn, early winter- and spring-month periods that coincide with peak crucifer production.
Biological Control. Several microbes have been investigated for control of imported cabbageworm, and have the potential to be developed as microbial insecticides. The imported cabbageworm granulosis virus (Pieris rapae GV) suppressed cabbageworm larvae in the laboratory (Payne et al., 1981) and in a field test, but required 4-10 days to inflict mortality, and was not superior to control provided by Bacillus thuringiensis (Jaques, 1973). The nuclear polyhedrosis virus (NPV) from alfalfa looper (Autographa californica NPV), a granulosis virus from cabbage butterfly (Pieris brassi-cae GV), and the microsporidian Vairimorpha necatrix were shown by Jaques (1977) and Tompkins et al. (1986) to suppress imported cabbageworm, but the population reduction was not superior to that achieved using Bacillus thuringiensis. These pathogens have commercial potential because they are not very host specific, and also suppress another important crucifer pest, cabbage looper, Trichoplusia ni (Hübner). Home gardeners sometimes collect dead or dying caterpillars, macerate them in water, and spray the resulting suspension onto foliage as a home-made biological insecticide.
Flower flies (Diptera: Syrphidae) consume the eggs and small larvae of imported cabbageworm and numerous other insects. Populations of flower flies have been manipulated to increase predation of cabba-geworm by interplanting cabbage with pollen-rich flowering plants. Although aphid populations on cabbage were suppressed, imported cabbageworm populations were not decreased significantly (White et al., 1995).
Host-Plant Resistance. Crucifer crops differ in their susceptibility to attack by imported cabbageworm. Chinese cabbage, turnip, mustard, rutabaga, and kale are less preferred than cabbage, collards, Brussels sprouts, broccoli, and cauliflower (Harrison and Bruba-ker, 1943; Radcliffe and Chapman, 1966). Some culti-vars of certain crops also have moderate levels of resistance to infestation by imported cabbageworm. One resistance character is due to, or correlated with, dark green, glossy leaves. This character imparts resistance to imported cabbageworm and other caterpillars, but increases susceptibility to flea beetle injury (Dickson and Eckenrode, 1980). The red color found in many crucifer varieties also affects imported cabbageworm. Cabbage butterflies avoid ovipositing on red cabbage varieties (Radcliffe and Chapman, 1966). However, larval survival is favored by red cabbage. Thus, while important genetic material has been identified, in most cases existing varieties are not a practical solution to caterpillar problems. Research conducted in Virginia on susceptibility of broccoli cultivars, and where pest abundance was relatively low, indicated that the principal factor in susceptibility to attack was the date of plant maturity; early maturing varieties were less infested (Vail et al., 1991).
Cultural Practices. Herbs are sometimes recommended as companionate or repellent plants for vegetable cultivation. Herbs are hypothesized to give off odors that repel ovipositing vegetable pests, or prevent them from locating the vegetables. In most cases this has not been investigated critically. However, for imported cabbageworm, there is ample evidence that herbs impart no benefit, and some herbs are associated with increased cabbageworm infestation (Latheef and Ortiz, 1983a,b). Paper caps early in the season, and row covers later, are effective in preventing oviposition by imported cabbageworm butterflies.
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