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Persistence of a granulosis virus of Pieris rapae in soil
NOTES Persistence
of a Granulosis
Detailed population studies on the imported cabbageworm, Pieris rapae, have shown that a granulosis of the larvae is the principal ‘key factor” governing survival of the insect in eastern Ontario (D. G. Harcourt, Can. Entomologist, 98, 653-662, 1966). The disease is caused by a capsule virus (apparently Bergoldiavirus virulenturn) which manifests itself during the last three instars when the caterpillars congregate in the central parts of the plant to feed on the head and wrapper leaves. The dying larvae become flaccid and usually fall from the plant or lodge at the base of a wrapper leaf. After death, the body contents liquefy and the integument ruptures, allowing the virus particles to ooze onto the leaves and soil. Life tables have been compiled for 27 generations of the insect during the past 9 years. In long-term study plots at Merivale, now in their 15th year of cropping to cabbage and other crucifers, the incidence of virus was moderate in the first generation each year and increased steadily to epizootic levels in the second and third (Fig. 1). By contrast, populations of the insect at similar levels of density in three plots with no previous history of cole crops at Ottawa (Central Experimental Farm), and in another at Richmond, were unaffected by virus in the first generation. Mortality rates increased in succeeding generations but severe epizootics did not occur. These data suggest that the virus particles persist in the environment, perhaps in the soil, from one season to the next. Two experiments were recently carried out to shed light on this hypothesis. In the first test, small plantings of late cabbage, variety Penn State Ballhead, were made in the plots at Merivale and Ottawa at the end of June, 1966. A month later, 10 plants from each location were selected at ran-
Virus of Pi&s
rapm
in Soil
dom, potted and placed in a field insectary. The following day a third-instar larva from a healthy laboratory culture of P. rapae was placed on each plant. The larvae were observed twice daily and their condition noted for expression of symptoms. Table 1 shows that the mortality rate from virus in larvae on the plants from Merivale was considerably higher than on the plants from Ottawa, and that these values were very similar to those obtained in nature, as found in the life table studies. Nearly equal amounts of precipitation occurred at the two sites (Table 1) . This test was repeated in 1967 with similar results. In the second experiment, a sample of soil was collected in the fall of 1965 from the Merivale plots, the top inch of soil being taken from beneath the rows of the crops. The soil was boxed and placed in a darkened storage at 4°C. In December 1967, some 26 months later, the soil was removed from storage and a sample mixed with distilled water to form a thin slurry. No surfactant was added. After some settling had occurred, two potted cabbage plants were inverted and dipped in the supematant liquid. Ten third-instar larvae from a healthy laboratory culture were placed on each plant and observed twice daily in the greenhouse. Within 2 weeks, seven of the larvae on the treated plants developed granulosis and died in the fourth or fifth instar. No disease symptoms or signs appeared in a control group. Very little data have been published on the persistence of virus in soil. W. A. L. David and B. 0. C. Gardiner (J. Invertebrate Pathol., 9, 342-347, 1967) treated garden soil and sand with a suspension of crude granulosis virus and found that it was still virulent when extracted and fed to P. brassicae after 2 years of storage. R. P. Jaques (Can. Entomologist 99, 820-829, 1967) 142
143
NOTES
FIG.
1.
Mortality
of Pieris
rqxxc
from
granulosis
a.t three
TABLE INCIDENCE
OF VIRUS
Source of plants Merivale Ottawa
locations
No. of larvae dying
2.10 2.03
8 3
carried out similar tests with a polyhedrosis virus of Trichoplusia ni. Although transovarial transmission of the granulosis virus undoubtedly occurs (K. M. Smith and C. F. Rivers, Parasitology, 46, 235242, 1956; W. A. L. David, Proc. Assoc. Appl. Biol., 56, 331-334, 1965), as does spread by parasites (David, lot. cit.; J. M. Kelsey, 11th Intern. Congr. Entomol., Viennu, 1960,790-796,1962), the present data suggest that soil-borne virus particles play an important role in the initiation of epizootics in established garden crop areas. The virus particles apparently accumulate near the surface of the soil and are spread
eastern
Ontario,
1959-1967.
