ENTOMOPHAGA, 22 (2~, 1977, 187-192
USE OF SPECIFICITY DIFFERENCE INDICES FOR THE IDENTIFICATION
OF NUCLEAR POLYHEDROSIS VIRUSES (BACULOVIRUS) OF INSECTS
A. BURGERJON
INRA, Station de Recherches de Lutte Biologique, La Minidre, 78000 Versailles, France
The specificity difference indices (SDI) are a quantitative comparison of
the degree of virulence shown by 2 viruses when assayed on 2 species of insect
hosts. The establishment of these indices permits the choice of better strains for
the development of preparations for use in the control of crop or forestry pests
with viruses. We have demonstrated a specificity difference index which is relatively high for
2 viruses of different origins but which are indistinguishable by serology. The
SDI thus can be used as a supplementary method for the identification of virus
samples particularly those useful ir~ mass culturing and biological control. In
this area, the identification of the viruses is very important and bioassays, neces-
sary for the establishment of the SDI, are already an ongoing activity for other
purposes.
It was again shown that the viruses isolated from other species have a lower
virulence than those isolated from the homologous insect species.
The diagnosis of diseases and identification of pathogens are very important
factors for the research on the microbial control of insects.
The standardisation of preparations begins with a precise identification of the
pathogen. This is illustrated by the ease of Bacillus thuringiensis. A precise diagnosis
is important as stressed by the objective of the recent symposium of the" Commission
de Pathologie des Insectes et de Lutte Microbiologique " of the O.1.L.B./S.R.O.P.,
on 21-22 March 1975 at St Christol les Al+s, France. The subject of the symposium
was" Methods in diagnosis of insects diseases in view of their utilisation in microbial
control "
In the case of nuclear polyhedrosis (Baculovirus), the inclusion bodies are easily
recognised using the light microscope. The identification of different samples, however,
requires more complex methods, as for example, histology, histochemistry, serology,
electron microscopy and tissue culture, taking into consideration their advantages and
limitations.
For a virus which is highly specific, the designation of the original host or the
result of a bioassay may be sufficient for using it in biological control and for une-
quivocal characterisation and identification. However, some nuclear polyhedrosis
viruses attack several hosts and alterrtatively the same host can be affected by different
viruses. The identification of virus samples has thus become a current preoccupation
in the work on microbiological control.
If the spectrum of oiological activity proves to be a stable characteristic of the
different viruses (which would, in fact, be desirable and necessary for biological control),
the determination of activity spectra of Baculovirus would be an important diagnostic

188 A. BURGERJON
tool to be used together with other diagnostic procedures. This is showrt by the
example presented in this paper where 2 serologically and morphologically indistin-
guishable Baculovirus are differerttiated by their activity spectra, based on the Speci-
ficity Difference Indices (SDI) which were previously defined (BURGE~JON et aL, 1975).
MATERIALS AND METHODS
Two samples of Baculovirus have heen examined: one of American origin
(Dr VAIL) isolated from Aut~grafa californica SPEYER and the other of Russian origin
(Dr GZRSHENSON) isolated from Galleria mellonella L. Despite the great difference in
geographical and biological origin of these viruses, CROIZIER & MEYNADIER (1975)
were not able to distinguish them by serological analysis, even though this method
gives positive results for the characterisation f other Baculovirus. Both these viruses
possess cubic polyhedra (VAIL et al., 1970; GERSHENSON, 1957; VAGO et al., 1970) which
is rare for this class of viruses. VAIL et al. (1970) and GERSHENSON (1957) have demon-
trated that the 2 viruses are virulent for several Lepidoptera, such as: Spodoptera exigua HB.,
Trichoplusia ni HB. (Noctuidae), Plutella maculipennis CURT. (Plutellidae), Estigmene
acrea DRURY (Arctiidae), Bucculatrix thurberiella BUSCK L. (Lyonetiidae) for the virus
from Autographa californica and Vanessa urticae L. (Nymphalidae), Pyrameis cardui L.
(Nymphalidae), Achroia grisella F. (Pyralidae) for the virus from Galleria mellonella.
We have tested the virus sample from Galleria mellonella (GM) as an aqueous
suspension of purified polyhedra described by CROIZIER & MEYNADIER (1975) and
used for their serological analysis. The virus sample from Autographa calfornica
(ACSE) was tested as a powder as previously used by BURGVRJON et al. (1975).
Another preparation of nuclear polyhedrosis virus from Mamestra brassicae L. (M.B.)
was used. for comparison. This virus however cart be distinguished from the two
others by serological analysis (CRoIZXER & MEYNADIER, 1975).
The bioassays were carried out with newly hatched caterpillars fed during the 1st
larval instar on a simple artificial medium described by PO~TOUT & BuYs (1970). A
plastic dish (41 mm diam. and 11 mm high) contained 2ml medium. The diet was
inoculated in a treatment ower apparatus (BuRGERJON, 1956) SO that the fine layer
of 1,5 mm medium was evenly sprayed with the suspension of polyhedra. Spraying
of 10 ml in the treatment apparatus results in a deposition of 0,005 ml of suspension
per cm 2 of medium. The doses shown along the abcissa of the graphs are expressed
as number of polyhedra per ram2 of medium. The following species of Lepidoptera
were examined : Mamestra brassicae, Spodoptera exigua (Noetuidae), Chilo suppres-
salis WLK. (Pyralidae) and Plutella maculipennis (Tineidae).
For P. maculipennis discs 12 cmz of cabbage leaf were sprayed instead of the arti-
ficial medium. The number of caterpillars used per dose were 50 for P. maculipennis,
40 for C. suppressalis, 50 for S. exigua and 90 for the tests with M. brassicae.
RESULTS
Two preliminary bioassays carried out on P. maculipennis and C. suppressalis
indicated that the GM preparation was more infectious than the ACSE preparation
(table 1), but there was no tendency towards a specifcity difference. A bioassay using
the same preparations plus the MB preparation carried out with three doses on S. exigua
(fig. 1) demonstrated the superiority of GM over ACSE, whereas MB is inferior to
both.