1
IN 20 LARVAE OF P. mpue ON POTTED CABBAGE PLANTS,
Rainfall in growth period ( inches )
in
PLACED 1966 Percent
In
pots 80 30
INDIVIDUALLY
mortality In
nature 85.1 26.2
to the outer Ieaves of the growing crop by cultivation as well as the action of wind and rain. Here, the virus is ingested by early-instar caterpillars during feeding; rapid dissemination occurs when the infected larvae crawl to the central parts of the plant after the second molt (Harcourt, lot. cit. ) . D. G. HARCOURT L. M. CASS Entomology Research Institute Canada Department of Agriculture Ottawa, Ontario, Canada Received February 27,1968
of a Granulosis
Detailed population studies on the imported cabbageworm, Pieris rapae, have shown that a granulosis of the larvae is the principal ‘key factor” governing survival of the insect in eastern Ontario (D. G. Harcourt, Can. Entomologist, 98, 653-662, 1966). The disease is caused by a capsule virus (apparently Bergoldiavirus virulenturn) which manifests itself during the last three instars when the caterpillars congregate in the central parts of the plant to feed on the head and wrapper leaves. The dying larvae become flaccid and usually fall from the plant or lodge at the base of a wrapper leaf. After death, the body contents liquefy and the integument ruptures, allowing the virus particles to ooze onto the leaves and soil. Life tables have been compiled for 27 generations of the insect during the past 9 years. In long-term study plots at Merivale, now in their 15th year of cropping to cabbage and other crucifers, the incidence of virus was moderate in the first generation each year and increased steadily to epizootic levels in the second and third (Fig. 1). By contrast, populations of the insect at similar levels of density in three plots with no previous history of cole crops at Ottawa (Central Experimental Farm), and in another at Richmond, were unaffected by virus in the first generation. Mortality rates increased in succeeding generations but severe epizootics did not occur. These data suggest that the virus particles persist in the environment, perhaps in the soil, from one season to the next. Two experiments were recently carried out to shed light on this hypothesis. In the first test, small plantings of late cabbage, variety Penn State Ballhead, were made in the plots at Merivale and Ottawa at the end of June, 1966. A month later, 10 plants from each location were selected at ran-
Virus of Pi&s
rapm
in Soil
dom, potted and placed in a field insectary. The following day a third-instar larva from a healthy laboratory culture of P. rapae was placed on each plant. The larvae were observed twice daily and their condition noted for expression of symptoms. Table 1 shows that the mortality rate from virus in larvae on the plants from Merivale was considerably higher than on the plants from Ottawa, and that these values were very similar to those obtained in nature, as found in the life table studies. Nearly equal amounts of precipitation occurred at the two sites (Table 1) . This test was repeated in 1967 with similar results. In the second experiment, a sample of soil was collected in the fall of 1965 from the Merivale plots, the top inch of soil being taken from beneath the rows of the crops. The soil was boxed and placed in a darkened storage at 4°C. In December 1967, some 26 months later, the soil was removed from storage and a sample mixed with distilled water to form a thin slurry. No surfactant was added. After some settling had occurred, two potted cabbage plants were inverted and dipped in the supematant liquid. Ten third-instar larvae from a healthy laboratory culture were placed on each plant and observed twice daily in the greenhouse. Within 2 weeks, seven of the larvae on the treated plants developed granulosis and died in the fourth or fifth instar. No disease symptoms or signs appeared in a control group. Very little data have been published on the persistence of virus in soil. W. A. L. David and B. 0. C. Gardiner (J. Invertebrate Pathol., 9, 342-347, 1967) treated garden soil and sand with a suspension of crude granulosis virus and found that it was still virulent when extracted and fed to P. brassicae after 2 years of storage. R. P. Jaques (Can. Entomologist 99, 820-829, 1967) 142
143
NOTES
FIG.
1.
Mortality
of Pieris
rqxxc
from
granulosis
a.t three
TABLE INCIDENCE
OF VIRUS
Source of plants Merivale Ottawa
locations
No. of larvae dying
2.10 2.03
8 3
carried out similar tests with a polyhedrosis virus of Trichoplusia ni. Although transovarial transmission of the granulosis virus undoubtedly occurs (K. M. Smith and C. F. Rivers, Parasitology, 46, 235242, 1956; W. A. L. David, Proc. Assoc. Appl. Biol., 56, 331-334, 1965), as does spread by parasites (David, lot. cit.; J. M. Kelsey, 11th Intern. Congr. Entomol., Viennu, 1960,790-796,1962), the present data suggest that soil-borne virus particles play an important role in the initiation of epizootics in established garden crop areas. The virus particles apparently accumulate near the surface of the soil and are spread
eastern
Ontario,
1959-1967.
1
IN 20 LARVAE OF P. mpue ON POTTED CABBAGE PLANTS,
Rainfall in growth period ( inches )
in
PLACED 1966 Percent
In
pots 80 30
INDIVIDUALLY
mortality In
nature 85.1 26.2
to the outer Ieaves of the growing crop by cultivation as well as the action of wind and rain. Here, the virus is ingested by early-instar caterpillars during feeding; rapid dissemination occurs when the infected larvae crawl to the central parts of the plant after the second molt (Harcourt, lot. cit. ) . D. G. HARCOURT L. M. CASS Entomology Research Institute Canada Department of Agriculture Ottawa, Ontario, Canada Received February 27,1968